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We thank Dr. Yurkin for his considered and constructive critique of our paper. We respond to his insightful comments below and hope that this discussion motivates further communication and cooperation and benefits to the scattering by small particles community. Key Points: Fenni et al. (2021, https://doi.org/10.1029/2020jd034172) main outcome is the enhancement in accuracy and computational cost of MIDAS with use of higher‐order quadrature schemesMajor conclusions of Fenni et al. (2021, https://doi.org/10.1029/2020jd034172) are validated on a large set of realistic arbitrarily‐shaped ice aggregates [ABSTRACT FROM AUTHOR]

The Stratospheric Total Aerosol Counter (STAC) is a lightweight balloon‐borne instrument that utilizes condensational growth techniques to measure the total aerosol concentration. STAC is a miniaturized version of the legacy Wyoming condensation particle counter that operated from 1974 through 2020 in the middle latitudes and polar regions, with a few measurements in the tropics. Here we provide a description of the STAC instrument and the total aerosol measurement record, demonstrating that typical total aerosol profiles exhibit a peak in number mixing ratio, with values between 800 and 2,000 particles per mg of air (mg−1), just below the lapse rate tropopause (LRT). In the tropics and middle latitudes, mixing ratios decrease above the LRT likely due to coagulation and scavenging that results in a transfer of mass to the fewer but larger aerosol particles of the Junge layer. Exceptions to this occur in the spring time in the middle latitudes where a new particle layer between 20 and 25 km is frequently observed. In the poles, total aerosol profiles exhibit two distinct features: new particle formation in austral spring, and an increasing mixing ratio above 17 km likely due to the presence of meteoric smoke that has been concentrated within the polar vortex. High observed stratospheric particle mixing ratios, in excess of 2,000 mg−1, are observed in the polar new particle layer and at the top of polar profiles. Plain Language Summary: Particles in the stratosphere range in size from a few nanometers to many micrometers in diameter. These particles are important to the radiation balance of Earth as well as chemical processes that occur in the atmosphere. Particles with diameters greater than about 50 nm are detectable by a wide variety of scientific instruments using various techniques to measure light scattered by the particles. Particles smaller than this need to be grown to larger diameters before their concentration can be measured, and therefore these measurements must be done in situ. This paper describes the design and capabilities of an instrument, the Stratospheric Total Aerosol Counter, that makes in situ measurements from high‐altitude balloons of the total aerosol concentration. Versions of this instrument have been flown around the globe since 1974. The second portion of this paper provides an overview of the total aerosol concentration record since 1989, comprising more than 250 profiles. The key findings are that in tropical regions there is a significant source of particles from the total aerosol concentration maximum at the top of the troposphere. In polar regions, a significant source of particles is from meteorites burning up when they strike the Earth's atmosphere. Key Points: The hardware and operation of a balloon‐borne Stratospheric Total Aerosol Counter (STAC) is detailed for measurements in the lower and middle stratosphereSome of the highest total aerosol mixing ratios are found in the polar mid‐stratosphere suggesting an under appreciation of this region's importanceVolcanic injections of sulfur have minimal impact on the total aerosol concentration, but significantly increase the mass of larger aerosol [ABSTRACT FROM AUTHOR]

This paper introduces the special collection in Geophysical Research Letters and Journal of Geophysical Research: Atmospheres on the exceptional stratospheric polar vortex in 2019/2020. Papers in this collection show that the 2019/2020 stratospheric polar vortex was the strongest, most persistent, and coldest on record in the Arctic. The unprecedented Arctic chemical processing and ozone loss in spring 2020 have been studied using numerous satellite and ground‐based data sets and chemistry‐transport models. Quantitative estimates of chemical loss are broadly consistent among the studies and show profile loss of about the same magnitude as in the Arctic in 2011, but with most loss at lower altitudes; column loss was comparable to or larger than that in 2011. Several papers show evidence of dynamical coupling from the mesosphere down to the surface. Studies of tropospheric influence and impacts link the exceptionally strong vortex to reflection of upward propagating waves and show coupling to tropospheric anomalies, including extreme heat, precipitation, windstorms, and marine cold air outbreaks. Predictability of the exceptional stratospheric polar vortex in 2019/2020 and related predictability of surface conditions are explored. The exceptionally strong stratospheric polar vortex in 2019/2020 highlights the extreme interannual variability in the Arctic winter/spring stratosphere and the far‐reaching consequences of such extremes. Plain Language Summary: The Arctic stratospheric polar vortex—a band of strong winds roughly encircling the pole at about 65°N latitude from about 15 to 50 km above the Earth's surface that forms every winter—was exceptionally strong during the 2019/2020 winter. The strong vortex in the stratosphere was linked to unusual conditions at both higher and lower altitudes. This collection of papers explores the far‐reaching consequences of the exceptionally strong stratospheric polar vortex in 2019/2020, including impacts on Arctic chemical ozone loss and on surface weather conditions. Chemical ozone loss in spring 2020 matched or exceeded the most previously on record (for 2011) and showed some features similar to the larger loss that occurs over the Antarctic every spring. The exceptionally strong stratospheric polar vortex was linked to weather extremes, including record heat, unusual patterns of precipitation, marine cold air outbreaks, and windstorms. Key Points: The stratospheric polar vortex in 2019/2020 was the strongest and longest‐lasting on record as described in this special collectionThis exceptionally strong and cold polar vortex led to unprecedented Arctic ozone loss, approaching that in some Antarctic wintersCirculation anomalies linked to the vortex spanned the mesosphere to the surface with implications for extreme weather and predictability [ABSTRACT FROM AUTHOR]

The Polar Mesospheric Cloud (PMC) Turbulence experiment performed optical imaging and Rayleigh lidar PMC profiling during a 6‐day flight in July 2018. A mosaic of seven imagers provided sensitivity to spatial scales from ∼20 m to 100 km at a ∼2‐s cadence. Lidar backscatter measurements provided PMC brightness profiles and enabled definition of vertical displacements of larger‐scale gravity waves (GWs) and smaller‐scale instabilities of various types. These measurements captured an interval of strong, widespread Kelvin‐Helmholtz instabilities (KHI) occurring over northeastern Canada on July 12, 2018 during a period of significant GW activity. This paper addresses the evolution of the KHI field and the characteristics and roles of secondary instabilities within the KHI. Results include the imaging of secondary KHI in the middle atmosphere and multiple examples of KHI "tube and knot" (T&K) dynamics where two or more KH billows interact. Such dynamics have been identified clearly only once in the atmosphere previously. Results reveal that KHI T&K arise earlier and evolve more quickly than secondary instabilities of uniform KH billows. A companion paper by Fritts et al. (2022), https://doi.org/10.1029/2021JD035834 reveals that they also induce significantly larger energy dissipation rates than secondary instabilities of individual KH billows. The expected widespread occurrence of KHI T&K events may have important implications for enhanced turbulence and mixing influencing atmospheric structure and variability. Key Points: First observation of unambiguous secondary Kelvin‐Helmholtz instabilities in high‐resolution images of the polar mesospheric cloud layerIdentification and quantification of Kelvin‐Helmholtz billow interactions leading to tubes and knots and accelerated billow breakdownEstimation of turbulence Reynolds number Returb ∼ 5,000 and νturb ∼ 3 times larger than the kinematic viscosity [ABSTRACT FROM AUTHOR]

In a recent paper Fenni et al. (2021, https://doi.org/10.1029/2020jd034172) compared the code MIDAS, based on the direct solution of the volume‐integral equation combined with advanced cubatures for orientation averaging, to the code DDSCAT, a state‐of‐the‐art implementation of the discrete dipole approximation. This comment highlights methodological issues in this comparison and shows that the quantitative claims of Fenni et al. (2021, https://doi.org/10.1029/2020jd034172), related to superiority of MIDAS over DDSCAT, are based on very specific test cases with respect to particle symmetries or initial orientation, as well as to the selected scattering quantity of interest. Thus, these claims are not expected to hold for other similar particles. Moreover, the detailed discussion of these issues is relevant for all light‐scattering simulation methods, except those allowing analytical orientation averaging. Thus, the comment constructs general guidelines for fair evaluation of orientation‐averaging techniques in a wide range of light‐scattering methods and computer codes. Plain Language Summary: The paper discusses several issues that appear when one is comparing different orientation‐averaging techniques (cubatures) in combination with the same or different light‐scattering simulation methods. Fair evaluation of cubature performance in realistic general scenarios is important both for practitioners (to choose the most efficient combination of the existing codes and cubatures) and for code developers (to set their priorities on the new features with the largest expected benefits). Unfortunately, the performance of the cubatures is complexly interwoven with the internals of the simulation methods and depends on specific test particles and computed scattering quantities. This questions the generality of conclusions in some previous publications. Based on this discussion, the paper ends with general guidelines for fair evaluation of cubatures, allowing future studies to arrive at general conclusions, so that they can be directly used by other researchers. Key Points: Quantitative conclusions of Fenni et al. (2021) are based on very specific test casesOrientation‐averaging techniques should be compared on non‐symmetric particles, and not with a special initial orientationAny comparison of simulated results should consider their uncertainties accounting for all sources of errors [ABSTRACT FROM AUTHOR]

The stepwise development of positive lightning leaders is still not well understood. A recent laboratory study indicated, at high absolute humidity, positive leaders can do steps due to the merging of a separate luminous structure and the primary leader channel, similar to the steps of negative leaders. The humidity may play a key role in the formation of positive leader steps, however, the humidity effect on the positive leader steps has never been explored. In this paper, we examine numerous positive long spark discharges at different humidity levels with the synchronized discharge current and high‐speed camera frames recording the evolution of leader channel. The positive leader propagation manners at different humidity levels are compared both morphologically and electrically. The effect of humidity on steps is further analyzed statistically. We found that the positive leader steps characterized by steep‐rise current pulse and abrupt channel elongation, which may be led by separate luminous structures, only appear under the condition that high absolute humidity is above a certain threshold. As the ambient humidity increases, these positive leader steps occur more frequently. Plain Language Summary: Lightning is driven by the propagation of positively and negatively charged, thermally‐ionized plasma channels, known as positive and negative leaders. Leaders propagate in a continuous or stepwise manner, characterized by intense reilluminations and abrupt elongations. The physics underlying the positive leaders is still mysterious. Previously, it was commonly accepted that the physics underlying the positive and negative leader steps differed. Recently, researchers found that, at least at high absolute humidity, positive leaders can form steps due to the merging of a separate luminous structure and the primary leader channel, similar to the steps of negative leaders. It seems that humidity plays an important role in the formation of positive leader steps, but the humidity effect on the positive leader steps has never been explored. In this paper, we reported the effect of humidity on the steep‐rise type steps. We found that the positive leader steps characterized by steep‐rise current pulse and abrupt channel elongation, which may be led by separate luminous structures, only appear under the condition that high absolute humidity is above a certain threshold. As the ambient humidity increases, these positive leader steps occur more frequently. Key Points: Humidity plays a key role in the formation of positive leader stepsThese steps only occur under high humidity above a certain thresholdAs the ambient humidity increases, these positive leader steps occur more frequently [ABSTRACT FROM AUTHOR]

In this paper, we examine differences in cloud adjustments (often called rapid adjustments) that occur as a direct result of abruptly increasing the solar constant by 4% or abruptly quadrupling of atmospheric CO2. In doing so, we devise a novel method for calculating the cloud adjustments for the abrupt solar forcing simulations that uses differences between coupled model simulations with abrupt solar and CO2 forcing, in combination with uncoupled, atmosphere‐only, abrupt CO2 forced experiments that have prescribed sea‐surface temperature. Our main findings are that (a) there are substantial differences in the responses of stratocumulus and cumulus clouds to solar and CO2 forcing, which follow the differences in the direct radiative effect that solar and CO2 forcing have at cloud top, and (b) there are differences in the adjustment of the average optical depth of high clouds to solar and CO2 forcing that we speculate are driven by the differences in the vertical profile of radiative heating and differences in the pattern of sea‐surface temperature change (for a fixed global mean temperature). These cloud adjustments contribute significantly to the total net cloud radiative effect, even after 150 years of simulation. Plain Language Summary: In climate change, clouds change due to a variety of mechanisms including surface temperature, dynamical circulations, and radiative forcing. In this paper, we examine the latter: how clouds respond to radiative forcing. We study this topic using climate model simulations where the brightness of the sun is abruptly increased by 4% and compare those with simulations where CO2 concentration is abruptly quadrupled. In doing so we find that, there are differences in the cloud response to changes in solar and CO2 forcing which include the occurrence of thick and thin high cloud, as well as the amount and height of low and mid‐level clouds. Key Points: Increasing CO2 causes a reduction and lowering of mid‐level and low‐level clouds which does not occur from solar forcingThere is large reduction in optically thin high clouds from solar forcing, especially as compared with CO2Even after 150 years adjustments make a significant contribution to the total net cloud radiative effect [ABSTRACT FROM AUTHOR]

There are many uncertainties in future climate, including how the Earth may react to different types of radiative forcing, such as CO2, aerosols, and even geoengineered changes in the amount of sunlight absorbed by Earth's surface. Here, we analyze model simulations where the climate system is subjected to an abrupt change of the solar constant by ±4%, and where the atmospheric CO2 concentration is abruptly changed to quadruple and half its preindustrial value. Using these experiments, we examine how clouds respond to changes in solar forcing, compared to CO2, and feedback on global surface temperature. The total cloud response can be decomposed into those responses driven by changes in global surface temperature, called the temperature mediated cloud feedbacks, and responses driven directly by the forcing that are independent of the global surface temperature. In this paper, we study the temperature mediated cloud changes to answer two primary questions: (a) How do temperature mediated cloud feedbacks differ in response to abrupt changes in CO2 and solar forcing? And (b) Are there symmetrical (equal and opposite) temperature mediated cloud feedbacks during global warming and global cooling? We find that temperature mediated cloud feedbacks are similar in response to increasing solar and increasing CO2 forcing, and we provide a short review of recent literature regarding the physical mechanisms responsible for these feedbacks. We also find that cloud responses to warming and cooling are not symmetric, due largely to non‐linearity introduced by phase changes in mid‐to‐high latitude low clouds and sea ice loss/formation. Plain Language Summary: As the global mean temperature changes, there are changes in cloud amount, location, and thickness, which can all impact the radiative balance of the Earth. Cloud changes driven directly by global temperature change are called temperature mediated cloud feedbacks. In this paper, we study the temperature mediated cloud feedbacks that occur in model simulations where the amount of sunlight incident upon the Earth is increased or decreased abruptly, and then held constant for 150 years. We compare the cloud changes in these experiments with experiments where the CO2 concentration is similarly increased or decreased abruptly and held constant for 150 years. In doing so we find that the temperature mediated cloud feedbacks following abrupt changes in solar radiation are characteristically similar to those occurring following CO2 increase. There are however substantial differences in the temperature mediated cloud feedbacks that occur while the climate is warming versus cooling. Key Points: The temperature mediated cloud changes and feedbacks incurred by changes in solar and CO2 forcing are similarOptical depth changes at high latitudes produce substantial differences in cloud feedbacks in cooling and warming experimentsLikewise, tropical circulations respond differently in models to cooling and warming, with a stronger change in the Walker circulation in warming experiments [ABSTRACT FROM AUTHOR]

The divergence method, a lightweight approach for estimating emission fluxes from satellite images, rests on a few implicit assumptions. This paper explicitly outlines these assumptions by deriving the method from first principles. The assumptions are: the enhanced mass flux is dominated by advection, normal fluxes vanish at the top and bottom of the atmosphere, steady‐state conditions apply, sources are multiplications of temporal and spatial functions, sinks are described as first‐order reactions, and effective wind fields are concentration‐weighted wind fields. No such assumptions have to be made for the background field. A "topography correction term" does not follow from the theory, but is rather shown to be a practical correction for topography‐dependent effective wind speed errors. The cross‐sectional flux method follows naturally from the derived theory, and the methods are compared. Effects of discrete pixels and finite‐difference operations are explored, leading to recommendations, primarily the recommendation to integrate over small regions only to minimize the influence of noise. Numerical examples featuring Gaussian plumes and COSMO‐GHG simulated plumes are provided. The Gaussian plume example suggests that the divergence method might underestimate emissions when assuming only advection in the presence of cross‐wind diffusion. Conversely, the cross‐sectional flux method remains unaffected, provided fluxes are integrated across the entire plume. The COSMO‐GHG example reveals frequent violations of the steady‐state assumption, although the assumption remains valid proximal to the source (<20 km in this example). It is the hope that this paper provides a solid theoretical foundation for the divergence and cross‐sectional flux methods. Plain Language Summary: Power plants, megacities, and other regions can be places where considerable amounts of gases are emitted into the atmosphere. Satellite data is capable of recording the enhanced concentrations in the atmosphere due to these sources. How does one estimate emissions of these sources based on this data? Typically by running an atmospheric inverse model; but one computationally lightweight method gaining popularity is the "divergence method." With this method, one applies a simple mathematical operation to the satellite image multiplied by the horizontal wind speeds—the result of which is related to the emissions. The method has been used before, but here we systematically state the assumptions involved; factors like how gases are transported horizontally and over time, and what happens when we apply the method on pixelated data (such as satellite images). We find that some other analyses in the literature may be partially incorrect. Another popular method (the "cross‐sectional flux") is easily derived from the same theory. Synthetic examples are used to illustrate that various assumptions are not met in realistic data; although the examples also show that one can work around these limitations by a careful application of the methods, and by acquiring more estimates over many days. Key Points: The divergence method for estimating emissions from satellite images is derived from first principlesAssumptions made on the way are made explicit, highlighting discrepancies with existing literatureA comparison is made against the cross‐sectional flux method which follows from the same theory [ABSTRACT FROM AUTHOR]

The paper presents a new method for the decomposition of the horizontal wind divergence among linear waves on the sphere: inertia‐gravity (IG), mixed Rossby‐gravity (MRG), Kelvin and Rossby waves. The work is motivated by the need to quantify the vertical velocity and momentum fluxes in the tropics where the distinction between the Rossby and gravity regime, present in the extratropics, becomes obliterated. The method leads to divergence power spectra as a function of latitude and pressure. The spectra follow the same power laws as the vertical kinetic energy spectra for different wave types. The key novel aspect of the work is the coexistence of wave types at the same zonal wavenumbers. Applied to ERA5 data in August 2018, the new method reveals that the zonally‐integrated Kelvin wave divergence makes up about 19% of the upper‐troposphere divergence within 10°S–10°N. The MRG wave divergence has four times smaller magnitude. The relative roles of the two waves vary with scale. Overall small roles of the Kelvin and MRG waves in the tropical divergence is explained by decomposing their kinetic energies into rotational and divergent parts. The beta effects produces less then 5% of the tropospheric divergence associated with Rossby waves. The majority of divergence belongs to IG modes and is nearly equipartitioned between the eastward‐ and westward‐propagating IG modes in the upper troposphere, whereas stratospheric partitioning depends on the background flow. Plain Language Summary: The horizontal circulation is commonly discussed using divergence and vorticity instead of the two wind components. The advantage is that vorticity and divergence characterize the Rossby wave and inertia‐gravity (IG) wave dynamics, respectively, thereby offering an easier understanding of wave processes. The two wave regimes are well separated in extratropics also in the wavenumber and frequency space. However, there is no regime separation in the tropics where IG waves shape variability at all scales including low‐frequency variability. Furthermore, two additional waves are present, Kelvin waves and mixed Rossby‐gravity (MRG) waves that span the wavenumber and frequency space between Rossby and IG waves. This study aims at a better understanding of partitioning between divergent and rotational components of equatorial waves. In particular, Kelvin and MRG waves have significant variances at the same scales as Rossby and IG waves calling for an approach that quantifies their divergence at the same wavenumbers. This paper develops such an approach for the spherical atmosphere and demonstrates it on the ERA5 data in August 2018. One important result is that the Kelvin and MRG wave constitute up to approximately 25% of divergence in the tropical upper troposphere, mainly in Kelvin waves. Key Points: A new method couples divergence with wave circulation as a function of latitude and pressure levelDivergence quantification accounts for wave types sharing the same zonal scale, which is crucial for disentangling tropical flowsApplied to ERA5, the new method highlights vortical nature of mixed Rossby‐gravity waves and large‐scale Kelvin waves [ABSTRACT FROM AUTHOR]

The Western Pacific jet stream (WPJS) is an essential part of atmospheric circulation in winter, which significantly influences the weather and climate of the North Pacific and North America. In this paper, the characteristics and mechanism of WPJS subseasonal variation in winter are investigated. The upstream atmospheric disturbances in the East Asian polar‐front jet and subtropical jet merge over the Northwestern Pacific to form the subseasonal variability in WPJS, which has a significant period of 40–60 days. During the positive phase events of subseasonal WPJS, the convergence position of the upstream atmospheric disturbances shifts southwardly accompanied with the local enhancement and eastward extension of subseasonal WPJS. On the other hand, the subseasonal WPJS divides into the southern and northern westerly branches during the negative phase events. By the horizontal propagation of local Eliassen‐Palm fluxes in the upper atmosphere, the northward drift of the upstream atmospheric disturbances convergence dominates the delayed acceleration of the northern upper westerly branch. However, the intensification of atmospheric baroclinicity and upward baroclinic energy caused by the leading strong subtropical frontal zone determine the acceleration of the southern upper westerly branch. Plain Language Summary: As an important part of the atmospheric circulation in the Northern Hemisphere, the Western Pacific jet stream (WPJS) has significant impacts on the climate and weather of the North Pacific and North America. However, previous studies more focused on the interannual or synoptic variability of WPJS. The characteristics and mechanism of the subseasonal variability of WPJS have not been appropriately explained, which are very important for the persistent disastrous weather. In this paper, the subseasonal variability characteristics and mechanism of WPJS in winter are investigated by using the daily average reanalysis data from year 1979 to 2022. About 130 WPJS persistent positive and negative phase events are detected, each of which can sustain more than 20 days. Further results show that the subseasonal variability of WPJS is determined by the synergistic effects of upstream atmospheric disturbances and subtropical frontal zone (STFZ). With the southward (northward) convergence of the strong upstream atmospheric disturbances and weaker (stronger) STFZ, the lagging positive (negative) subseasonal phase events of WPJS is more likely to occur and persist. This study is beneficial to the weather and climate forecasts of North Pacific and North America. Key Points: About 130 subseasonal events of winter Western Pacific jet stream (WPJS) are detected from year 1979–2022, each of which persists more than 20 daysWPJS shows local enhancement and extension during the positive subseasonal phase, but meridional separation in the negative oneThe synergistic effects of the upstream atmospheric disturbances and oceanic subtropical front on the subseasonal WPJS are revealed [ABSTRACT FROM AUTHOR]

Coherent structure is an important phenomenon in the fluid field, and also exists in atmospheric boundary layer. For example, after the passage of a cold front in spring in northern China, there is a rather regular gusty wind with a period of approximately 3 min superimposed on the basic strong wind, and the gusty wind possesses a coherent structure: the vertical velocity is upward when horizontal velocity is in the valley phase, but downward when horizontal velocity is in the peak phase. The coherence makes transport of momentum and matter more effective. It was later found that this gusty coherent structure is a phenomenon of mechanical turbulence, which occurs in weakly stable, neutral and unstable stratification. However, research on the physical mechanism of this coherent structure is still lacking. This paper shows that the gusty coherent structure can be explained by quasi‐streamwise vortex pairs and high and low speed streaks induced by quasi‐streamwise vortexes. The coherent structure can be obtained from the three‐dimensional Navier‒Stokes (N‒S) equations. The maximum intensities of vortices and streaks are located at 20.7% and 12.1% of the boundary layer heights respectively, which are consistent with the observed results at 200 and 120 m. Plain Language Summary: After the passage of a cold front in spring in northern China, there is a rather regular gusty wind superimposed on the basic strong wind, and its horizontal and vertical component are negatively correlated and make downward transport of momentum and matter more effective. It was later found that this gusty coherent structure is a phenomenon of mechanical turbulence. This paper points out the possible physical phenomenon underlying this negative coherence, gives a dynamic explanation of this phenomenon by the hydrodynamic stability method, and compares the solutions with the observation results. Key Points: The gusty coherent structure can be explained by quasi‐streamwise vortex pairs and high and low speed streaksThe physical model of the gusty coherent structure can be obtained by the hydrodynamic stability analysis methodThe position of maximum intensity of vortexes and streaks obtained from the solution is consistent with the observation [ABSTRACT FROM AUTHOR]

The expression "lightning never strikes twice" is questioned in this paper because, among the randomness of lightning impacts, some spots are hit even more than twice year after year. This article introduces the recurrent lightning spots (RLS) concept, which are locations periodically impacted by cloud‐to‐ground lightning every consecutive year over a certain period. RLS are investigated in two regimes, with markedly different lightning climatology but similar orography, for 10 consecutive years: Catalonia (North East of Spain, Europe) and Barrancabermeja (North Central Colombia, South America). Results revealed 148 and 916 RLS in Catalonia and Barrancabermeja, respectively. RLS in both regions are typically found to be related to tall structures, mountain peaks, and steep terrain. The method allowed us to identify those tall towers and orographic relief frequently affected by lightning that are not detected with the mere computation of the ground flash density. In the case of Catalonia, some RLS are found offshore. Besides the scientific interest in understanding lightning, the new concept of RLS provides additional and valuable information applicable to lightning protection engineering. Plain Language Summary: The expression "lightning never strikes twice" is questioned in this paper because it shows that some spots are hit even more than twice: year after year. The RLSs (RLS) concept is presented, which are locations where lightning strike them periodically. The two regions of study with markedly different lightning climatology but similar orography are Catalonia (North East of Spain, Europe) and Barrancabermeja (in North Central Colombia, South America). RLS in both regions are typically found to be related to tall structures, mountain peaks and steep terrain. In the case of Catalonia, some RLS are found offshore. RLS new concept provides valuable information to lightning protection engineering. Key Points: Recurrent lightning spots (RLS) are targets with at least one cloud‐to‐ground lightning per year during a certain number of consecutive yearsIn Catalonia (Spain), 13% of the RLS are tall towers, 72% are mountainous peaks and 5% are offshore sitesThe method effectively finds tall structures affected by lightning in tropical climates [ABSTRACT FROM AUTHOR]

Wildfire‐related aircraft field campaigns frequently offer opportunities to validate remote‐sensing retrievals of aerosol properties and other quantities derived from satellite‐borne‐instrument observations. Satellite instruments often provide regional context‐imagery for more sparsely sampled aircraft and surface‐based measurements. However, aerosol amount, particle type, aerosol plume height and the associated wind vector products retrieved from the NASA Earth Observing System's Multi‐angle Imaging SpectroRadiometer (MISR) instrument have matured sufficiently that these quantities can also contribute substantially to a campaign data set, in regional context. This is especially useful when such measurements are not acquired at all from the suborbital platforms. During NOAA's California Fire Dynamics Experiment (CalFiDE), aircraft operations were coordinated with MISR overpasses on two occasions: for the Rum Creek fire on 30 August 2022, and for the Mosquito fire on 08 September. MISR‐retrieved aerosol properties show distinctly different patterns of black and brown smoke particle distributions and inferred plume evolution in the two cases. This paper presents the satellite‐retrieved results that complement the field observations, demonstrating what such measurements can offer, and contributing material for detailed fire dynamics and chemistry studies when combined with the CalFiDE suborbital observations and models in continuing studies. Plain Language Summary: A common use of aircraft field campaigns is to validate the radiances measured by space‐based instruments and the geophysical quantities derived from the satellite observations. However, satellite aerosol amount and properties derived from the NASA Earth Observing System's Multi‐angle Imaging SpectroRadiometer (MISR) instrument are sufficiently mature that they can also contribute directly to field‐campaign data sets. During NOAA's CalFiDE campaign is summer 2022, on two occasions the aircraft observed wildfire smoke plumes coordinated with MISR overpasses: for the Rum Creek fire on 30 August 2022, and for the Mosquito fire on 08 September. In addition to providing broad spatial context to the much more spatially limited aircraft measurements, the MISR results offer geometrically‐derived smoke‐plume height and plume‐level motion vectors from which smoke age can be estimated downwind along the plume. From MISR‐retrieved constraints on particle size, shape, and light‐absorption properties, the distribution of black and brown smoke can be inferred, along with the underlying processes responsible for plume‐particle evolution. This paper presents the satellite‐retrieved results that complement the suborbital data acquired for the CalFiDE campaign and associated modeling, for use in continuing studies of fire dynamics. Key Points: NASA's MISR multi‐angle imagery allows aerosol plume‐height, associated motion vector, and particle property retrievals from spaceTypically, field data are acquired to validate satellite data, but MISR data is mature enough to contribute directly to the CalFiDE campaignAircraft and MISR overflights were coordinated twice, yielding MISR aerosol context and detail for joint smoke‐plume dynamics and chemistry study [ABSTRACT FROM AUTHOR]

Volcanic ash clouds are carefully monitored as they present a significant hazard to humans and aircraft. The primary tool for forecasting the transport of ash from a volcano is dispersion modeling. These models make a number of assumptions about the size, sphericity and density of the ash particles. Few studies have measured the density of ash particles or explored the impact that the assumption of ash density might have on the settling dynamics of ash particles. In this paper, the raw apparent density of 23 samples taken from 15 volcanoes are measured with gas pycnometry, and a negative linear relationship is found between the density and the silica content. For the basaltic ash samples, densities were measured for different particle sizes, showing that the density is approximately constant for particles smaller than 100 μm, beyond which it decreases with size. While this supports the current dispersion model used by the London Volcanic Ash Advisory Centre (VAAC), where the density is held at a constant (2.3 g cm−3), inputting the measured densities into a numerical simulation of settling velocity reveals a primary effect from the silica content changing this constant. The VAAC density overestimates ash removal times by up to 18%. These density variations, including those varying with size beyond 100 μm, also impact short‐range particle‐size distribution measurements and satellite retrievals of ash. Plain Language Summary: Volcanic ash clouds are carefully monitored as they present a significant hazard to humans and aircraft. Dispersion modeling is a primary tool used to forecast ash flows from volcanoes. These models make a number of assumptions about the size, sphericity (roundness) and density of the ash particles. Few studies have measured the density of ash particles or explored the impact that the assumption of ash density might have on the dispersion forecasts. In this paper, the density of 23 samples taken from 15 volcanoes are measured, and a negative linear relationship is found between the density and the silica content. For the basaltic ash samples (the most common type of ash), densities were measured for different particle sizes, showing that the density is approximately constant for particles smaller than 100 μm, beyond which it decreases with size. This supports the London Volcanic Ash Advisory Centre keeping density constant in their current model, but in fact this constant changes with silica content, leading to an overestimation of ash removal times by up to 18%. These density deviations also impact short‐range particle‐size distribution measurements and satellite retrievals of ash. Key Points: The density of volcanic ash is measured as a function of particle size for a range of eruptionsSilica content and particle size negatively correlate with densityThe density of particles smaller than 100 μm is approximately constant but is dependent on silica content [ABSTRACT FROM AUTHOR]

Spatial climate data sets that extend back in time over many decades are an important resource for climate monitoring. The long‐term consistency of such data sets is, however, compromised by changes in the measurement systems over time. In this paper, we introduce a data set of monthly precipitation on a 5‐km grid over the European Alps that extends back to the late 19th century. In deriving the "long‐term Alpine precipitation reconstruction" (LAPrec), special care is taken of variations in the station network, in order for the data set to satisfy high standards in long‐term consistency. LAPrec builds on a reconstruction method that integrates the available information in two portions: The first is a set of high‐quality homogenized station series, taken from the HISTALP data archive, covering the entire period almost continuously. The second is a high‐resolution gridded precipitation analysis, taken from the "Alpine Precipitation Grid Data Set," constructed from thousands of rain‐gauges but covering a few decades only. We demonstrate how the reconstruction approach successfully introduces mesoscale structures that are not resolved by the available long‐term station series, more plausibly so than a predecessor data set using conventional interpolation. We also illustrate that LAPrec reveals long‐term precipitation trends that are spatially more consistent and more detailed than the trends in popular climate monitoring data sets. Over the period 1871–2017 a statistically significant increase is found in winter over the northern parts of the Alps (1%–2% per 10 years). LAPrec is available in two versions (back until 1871 and 1901 respectively) from the Copernicus climate data store. Plain Language Summary: Data sets extending back in time over many decades are an important resource for climate monitoring. In this paper, we introduce a data set on a regular grid of monthly precipitation over the European Alps that extends back to 1871. In deriving the "long‐term Alpine precipitation reconstruction" (LAPrec), special care is taken to keep the station network constant and make use of high‐quality homogenized time series, in order for the data set to be as consistent as possible over time. LAPrec builds on two sources of information: The first is a set of high‐quality station series from the HISTALP data archive, covering the entire period almost continuously. The second is a high‐resolution gridded precipitation analysis, the "Alpine Precipitation Grid Data Set," constructed from thousands of rain‐gauges but covering a few decades only. We demonstrate how this approach successfully introduces details that are not resolved by the available long‐term station series. We also illustrate that LAPrec reveals long‐term precipitation trends that are spatially more consistent and detailed than the trends in popular climate monitoring data sets. Over the period 1871–2017 a statistically significant increase in winter precipitation is found over the northern parts of the Alps. LAPrec is available from the Copernicus climate data store. Key Points: Long‐term spatial analyses of precipitation in the European Alps since 1871For applications such as climate monitoring and trend analysis, the data set meets high standards in long‐term consistency and homogeneityLong‐term Alpine precipitation reconstruction detects a significant trend in winter precipitation over the northern part of the Alps in the period starting from 1871 [ABSTRACT FROM AUTHOR]

An international joint research project, entitled Interhemispheric Coupling Study by Observations and Modelling (ICSOM), is ongoing. In the late 2000s, an interesting form of interhemispheric coupling (IHC) was discovered: when warming occurs in the winter polar stratosphere, the upper mesosphere in the summer hemisphere also becomes warmer with a time lag of days. This IHC phenomenon is considered to be a coupling through processes in the middle atmosphere (i.e., stratosphere, mesosphere, and lower thermosphere). Several plausible mechanisms have been proposed so far, but they are still controversial. This is mainly because of the difficulty in observing and simulating gravity waves (GWs) at small scales, despite the important role they are known to play in middle atmosphere dynamics. In this project, by networking sparsely but globally distributed radars, mesospheric GWs have been simultaneously observed in seven boreal winters since 2015/16. We have succeeded in capturing five stratospheric sudden warming events and two polar vortex intensification events. This project also includes the development of a new data assimilation system to generate long‐term reanalysis data for the whole middle atmosphere, and simulations by a state‐of‐the‐art GW‐permitting general circulation model using the reanalysis data as initial values. By analyzing data from these observations, data assimilation, and model simulation, comprehensive studies to investigate the mechanism of IHC are planned. This paper provides an overview of ICSOM, but even initial results suggest that not only GWs but also large‐scale waves are important for the mechanism of the IHC. Plain Language Summary: In the late 2000s, an interesting form of the coupling between the Northern and Southern Hemispheres was discovered: when the winter polar stratosphere warms, the upper summer mesosphere also warms several days later. An international research project called Interhemispheric Coupling Study by Observations and Modelling (ICSOM) is ongoing to examine the mechanism of this interhemispheric coupling (IHC). This IHC phenomenon is thought to be the connection in the middle atmosphere (i.e., stratosphere, mesosphere, and lower thermosphere). Several promising mechanisms have been proposed, but they remain controversial. This is because gravity waves (GWs) having small scales, which are difficult to observe and simulate, are thought to play a crucial role in the coupling. So, we have performed observations of GWs by networking radars over seven Northern Hemisphere winters, and succeeded in capturing five stratospheric warming events and two opposite events. We also developed a new data assimilation system for the entire middle atmosphere and used the global data produced by the system to simulate GWs with a high‐resolution global model. By combining these research tools, we plan to elucidate the mechanism of IHC comprehensively. This paper presents an overview of ICSOM. Initial results show that not only GWs but also large‐scale waves are important for the IHC mechanism. Key Points: An international project is ongoing to elucidate the mechanism of interhemispheric coupling (IHC) in the middle atmosphereGravity waves (GWs), which are thought to play a key role in IHC, were observed by a radar network and simulated by high‐resolution global modelInitial results suggest that not only GWs but also large‐scale waves are important for the IHC mechanism [ABSTRACT FROM AUTHOR]

Changes in the frequency of temperature extremes are often attributed to global warming. The recent availability of near‐surface temperature data records from reference networks, such as the U.S. Climate Reference Network (USCRN), enables the quantification of measurement uncertainties. Within an activity of the Copernicus Climate Change Service, the estimation of the measurement uncertainty has been provided for USCRN temperature data, using metadata made available by the National Oceanic and Atmospheric Administration (NOAA). In this paper, four climate extreme indices (Frost Days, Summer Days, Ice Days, Tropical Nights) and the related uncertainties are calculated for the period 2006–2020 from the USCRN data set and compared with traditional indices. Moreover, the asymmetric USCRN measurement uncertainties are propagated to estimate the uncertainties of climate indices. The comparison shows expanded uncertainties homogeneously distributed with the latitude and typically within 15 days per year for Frost Days and within 10 days for Ice Days, while smaller uncertainties are estimated for Summer Days and Tropical Nights, with values typically within six to seven days per year. Positive uncertainties are typically larger than negative ones for all the indices. The values of Frost and Ice Days with the related uncertainties for USCRN have also been compared with the corresponding values calculated from reanalyses data, showing differences typically within 60 days for median values, quite often smaller than USCRN and inconsistent within the related uncertainties, Overall, the results show that USCRN measurement uncertainties increase confidence in the estimation of climate extreme indices and decisions for adaptation. Plain Language Summary: The relationship between the intensity and frequency of extremes and climate change as well as their attribution to human activities is fundamental for improving the assessment of risk and the elaboration of adaptation strategies. Temperature extremes are often reported and estimated using observations or model data using indices, which are widely adopted in the research community and by decision‐makers. However, the number of temperature extremes is quantified assuming input observations as perfect, whereas these are always affected by uncertainties due to instrumental noise and systematic effects that cannot be always properly accounted for. This also implies that climate extreme indices may under or over‐represent the number of temperature extremes. The advent of reference measurement networks, as well as the overall increase in observational data quality due to recent technological improvements, allows us to quantify measurement uncertainties in detail. In this paper, temperature extremes over the US are estimated from near‐surface temperature measurements provided by the USCRN network in the period 2006–2020 with related uncertainties. The use of uncertainty illustrates the range of values that climate extreme indices may assume. Possible sources of uncertainties and comparisons with data from atmospheric reanalysis are also discussed. Key Points: An extensive assessment of uncertainties for four climate extreme indices is provided using reference near‐surface temperaturesEstimate uncertainties of climate indices for reanalysis validation and quantification of extremes by propagating measurement uncertaintiesUSCRN traceable measurements with quantified uncertainties increase confidence in estimating extreme indices and decisions for adaptation [ABSTRACT FROM AUTHOR]

A new satellite observation concept to estimate upward air mass flux in convective storms has been proposed using a convoy of small radars that measure reflectivity only. The approach is to analyze a pair of radar reflectivity profiles obtained within 30–120 s of one another, and infer the upward transport above the freezing level and at a nominal 3‐km horizontal resolution from the reflectivities and their temporal change. This paper summarizes the theoretical basis for this approach, with quantitative justification using sensitivity analyses of convection‐permitting model simulations and from ground‐based zenith profiler data. We then describe and quantify the performance of the approach to detect updrafts from the radar observations. Finally, we illustrate the expected performance of retrievals of the vertical transport and evaluate their ability to meet the objectives of the "Investigation into Convective Updrafts" mission, selected by NASA to place in orbit in 2027–2028 a convoy of three identical Ka‐band radars measuring radar reflectivity within their common swath. Plain Language Summary: This paper describes a new concept to measure the vertical transport in updrafts in convective storms, from space, using pairs of small radars that measure radar reflectivity only. The paper first presents the theoretical relation between the evolution of reflectivity from the time of the first radar observation to the time of the last (up to 2 min later), on one hand, and the underlying vertical velocities in the updraft. The paper then demonstrates the forward‐sensitivity of reflectivity, under the practical radar constraints, to the vertical velocities. And finally the paper demonstrates the ability to invert this forward sensitivity and produce retrievals of vertical velocity with low uncertainty. Key Points: A new satellite observation concept to estimate air mass transport in convective updrafts is explainedIt is shown that chronological sequences of radar reflectivities are sensitive to the characteristics of vertical transportIt is shown how the characteristics of the vertical transport can be derived from the radar observations [ABSTRACT FROM AUTHOR]

Atmospheric electricity parameters (AEP) measurements from Antarctica predominantly feature either the potential gradient (PG) and/or air‐Earth current (AEC) density. We report for the first time simultaneous measurements of the bipolar ions concentration/conductivity, PG, and AEC density. AEP measurements were carried out at Maitri (70.8°S, 11.8°E) from December 2018 to November 2019. We formulated a few criteria, irrespective of the weather conditions, to select the electrically quiet days and some additional criteria based on the conductivity measurements to discern globally representative data (GRD) from such days. The measurements of the PG and AEC density over the Antarctic plateau demonstrated the diurnal curves similar to the Carnegie pattern, which represents the global thunderstorms and electrified shower clouds (ESCs) occurring on different continents and oceans, we regard the data having such trend as GRD. We found significant variability in the concentration of small bipolar ions/conductivity in the austral summer which in turn affects GRD. However, the concentration of bipolar ions is nearly consistent at ∼250 negative ions cm−3 and ∼300 positive ions cm−3 in winter and enhances the probability of GRD. Such differences can arise out of the prevalent planetary boundary layer processes in the two seasons. When the PG varied between ∼50 Vm−1 and ∼150 Vm−1 and the maximum range of conductivity variations was ∼0.2 × 10−14 ℧ m−1, the AEPs represented the signatures of the global thunderstorm and ESC activities. Plain Language Summary: Monitoring of the atmospheric electricity parameters is a simple technique to monitor global thunderstorm activity and electrified shower clouds. For this, the data need to be free from local disturbances. Obtaining such data in Antarctic Plateau was found to be successful. On the other hand, the coastal Antarctic stations, experience local or regional contributions in it. This paper attempts to provide some techniques to obtain globally representative data (GRD). This paper suggests that the diurnal variation of the concentration of bipolar small ions strongly impacts the GRD. Therefore a day free from the diurnal variation of the concentration of bipolar ions is essential to discern the global signals. The winter season appears to be a better season for this as the summer season experiences mild convection activity that causes local and regional electrical signals that contaminate the data. Key Points: Atmospheric electrical conductivity is the key parameter to discern globally representative data (GRD) over the Maitri, AntarcticaGRD is discernible on a day when conductivity is consistent, and such days are most common in local winterIn the austral summer, the planetary boundary layer (PBL) processes produce local electrical signals that interfere with the global signals [ABSTRACT FROM AUTHOR]

The editorial board of JGR Atmospheres thanks reviewers who refereed papers in 2022. Plain Language Summary: The editorial board of JGR Atmospheres thanks reviewers who refereed papers in 2022. Key Point: The editors thank the 2022 peer reviewers [ABSTRACT FROM AUTHOR]

This paper makes use of a combination of FLEXPART‐WRF simulations forced with ERA5 and the CESM2 model—incorporated in the CMIP6 project—to infer a series of changes over the present century in the behavior of the landfalling atmospheric rivers (ARs) arriving to the Iberian Peninsula. In addition, future changes in the intensity and position of their main moisture sources are studied. In overall terms, there is a noticeable increase in the amount of moisture transported by ARs in the study region, particularly accentuated by the end of the century. However, no significant changes in the number of events are observed. A northward shift of both the mean position of the ARs as well as their main sources of moisture is also detected, particularly for the end of the century, and in the summer and fall months. In relation to the latter, an increase in the contribution of moisture contribution is also observed, quantitatively compatible with Clausius‐Clapeyron amplification. Plain Language Summary: This paper makes use of a combination of simulations forced with reanalysis data and a climate model to infer a series of changes over the present century in the behavior of the landfalling atmospheric river—ARs, regions of intense moisture transport located in the lower layers of the atmosphere—arriving at the Iberian Peninsula. In addition, future changes in the intensity and position of their main moisture sources are studied. In overall terms, there is a noticeable increase in the amount of moisture transported by ARs in the study region, particularly accentuated by the end of the century. However, no significant changes in the number of events are observed. A northward shift of both the mean position of the ARs as well as their main sources of moisture is also detected, particularly for the end of the century, and in the summer and fall months. In relation to the latter, an increase in the contribution of moisture contribution is also observed, in a ratio similar to that expected. Key Points: FLEXPART‐WRF forced with CESM2 model has been able to reproduce the historical conditions of Atmospheric River over the Iberian PeninsulaA northward shift of the main source regions is projected, notable in summer and fall and particularly by the end of the centuryGradual strengthening in the intensity of Atmospheric Rivers is expected, observable from an increase in the amount of moisture transported [ABSTRACT FROM AUTHOR]

In this paper we analyze electric‐field and current measurements of upward leaders induced by a downward negative lightning flash that struck a residential building. The attachment process was recorded by two high‐speed cameras running at 37,800 and 70,000 images per second and the current measured in two lightning rods. Differently from previous works, here we show, for the first time, current measurements of multiple upward leaders that after initiation propagate to connect the negative downward moving leader. At the beginning of the propagation of the leaders that initiate on the instrumented lightning rods, current pulses appear superimposed to a steadily increasing DC current. The upward leader current pulses increase with the approach of the downward leader and are not synchronized but present an alternating pattern. All 2D leader speeds are approximately constant. The upward leaders are slower than the downward leader speed. The average time interval between current pulses in upward leaders is close to the interstep time interval found by optical or electric field sensors for negative cloud‐to‐ground stepped leaders. The upward leaders respond to different downward propagating branches and, as the branches alternate in propagation and intensity, so do the leaders accordingly. Right before the attachment process the alternating pattern of the leaders ceases, all downward leader branches intensify, and consequently upward leaders synchronize and pulse together. The average linear densities for upward leaders (49 and 82 μC/m) were obtained for the first time for natural lightning. Plain Language Summary: The effectiveness of a lightning protection system depends on its efficiency to intercept the down coming leader of a cloud‐to‐ground lightning flash. The interception is usually done by an upward connecting leader that initiates from grounded structures, humans, or living beings that protrude from nearby ground. The understanding of the upward connecting leader and of the attachment process with the downward leader plays an important role in the determination of the zone of protection and therefore in the improvement of a lightning protection system. Unconnected upward leaders, that is, upward leaders that fail to connect the downward leader, are also of great importance in lightning protection. They can be large enough to cause damage to equipment vulnerable to sparks or induced currents, and enough to injure people from who it initiates. In this paper we analyze electric‐field, speed, and current measurements of upward leaders induced by a downward negative lightning flash that struck a residential building. The attachment process was simultaneously recorded by two high‐speed cameras, an electric‐field sensor, and current sensors installed on two lightning rods. Differently from previous works we show, for the first time, current measurements of multiple upward leaders induced by the negative downward moving leader. Key Points: Current and charge density measurements of two upward leaders induced by the same downward leaderUpward leaders alternate their progression during initial propagationCurrent pulses of upward leaders increase intensity and synchronize right before attachment [ABSTRACT FROM AUTHOR]

Volcanic aerosols change the atmospheric composition and thereby affect weather and climate. Aerosol dynamic processes such as nucleation, condensation, and coagulation modify the shape, size, and mass of aerosol particles, which influence their atmospheric lifetime and radiative properties. Nevertheless, most models omit these processes for ash particles. In this work, we explore the ash aerosol aging and sulfate production during the first 4 days following the 2021 La Soufrière (St. Vincent) eruption with the ICON‐ART model (ICOsahedral Nonhydrostatic model with Aerosol and Reactive Trace gases). Online coupling of ICON‐ART with a one‐dimensional volcanic plume model calculates volcanic emission, which makes it possible to resolve the different eruption phases of the noncontinuous La Soufrière eruption. We compared our simulated aerosol distribution and composition with observations from the Cloud‐Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument, the Multiangle Imaging SpectroRadiometer (MISR) Research Aerosol (RA) Algorithm, and the Barbados Cloud Observatory (BCO). We show that online coupling is essential to adequately model the emissions and plume development close to the volcano. The modeled aerosol aging is in very good agreement with observations from MISR near the emission source and with CALIOP at larger distances. Furthermore, particle aging occurs faster in the troposphere than in the stratosphere due to the availability of water vapor and OH, but a layer of coated ash appears at the plume top due to faster oxidation of SO2 and lofting by aerosol‐radiation interaction. This paper gives the first direct comparison of aerosol aging in volcanic eruption plumes between simulations and observations. Plain Language Summary: Large volcanic eruptions can influence weather and climate, and endanger aviation and public health. To constrain these effects and risks, it is critical to reliably predict the volcanic plume dispersion. However, the atmospheric lifetime of the ash released during an eruption is influenced by many factors, such as emission height, meteorology, and aerosol dynamical processes. Aerosol dynamical processes lead to growth and aging of volcanic plume particles. They include the formation of new particles from precursor gases, condensation on existing particles, and coagulation of particles. This paper investigates the formation of aged ash particles in model simulations and observations following the 2021 La Soufrière eruption. We consider ash aging both close to the volcano and during further transport. We found that ash aging takes place already close to the volcano and the fraction of aged particles increases with distance from the source. During further transport, a layer of aged ash particles forms at the plume top due to interaction of these particles with radiation and subsequent warming of the plume. Key Points: Resolving individual eruption phases is essential for modeling the 2021 La Soufrière eruptionCombination of modeling and satellite data confirm, for the first time, the aging of volcanic ashAsh aging and sulfate production rates depend on distance from the source and altitude [ABSTRACT FROM AUTHOR]

This paper examines the empirical relationship between the Madden–Julian oscillation (MJO), the quasi‐biennial oscillation (QBO), and atmospheric river (AR) activity and precipitation in California on subseasonal time scales. We introduce an experimental forecast tool that uses observed anomaly patterns during a 38 yr period to predict the probability of above‐ and below‐normal AR activity and precipitation at lead times of 1–6 weeks based on the phase and amplitude of the MJO and QBO. The hindcast prediction skill of probabilistic AR activity and precipitation forecasts is evaluated for Northern, Central, and Southern California, as well as two sets of smaller geographical domains. These smaller domains are more relevant for water resource management and allow us to investigate the sensitivity of prediction skill to domain size. Consistent with previous studies, our results demonstrate that subseasonal AR activity and precipitation in California are strongly modulated by the MJO and QBO. The anomaly patterns of AR activity and precipitation vary considerably throughout the cool season, with a tendency toward below‐normal AR activity and precipitation during easterly QBO and above‐normal AR activity and precipitation during westerly QBO in JFM. The opposite patterns are generally observed in OND, but the anomaly signals are weaker and less coherent for AR activity. Certain combinations of MJO phase, QBO phase, lag time, and season yield notably higher skill scores, reinforcing the notion of "windows of opportunity" for skillful subseasonal‐to‐seasonal predictions. In California, these forecasts of opportunity are predominantly associated with easterly QBO in JFM and FMA. Plain Language Summary: This paper examines the relationship between the Madden–Julian oscillation (MJO), the quasi‐biennial oscillation (QBO), and atmospheric river (AR) activity and precipitation in California. We introduce an experimental forecast tool that shows the likelihood of above‐normal and below‐normal AR activity and precipitation based on the phases of the MJO and QBO. Consistent with previous studies, our results demonstrate that AR activity and precipitation in California are strongly influenced by the MJO and the QBO. There is a tendency for below‐normal AR activity and precipitation during easterly QBO conditions in January–March. The opposite pattern (above‐normal AR activity and precipitation) generally occurs during westerly QBO conditions. The results also suggest that our forecast tool has some potential to improve the prediction of AR activity and precipitation. The reliability and usefulness of this forecast tool depend on multiple factors, including the MJO phase, the QBO phase, and the time of year. In California, this forecast tool is likely to provide the most beneficial value during easterly QBO conditions in mid‐winter through early spring. Key Points: The modulation of atmospheric river (AR) activity and precipitation in California by the Madden‐Julian oscillation and quasi‐biennial oscillation is quantifiedA hindcast skill assessment of probabilistic AR activity and precipitation forecasts is presentedAn experimental subseasonal AR activity and precipitation forecast tool is introduced [ABSTRACT FROM AUTHOR]

This paper utilizes Aerosol Index (AI) data from the Total Ozone Mapping Spectrometer (TOMS) instrument, along with ERA5 reanalysis data, to identify atmospheric processes contributing to the uplift of dust from the Etosha Pan through the annual cycle. Etosha is one of the most prominent source areas in the Southern Hemisphere, although very little is known about its meteorology outside of the peak dust season (August–October). Emissions in December (AI = 1.6) are shown to be comparable to those in September (AI = 1.7), the dustiest month in the TOMS record. Unlike in September however, when a nocturnal low‐level jet is the primary emission mechanism, uplift in December is associated with an anabatic‐sea breeze that develops along the Namib coast, and propagates inland to reach Etosha during the evening. The system is a response to the thermal contrast between the elevated interior plateau and the adjacent waters of the cool Benguela Upwelling System, and so is at its strongest during austral summer, when the area of maximum diabatic heating shifts south over southern Africa. Topographic channeling of the flow through the east‐west orientated Hoanib River valley is shown to facilitate the inland propagation of the anabatic‐sea breeze, and explains the persistence of the system at Etosha's latitude. Evening surface winds at Etosha, associated with the anabatic‐sea breeze, are significantly stronger in the dustier December months, when diabatic heating over the subcontinent and hence the zonal thermal gradient are enhanced. Plain Language Summary: This paper uses satellite and meteorological data to identify the features of southern Africa's weather and climate that contribute to dust emission from the Etosha Pan throughout the year. Etosha is an important source of dust in the Southern Hemisphere, however very little work has been conducted there outside of the winter season, when emissions are at their highest. From the satellite data, it is shown that December is just as dusty as some of the winter months, however there is a difference in the low‐level winds between the two seasons; in winter, emissions are driven by a morning peak in surface wind speeds, whereas emissions in December are driven by maximum surface winds during the evening. This evening peak in surface winds is shown to coincide with the arrival of a sea breeze at Etosha, with the system a response to the strong heating of the southern African plateau at this time of year. Evening surface winds associated with the sea breeze are stronger during the dustiest December months at Etosha, and are driven by enhanced heating over the subcontinent. Key Points: An anabatic‐sea breeze helps to drive austral summer dust emissions from one of the Southern Hemisphere's most prominent source areasThe anabatic‐sea breeze is present throughout the year, however is at its strongest, and propagates furthest inland during austral summerVariability in the strength of the system is driven by changes in the pattern of diabatic heating over the interior of southern Africa [ABSTRACT FROM AUTHOR]

Following on our earlier single‐station, 2‐dimensional (2D) broadband interferometric mapping and polarization (BIMAP) observations of lightning discharges, we recently deployed two BIMAP stations at Los Alamos National Laboratory to map the lightning sources and their polarization in full 3‐dimensional (3D) space (BIMAP‐3D). The two stations are separated by 11.5‐km and each station consists of four antenna sets (instead of three for the original BIMAP) that form a Y‐shaped array for improved interferometric performance. In this paper, we report the BIMAP‐3D system design, a generalized and analytical 2D interferometry technique for noncoplanar antenna array, a two‐stage 3D mapping technique based on geometric triangulation and baseline‐based differential time of arrival, and a technique to reconstruct the polarization orientation in 3D space by combining the 2D polarization results from the two‐station observations. Along with description of the techniques, we demonstrate and discuss the initial lightning results, including 3D maps for a hybrid intracloud and cloud‐to‐ground flash and for a normal intracloud flash, development of abnormal K‐change leaders, and polarization signatures for a K‐change leader. We find that with the two‐stage 3D mapping techniques, the sources can be located to meters accuracy for a favorable event that occurs between the two stations. We also find the polarization vectors for the example K leader are mostly orthogonal to the leader channel after the full 3D polarization analysis. The main purpose of this paper is to report the BIMAP‐3D techniques and capabilities. Detailed analysis of more specific discharge processes will be reported in later studies. Plain Language Summary: A new 3‐dimensional broadband radio frequency interferometric mapping and polarization system (BIMAP‐3D) is developed and deployed at Los Alamos National Laboratory for lightning research. BIMAP‐3D provides an unprecedented capability in high‐resolution, time‐evolving 3D lightning source mapping and 3D source polarization detection for detailed study of lightning discharge physics. In this research, we described the BIMAP‐3D system, introduced a suite of advanced data processing techniques, and demonstrated BIMAP‐3D's capabilities with actual lightning observations. This new capability is expected to lead to a range of new understandings and discoveries for a variety of lightning discharge processes. Key Points: A new 3D broadband interferometric mapping and polarization system for lightning study is introducedA suite of new data process algorithms is reported, and 3D lightning results for overall flashes and K‐change leaders are demonstratedPolarization orientations for a K‐change leader are found mostly orthogonal to the leader channel [ABSTRACT FROM AUTHOR]

Lidar and radar observations of persistent atmospheric wave activity in the Antarctic atmosphere motivate investigation of generation of acoustic‐gravity waves (AGWs) by vibrations of ice shelves and exploiting their possible ionospheric manifestations as a source of information about the ice shelves' conditions and stability. A mathematical model of the waves radiated by vibrations of a finite area of the lower boundary of the atmosphere is developed in this paper by extending to AGWs an efficient, numerically exact approach that was originally developed in seismology and underwater acoustics. The model represents three‐dimensional wave fields as Fourier integrals of numerical or analytical solutions of a one‐dimensional wave equation and accounts for the source directionality, AGW refraction and diffraction, and the wind‐induced anisotropy of wave dissipation. Application of the model to the generation of atmospheric waves in Antarctica by free vibrations of the Ross Ice Shelf reveals a complex three‐dimensional structure of the AGW field and elucidates the impact of various environmental factors on the wave field. The intricate variation of the wave amplitude with altitude and in the horizontal plane is shaped by the spatial spectrum of the ice surface vibrations and the temperature and wind velocity stratification from the troposphere to the mesosphere. It is found that the waves due to the low‐order modes of the free oscillations of the Ross Ice Shelf, which have periods of the order of several hours, can transport energy to the middle and upper atmosphere in a wide range of directions from near‐horizontal to near‐vertical. Plain Language Summary: This research paper paves the way to infer the conditions and stability of ice shelves in Antarctica by looking at unusual wave activity in the atmosphere. The researchers have developed a mathematical model to understand how these waves, called acoustic‐gravity waves (AGWs), are created by the vibrations of ice shelves. The model is based on a method previously used in seismology and underwater acoustics and accounts for various factors that affect the waves, such as the properties of the source, the way the waves bend and spread due to wind, and how they dissipate. The researchers applied this model to study the atmospheric waves generated by vibrations of the Ross Ice Shelf in Antarctica. The results show a complex 3D structure of the AGW field, highlighting the impact of different environmental factors on the wave activity. The variation in wave amplitude depends on the ice surface vibrations and the temperature and wind conditions at different heights in the atmosphere. The study found that waves with periods of several hours can transfer energy from the ice shelf to the middle and upper atmosphere in various directions. This new approach could help scientists better understand the conditions and stability of ice shelves in the future. Key Points: Vibrations of large ice shelves radiate atmospheric waves that can travel in a wide range of directions from near‐horizontal to near‐zenithSimple, numerically efficient model is developed of atmospheric wave generation by finite sources at the ground levelWith the surface vibrations' spectrum as the input, the model quantifies atmospheric perturbations from the troposphere to the thermosphere [ABSTRACT FROM AUTHOR]

Multi‐resolution analysis methods can reveal the underlying physical dynamics of nonstationary signals, such as those from lightning. In this paper we demonstrate the application of two multi‐resolution analysis methods: Ensemble Empirical Mode Decomposition (EEMD) and Variational Mode Decomposition (VMD) in a comparative way in the analysis of electric field change waveforms from lightning. EEMD and VMD decompose signals into a set of Intrinsic Mode Functions (IMFs). The IMFs can be combined using distance and divergence metrics to obtain noise reduction or to obtain new waveforms that isolate the physical processes of interest while removing irrelevant components of the original signal. We apply the EEMD and VMD methods to the observations of three close Narrow Bipolar Events (NBEs) that were reported by Rison et al. (2016, https://doi.org/10.1038/ncomms10721). The ΔE observations reveal the occurrence of complex oscillatory processes after the main NBE sferic. We show that both EEMD and VMD are able to isolate the oscillations from the main NBE, with VMD being more effective of the two methods since it requires the least user supervision. The oscillations are found to begin at the end of the NBEs' downward fast positive breakdown, and appear to be produced by a half‐wavelength standing wave within a weakly‐conducting resonant ionization cavity left behind in the wake of the streamer‐based NBE event. Additional analysis shows that one of the NBEs was likely initiated by an energetic cosmic ray shower, and also corrects a misinterpretation in the literature that fast breakdown is an artifact of NBE‐like events in interferometer observations. Plain Language Summary: This paper investigates the application of mode decomposition techniques to the analysis of Narrow Bipolar Events (NBEs). NBEs are high‐power discharges that often occur as the initiating event of lightning flashes, and are produced by streamer‐based activity called fast positive breakdown. We apply Ensemble Empirical Mode Decomposition (EEMD) and Variational Mode Decomposition (VMD) methods to three NBEs that were observed at Langmuir Laboratory in New Mexico to extract and interpret oscillatory behavior that occurred following the NBEs. We show that both EEMD and VMD separate the oscillations from the strong electric field change of the parent NBE, with VMD being the preferred choice. The resulting waveforms are indicative of a shock‐excited residual process that lasts for tens of microseconds, beginning at the end of the downward fast breakdown activity. Although questions remain about the physical mechanism of the oscillations, they appear to be caused by the NBE's streamers creating a weakly‐conducting resonant cavity in its wake that supports half‐wavelength standing wave oscillations, analogous to the vibrations of a plucked guitar string. Key Points: Ensemble Empirical Mode Decomposition and Variational Mode Decomposition separate out Narrow Bipolar Event (NBE) oscillations for further studies of physical mechanisms responsible for oscillationsThe results show the oscillations initiate not during the NBE but at the end of the fast positive breakdown processThe oscillatory behavior indicates that residual ionic conductivity creates a resonant cavity which emits half‐wavelength radiation [ABSTRACT FROM AUTHOR]

The Guangdong‐Hong Kong‐Macao Greater Bay Area (GBA), a cluster of world‐class cities, is undergoing rapid urbanization. However, the heterogeneity of the urban thermal environment resulting from the diversity of urban forms is not yet fully understood. This paper assesses the heterogeneity of the urban heat island (UHI) effect in the GBA using the coupled Weather Research and Forecasting (WRF) model/multi‐layer urban canopy and building energy model (BEP/BEM), with high‐resolution local climate zone (LCZ) map as urban land use/land cover data. The average UHI intensity is found to peak at 1.8 ± 0.4°C in the evening, when the average UHI intensity of LCZ 2 can reach a maximum of 2.4 ± 0.58°C. Properly setting air‐conditioning temperatures can effectively prevent the enhancement of the UHI phenomenon at night by the anthropogenic heat (AH) released from air‐conditioning. The UHI‐induced local circulations and enhanced surface roughness inhibit the penetration of sea breezes inland, and surface wind speed decreases in all LCZs, with a maximum change of more than 0.8 m s−1. However, the increased thermal difference between land and sea leads to enhanced sea breezes offshore, especially in the Pearl River estuary. In addition, a series of sensitivity experiments have been conducted in this paper on initial and boundary conditions, building drag coefficients and urban fractions, which paves the way for further analyzing urban climate in GBA using WRF model and LCZs. Plain Language Summary: With the rapid urbanization of the world, the demand for functional buildings has increased. Along with the diversification of urban forms, the differences in the thermal environment within cities are becoming more and more significant. This study therefore provides an in‐depth study of the urban thermal environment in the Guangdong‐Hong Kong‐Macao Greater Bay Area (GBA) based on numerical simulation and local climate zones (LCZs). It was found that the urban heat island (UHI) intensity in different urban forms has obvious differences, and may vary by 1°C. However, the daily variation trends are similar, all showing a stronger UHI intensity at night than during the day, and reasonable setting of air‐conditioning temperature can effectively mitigate the UHI intensity at night. The UHI‐induced local circulations and enhanced surface roughness weaken the surface wind speed and inhibit the penetration of sea breeze inland, but enhance the sea breezes offshore, especially in the Pearl River estuary. This study provides references for urban planning and future sustainable development, especially for areas located along the coast that are undergoing rapid development. In addition, a series of sensitivity experiments on initial and boundary conditions, building drag coefficients and urban fractions provide useful suggestions for numerical model configuration in the GBA. Key Points: Tests of initial and boundary conditions, building drag coefficient and urban fractions provide recommendations for Weather Research and Forecasting configurationUrban heat island (UHI) varies between different local climate zones, but all peak in evening, and proper setting of air‐conditioning temperatures can mitigate UHI at nightUrbanization weakens surface wind speeds and inhibits the penetration of sea breezes inland, but strengthens the sea breezes offshore [ABSTRACT FROM AUTHOR]

A method to identify the turbulent mixing sources within the marine atmospheric boundary layer (MABL) based on the shipborne coherent Doppler lidar measurements is introduced in this paper. Combining with the coherent Doppler lidar signal‐to‐noise ratio, vertical velocity skewness, turbulence kinetic energy dissipation rate, and wind shear intensity, the categories of turbulent mixing sources and the specific turbulent mixing sources could be determined. The method is applied into two voyages of MABL observation during May 2021 in the South China Sea and during April 2022 in the Bohai Sea and Yellow Sea. The turbulent mixing processes are captured and the classification of the turbulence driven sources within the MABL are realized. The temporal‐spatial evolution characteristics of the turbulence mixing process in the MABL are investigated under different weather conditions containing clear‐sky day, cloudy‐sky day, and sea‐fog day. The convective mixing process is recognized in the daytime of the clear‐sky day and the intermittent cloud‐driven turbulence exists below the cloud layer. Additionally, the turbulent mixing is weak which could not act as the main driven source during the sea‐fog day. Furthermore, the dominant turbulence scale analyses of different turbulence sources are conducted based on the cospectra of the vertical velocity and the horizontal speed measurements. The turbulence parameters of different turbulence sources are statistical analyzed and compared in different sea areas. The classification method has the broad application prospects on the study of the air‐sea interaction. Plain Language Summary: The redistribution of substance and energy within the atmospheric boundary layer is achieved through the turbulent mixing in most cases. When the underlying surface is ocean, the complex turbulent mixing process within the marine atmospheric boundary layer (MABL) is the key topic in the air‐sea interaction research because it would affect the atmosphere circulation through changing the momentum, heat and water vapor distributions. Hence the quantitative measurements of the vertically resolved turbulence parameters and understanding of the main sources of the turbulent mixing are crucial. In this paper, the turbulent mixing driven sources are identified based on the high‐accuracy turbulence parameters measured by coherent Doppler lidar. Through the two voyages of MABL observation in the South China Sea, Bohai Sea, and Yellow Sea, the temporal‐spatial evolution characteristics of the turbulence mixing process are investigated under different weather conditions including clear‐sky day, cloudy‐sky day, and sea‐fog day. Additionally, the scale analysis focuses on the dominant turbulence and the statistical analysis of turbulence parameters of different turbulent driven sources are conducted in different sea areas. This classification method has the great potential and broad application prospects on the study of the turbulent mixing characteristics and air‐sea interaction. Key Points: A method to identify the turbulent mixing sources in marine atmospheric boundary layer based on coherent Doppler lidar is introducedThe classification method is first demonstrated with the lidar measurements over the South China Sea, Bohai Sea, and Yellow SeaThe temporal‐spatial evolution characteristics of the turbulence mixing process are investigated under different weather conditions [ABSTRACT FROM AUTHOR]

E. Williams et al. (2022, commented paper) questioned electron energy spectra derived from thunderstorm ground enhancements (TGEs) measured on Aragats; they concluded that "A more likely origin for any detected electrons at 3.2 km above sea level is Compton scattering and pair production activated by longer‐range bremsstrahlung gamma rays, themselves produced by runaway electron encounters with nuclei in the breakeven field at higher altitude." In this comment, we show that the selection criteria of "electron" TGEs unambiguously reject the assumption of the origination of TGE electrons measured on Aragats from the Compton and pair production processes. Thus, the strong accelerating electric field above the earth's surface can be significantly lower (25–150 m) than derived in the commented paper 500 m altitude. Plain Language Summary: Electron accelerators operate in the thunderous atmosphere, sending copious particles to the Earth's surface. To get inside the models of electron acceleration and multiplication by strong atmospheric fields, the critical problem is the measurement of electrons and their energies as they arrive at the earth's surface. It is rather tricky because electrons are fast attenuated in the air, and the flux of accompanied gamma rays is attenuated much less and reaches the ground in overwhelming amounts. We developed special hardware and software methods to prove electrons' existence in the vast particle fluxes reaching the ground and to measure their energies. Simulations and careful examination of the registered particle fluxes check these methods. Key Points: The contribution of the Compton scattered and pair‐production electrons to TGE flux is negligible and cannot "mimic" the TGE electron fluxThe criteria used in the energy spectrum recovery from Aragats Solar Neutron Telescope (ASNT) reliably select "electron" TGE events and reject TGE events with small electron contentIf the strong accelerating electric field terminates low above the earth's surface (25–100 m), electrons from the large RREAs reach ASNT, and their energy spectrum can be reliably recovered [ABSTRACT FROM AUTHOR]

Based on directly measured triggered lightning currents, the characteristics of 70 continuing current (CC) waveforms and 106 M‐component waveforms were analyzed. The durations of CC without M‐component are all less than 10 ms, which are significantly less than those of CCs with M‐components. The first M‐component always appears at no more than 4 ms after the return stroke. The characteristics of the superimposed M‐components will be different for CC of different durations. The M‐components superimposed on CCs with duration greater than 10 ms are 3–4 times larger in terms of risetime, half‐peak width, duration, and transfer charge than that superimposed on CCs with duration less than 10 ms. The time difference (TD) between the peak electric field and the peak current of the M‐component is affected by both the continuing current level (ICC) and the magnitude of the M‐component (IM). TD and ICC (or IM) will not be large at the same time. The duration of a CC following a return stroke with a peak current greater than 21.2 kA will not exceed 46.0 ms. Key Points: The characteristics of continuing current and M‐component current waveforms from triggered lightning were analyzedThe characteristics of the M‐components superimposed on continuing currents with different durations are differentThe time difference between the peak electric field and the peak current of the M‐component is related to two current parameters [ABSTRACT FROM AUTHOR]

We examine the thermal structure of the mesosphere and lower thermosphere (MLT) using observations from 2002 through 2021 from the SABER instrument on the NASA TIMED satellite. These observations show that the MLT has significantly cooled and contracted between the years 2002 and 2019 (the year of the most recent solar minimum) due to a combination of a decline in the intensity of the 11‐year solar cycle and increasing carbon dioxide (CO2.) During this time the thickness of atmosphere between the 1 and 10−4 hPa pressure surfaces (approximately 48 and 105 km) has contracted by 1,333 m, of which 342 m is attributed to increasing CO2. All other pressure surfaces in the MLT have similarly contracted. We further postulate that the MLT in the two most recent solar minima (2008–2009 and 2019–2020) was very likely the coldest and thinnest since the beginning of the Industrial Age. The sensitivity of the MLT to a doubling of CO2 is shown to be −7.5 K based on observed trends in temperature and growth rates of CO2. Colder temperatures observed at 10−4 hPa in 2019 than in the prior solar minimum in 2009 may be due to a decrease of 5% in solar irradiance in the Schumann‐Runge band spectral region (175–200 nm). Plain Language Summary: The region of the atmosphere from 48 to 105 km (30 miles to 65 miles) above the Earth's surface is called the mesosphere and lower thermosphere or MLT. In this paper we show that the MLT has cooled dramatically from the year 2002–2019, by as much as 1.75 to 19 K (3.1–34.2 degrees Fahrenheit), depending on altitude. These results are obtained from temperatures measured by an instrument on a satellite in low Earth orbit since 2002. The observed cooling is attributed to a reduction in solar activity that influences the MLT temperature and to an increase of carbon dioxide (CO2) in the MLT. The observed cooling of the MLT due to increasing CO2 is an expected result. Between 2002 and 2019 the MLT contracted or shrank by up to 1,333 m or over 4,700 feet. A total of 342 m (1,142 feet) of this shrinkage is attributed to increasing CO2 and is considered a permanent change to the atmosphere. Our results also show that the MLT is expected to cool by 7.5 K (13.5 degrees Fahrenheit) due to a doubling of CO2 which is a benchmark calculation often used for comparison with other results. Key Points: Satellite measurements of temperature, geopotential height, and thickness reveal a cooling, contracting mesosphere and lower thermosphere (MLT)MLT temperatures in the two recent solar minima (2008–2009 and 2019–2020) were likely the coldest since the start of the Industrial AgeCold temperatures at 10−4 hPa in 2019 may be the result of lower solar irradiance in the Schumann–Runge bands (175–200 nm) [ABSTRACT FROM AUTHOR]

Subsidence in atmospheric boundary layer (ABL) appears to be rare and short‐lived, so it is very difficult to observe. In this paper, field experimental data from a large tethered balloon (1,900 m3) synchronously measuring meteorological parameters and air pollutants, a ground‐based aerosol lidar, a Doppler wind lidar, and some other ground observations were used to analyze the formation mechanism of subsidence and its influences on the variation of meteorological parameters and pollutant concentrations of the ABL. Results show that the occurrence of subsidence was closely correlated with the cold advection accompanied by cold front. Pollutants were horizontally transported to the observation site driven by southwest low‐level jet. Under the influence of subsidence behind the cold front and turbulent kinetic energy transported toward the surface, the vertical downward transport of pollutants aloft lasted for nearly 5 hr. The observation data of the tethered balloon suspending at 500 m show that the concentrations of PM2.5, NO3−, SO42−, and NH4+ increased simultaneously. O3 concentration in the lower atmosphere also increased. Subsidence had significant effects on the secondary inorganic aerosol concentrations but the concentration of organic aerosols was not affected. Subsidence led to a stronger inversion layer, further suppressing the vertical dispersion of pollutants. Both the stronger subsidence‐induced inversion layer and the downward transport of pollutants have led to an increase in the ground PM2.5 concentration. Our findings demonstrate the close connection between subsidence and air pollution process and may provide the scientific reference for air quality potential forecast. Plain Language Summary: Subsidence motion is an important kind of vertical exchange process and is usually known to exert great influences on the local air quality. The study on subsidence is poorly understood because of the less observation. Here, we utilized the multisource observation data of a tethered balloon platform and remote sensing measurements in Wangdu County located in the North China Plain to comprehensively study a subsidence case and investigate its impacts on the meteorological parameters and pollutants of the atmospheric boundary layer (ABL). We found that the subsidence located behind the cold front and the observation site has been controlled by cold advection. Subsidence stabilized the lower layer of the ABL and the inversion intensity was strengthened. Subsidence also contributed to the vertical transport of pollutants advected under the influence of low‐level jet. The vertical transport of pollutants had more significant effects on secondary inorganic aerosols, while the concentration of organics was generally not affected. Our study provides the observation evidence of the formation and maintenance of subsidence and highlights the impacts of subsidence to the variation of meteorological parameters and pollutant concentrations of the ABL. Key Points: Data from large tethered balloon (1,900 m3), aerosol lidar, and Doppler wind lidar observations were analyzedSubsidence led to the increase of surface inversion intensity and a reduction of the core of the low‐level jet (LLJ) and aggravated ground pollutionThe downdrafts and the negative vertical turbulent kinetic energy flux associated with LLJ enhanced the downward diffusion of pollutants [ABSTRACT FROM AUTHOR]

Over the past decades, various experimental and numerical model studies have indicated cooling trend in the mesosphere and lower thermosphere (MLT), while the magnitude of the trend varies noticeably. Previous studies using the lidar observations derived the temperature trends and solar responses solely from the traditional nocturnal measurements. While these archived results are more or less in agreement with modeling studies, one of the main uncertainties in these studies is the potential biases induced by the trends of the diurnal tide forced in the lower atmosphere, and that of the in situ exothermal reactions involving the photolysis. In the MLT, the diurnal tide has significant seasonal variations, considerable amplitude and is one of the dominant dynamic sources. However, its potential effects in the trend studies have rarely been discussed. In this paper, we present and compare the long‐term temperature trends in the upper mesosphere utilizing the daily mean and nightly mean temperature profiles measured by a Sodium (Na) Doppler lidar at midlatitude. The system was operating routinely in full diurnal cycles between 2002 and 2017, obtaining a unique multi‐year temperature data set. A customized multi‐linear regression (MLR) model is applied to determine the linear trends and the other fitting parameters, such as ENSO and solar F10.7 responses in the upper mesosphere. This study indicates the daily mean cooling trend between 84 and 98 km is larger than that of nightly mean trend by ∼−1 K/decade, while differences in the solar response are within the fitting uncertainties. Plain Language Summary: The upper atmosphere is sensitive to both the climate change in the lower atmosphere and the solar activity. Studies have demonstrated this region has been experiencing long‐term cooling trend over the past several decades. Most of the previous ground‐based investigations utilized nighttime temperature observations to derive the long‐term changes in the mesosphere and lower thermosphere. However, these nighttime trend results can be biased due to the lack of daytime temperature measurements, leading to the uncertainty in these achieved results. In this study, the daily mean temperatures measured by a lidar at middle latitude between 2002 and 2017 are used to determine the trend and the upper atmosphere response to solar activity. The customized algorithm includes new and important climate parameters. This investigation eliminates such potential bias in the temperature, and the results demonstrated noticeably larger cooling trend in this atmospheric region when compared with those derived from the nightly temperature observations. In addition, the solar response derived from these daily mean lidar temperatures are positive but less than those based on nightly mean observations. The revealed differences will lead to future investigations on the underlying mechanisms and, thus, uncover new aspect of the long‐term studies. Key Points: The temperature in the mesosphere and lower thermosphere is coolingThe cooling trend derived from the daily mean temperatures is noticeably larger than that revealed by the nightly average temperaturesThe WACCM‐X simulations also indicate the differences between the daily mean trend and the nightly mean trend with less magnitude [ABSTRACT FROM AUTHOR]

Increasing impacts of wildfires on Western US air quality highlights the need for forecasts of smoke emissions based on dynamic modeled wildfires. This work utilizes knowledge of weather, fuels, topography, and firefighting, combined with machine learning and other statistical methods, to generate 1‐ and 2‐day forecasts of fire radiative energy (FRE). The models are trained on data covering 2019 and 2021 and evaluated on data for 2020. For the 1‐day (2‐day) forecasts, the random forest model shows the most skill, explaining 48% (25%) of the variance in observed daily FRE when trained on all available predictors compared to the 2% (<0%) of variance explained by persistence for the extreme fire year of 2020. The random forest model also shows improved skill in forecasting day‐to‐day increases and decreases in FRE, with 28% (39%) of observed increase (decrease) days predicted, and increase (decrease) days are identified with 62% (60%) accuracy. Error in the random forest increases with FRE, and the random forest tends toward persistence under severe fire weather. Sensitivity analysis shows that near‐surface weather and the latest observed FRE contribute the most to the skill of the model. When the random forest model was trained on subsets of the training data produced by agencies (e.g., the Canadian or US Forest Services), comparable if not better performance was achieved (1‐day R2 = 0.39–0.48, 2‐day R2 = 0.13–0.34). FRE is used to compute emissions, so these results demonstrate potential for improved fire emissions forecasts for air quality models. Plain Language Summary: Increasing wildfire smoke is undoing decades of air quality progress, yet air quality forecasts often miss the most intense smoke events. This is because forecasted smoke is released at constant rates whereas the rate of smoke release from real wildfires varies in time. In this work we teach a machine learning algorithm to predict the daily change in fire heat output, a quantity that is used to calculate wildfire emissions. The machine learning algorithm uses information regarding weather, fuel moisture and amount, and firefighting efforts to make its predictions. We also tried to predict the daily change in fire heat output using only weather information but found the machine learning method to be more successful. Many federal agencies have their own ways of tracking fire weather and fuel moisture, and in this paper, we show that we can apply machine learning to the data from any of several agencies and get the same level of forecasting skill. Key Points: Random forest models trained on weather, fuel, and firefighting data surpass persistence and weather‐based methods to predict fire energyRandom forest models beat persistence across states, for most days in the 2020 fire season, and across levels of fire severityFire weather and latest fire energy predictors add most skill to the random forest; models using agency weather data perform similarly [ABSTRACT FROM AUTHOR]

Satellite remote sensing of aerosol is largely conducted at moderate or coarse spatial resolution around 1–10 km. Nevertheless, at urban areas with high human activity, aerosol can originate from complex emission sources and may also vary strongly in space. Therefore, aerosol characterization at fine spatial resolution is essential for air quality study and assessment of anthropogenic pollution as well as climate effects. However, space‐borne instruments with high spatial resolution are usually limited in swath width or spectral coverage which result in lowering information content required for aerosol and surface retrieval. Based on the Generalized Retrieval of Atmosphere and Surface Properties (GRASP) algorithm, we propose a hybrid approach by combining fine and coarse spatial resolution measurements to retrieve aerosol and surface properties simulataneously at fine spatial resolution. The instruments with coarse spatial resolution and high revisting time can provide advanced aerosol characterization. At the same time, the instruments with fine spatial resolution are sensitive to spatial variability of aerosol nearby sources. In this study, the GRASP/Hybrid approach is demonstrated and tested based on the European Space Agency Sentinel‐5p/TROPOMI together with the Italian Space Agency PRISMA satellite data. Specifically, the detailed aerosol microphysical properties from Sentinel‐5p/TROPOMI 10 km retrievals are used as a priori information for PRISMA to derive aerosol loading and surface properties at 100 meter (m) spatial resolution. The PRISMA 100 m aerosol and surface retrieval based on the developed GRASP/Hybrid approach are evaluated using available ground‐based and satellite measurements, including AERONET, VIIRS/DB aerosol and PRISMA Level 2 surface reflectance products. Plain Language Summary: Both aerosol and surface characterization at high spatial resolution are always highly demanded especially over densely populated urban areas. However, the high spatial resolution space‐borne instruments typically have small swaths that, in general, reduce the possibility of aerosol and surface decoupling. In this paper, we develop a hybrid approach based on GRASP algorithm (GRASP/Hybrid) to retrieve aerosol and surface properties at fine spatial granularity by combining fine and coarse spatial resolution satellite data. The GRASP/Hybrid approach is demonstrated using European Space Agency (ESA) Sentinel‐5p/TROPOMI and Italian Space Agency (ASI) PRISMA (Hyperspectral Precursor of the Application Mission) measurements. Specifically, the detailed aerosol microphysical properties from Sentinel‐5p/TROPOMI 10 km retrievals are used as a priori information to derive aerosol loading and surface properties exploiting PRISMA at 100 m spatial resolution. Overall, the results obtained in this study demonstrate the big potential of the approach based on the combination of instruments for a number of climate and environment monitoring studies. Key Points: Generalized Retrieval of Atmosphere and Surface Properties (GRASP)/Hybrid approach is developed and demonstrated using Sentinel‐5p/TROPOMI and PRISMA measurementsPRISMA 100 m aerosol and surface properties are retrieved based on GRASP/Hybrid approach and evaluated using reference data setsWe demonstrate the big potential of hybrid approach based on the combination of instruments for climate and environment studies [ABSTRACT FROM AUTHOR]

Narrow bipolar events (NBEs) are impulsive and powerful intracloud discharges. Recent observations indicate that some NBEs exhibit a slanted orientation rather than strictly vertical. This paper investigates the effect of the slanted NBEs using a newly developed rebounding‐wave model. The modeling results are validated against the full‐wave Finite‐ Difference Time‐Domain method and compared with measurements for both vertical and slanted NBE cases. It is found that the inclination of the NBEs affects both the waveforms and amplitudes of the electrostatic, induction and radiation components of the electric fields at close distances (≤10 km). However, it primarily influences the amplitudes of the fields for distances beyond 50 km, where the radiation component dominates, resulting in changes of ≥30% when the slant angle exceeds 30°. The slanted rebounding‐wave model improves the agreement with respect to a purely vertical channel and can be extended to any discharge geometry at arbitrary observation distances. Plain Language Summary: Narrow Bipolar Events (NBEs) are unique intracloud discharges that occur either individually or as the initiation event for lightning flashes inside thunderstorms. Knowing the physical mechanisms of NBEs will help us to better understand how lightning initiates inside thunderstorms. Recent studies indicated that NBEs could exhibit a slanted orientation rather than being strictly vertical. However, the inclination of NBEs has not been considered in previous transmission line models, leading to uncertainty when evaluating their characteristics based on electromagnetic fields. Here, in the light of recent observations, we analyze the propagation effect of the slanted NBEs by using a newly developed slanted rebounding‐wave model, and we compare the modeling results with observations. This study contributes to a better understanding of the physical mechanism of NBEs and provides a reference for accurately characterizing NBEs based on their electromagnetic fields. Key Points: The propagation effect of slanted Narrow bipolar events (NBEs) at different distance is investigated and compared with the observationsThe inclination of the NBEs could significantly affect the electromagnetic fields in the close distanceThe proposed equations will improve the quality of inferred features of slanted NBEs and can be extended to any discharge shape [ABSTRACT FROM AUTHOR]

A three‐dimensional winter (DJF) climatology of Lagrangian diffusivity characterizing eddy mixing and transport in the northern middle atmosphere is presented. To emphasize aspects other than zonal averages, we use the theory of Lagrangian diffusivity (κyy) hitherto not applied in stratospheric contexts to our knowledge. Our formulation of Lagrangian diffusivity requires the calculation of parcel trajectories, which is made on isentropic surfaces. A Lagrangian descriptor is used to estimate the boundary of the stratospheric polar vortex (SPV). To characterize quasi‐geostrophic motions and their influence on the SPV we apply the wave activity flux (W) and Local Wave Activity (A) $(\mathcal{A})$. Our data set is the ERA5 reanalysis for the period 1979–2013. Results for κyy show important zonal asymmetries. In the lower and middle stratosphere, κyy is highest at midlatitudes, particularly around the prime meridian. This location is surrounded by manifolds associated with hyperbolic trajectories emanating from the outer SPV boundary. κyy is also high within the SPV, and near the locations where the SPV boundary is open. Zonal asymmetries are also clear in W at midlatitudes. The larger values of A $\mathcal{A}$ are at high latitudes and upstream of the opening of the vortex boundary. The role of quasi‐geostrophic waves on the south‐north shift of the midlatitude westerlies is highlighted. In particular, the waves contribute to open the SPV boundary at around 90W. The interannual variability of κyy is explored by contrasting winters with positive‐negative Northern Annular Mode index, and Sudden Stratospheric Warmings of displacement‐split type. Plain Language Summary: This paper applies a novel approach to studying eddy mixing and transport in the northern middle atmosphere during winter (December–January–February). The approach is based on the concept of Lagrangian diffusivity, a measure of how quickly air parcels mix together. Unlike traditional diagnostics that rely on longitudinal or contour‐based averages, Lagrangian diffusivity provides a detailed three‐dimensional view of the mixing. This approach, hitherto restricted to oceanographic applications, is a significant contribution of the present work. We employ a Lagrangian descriptor, a tool based on parcel trajectory length, to locate the stratospheric polar vortex SPV boundary. Also, we investigate the influence of quasi‐geostrophic motions on the SPV using wave activity flux and local wave activity (LWA). The data set is the ERA5 reanalysis from 1979 to 2013. The results reveal pronounced zonal asymmetries in Lagrangian diffusivity and wave activity flux. Mixing is highest at midlatitudes around the prime meridian and at locations within the SPV. LWA is elevated at high latitudes and upstream of the climatological vortex boundary opening, highlighting the role of quasi‐geostrophic waves in the southward displacement of midlatitude westerlies. Comparisons are made of diffusivity during weak and strong vortex winters, as well as of during winters with and without Sudden Stratospheric Warmings. Key Points: Locally, the largest values of Lagrangian diffusivity around the vortex are along the prime meridian at all levels in the stratosphereThe location of maximum diffusivity is bounded by the edge of the climatological displaced vortex and a manifold emanating from the vortex edgeWhen the Northern Annular Mode is positive, the vortex boundary closes and diffusivity is much lower inside [ABSTRACT FROM AUTHOR]

In this paper the meteorological drivers of North American Monsoon (NAM) extreme precipitation events (EPEs) are identified and analyzed. First, the NAM area and its subregions are distinguished using self‐organizing maps applied to the Climate Prediction Center global precipitation data set. This reveals distinct subregions, shaped by the inhomogeneous geographic features of the NAM area, with distinct extreme precipitation character and drivers. Next, defining EPEs as days when subregion‐mean precipitation exceeds the 95th percentile of rainy days, five synoptic features and one mesoscale feature are investigated as potential drivers of EPEs. Essentially all EPEs can be associated with at least one selected driver, with only one event remaining unclassified. This analysis shows the dominant role of Gulf of California moisture surges, mesoscale convective systems and frontal systems in generating NAM extreme precipitation. Finally, a frequency and probability analysis is conducted to contrast precipitation distributions conditioned on the associated meteorological drivers. The findings demonstrate that the co‐occurrence of multiple features does not necessarily enhance the EPE probability. Plain Language Summary: Extreme precipitation is of great importance for both geophysical and socioeconomic reasons. This study first identifies geographic subregions of the North American Monsoon within which extreme precipitation exhibits distinct characteristics. The extreme precipitation events in each subregion are then associated with at least one candidate atmospheric driver, revealing the dominant precipitation drivers among subregions. Depending on the subregions and driver, precipitation rates may increase or decrease when two candidate factors co‐occur. Several such double driver combinations are examined. Key Points: Seven subregions of the North American Monsoon with distinct precipitation characters are identifiedGulf of California moisture surges, mesoscale convective systems and frontal systems are primary drivers of extreme precipitationA probability analysis reveals the nonlinear and non‐additive characters of the interactions among extreme precipitation event drivers [ABSTRACT FROM AUTHOR]

The Great Arctic Cyclone 2012 (AC12) is used to understand the role of initial condition errors in the predictability at 2–3‐day forecast range of a high‐impact summer Arctic Cyclone (AC). Ensemble sensitivity analysis (ESA) is first performed to identify potentially sensitive regions of the cyclone evolution using an ensemble baseline forecast with conventional in situ observations assimilated. A pseudo‐observation method is then introduced to investigate impacts of hypothetical observations in these sensitive but unobserved regions. In the baseline experiments with in situ observations assimilated, the forecasted AC12 reaches its peak intensity 18 hr earlier than in the verifying Global Forecast System Analysis (GFS‐ANL) and the cyclone track is biased toward the southwest. Using ESA, the time of peak intensity and the cyclone track error are identified to be sensitive to the upstream trough, downstream ridge, and the tropopause polar vortex (TPV) to the northeast (NE TPV) of the AC12. These features were not observed by the in situ observation networks. To examine the impact of the observation gaps, pseudo‐observations drawn from GFS‐ANL are assimilated. Pseudo‐observations sample the three features separately to study the impact of the initial condition error on the predictability of AC12. The cyclone peak intensity timing error and track error are greatly reduced when the initial condition error is reduced near the NE TPV. A southward expansion of the NE TPV and the corresponding southward shifting low‐level front lead the forecasted AC12 to progress to the east, which better agrees with the verifying GFS‐ANL. Plain Language Summary: Accurate prediction of strong Arctic cyclones is critical to the scientific and economical activities in the Arctic. However, the sparse observation networks in the Arctic pose challenges to the data assimilation system to constrain the initial conditions in various features of the Arctic Cyclones. This paper used a case study approach to investigate the impact of initial condition errors in different features on the development of the 2012 Great Arctic Cyclone. Synoptic features that are sensitive to the track and timing of the cyclone were first identified using ensemble sensitivity analysis. Assimilating in situ conventional observations did not sufficiently constrain the initial conditions in the sensitive features. Hypothetical pseudo‐observations from an external analysis system were then designed and assimilated to study how each sensitive feature affects the prediction of the physical processes of cyclone development. The results showed that more observations inside the TPV to the northeast of the cyclone brought the most significant forecast improvements at a 2‐day forecast lead time. Key Points: Ensemble sensitivity analysis reveals track and deepening trend forecast biases of Arctic Cyclone 2012 are sensitive to unobserved synoptic scale featuresPseudo‐observation experiments show the northeast tropopause polar vortex (TPV) is the most impactful feature among sensitive featuresSouthward expansion of the northeast TPV impacts the cyclone evolution by shifting the low‐level front southward [ABSTRACT FROM AUTHOR]

Kjellstrand et al. (2022), https://10.1029/2021JD036232 describes the evolution and dynamics of a strong, large‐scale Kelvin‐Helmholtz instability (KHI) event observed in polar mesospheric clouds (PMCs) on 12 July 2018 by high‐resolution imagers aboard the PMC Turbulence (PMC Turbo) stratospheric long‐duration balloon experiment. The imaging provides evidence of KH billow interactions and instabilities that are strongly influenced by gravity waves at larger scales. Specific features include initially separated regions of KHI, secondary convective and KH instabilities of individual billows, and "tubes" and "knots" that arise where billow cores are mis‐aligned or discontinuous along their axes. This study describes a direct numerical simulation of KH billow interactions in a periodic domain seeded with random initial noise that enables excitation of multiple KH billows exhibiting variable phase structures that capture multiple features of the observed KHI dynamics. Variable KH billow phases along their axes yield initial vortex tubes having diagonal alignments that link adjacent, but mis‐aligned, billow cores. Weak initial vortex tubes and billow cores having nearly orthogonal alignments amplify, interact strongly, and drive intense vortex knots at these sites. These vortex tube and knot (T&K) dynamics excite "twist waves" that unravel the initial vortex tubes, and drive increasingly strong vortex interactions and a cascade of energy and enstrophy to successively smaller scales in the turbulence inertial range. The implications of T&K dynamics are much more rapid and intense breakdown and decay of the KH billows, and significantly enhanced energy dissipation rates, where these interactions occur. Plain Language Summary: Kelvin‐Helmholtz instabilities (KHI) are ubiquitous throughout the atmosphere (and oceans) and have been studied for many years. Interactions between adjacent KH billows seen in early laboratory experiments named "tubes and knots" by Steve Thorpe were only recently recognized in imaging in the mesosphere. These KHI interactions were seen in the laboratory to lead to turbulence faster than secondary instabilities of individual billows. Despite very many papers describing KHI modeling, none have addressed tube and knot dynamics prior to their recent identification in the mesosphere. This paper describes modeling performed to explore the tube and knot dynamics seen during the PMC Turbo experiment and described in the companion paper. Results reveal that tube and knot dynamics yield dramatic increases in energy dissipation that may have important influences in the atmosphere and oceans. Key Points: Multi‐scale Kelvin‐Helmholtz (KH) instability dynamics arise due to natural variations in background flows and initial conditionsInteracting KH billows induce "tubes" and "knots" that form rapidly and are distinct from secondary instabilities of individual billowsTube and knot dynamics evolve more rapidly and yield larger energy dissipation rates than those of individual KH billows [ABSTRACT FROM AUTHOR]

Comprehensive marine atmospheric turbulence observation data, meteorological sounding and sea surface conditions in the South China Sea were employed to analyze and parameterize the vapor profile in the stable marine atmospheric boundary layer. The observations involved a three‐dimensional ultrasonic anemometer, water vapor carbon dioxide analyzer, radiosonde, and buoy. This paper theoretically determined that the water vapor profile function φq differs from the temperature profile function φh and that φq should be independently parameterized. A linear relationship existed between the dimensionless water vapor gradient and stability parameters based on the observation results, and φq was then obtained as φq(z/L) = az/L + b, in which the stability covered the stability range (z/L > 1). This result was applied in the Tropical Ocean‐Global Atmosphere Coupled‐Ocean Atmosphere Response Experiment bulk flux algorithm, and the simulation of the latent heat flux was improved. Plain Language Summary: Monin and Obukhov established the relationship between momentum flux and temperature flux and their respective vertical gradients. Using these two relations and some parameterization methods, we can calculate momentum flux and sensible heat flux. However, there is no physical basis for the above relationship for latent heat flux, and the effect of vertical distribution of water vapor on latent heat flux is not taken into account. In this paper, the relationship between water vapor flux and water vapor gradient is established and verified by marine observation data. Thus it can have more physical meaning and calculate the latent heat flux more accurately. This study is useful for understanding the boundary layer and model development. Key Points: The linear relationship between the dimensionless specific humidity gradient and the stability parameter is establishedThe temperature mixing length is longer than the specific humidity, and water vapor is more difficult to transfer in the vertical directionThe vertical distribution of water vapor has an important influence on the parameterization of water vapor flux [ABSTRACT FROM AUTHOR]

This paper investigates the lower‐to‐upper atmosphere coupling at high latitudes (>60°N) during the northern winter months of 2012–2013 years, which includes a period of major Sudden "Stratospheric" Warming (SSW). We perform statistical analysis of thermosphere wind disturbances with periods of 30–70 min, known as the medium scale traveling atmospheric disturbances (MSTADs) in atomic oxygen green line (557.7 nm) near ∼120 km and red line (630.0 nm) emissions near ∼250 km observed from Scanning Doppler Imagers (SDIs) over Alaska. The SDI MSTADs observations (60°–75°N) are interpreted in conjunction with the previous daytime medium‐scale traveling ionospheric disturbance (MSTID) observations by SuperDARN midlatitudes (35°–65°N) radars in the F‐region ionosphere and western hemisphere, which confirm findings from the SDI instruments. Increases in MSTAD activity from SDIs show correlations with the increasing meridional planetary wave (PW) amplitudes in the stratosphere derived from MERRA2 winds. Furthermore, a detailed study of the lower atmospheric conditions from MERRA2 winds indicates that the lower atmospheric sources of MSTADs are likely due to the stratospheric generated Gravity Waves (GWs) and not orographic GWs. Favorable stratospheric propagation conditions and polar vortex disturbances resulting from the increased PW activity in the stratospheric region both appear to contribute to increased MSTAD activity in the thermosphere. Additionally, the results show that the MSTID activity from SuperDARN HF radars at mid latitudes during the January 2013 SSW is lower than the MSTAD activity in SDI winds at high latitudes. Plain Language Summary: The objective of this paper is to investigate how Atmospheric Gravity Waves (GWs) contribute to the vertical coupling of the atmosphere‐ionosphere system. When propagating from below, GWs can create medium‐scale traveling atmospheric disturbances (MSTADs) in the thermosphere, which in turn generate medium‐scale traveling ionospheric disturbances (MSTIDs) in the ionosphere. This study examines the coupling between the lower and upper atmosphere at high latitudes (>60°N) during the winter of 2012–2013, including a period of Sudden "Stratospheric" Warming (SSW) and quiet geomagnetic conditions. The analysis of MSTAD day‐to‐day activity observed using Scanning Doppler Imagers that measure airglow emissions in Alaska confirms similar MSTID activity in observed ion density from SuperDARN mid‐latitudes radars. The study also confirms that the increased MSTAD activity in the lower‐to‐upper thermosphere is due to stratospheric‐generated GWs, rather than orographic GWs, and is linked to increased planetary wave activity in the stratospheric meridional winds and polar vortex disturbances. Therefore, the paper not only highlights the global characteristics of thermosphere GW‐like variations in the western hemisphere but also their connection to lower atmosphere dynamics. Key Points: Variations of medium scale traveling atmospheric disturbances (MSTADs) observed in Scanning Doppler Imager (SDI) winds near ∼250 km agree with SuperDARN HF radars medium‐scale traveling ionospheric disturbances observations over western hemisphereMSTAD variability in the thermosphere over Alaska during winter months demonstrates a correlation with dynamics in the stratosphereIncreased MSTAD activity in the thermosphere correlates with increasing meridional planetary wave amplitudes in the stratosphere [ABSTRACT FROM AUTHOR]

The most serious heavy precipitation in the past 43 years occurred over Fujian‐Jiangxi region from late May to early June of 2006, causing significant economic losses. Using the ERA5 reanalysis database and daily precipitation collected at 2,479 surface meteorological stations in China, the present study investigates the relationship between the heavy precipitation over Fujian‐Jiangxi region in late spring‐early summer of 2006 and baroclinic wave packets in the upper troposphere. Information flow between the two systems has been diagnosed. Results indicate that different from the disturbance source for Meiyu in the middle and lower reaches of the Yangzi River, the disturbance source for this heavy precipitation originated from areas near the Syrian Desert to the north of the Arabian Peninsula and propagated along the northwest‐southeast direction, reaching Fujian‐Jiangxi region 4 days later. This kind of baroclinic wave packets provide the necessary energy for the occurrence and persistence of heavy precipitation. Analysis of wave activity flux vectors indicates that during the heavy precipitation period, disturbance energy was transported from the upstream westerly belt to Fujian‐Jiangxi region almost every day. Obviously, there existed information transfer between the two regions, re‐confirming that the upstream Rossby wave packets affect the Fujian‐Jiangxi precipitation. The above results provide helpful hints for a better understanding of the mechanisms for heavy precipitation in this region and will be helpful for its effective prediction. Plain Language Summary: In the late spring and early summer of 2006, the strongest precipitation occurred in Fujian‐Jiangxi in China from 1979 to 2021 at the same time. It is of significance to study the causes of such extremely heavy precipitation events to predict future similar events in advance to reduce casualties and economic losses. Atmospheric motions could be regarded as wave motions, and Rossby wave is one of the major atmospheric waves. Multiple Rossby waves together could be simply regarded as Rossby wave packets (RWPs). In this paper, we have used as many methods as possible to find whether RWPs have influence on precipitation and how. A large amount of evidence shows that RWPs have significant influence on this extreme precipitation, especially by providing energy. Four days before the beginning of the heavy precipitation, RWPs began to spread from near the Syrian desert to Fujian‐Jiangxi region. Moreover, a new mathematical method is used to prove that there is a causal relationship between wave packet activities and this heavy precipitation. Nevertheless, the above research is only for such an extreme precipitation event. Whether each extreme precipitation event corresponds to obvious wave packet activities needs further study. Key Points: Rossby wave packets (RWPs) propagated from areas near the Syrian Desert to Fujian‐Jiangxi region before the heavy precipitation beganRWPs at 300 hPa provide disturbance energy daily to heavy precipitation over Fujian‐Jiangxi region during the precipitation periodThere is evident information transfer between RWPs activities from the upstream area with the heavy precipitation in Fujian‐Jiangxi region [ABSTRACT FROM AUTHOR]

The ash and gas released by large explosive volcanic eruptions rises to its neutral buoyancy level in the atmosphere, then spreads laterally to form an umbrella cloud. Density stratification of the atmosphere generates buoyancy forces in the cloud, which drive the outward spread. Although umbrella clouds are often modeled as circular axisymmetric structures, in practice they are usually influenced quite strongly by the meteorological wind, with spread in the upwind direction halted by the oncoming wind, and different rates of spreading in the downwind and crosswind directions. In this work, we derive a simple parametrization of non‐axisymmetric umbrella cloud spreading from a much more complex physically based shallow‐layer intrusion model. The new parametrization is quick to evaluate and so is suitable for use in operational Volcanic Ash Transport and Dispersion Models (VATDMs). In contrast to previous parametrizations, in which there is assumed to be no interaction between a circular umbrella cloud and the meteorological wind, here the umbrella cloud is influenced by the wind and adopts a shape determined by the balance of buoyant spreading and downwind drag forces. We apply the new scheme to four historical case studies of eruptions at Puyehue 2011, Pinatubo 1991, Ulawun 2019, and Calbuco 2015. The results are compared with VATDM simulations using a conventional circular umbrella cloud parametrization. Using the new scheme, good descriptions of cloud spread are recovered and the prediction of horizontal ash distribution is improved relative to the axisymmetric parametrization. Plain Language Summary: Erupting volcanoes eject a mixture of fine ash and gas, which rises into the upper atmosphere, where the ash presents a major hazard to jet aircraft. Large volcanic eruptions produce a giant cloud of ash, known as an umbrella cloud, which spreads horizontally. The spread of this umbrella cloud is driven by the variation in atmospheric density across its depth. Umbrella clouds are commonly assumed to be cylindrical, but in reality, they often assume a more complex shape, due to the influence of wind. In this paper, we derive a new mathematical model of umbrella cloud spreading that includes the effect of wind, which can be simulated on a computer quickly enough to produce practical forecasts of volcanic ash transport. The new model is applied to four historical eruptions and shown to produce good predictions of the umbrella cloud shape and extent in each case. Compared to existing cylindrical models, the distribution of ash inside the cloud is much improved. Key Points: A non‐axisymmetric parametrization of umbrella cloud spreading in wind is derived from a shallow‐layer intrusion modelThe new parametrization describes buoyancy‐driven spreading in both near‐circular umbrella clouds and in strongly wind‐blown plumesThe predictions of a Volcanic Ash Transport and Dispersion Model are improved by the new parametrization, compared to axisymmetric models [ABSTRACT FROM AUTHOR]

Shallow cloud decks residing in or near the boundary layer cover a large fraction of the Southern Ocean (SO) and play a major role in determining the amount of shortwave radiation reflected back to space from this region. In this article, we examine the macrophysical characteristics and thermodynamic phase of low clouds (tops <3 km) and precipitation using ground‐based ceilometer, depolarization lidar and vertically‐pointing W‐band radar measurements collected during the Macquarie Island Cloud and Radiation Experiment (MICRE) from April 2016 to March 2017. During MICRE, low clouds occurred ∼65% of the time on average (slightly more often in austral winter than summer). About 2/3 of low clouds were cold‐topped (temperatures ≤0°C). These were thicker and had higher bases on average than warm‐topped clouds. 83%–88% of cold‐topped low clouds were liquid phase at cloud base (depending on the season). The majority of low clouds had precipitation in the vertical range 150–250 m below cloud base, a significant fraction of which did not reach the surface. Phase characterization is limited to the period between April 2016 and November 2016. Small‐particle (low‐radar‐reflectivity) precipitation (which dominates precipitation occurrence) was mostly liquid below‐cloud, while large‐particle precipitation (which dominates total accumulation) was predominantly mixed/ambiguous or ice phase. Approximately 40% of cold‐topped clouds had mixed/ambiguous or ice phase precipitation below (with predominantly liquid phase cloud droplets at cloud base). Below‐cloud precipitation with radar reflectivity factors below about −10 dBZ were predominantly liquid, while reflectivity factors above about 0 dBZ were predominantly ice. Plain Language Summary: The Southern Ocean is covered by low altitude cloud decks the majority of the time. Properties like cloud occurrence frequency, particle phase and precipitation habits determine how much solar radiation clouds reflect and how much infrared radiation they emit, which in turn affects the balance of the planet's incoming and outgoing radiation. In this paper, we examine low cloud properties observed from the ground at Macquarie Island, including how frequently they occur and at what temperatures. We study particle thermodynamic phase (liquid, ice or mixed) at cloud base and in precipitation below‐cloud. A majority of low clouds are predominantly composed of liquid phase droplets, although frozen precipitation is frequently found below cloud base. Low clouds form precipitation more often than not, much of which evaporates before reaching the ground. In below‐freezing low clouds, the majority of large raindrops & snowflakes that do reach the ground originate as frozen precipitation directly below cloud base. This indicates that ice formation is frequently active in clouds composed predominantly of liquid‐phase droplets. Lastly, we build upon an established radar‐lidar relationship that particles with radar reflectivity factors below −10 dBZ are generally liquid, whereas above 0 dBZ are most often ice phase. Key Points: Ground observations at Macquarie Island indicate low clouds occur ∼65% of the time, with about 2/3 having cloud top temperatures below 0°CCold‐topped low clouds have liquid‐phase bases ∼85% of the time & precipitate ∼3/4 of the time; below‐cloud precip. is 40% ice/mixed phaseBelow‐cloud radar reflectivity factors <−10 dBZ are predominately due to liquid phase precipitation, while >0 dBZ are predominantly ice [ABSTRACT FROM AUTHOR]

Based on potential vorticity (PV) thinking, northeast China cold vortex (NCCV) corresponds to an upper‐level high PV anomaly from stratospheric PV downward intrusion. Within a vortex, latent heating from precipitation would produce a vertical dipole of PV anomalies that would affect structures and evolution of the vortex. In this paper, three‐dimensional structures and evolution of NCCV and, effects of latent heating from precipitation along bent‐back, cold and warm fronts on them are investigated based on convection‐allowing simulations for an intense NCCV case during 8–17 June 2012. Trajectory analysis shows that the negative upper‐level diabatic PV anomaly from bent‐back frontal precipitation, near the vortex center, is the dominant contributor to erosion of high PV in the vortex core region as it is advected in, leading to the weakening of the vortex. The negative PV anomalies along the cold and warm fronts, at the east‐to‐southeast side of the vortex, are mostly advected downstream away from the NCCV. In the middle troposphere, positive PV anomalies are primarily generated along fronts and the accumulated positive PV anomalies filling the vortex region help to reinforce the low‐level cyclonic circulation. The lower‐level PV is affected by surface heating and cooling through their effects on static stability, but such effects are periodic and create mainly diurnal variations. The NCCV eventually decays as the upper‐level vortex weakens due to significant PV erosion. Plain Language Summary: Northeast China cold vortex (NCCV) is responsible for much of the warm season precipitation in northeast China. Based on convection‐allowing simulations of a characteristic case, the distribution and effects of latent heating release from precipitation in bent‐back, cold and warm frontal regions on the structures and evolution of potential vorticity (PV) within the NCCV are studied. Negative upper‐level PV anomalies generated at the bent‐back front are primarily responsible for the filamentation and erosion of high PV in the core of NCCV, leading to the weakening of the vortex. At middle‐to‐low levels, positive PV anomalies generated reinforce the low‐level circulations. However, the lower‐level PV exhibits a significant diurnal cycle, which is mainly affected by the surface heating and cooling through reducing static stability at daytime and increasing static stability during night. The overall system eventually decays as the upper‐level vortex weakens. Key Points: Precipitation in northeast China cold vortex produces negative upper‐level potential vorticity (PV) anomaly that weakens the upper level cold vortexNegative PV anomaly produced at the bent‐back front is the dominant contributor to the erosion of upper‐level high PV in the vortex centerLower‐level vortex is enhanced by positive PV anomaly from precipitation, and the PV has diurnal variations due to surface heating/cooling [ABSTRACT FROM AUTHOR]

This paper examines precipitation retrievals from Kidd et al. (2022, https://doi.org/10.3390/rs14132992) by applying the Precipitation Retrieval and Profiling Scheme to data observed by the NASA Time‐Resolved Observations of Precipitation structure and storm Intensity with a Constellation of Smallsats (TROPICS) Millimeter‐wave Sounder (TMS) onboard the TROPICS Pathfinder satellite, a precursor to the TROPICS mission. We first compare the TMS precipitation retrieval performance over ocean with those from seven current operational passive microwave radiometers, including four conical scanning sensors [Special Sensor Microwave Imager/Sounders (SSMISs) onboard F16/F17/F18 satellites, and Advanced Microwave Scanning Radiometer 2 (AMSR2)] and three cross track scanning sensors [Microwave Humidity Sounders onboard NOAA19 and MetOpB satellites, and the Advanced Technology Microwave Sounder onboard the National Polar‐orbiting Partnership satellite], using precipitation estimates from Global Precipitation Measurement Mission Microwave Imager (GMI) as the reference. We show that TMS performs similarly to the current operational cross track scanning sensors, while worse than conical scanning sensors largely due to the lack of low frequency channels (e.g., 19 GHz). Second, we create an adjusted TMS precipitation retrievals by combining the original TMS precipitation and the precipitation propagated from AMSR2 and SSMISs to the time/location of the TMS overpass, using our previously developed microwave‐to‐microwave propagation technique. Results show that the adjusted TMS precipitation performs better than the original TMS estimates. For example, the correlation of the original TMS and GMI estimates is 0.35 but increases to 0.49 when the adjusted TMS result is compared to GMI. The propagation technique lays the foundation to improve TMS precipitation when incorporating them into level‐3 merged products. Plain Language Summary: Passive microwave radiometers provide the essential inputs for generating global high‐quality precipitation estimates and can be classified into cross track scanning and conical scanning sensors. While radiometers onboard large satellites currently dominate in providing observations, small satellites are rapidly emerging with novel designs and high‐performance instruments at a much lower cost. The Time‐Resolved Observations of Precipitation structure and storm Intensity with a Constellation of Smallsats (TROPICS) is a small satellite mission funded by NASA and developed at Massachusetts Institute of Technology, Lincoln Laboratory, with its first satellite, the TROICS Pathfinder launched in June 2021. The payload is the TROPICS Millimeter‐wave Sounder (TMS), a cross‐track scanning radiometer. The performance of TMS precipitation retrieval by Precipitation Retrieval and Profiling Scheme has been evaluated over ocean. Using TROPICS Pathfinder observations, we show that TMS performs similarly to current operational cross‐track scanning sensors, while performing worse than conical scanning radiometers. Additionally, we improve TMS's precipitation retrievals by combining the original TMS estimates with precipitation estimates from high‐information‐content conical scanning radiometers propagated to the time/location of the TMS overpass. This study may enhance the usefulness of the TMS precipitation products and is especially useful when incorporating them into widely used global gridded precipitation products. Key Points: TMS retrievals are comparable to current operational cross‐track scanning radiometersTMS retrievals are worse than those from conical scanning radiometersTMS retrievals can be improved by incorporating retrievals from conical scanning radiometers [ABSTRACT FROM AUTHOR]