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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 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]

The Journal of Geophysical Research: Atmospheres expresses gratitude to the 2854 scientists who reviewed manuscripts for the journal in 2023. Peer review is essential for maintaining the integrity and rigor of scientific research. The reviews have contributed to the improvement of paper quality, the generation of new ideas, and the advancement of young scientists' careers. The journal acknowledges the reviewers' selfless service and dedication to the scientific community and looks forward to their continued support. [Extracted from the article]

Land surface temperature anomalies can be linked to changes in local surface energy balance, although the relationship between surface temperature variability and individual radiative processes remains unclear. In this paper, we quantify the contributions of surface radiative effects and non‐radiative heat fluxes to the variance of monthly land surface temperature using European Centre for Medium‐Range Weather Forecasts Reanalysis v5 data and Coupled Model Intercomparison Project Phase 6 simulations. The surface energy budget equation is used to link changes in surface radiation, surface heat fluxes and land surface temperature. Subsequently, surface radiation is decomposed into the radiative effects of clouds, air temperature, surface albedo and relative humidity using radiative kernels. The contributions of these radiative processes, including their coupling effects, are quantified using covariance matrices. The results reveal the air temperature radiative effect to be the most significant contributor to the variability of land surface temperature. In addition, the covariance terms reveal important coupling effects. For example, the contribution from the cloud radiative effect is found to be substantially dampened by its coupling with surface heat fluxes. The air temperature radiative effect is further decomposed into a forcing component and a feedback component using different regression methods, in an attempt to separate the air temperature radiative effect as the driver of the surface temperature variability. The cloud radiative effect becomes the primary contributor to the variance of surface temperature after separating the air temperature feedback, while the contribution of the air temperature radiative forcing remains important. Plain Language Summary: Land surface temperature is critical environmental variable. Through a statistical analysis of the European Centre for Medium‐Range Weather Forecasts Reanalysis v5 data and Coupled Model Intercomparison Project Phase 6 multi‐model simulations, we have explained the land surface temperature variability in relation to the surface energy fluxes. Specifically, we have attributed the variability of monthly surface radiation to the effects of different meteorological variables such as clouds, air temperature, surface albedo and relative humidity. Our findings suggest that the radiative effect of air temperature is the primary contributor to the variance of land surface temperature in most regions, although this effect includes a strong feedback effect of air temperature to surface temperature changes. After separating this feedback effect, the cloud radiative effect becomes the primary contributor. On the other hand, the contribution from cloud radiative effect is significantly counteracted by its coupling with surface heat fluxes. Key Points: The variance of land surface temperature is decomposed to contributions from surface radiative effects and heat fluxesThe radiative effect of air temperature is found to contribute the most to surface temperature variabilityThe cloud radiative effect is found to be the primary contributor after decoupling the air and surface temperature variations [ABSTRACT FROM AUTHOR]

In this paper, we applied a variety of statistical methods to study gravity waves in the troposphere and lower stratosphere in the Brazilian sector, using a large database from Instituto de Controle do Espaço Aéreo (ICEA) of radiosonde measurements carried out in 2014 at 32 locations in the Brazilian territory totaling 49,652 wind and temperature profiles. The average wind profiles were computed and classified by means of a hierarchical cluster analysis. The kinetic and potential energy densities of gravity waves were determined using a detrending technique based on the Least Squares Method and the Fast Fourier Transform. By analyzing the energy density time series it was found that tropospheric average values are consistently larger in the months of winter, late autumn and early spring. Stratospheric average values of variability and kinetic energy density are also consistently larger in this period. A systematic search for quasi monochromatic waves was carried out and their main characteristics such as horizontal/vertical wavelengths and velocities were determined both in the troposphere and lower stratosphere. A correlation analysis between the troposphere and the lower stratosphere based on the measured parameters was used to investigate the wave coupling between the two layers, and no significant correlation was found. Finally, a spatial correlation analysis between energy densities measured at different aerodromes in the same atmospheric layer was carried out, showing that energy densities are spatially correlated for distances less than 3,000–4,000 km. Plain Language Summary: Like waves in the ocean that can be easily seen by any observer in the beach, the atmosphere is also permeated by waves of similar nature, called Gravity Waves (GWs). These waves transport energy through the atmosphere, eventually breaking, reflecting or dissipating at some point. In this work we investigated the characteristics of these waves using weather data retrieved by weather balloons released from several locations in the Brazilian territory in 2014. By analyzing the measurements, we quantified parameters related to GWs, such as the kinetic and potential energy densities. We also investigated GWs that have well defined frequencies, called monochromatic waves, and determined their wavelengths, phases, amplitudes and phase velocities. We did not find correlations between the wave energies in the troposphere and the low stratosphere, which is an evidence of weak coupling between both layers. This result suggests that GWs characteristics are substantially modified in the perturbed, turbulent and windy region between the troposphere and low stratosphere. Moreover, we also identified the prevailing behavior of the winds in each of the studied locations. Key Points: A comprehensive survey on gravity waves using radiosonde data from 32 locations in the Brazilian territory totaling 49,652 profilesThe energy densities of gravity waves are spatially correlated within a region of approximately 3,000 km of radiusIt was found that gravity waves propagating in the troposphere and low stratosphere are uncorrelated in the studied locations [ABSTRACT FROM AUTHOR]

Seasonal forecasts are commonly issued in the form of anomalies, which are departures from the average over a specified multiyear reference period (climatology). The model climatology is estimated as the average of the retrospective forecasts over the hindcast period. However, different operational centers that provide seasonal ensemble predictions use different hindcast periods based on their model climatology. Additionally, the hindcast periods of recently developed and upgraded newer models have shifted in the recent years. In this paper, we discuss the recent challenges faced by APCC multi‐model ensemble (MME) operations, especially changes in the hindcast period for individual models. Based on the results of various experiments for MME prediction, we propose changing the hindcast period, which is the most appropriate solution for APCC operation. This makes the newly developed models join the MME and increases the total number of participating models, which facilitates the skill improvement of the MME prediction. Plain Language Summary: In seasonal forecasting, it is well known that the MME, which combines different single‐model predictions from various operational and research centers, is a more effective way to improve forecast skill. Since 2005, the APCC has provided the MME seasonal forecasts, and the models participating in the APCC MME operations have been continuously changing. In particular, as the hindcast periods of newly developed models shift to the latest, they cannot participate in operational MME forecasts because of climatological discrepancies. However, over time, as the number of new models expected to provide skillful forecasts gradually increases, the APCC faces the challenge of continuously reducing the number of participating models or changing the hindcast period to more recent years. Considering various aspects such as the number of participating models, skills, and climatology period, we selected the most appropriate method for APCC operation. Thus, the MME prediction skill has improved over most of the globe and seasons because of the increase in the number of participating models, particularly the inclusion of newer models. Key Points: APCC, which combines all the information from different ensemble prediction systems, recently faced challenges in hindcast period issuesThe proposed solution leads to an increase in the number of models contributing to MME prediction, particularly recently developed modelsIt shows improved skills for both temperature and precipitation predictions over most of the globe and seasons [ABSTRACT FROM AUTHOR]

An analysis is presented of the precipitation bias and change signal in an ensemble of regional climate model (RCM) (RegCM4) projections driven by multiple general circulation models (GCMs) over China. RegCM4 is driven by five different GCMs for the 120‐year period 1979–2099 at 25 km grid spacing, under the representative concentration pathway RCP8.5. We find that the GCMs and RegCM4 reproduce the general spatial pattern of precipitation over China in all four seasons, with RegCM4 providing greater spatial detail, especially over areas with complex terrain. The spatial patterns of precipitation bias show common features between the GCMs and RegCM4, characterized by an underestimation in the wetter regions, and an overestimation in the drier ones. Systematic increases of precipitation are projected in northern China, most pronounced in the Northwest basins, by both the GCMs and RegCM4 in all seasons except summer, when more mixed results are found. In addition, weak correlations of the projected change patterns are found in summer between the GCMs and nested RegCM4, indicating the greater role played by the representation of local convection processes during this monsoon season. The projections across the RegCM4 experiments show higher consistency and lower spread compared to the GCM ensemble, again indicating that the nested model physics significantly modulates the change signal deriving from the GCM boundary forcing. Plain Language Summary: China is a vulnerable country to climate change due to its dense population, unbalanced social and economic development, shortage of water resources, and fragile ecosystems. How future precipitation will change over the region is of great concern for the general public and decision makers. This paper presents a first analysis of precipitation simulations from a set of five RCM (RegCM4) 21st century climate change projections, driven by coarse resolution general circulation models (GCMs) over China. We find that the spatial patterns of precipitation bias show common features between the GCMs and RegCM4, characterized by a precipitation underestimation in the wetter regions, and an overestimation in the drier ones. Systematic increases of precipitation are projected in north China by both the GCMs and RegCM4 in all seasons except summer, when, weak correlations of the projected change patterns are found between the GCMs and nested RegCM4, indicating the greater role of the representation of local convection processes during this monsoon season. The projections across the RegCM4 experiments show higher consistency and lower spread compared to the GCM ensemble, again indicating that the nested model physics significantly modulates the change signal deriving from the GCM boundary forcing. Key Points: The spatial patterns of bias show common features between the GCMs and RegCM4RegCM4 provides greater spatial detail of present day precipitation simulation compared to the GCMs and finer structures of future changesThe change patterns across the RegCM4 projections show a high correlation, but not always between each pair of driving GCM and RegCM4 [ABSTRACT FROM AUTHOR]

In this paper, the effects of spring soil moisture (SM) anomalies in the mid‐latitudes on the atmospheric circulation in summer over the Northern Hemisphere (NH) are investigated. The results show that there are two regions of maximum interannual variability of the spring SM in the mid‐latitudes, which are located in central North America (CNA) and Europe and central Asia (ECA). In addition, the interannual variation of spring SM anomalies between CNA and ECA exhibits a seesaw pattern. The CNA–ECA seesaw pattern of the spring SM anomalies leads to the surface heat anomalies having opposite phases in CNA and ECA from spring to summer, which subsequently cause the opposite phase of baroclinicity anomalies in spring. The anomalous meridional temperature advections in spring cause the baroclinicity anomalies to have the same phase around CNA and ECA in summer. Corresponded with the same phase of baroclinicity anomalies, the anomalous centers of the stationary Rossby wave train (RWT) and Rossby wave source (RWS) have the same phase in CNA and ECA in summer. Through analysis of the vorticity budgets, the maintenance mechanism of the RWT in summer is considered as a positive feedback that anomalous meridional winds characterized by a RWT, transport the mean absolute vorticity and subsequently lead to an anomalous RWS, which in turn maintains the stationary RWT. Numerical experiments further demonstrate the effects of CNA–ECA seesaw pattern of spring SM anomalies on stationary RWT and RWS in summer. Plain Language Summary: The interannual variation of soil moisture (SM) anomalies in mid‐latitudes exhibits a seesaw pattern in central North America (CNA) and Europe and central Asia (ECA). When spring SM anomalies are negative in CNA and positive in ECA, positive (negative) sensible heat flux anomalies are observed in CNA (ECA) while negative (positive) latent heat flux anomalies are observed in CNA (ECA) from spring to summer, or vice versa. Influenced by surface heat anomalies, baroclinicity anomalies exhibit a seesaw pattern in CNA and ECA in spring, and have same phase in CNA and ECA in summer due to anomalous meridional temperature advections. The baroclinicity anomalies in CNA and ECA in summer contribute to the same variation of wave‐flow interactions. The Rossby wave train (RWT) induced and maintained by CNA–ECA seesaw pattern of spring SM anomalies in spring can be maintained by its meridional transportation of the mean absolute vorticity in summer. Apparently, spring SM anomalies in the middle latitudes can persistently influence the large‐scale atmospheric circulations over the Northern Hemisphere by maintaining RWT. Key Points: Seesaw pattern of spring soil moisture anomalies in central North America (CNA) and Europe and central Asia (ECA)Baroclinicity anomalies change from opposite to same phase in CNA and ECA during the transition from spring to summerRossby wave train induced by spring CNA–ECA seesaw pattern is maintained by its meridional mean absolute vorticity transportation in summer [ABSTRACT FROM AUTHOR]

The National Oceanic and Atmospheric Administration (NOAA)'s National Water Model (NWM) provides analyses and predictions of hydrologic variables relevant to drought monitoring and forecasts at fine time and space scales (hourly, 0.25–1 km). We present results exploring the potential for NWM soil moisture and streamflow analyses to inform operational drought monitoring. Both agricultural and hydrologic drought monitoring rely either explicitly or implicitly on an accurate representation of anomalous soil moisture values. Much of our analysis focuses on comparisons of soil moisture anomalies in the NWM to those from in‐situ observations. To establish benchmarks for NWM soil moisture skill, we also include other gridded data sets currently used to inform the US Drought Monitor, specifically those from the North American Land Data Assimilation System phase 2 (NLDAS‐2) land surface models. We then compare NWM streamflow low flows with ∼500 stream gauges from the United States Geological Survey (USGS) Hydro‐Climatic Data Network of undisturbed basins. The NWM soil moisture simulation's skill parallels that from NLDAS‐2. The accuracy of drought condition identification from NWM streamflow exceeds that based on soil moisture as determined by Critical Success Index scores for extreme dry percentiles. Different meteorological forcings are used in the operational NWM cycles than those used in this retrospective analysis. This forcing disconnect, together with concerns about current‐generation land surface model soil moisture‐transport schemes, inhibit its current operational use for drought monitoring. Plain Language Summary: The National Oceanic and Atmospheric Administration's National Water Model offers output relevant for drought monitoring. This paper evaluates the National Water Model soil moisture and streamflow with drought applications in mind, and compares those evaluations to other modeling tools currently used to inform drought monitoring. We find the model's ability to estimate anomalous low flows exceeds its ability to estimate anomalously dry soils, and discuss its current potential to inform operational drought monitoring. Key Points: Retrospective NOAA National Water Model soil moisture and streamflow were evaluated for drought‐monitoring applicationsThe National Water Model was comparable in skill to land model guidance currently used to inform the US drought monitorThe National Water Model's operational forcing strategy currently limits its application for real‐time drought monitoring [ABSTRACT FROM AUTHOR]

The discovery of smoke‐induced dynamical anomalies in the stratosphere associated with the 2019/2020 Australian New Year pyrocumulonimbus (pyroCb) super outbreak initiated a new field of study involving aerosol/weather anomalies. This paper documents the dynamical anomalies associated with the February 2009 Australian Black Saturday pyroCb outbreak. Positive potential vorticity anomalies (indicating anticyclonic rotation) with horizontal extent ∼1000 km and vertical thickness ∼2 km are associated with the plume, which we classify as a Smoke With Induced Rotation and Lofting (SWIRL). The SWIRL initially formed east of Australia, but then moved westward, crossing over Australia, and continuing to Africa. The SWIRL lasted for nearly three weeks (13 February–4 March), traveling ∼27,000 km and rising from potential temperatures of ∼410–500 K (altitudes ∼18–21 km). The altitude of the SWIRL is corroborated with coincident satellite‐based profiles of H2O, CO, HCN, O3, and aerosol extinction. A vertical temperature dipole (±3 K) accompanied the PV anomaly, as verified with coincident Global Navigation Satellite System radio occultation temperatures. The SWIRL dissipated as it passed over Africa. Operational ECMWF forecasts with early initialization (13 February) and late initialization (21 February) are examined. In the early case, the forecasted PV anomaly disappeared within 4 days, as expected due to lack of smoke heating in the forecast model. In the late case, while the forecasted PV anomaly was weaker than in the reanalyzes, a remnant anomaly remained out to 10 days. Plain Language Summary: Large bushfires in February 2009 in Australia led to large thunderstorms called pyrocumulonimbus (pyroCb) that injected significant amounts of smoke high into the atmosphere. When this smoke was warmed by absorbing sunlight it became buoyant and started to rise and rotate. This feature has been named a Smoke With Induced Rotation and Lofting (SWIRL) and had previously been observed in large pyroCbs that occurred around New Year's Day 2020 and in Canadian fires that occurred in 2017. The February 2009 SWIRL was a pancake‐like structure 1000 km across and 2 km thick. It lasted for three weeks, traveling over 27,000 km, and rising from 18 to 21 km in altitude. The SWIRL was observed both in weather maps and in satellite observations of gases such as water vapor, carbon monoxide, and ozone. Forecasting the SWIRL is difficult because current numerical weather models either do not consider smoke heating or lack inputs of pyroCb events as they are occurring in real time. Key Points: The February 2009 Australian Black Saturday pyroCbs injected a smoke plume into the stratosphere that induced a mesoscale anticycloneThe anticyclone was 1000 km across and 2 km thick and traveled 27,000 km in three weeks as it rose from 18 to 21 km in altitudeSatellite trace gas and aerosol anomalies coincide with potential vorticity anomalies, confirming smoke‐induced dynamical perturbations [ABSTRACT FROM AUTHOR]

Turbulence in very stable boundary layers is typically unsteady and intermittent. The study implements a stochastic modeling approach to represent unsteady mixing possibly associated with intermittency of turbulence and with unresolved fluid motions such as dirty waves or drainage flows. The stochastic parameterization is introduced by randomizing the mixing lengthscale used in a Reynolds average Navier‐Stokes (RANS) model with turbulent kinetic energy closure, resulting in a stochastic unsteady RANS model. The randomization alters the turbulent momentum diffusion and accounts for sporadic events of possibly unknown origin that cause unsteady mixing. The paper shows how the proposed stochastic parameterization can be integrated into a RANS model used in weather‐forecasting and its impact is analyzed using neutrally and stably stratified idealized numerical case studies. The simulations show that the framework can successfully model intermittent mixing in stably stratified conditions, and does not alter the representation of neutrally stratified conditions. It could thus present a way forward for dealing with the complexities of unsteady flows in numerical weather prediction or climate models. Plain Language Summary: Limited computer resources lead to a simplified representation of unresolved small‐scale processes in weather forecasting and climate models, through parameterization schemes. Among the parameterized processes, turbulent fluxes exert a critical impact on the exchange of heat, water and carbon between the land and the atmosphere. Turbulence theory was, however, developed for homogeneous and flat terrain, with stationary conditions. At nighttime or in cold environment, turbulence is typically non‐stationary, weak and intermittent and the classical theory fails. Part of the intermittent mixing is due to turbulence enhancement by small‐scale wind variability. In the following, a random modeling approach is used to enhance turbulent mixing due to small‐scale wind variability and intermittency of mixing. The proposed approach is shown to be a viable approach to represent the effect of small‐scale variability of mixing for different atmospheric flow conditions. Key Points: A stochastic parameterization of turbulence is implemented in a Reynolds average Navier‐Stokes (RANS) model to represent unsteady mixingThe introduced stochastic perturbations of the mixing length enable the simulation of intermittent turbulence in the stable boundary layerThe stochastic unsteady RANS model does not alter the simulation of neutral conditions [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]

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]

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]

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]

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 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]

As a result of their important role in weather and the global hydrological cycle, understanding atmospheric rivers' (ARs) connection to synoptic‐scale climate patterns and atmospheric dynamics has become increasingly important. In addition to case studies of two extreme AR events, we produce a December climatology of the three‐dimensional structure of water vapor and O3 (ozone) distributions associated with ARs in the northeastern Pacific from 2004 to 2014 using MERRA‐2 reanalysis products. Results show that positive O3 anomalies reside in dry intrusions of stratospheric air due to stratosphere‐to‐troposphere transport (STT) behind the intense water vapor transport of the AR. In composites, we find increased excesses of O3 concentration, as well as in the total O3 flux within the dry intrusions, with increased AR strength. We find that STT O3 flux associated with ARs over the NE Pacific accounts for up to 13% of total Northern Hemisphere STT O3 flux in December, and extrapolation indicates that AR‐associated dry intrusions may account for as much as 32% of total NH STT O3 flux. This study quantifies STT of O3 in connection with ARs for the first time and improves estimates of tropospheric ozone concentration due to STT in the identification of this correlation. In light of predictions that ARs will become more intense and/or frequent with climate change, quantifying AR‐related STT O3 flux is especially valuable for future radiative forcing calculations. Plain Language Summary: Long filaments of rapidly moving water vapor in the atmosphere, known as atmospheric rivers (ARs), play a vital role in the Earth's water cycle. Because of this, research continues to expand into ARs' relationship with large‐scale climate patterns. In this paper, we use data from the Modern Era Retrospective analysis for Research Applications to examine several extreme ARs that made landfall on the U.S. West Coast and their relationship to the transport of ozone from the stratosphere to the troposphere. We then combine 11 years of December AR and ozone data in order to study the average trend of ozone transport in connection with ARs. We quantify the AR‐related ozone transport for the first time, and we find ARs with more intense water vapor transport result in the transport of higher concentrations of ozone. Quantifying ozone transport into the troposphere in connection with ARs is important as ARs may become more intense and/or more frequent with climate change, and ozone in the troposphere has consequences for the greenhouse effect. Key Points: Case studies and December climatology using MERRA‐2 reveal positive tropospheric ozone anomalies within dry intrusions associated with ARsAverage excess ozone concentrations are 10–13 ppbv at 400 hPa, and are even greater for increasing intensity of ARsSTT of ozone associated with ARs in the NE Pacific may account for (13 ± 2)% of the total December Northern Hemisphere STT ozone flux [ABSTRACT FROM AUTHOR]

The physicochemical properties and origins of atmospheric aerosols in the Tibetan Plateau (TP) region are a research topic of great interest, but an in‐depth understanding of this topic is challenging, partially due to a lack of intensive in situ observations. Thus, a field campaign was conducted over Yadong, a remote area on the south slope of the TP from June 11 to 31 August 2021. The aerosol loading was low, with a black carbon mass concentration of 147.4 ± 98.4 ng·m−3. Aerosol single‐scattering albedo was low (0.73 ± 0.11 at 550 nm) and increased from 450 to 700 nm wavelength. Organic matter (OM) accounting for 69.6% of the total aerosol mass and relatively high secondary organic carbon ratios, highlighting the importance of secondary formation. An interesting phenomenon observed was that the evolution of aerosols was mainly characterized by diurnal variation, which could not be explained by large‐scale atmospheric processes such as Indian summer monsoon. Instead, it was found that regional mountain‐valley winds between the Himalayas and South Asia transported polluted air masses toward the TP, especially in the afternoon when regional valley wind are expected to be the strongest and the boundary layer in South Asia is deepest. Additionally, daytime local valley wind further elevated these aerosols to higher altitudes on the TP. This paper provides insights into the transport mechanisms of aerosols from South Asia to the TP. These findings are of great importance since aerosols exhibit significant diurnal variations in the TP region. Plain Language Summary: Previous studies focused on the analysis of the physical or chemical properties of aerosols on the Tibetan Plateau, but this study provides a comprehensive examination of both. The findings reveal that aerosols on the southern slope of the Tibetan Plateau exhibit strong absorption efficiency. Aerosol single‐scattering albedo was low (0.73 ± 0.11 at 550 nm), which may be attributed to aerosol secondary generation and coating. Finally, the mechanism of pollutant transport from South Asia to the Tibetan Plateau was analyzed relies on site observations, satellite, and reanalysis data to highlight the link between diurnal variations of pollutants and transport mechanism. The specific transport mechanism be understood uniformly across different scales, including Indian summer monsoon, regional mountain‐valley winds between the Himalayas and South Asia, and local mountain‐valley winds circulation. Key Points: Aerosol single‐scattering albedo was low (0.73 ± 0.11 at 550 nm) and secondary organic matter was the major aerosol componentThe evolution of aerosols was mainly characterized by diurnal variation that was related to transport mechanism over YadongThe Himalayas‐South Asia regional mountain‐valley winds combined with local mountain‐valley winds transport aerosols to the Tibetan Plateau [ABSTRACT FROM AUTHOR]

Most tropical cyclone (TC) forecasting centers have implemented a probabilistic circle to represent track uncertainty at a specified lead time. Recent studies suggest that probability ellipses constructed from ensemble prediction systems can convey the anisotropy of track predictability. In this study, a new probability ellipse model is developed to interpret the extent of forward speed and heading uncertainties in ensemble forecasts by selecting an equal proportion of members in the along‐ and cross‐track directions. This method is validated using the 2019–2021 western North Pacific (WNP) TC track forecasts from the ensemble predictions of the European Centre for Medium‐Range Weather Forecasts, the United States National Centers for Environmental Prediction, and the Korea Meteorological Administration. When the proportion of ensemble members in the ellipse is set to 70%, more than one‐half (50.0%–73.6%) of the forecasts, depending on the lead time, indicate reduced area compared with that of the circle. The mean areas of the probability ellipses are 4.9%, 7.0%, 10.0%, and 11.5% smaller than those of the circle in 48‐, 72‐, 96‐, and 120‐hr forecasts, respectively. The forward speed shows greater uncertainty than the heading, as evidenced by the along‐track radii being larger than the cross‐track counterpart in ∼60% of the samples, regardless of the lead time. In addition, the regional distribution of the along‐track/cross‐track ratio in the probability ellipses can explain the dominant direction of the track error in a particular location. The proposed probability ellipse shows potential for application in operational TC track predictions. Plain Language Summary: Operational tropical cyclone (TC) forecasting centers usually represent the uncertainty of a TC track forecast with a circle, namely the probabilistic circle. In this paper, the circle is generalized to a probability ellipse directly using the output of ensemble prediction systems. The key to the new ellipse is that an equal proportion of ensemble members are selected in the along‐ and cross‐track directions to determine the two elliptical radii. This probability ellipse is smaller in size than the circle when representing the same level of uncertainty. The eccentric property of the probability ellipse allows for easy interpretation of the direction with larger ensemble‐forecast uncertainty. Such results can aid in operational TC track predictions and subsequent implementation of preventive measures. Key Points: The proposed probability ellipse explicitly shows the uncertainty of ensemble forecast tracks in the along‐ and cross‐track directionsThe probability ellipse is capable of explaining the dominant direction of error in track forecastsRegional distributions of uncertainty and error resemble each other in terms of magnitude and direction [ABSTRACT FROM AUTHOR]

We conduct a global assessment of the spatial heterogeneity of cloud phase within the temperature range where liquid and ice can coexist. Single‐shot Cloud‐Aerosol Lidar with Orthogonal Polarization lidar retrievals are used to examine cloud phase at scales as fine as 333 m, and horizontal heterogeneity is quantified according to the frequency of switches between liquid and ice along the satellite's path. In the global mean, heterogeneity is greatest between −15 and −4°C with a peak at −5°C, when small patches of ice are prevalent within liquid‐dominated clouds. Heterogeneity "hot spots" are typically found over the extratropical continents, whereas phase is relatively homogeneous over the Southern Ocean and the eastern subtropical ocean basins, where supercooled liquid clouds dominate. Even at a fixed temperature, heterogeneity undergoes a pronounced annual cycle that, in most places, consists of a minimum during autumn or winter and a maximum during spring or summer. Based on this spatial and temporal variability, it is hypothesized that heterogeneity is affected by the availability of ice nucleating particles. These results can be used to improve the representation of subgrid‐scale heterogeneity in general circulation models, which has the potential to reduce longstanding model biases in cloud phase partitioning and radiative fluxes. Plain Language Summary: At temperatures where ice and liquid can coexist within clouds, climate models tend to produce too much ice and too little liquid compared to satellite observations. This bias is likely caused by the assumption that liquid and ice are uniformly mixed, which results in the rapid conversion of liquid to ice for thermodynamic reasons. To reduce this bias, models need to account for the spatial heterogeneity ("patchiness") of liquid and ice that exists in the real atmosphere. The goal of this paper is to quantify this spatial heterogeneity using satellite‐based lidar observations of cloud phase. We find small pockets of ice in liquid‐dominated clouds to be more common than small pockets of liquid in ice‐dominated clouds. The greatest heterogeneity is found over the midlatitude continents, whereas phase is relatively uniform over the Southern Ocean and other maritime regions with extensive low cloud cover. In the mid and high latitudes, cloud phase tends to be more heterogeneous during spring and summer and more homogeneous during autumn and winter. These results can be used in the future to improve model representations of the thermodynamic processes responsible for biases in cloud phase. Key Points: Cloud phase heterogeneity is greatest at −5°C, when small ice patches form in majority‐liquid cloudsCloud phase is relatively homogeneous over the Southern Ocean and heterogeneous over the northern continentsFor a fixed temperature, extratropical phase heterogeneity is generally greatest during local spring and summer [ABSTRACT FROM AUTHOR]

Arctic cyclone activity is an important component of the local climate, and the frequent occurrence of extreme summer storms has raised widespread scientific interest. In this paper, we investigated the distinctive structural characteristics of intense summer Arctic cyclones by utilizing ERA‐Interim reanalysis data and employing a deep learning algorithm for cyclone detection. We found that the northern edge of Eurasia (i.e., the marginal ice zone (MIZ)) and the Alpha Ridge of Arctic Ocean (AR, i.e. central Arctic) are the two most active regions for intense Arctic cyclone activities in summer (from June to September). However, the surface conditions and coupling frequency between surface cyclone and tropopause polar vortices (TPVs) are distinct over these two regions. By further analysis of 100 intense cyclone activities in these two areas, respectively, we found that cyclones in MIZ are often smaller in size but higher in intensity at their maximum intensity, and their life cycles are generally shorter. MIZ cyclones are typically accompanied by a large Eady growth rate and frontal structure in the lower troposphere and their intensification primarily attributed to the thermal‐baroclinic process. In contrast, cyclones in AR are more frequently associated with higher potential vorticity (PV) values and pronounced PV downward intrusion from the stratosphere, as well as notable "upper warm‐lower cold" structures. The downward intrusion of TPVs and stratosphere vortices contribute to a decrease in the upper and column air mass deficit, leading to the intensification of surface Arctic cyclones in these regions. Plain Language Summary: In this study, we researched intense summer storms in the Arctic. We found that there are two main areas where these storms occur: the marginal ice zone (MIZ) near Eurasia and the Alpha Ridge (AR) in the central Arctic. However, storms in these two areas have different characteristics. In the MIZ, the storms are smaller but stronger, and they do not last as long. They are mainly driven by instability in the lower troposphere. On the other hand, the storms in AR respond more to the downward intrusion of potential vorticity from the stratosphere. These storms have a unique structure where the upper air is warmer than their surroundings, and the lower air is colder than their surroundings, especially in AR. This structure makes them more intense and longer‐lasting. Exploring these differences helps us understand how Arctic storms work, and how they might be affected by climate change. Key Points: Both the marginal ice zone (MIZ) and Alpha Ridge exhibit active summer Arctic cyclone activities, especially for intense stormsIn the MIZ, baroclinic instability plays a more prominent role in the intensification and maintenance of cyclonesCyclones in Alpha Ridge are more commonly accompanied by potential vorticity downward intrusion, and "upper warm‐lower cold" structures [ABSTRACT FROM AUTHOR]

This study explores the impact of rising CO2 levels on the Atlantic meridional overturning circulation's (AMOC) interdecadal variability within the context of the Last Glacial Maximum (LGM) background climate. Under heightened CO2 concentrations, the AMOC interdecadal variability intensifies dramatically, which is very different from the future warming case that shows a weakening of AMOC interdecadal variability in response to increased CO2 concentration. This unexpected phenomenon primarily results from the extensive retreat of sea ice, which exposes a larger portion of the ocean surface to efficiently feel the heat flux fluctuations from atmospheric processes. These findings underscore the significance of background climate conditions in shaping AMOC responses to increased CO2 and emphasize the necessity of considering these nuances to develop a more accurate understanding of AMOC dynamics in an evolving climate. Plain Language Summary: The Atlantic meridional overturning circulation (AMOC) is an important component of the Earth system, and its interdecadal variability is predicted to be significantly weakened under future warming scenarios. In this paper, we analyze the response of AMOC interdecadal variability to rising CO2 levels under the background of the Last Glacial Maximum (LGM) and find that the AMOC interdecadal variability is intensified under increased CO2, which is totally different from its response at the background of modern climate. Analyses suggest that this unexpected result is mainly caused by dramatic sea ice retreat, which exposes much seawater to efficiently receive large fluctuations of heat flux from atmospheric forcing. The findings reveal that the response of AMOC to increased CO2 and relevant dominant mechanism differs significantly under different climate conditions. Key Points: The Atlantic meridional overturning circulation (AMOC) interdecadal variability is intensified with increased CO2 under the Last Glacial Maximum (LGM) background climate, diverging from that in future warmingThe intensified AMOC variability cannot be explained by ocean dynamics as shown in future warming casesLarge sea ice retreat drives the intensification of AMOC interdecadal variability under the LGM warming [ABSTRACT FROM AUTHOR]

More and more optical records have exhibited that multiple upward leaders (MULs) occur frequently on a building in the flash attachment process. An interesting issue is why a building can continue to launch upward leader (UL) after the first one appears. This phenomenon is analyzed in the present paper. Considering the influence of the leader behaviors on the ambient electric field, an improved 3‐D fine‐resolution lightning attachment model with MULs is established to simulate cloud‐to‐ground flash events with diverse leader spatial morphologies. The simulation results show that MULs may initiate almost simultaneously or with an obvious delay and the variation range of UL length is large. From this, the flash events of lightning terminating on a building are divided into four scenarios and each scenario is analyzed. It was found that the spatial location of downward leader, the length and propagation direction of the first UL and the time interval from the inception of the first UL to final jump significantly affect the electric fields at top corners of building and further affect the inception of the second UL. Based on qualitative analysis, four factors are proposed to explain why the above four scenarios happen. Plain Language Summary: This research focuses on understanding the process of cloud‐to‐ground (CG) lightning, which can cause significant harm to society. Specifically, the study investigates the initiation of multiple upward leaders in the CG lightning process. By considering the impact of lightning leader behaviors on the surrounding electric field, the researchers develop an improved lightning attachment model. Using this model, we simulate the development of leaders and identify factors that explain why one or more leaders originate from an isolated building. The results highlight the importance of the location of the lightning, the characteristics of the first upward leader, and the timing of the lightning strike in influencing the initiation of multiple upward leaders. Future studies will explore CG lightning within groups of buildings, contributing to our understanding of this phenomenon and providing insights for protecting buildings from lightning strikes. Key Points: An improved 3‐D fine‐resolution stochastic discharge model is developedThe spatial location of lightning, as well as the length and propagation direction of the first upward leader, has an impact on the initiation of multiple upward leadersThe time interval between the inception of the first upward leader and the final jump affects the initiation of multiple upward leaders [ABSTRACT FROM AUTHOR]

Our basic knowledge of downward positive lightning leaders is incomplete due to their rarity and limited ability of VHF mapping systems to image positive streamers. Here, using high‐speed optical records and wideband electric field and magnetic field derivative signatures, we examine in detail the development of a descending positive leader, which extended intermittently via alternating branching at altitudes of 4.2 to 1.9 km and involved luminosity transients separated by millisecond‐scale quiet intervals. We show that the transients (a) are mostly initiated in previously created but already decayed branches, at a distance of the order of 100 m above the branch lower extremity, (b) extend bidirectionally with negative charge moving up, (c) establish a temporary (1 ms or so) steady‐current connection to the negative part of the overall bidirectional leader tree, and (d) exhibit brightening accompanied by new breakdowns at the positive leader end. One of the transients unexpectedly resulted in a negative cloud‐to‐ground discharge. Both positive and negative ends of the transients extended at speeds of 106–107 m/s, while the overall positive leader extension speed was as low as 103–104 m/s. Wideband electric field signatures of the transients were similar to K‐changes, with their millisecond‐ and microsecond‐scale features being associated with the steady current and new breakdowns, respectively. For transients with both ends visible in our optical records, charge transfers and average currents were estimated to be typically a few hundreds of millicoulombs and some hundreds of amperes, respectively. Plain Language Summary: Our knowledge of downward positive lightning is very limited. Positive lightning constitutes only about 5% of the global lightning activity, but it causes the most severe damage to various objects and systems, as well as most transient luminous events in the mesosphere. The leader processes in positive lightning remain a mystery and are widely debated in the atmospheric electricity community. In this paper, we present two new findings on propagation mechanisms of branched downward positive leader: (a) its ability to produce an opposite‐polarity (negative) lightning discharge to ground and (b) its unusual mode of propagation involving bidirectional transients, which temporarily reactivate individual leader branches and facilitate their alternating extension. Key Points: Branched positive leader extending at an average speed of 103–104 m/s resulted in a three‐stroke negative cloud‐to‐ground flashPositive leader extension involved bidirectional transients, moving negative charge up, separated by millisecond‐scale quiet intervalsElectric field signatures of the transients are similar to K‐changes and are associated with steady currents and new breakdowns at far end [ABSTRACT FROM AUTHOR]

In this paper, we present XStorm, a gamma ray spectrometer developed to detect gamma ray glows and terrestrial gamma ray flashes (TGFs) in close proximity. Measurements are mostly planned to take place on balloon campaigns but also on the ground using bigger detectors. The main aim in developing XStorm is to perform new in situ and close proximity measurements of those events to improve the understanding of the physical processes involved. For that, we ensured XStorm reached performances adapted to glow and TGF detections. It detects photons with energy between ∼400 keV and ∼20 MeV. Detected particles are timetagged with a 600 ns precision with respect to UTC. Using two types of scintillator, Bismuth Germanium Oxide and EJ‐276 plastic associated with SiPMs, the instrument is able to discriminate three types of particles involved in those events: photons, neutrons, and electrons. The behavior of the detector under high particle fluxes has been quantified through ground testing using a pulse generator. A triggered detection system has been developed, with different thresholds depending on the target of study. First measurements have been carried out with test flights in fair weather conditions and are presented here. Estimations of the configurations in which a gamma ray glow can be detected by XStorm and of the number of TGFs that could be detected in specific campaigns are also addressed. Plain Language Summary: Terrestrial gamma ray flashes (TGFs) are bursts of high‐energy photons generated in thunderstorms in less than 100 μs, whereas gamma ray glows are enhancement of the high‐energy radiation background in thunderstorms, lasting from seconds to minutes. We present a gamma ray spectrometer, XStorm, designed to detect terrestrial gamma ray flashes (TGFs) and gamma ray glows in close proximity. It is composed of two scintillators of different kinds to allow the detection of TGF and gamma ray glow photons (energies between 400 keV and 20 MeV, with a time precision of 600 ns UTC). XStorm measurements are mainly planned to take place on board balloons, but can also be performed at ground level using bigger scintillators. XStorm is able to discriminate photons, electrons and neutrons, that are particle types involved in TGFs. We show the first measurements in fair weather and configurations allowing the detection of gamma ray glows. We estimate that XStorm will detect ∼0.5 TGF on average over one balloon flight of Stratéole‐2 campaign presented in the scientific objectives. Key Points: Development of a gamma ray spectrometer to detect gamma ray glows and terrestrial gamma ray flashes (TGFs), able to discriminate photons, electrons, and neutronsResults from balloon test flights performed in fair weather conditions, detecting the background radiation level as a function of altitudeEstimation of the detectability of gamma ray glows and TGFs as a function of altitude with XStorm [ABSTRACT FROM AUTHOR]

The role of free passage distance (FPD: the distance between the avalanche region and surface detectors) in influencing the relative numbers of energetic electrons and gamma rays in Thunderstorm Ground Enhancements (TGEs) is reconsidered and focuses on the contrast between long (>100 m) versus short (<100 m) FPDs, respectively. Estimates of FPD are based on information from published balloon soundings of the electric field, from published profiles of radar reflectivity in TGEs, and from analyses of Japan winter storms. All these data sources support typical values of FPD >100 m. Neither the shortcomings of present particle detectors in distinguishing electrons from gamma rays, nor the dominance of gamma rays over electrons, are sufficient evidence to deny the robust presence of Compton electrons at FDP values greater than 100 m that have also been shown in earlier simulations as well as the present Comment. Problems with having sustained electric fields of breakeven magnitude within 100 m of the Earth's surface (in relatively rare TGEs) are identified. The resolution of these problems, and the prominent nocturnal presence of these rare events, may possibly be explained by the descent of a strong field region in a collapsing storm, and by a low cloud base that intercepts and immobilizes fast corona ions, thereby preserving the intense electric field. Plain Language Summary: Thunderstorms are capable of accelerating electrons to large energy by a process called electron runaway. This process is often confined to the cold portion of the thunderstorm at higher altitude where ice particles are available to separate electric charge to produce the necessary electric field, and where so‐called avalanche electrons are present. As a result, the high field region in the storm is removed from the ground where measurements of energetic radiation are usually undertaken to diagnose electron acceleration aloft. Gamma rays are produced when the energetic electrons are decelerated in coming in contact with heavy nuclei in a process called bremsstrahlung. Electrons unaided by strong field have short range in the atmosphere: tens of meters and less, whereas gamma rays have larger range (hundreds of meters). Accordingly, energetic electrons cannot be expected far (>200 m) below the high field region. One possible scenario for reducing this gap is the descent of strong field to near cloud base in a collapsing storm and the protection of field dissipation by the capture of small corona ions by cloud droplets. Evidence from several research areas in the literature is used to support the arguments in this paper. Key Points: Energetic Compton electrons are an inevitable accompaniment of the gamma ray flux of Thunderstorm Ground Enhancements (TGEs) but in numbers too small to be readily distinguished from the gamma rays with typical detectorsThe observations of avalanche electrons in TGEs is surprising, given the small free passage distance (FPD) (<50–100 m) requiredOne suggested scenario for creating small FPD is the lowering of negative charge in storm collapse and the capture of fast corona ions by cloud droplets [ABSTRACT FROM AUTHOR]

The sensitivity of tropical cyclone (TC) intensification to its inner size in a sheared environment is investigated in this study. Previous research indicated that TCs with smaller sizes spin up more quickly in a quiescent environment. In contrast, our idealized numerical simulations show that TCs with larger inner‐core sizes experience faster growth within a certain size range under the vertical wind shear (VWS) because stronger upper‐level outflows are established quickly for larger TCs. The presence of strong outflow diminishes the impact of VWS, causing the TC re‐alignment. In more detail, the stronger outflow locally reduces the shear, allowing the convective asymmetry to propagate to the upshear side and migrate inward toward the TC center more rapidly. The upshear convection leads to a stronger outflow and thus a greater blocking effect on the upper‐level wind, effectively reducing the VWS and thus allowing subsequent faster TC growth. Our analysis reveals that the TC re‐alignment at an earlier stage allows for significant differences in surface heat flux (surface latent heat flux [SLHF]) distribution based on size. Larger TCs exhibit larger areas of high SLHF, which create favorable thermodynamic conditions for TC developments. Conversely, smaller vortices have limited SLHF underneath, resulting in a prolonged intensification process. Furthermore, the boundary layer recovery mechanism effectively counteracts the low‐level ventilation pathway imposed by the shear. This mechanism supports the downstream deep convection development on the upshear side. This study presents a new perspective, highlighting that the impact of shear on TCs is contingent upon their sizes upon entering a sheared environment. Plain Language Summary: The paper shows that a tropical cyclone (TC) with a larger inner‐core size experiences rapid intensification in the presence of vertical wind shear (VWS). Our findings indicate that when the TC has a larger inner‐core size, the enhanced inner‐core convection can increase the upper‐level outflow, which helps to resist the upper‐level environmental wind and reduce the VWS. This leads to the TC re‐alignment and allows for faster development. The large‐size TC possesses a larger area of high surface heat flux that provides abundant energy for TC development. This finding emphasizes a new perspective, highlighting that the impact of shear on TC development is contingent upon the size of the TC itself. Key Points: In the presence of vertical wind shear (VWS), tropical cyclone (TC) with a larger size is apt to experience a high intensification rateTC with a larger size quickly develops stronger upper‐level outflow, which helps decrease VWS. This leads to TC re‐alignment and faster developmentThe TC with a larger sizes possesses a larger area of high surface heat flux, favoring the thermodynamic forcing for TC rapid development [ABSTRACT FROM AUTHOR]

Polar stratospheric clouds (PSCs) play a key role in Arctic amplification and stratospheric ozone destruction in polar regions. In this paper, we used the CALIPSO data to analyze the spatiotemporal distribution of Arctic PSCs from 2006 to 2021. We found that Arctic PSCs mainly appear in December, peak in late December and early January, disappearing in late February and early March. PSCs can extend from heights near the tropopause to over 25 km. However, there is the lowest fraction of PSCs in the 2014/2015 winter. This study found that the temperature in the 2014/2015 winter was warmer than the 15‐year average temperature, with the lowest temperature slightly below the PSCs formation temperature of about 5 K. The formation of the Ural blocking high accompanied by the poleward propagation of the planetary wave caused a sudden stratospheric warming (SSW) event on 3 January 2015, during which the warm air entered the polar vortex and divided it into two lobes. Additionally, a reduction in SO2 column mass density before the SSW event resulted PSCs occurring with a frequency of only 0.148 and dissipating rapidly in December. Moreover, the concentration of H2O and HNO3 in the gravitational settling process of PSCs decreased by 20–50%, the reduction of condensation nuclei made PSCs with the highest frequency of 0.074 in February appear briefly and then disappear. The chemical and dynamic analysis of PSCs formation is needed to further understand the spatiotemporal distribution of Arctic PSCs and to better predict future Arctic amplification and ozone destruction. Plain Language Summary: Polar stratospheric clouds (PSCs) influence polar ozone depletion by providing a reaction interface and also influencing surface temperature changes through longwave radiation effects. Previous studies on polar stratospheric clouds mainly focus on the Antarctic, and few studies on the temporal and spatial distribution characteristics of Arctic PSCs over long timescales. Therefore, by studying the spatial and temporal distribution of PSCs in the Arctic, we found that the spatial and temporal distribution of PSCs in the Arctic has obvious interannual variation compared with that in the Antarctic. The stratospheric sudden warming (SSW) events that occur almost every 2 years in the Arctic cause great interannual variations of the Arctic polar vortex and thus affect the distribution of Arctic PSCs. Moreover, there is the least and almost none occurrence of PSCs in the 2014/2015 winter, the chemical and dynamic analysis found that SSW, decrease of SO2, H2O, and HNO3 concentration are not conducive to the formation of PSCs. It is of great significance to study the influencing factors of Arctic PSCs formation and provide a new basis for further prediction of Arctic amplification and ozone destruction. Key Points: The fraction of Arctic polar stratospheric clouds (PSCs) during 2014/2015 winter was the lowest observed in the past 15 yearsThe splitting of polar vortex and the rise in temperature caused by sudden stratospheric warming (SSW) inhibited the formation of PSCsThe decrease in stratospheric sulfur dioxide, nitric acid, and water was unfavorable for the formation of PSCs before and after the SSW [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]

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]

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]

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]

Coupled ocean‐atmosphere models still suffer from systematic biases in simulating the sea‐surface temperature (SST) in the equatorial Pacific. Like in many current generations of climate models, the Chinese Academy of Sciences (CAS) Earth System Model (CAS‐ESM2‐0) simulated warmer SST in the eastern Pacific and colder SST in the central to western Pacific at the equator. We analyzed the physical processes causing these biases. We find that model biases in surface heat flux are a consequence, not the cause, of the SST bias. Ocean currents are implicated as the cause. The biases in ocean currents come from both the ocean model itself and the bias in surface winds in the atmospheric model. We show that the bias of ocean currents in the standalone ocean model causes warming in the region of positive precipitation bias in the standalone atmospheric model. When the two models are coupled, the precipitation bias is greatly amplified. Anomalous equatorial winds as atmospheric Rossby‐gravity wave response to precipitation induce additional bias in ocean currents that transport heat in the upper ocean. The cold equatorial SST bias in the central and western Pacific is found to be due to temperature advection by the anomalous zonal currents, while the warm bias in the eastern Pacific is due to advection by the anomalous zonal and meridional currents as well as downwelling. Results from the analysis offer insights into the causes of the SST biases in coupled models in the equatorial Pacific. Plain Language Summary: The systematic bias of simulated sea‐surface temperature (SST) in the equatorial Pacific is a well‐known problem in coupled ocean‐atmosphere models. This paper uses CAS‐ESM2‐0 as a case study to investigate such a problem. We found that the SST bias in this model is caused by ocean dynamics rather than the surface heat flux. The bias in ocean currents comes from both the standalone ocean model and amplification by bias in the atmospheric model. The precipitation bias in the atmospheric model causes the Rossby‐gravity wave response of the surface wind to force additional bias in ocean currents. The cold equatorial SST bias in the central and western Pacific in CAS‐ESM2‐0 is found to be due to temperature advection by the anomalous zonal currents, while the warm bias in the eastern Pacific is due to advection by the anomalous zonal and meridional currents as well as downwelling. Key Points: The annual equatorial Pacific SST bias in CAS‐ESM2‐0 is caused by ocean dynamics rather than surface heat fluxBias in ocean currents is contributed by both the standalone ocean model and atmospheric surface windsBias in atmospheric precipitation impacts surface winds as equatorial Rossby‐gravity wave response to latent heating [ABSTRACT FROM AUTHOR]

An interpretable convolutional neural network model is proposed for the classification of very low frequency and low frequency lightning electric field waveforms. This model adopts multi‐scale convolutional kernels and shortcut connections to enhance the ability of lightning waveform classification. Based on the data recorded from five provinces in China, the proposed model achieves an accuracy of 98.56% for a four‐type classification task including return strokes, the intra‐cloud lightning, preliminary breakdown, and narrow bipolar events. The proposed model is validated with another open‐source data set from Argentina with an accuracy of 98.45%, which shows good robustness. To ensure the classification, the features learned by the model are visualized. The class activation mapping (CAM) method is adopted to visualize the class‐specific contribution of different waveform parts by using the feature maps of the final convolutional layer. It is highlighted by the CAM method that the proposed model focuses on waveform parts that align with those areas of interests identified by human experts. The high‐contribution waveform parts are furtherly analyzed, which indicate that the proposed model possesses the capability to associate waveform features with the corresponding lightning discharge processes. Plain Language Summary: Electromagnetic waveforms in very low frequency and low frequency bands are usually used to detect and locate lightning activities. Traditional waveform classification methods have difficulties in distinguishing multiple types of lightning waveforms. Although machine learning models have great potential in multi‐type waveform classification tasks, these models rely on the features proposed by human experts and cannot capture the features of different scales in lightning waveforms. To this end, this paper proposes an improved convolution neural network model, which incorporates modifications to the model structure to better suit the lightning waveform classification task. The data set for model training comes from five provinces in China and contains different meteorological conditions. The proposed model achieves a classification accuracy of 98.56% on this data set and 98.45% on an open‐source data set from Argentina. Meanwhile, the classification process is interpretable by visualizing the convolution outputs. The analysis of the visualization results shows that the high performance of the proposed model is reliable for its ability to focus on waveform parts that align with areas of interests identified by human experts. A closer inspection of these waveform parts suggest that the proposed model possesses the capability to associate waveform features with the corresponding lightning discharge processes. Key Points: The proposed model achieves an accuracy of 98.56% for a four‐type lightning waveform classification task and shows good robustnessThe model is interpretable by visualizing the contribution of different waveform parts to the classification resultsThe proposed model aligns with human‐expert classifications and suggests potential to link waveform features with lightning processes [ABSTRACT FROM AUTHOR]

Volcanic activity and the associated gas emissions into the atmosphere often result in adverse air quality conditions and present a hazard to human health and the environment. Building on a decade‐long effort to provide operational surface sulfur dioxide and sulfate aerosol forecasts for the State of Hawai'i, we present an air quality modeling framework called VogCast. VogCast is designed to simplify ensemble air quality prediction on a regional scale by linking together multiple state‐of‐the‐art models of meteorology, emissions, and dispersion. The framework is open‐source and introduces a new dynamic plume‐rise algorithm for distributing pollutants vertically. Using radar and satellite data, we demonstrate that VogCast reasonably captured the mean injection height, the location, and the general envelope of the vog plume during Mauna Loa's 2022 eruption. The results suggest that during the 12‐day eruption period model performance varied between days with trade and non‐trade wind conditions. Our findings also highlight the importance of sulfur dioxide emission rate and vent parameter inputs for improving forecast accuracy. The broad goal of this work is to better our understanding of vog dispersion and improve air quality prediction for impacted communities. Plain Language Summary: Volcanic activity often results in poor air quality for communities downwind of active eruptions. In Hawai'i, forecasts of volcanic air pollution (vog) are used to provide early warning and help reduce negative impacts from vog exposure. In this paper, we introduce a new air quality modeling framework called VogCast designed to simplify and improve the accuracy of existing surface air quality forecasts in Hawai'i. We use radar and satellite data to assess the ability of VogCast to capture vog dispersion during the 2022 Mauna Loa eruption. Our findings suggest VogCast is able to reasonably predict the height and general location of the vog plume. We also show that both volcanic gas emission rates and meteorology have a strong impact on model performance. The broad goal of this work is to better our understanding of vog dispersion and improve air quality prediction for impacted communities. Key Points: VogCast is a Python‐based modular open‐source air quality modeling framework for volcanic gas emissionsVogCast incorporates a dynamic plume‐rise scheme for estimating plume injection heightComparison with remotely sensed data from 2022 Mauna Loa eruption suggests VogCast is able to accurately capture regional‐scale vog dispersion [ABSTRACT FROM AUTHOR]

High‐speed video records of a single‐stroke positive cloud‐to‐ground (+CG) flash were used to examine the evolution of eight needles developing more or less radially from the +CG channel. All these eight needles occurred during the later return‐stroke stage and the following continuing current stage. Six needles, after their initial extension from the lateral surface of the parent channel core, elongated via bidirectional recoil events, which are responsible for flickering, and two of them evolved into negative stepped leaders. For the latter two, the mean extension speed decreased from 5.3 × 106 to 3.4 × 105 and then to 1.3 × 105 m/s during the initial, recoil‐event, and stepping stages, respectively. The initial needle extension ranged from 70 to 320 m (N = 8), extension via recoil events from 50 to 210 m (N = 6), and extension via stepping from 810 to 1,870 m (N = 2). Compared with needles developing from leader channels, the different behavior of needle flickering, the longer length, the faster extension speed, and the higher flickering rate observed in this work may be attributed to a considerably higher current (rate of charge supply) during the return‐stroke and early continuing‐current stages of +CG flashes. Plain Language Summary: This paper presents the dynamics of eight needles appearing on the lateral surface of the channel of a single‐stroke positive cloud‐to‐ground lightning flash. Six needles, after their initial extension, elongated via recoil events, which are responsible for flickering, and two of them evolved into negative stepped leaders. This study helps to improve our understanding of the dynamics of needles and explain the mechanism of recently discovered initiation of negative leaders from positively charged channels. Key Points: Dynamics of two needles developing from a +CG channel and evolving into negative stepped leaders were studied in detailAfter initial extension from +CG channel, needle channels first elongated via bidirectional recoil events and then via steppingExtension speeds were 5.3 × 106, 3.4 × 105, and 1.3 × 105 m/s during the initial, recoil‐event, and stepping stages, respectively [ABSTRACT FROM AUTHOR]

The surface air temperature (SAT) trend on the Tibetan Plateau (TP) was 3.45°C 100 years−1 from 1961 to 2014. The multi‐model ensemble (MME) of 33 coupled models participated in the Coupled Model Intercomparison Project phase six (CMIP6) was about 1°C 100 years−1 lower than the observation. Although MME generally shows better skill in reproducing the distribution of SAT trend over TP than most of the CMIP6 models, its performance is greatly degraded by a small group of models, about 12% on average, with large biases. In this paper, the constrained multi‐model ensemble (CMME) based on a certain observation‐based threshold is used to constrain future projections of the SAT trend over TP. Compared with the MME results, the improvements in CMME are mainly over the eastern plateau in historical simulation and are relative to the reduction of the model biases to carbon dioxide (CO2) forcing. Under the high‐emission SSP5‐8.5 scenario, SAT increases significantly over the entire TP. The constraint of CMME on the MME is mainly over the eastern plateau with a difference of 0.5°C 100 years−1, about 6% of the MME results. Under the intermediate‐emission scenario SSP2‐4.5, the effect of CMME is relatively smaller, but the corresponding spatial distribution is similar to that under the SSP5‐8.5 scenario. The CMIP6 models tend to underestimate the warming trend projections over the water source regions in the northeastern plateau and should be noticed. Key Points: The Coupled Model Intercomparison Project phase six model ensemble in surface air temperature trend reproduction on the Tibetan Plateau are greatly degraded by a small group of models with large biasesThe impact of constrained ensemble (CMME) is mainly over the eastern plateau in relative to the reduction of model biases to CO2 forcingThe CMME results indicate the underestimated warming projections in multi‐model ensemble over the northeastern plateau [ABSTRACT FROM AUTHOR]

Self‐organizing maps (SOMs) represent a popular tool for classifying atmospheric circulation patterns. One of their traditional applications has been to link typical synoptic‐scale patterns to large‐scale teleconnections, or modes of low‐frequency circulation variability. However, recently there have been attempts to interpret an array of SOM nodes directly as a continuum of teleconnections, grounded in SOMs' ability to combine two otherwise distinct approaches to data analysis, that is, exploratory projection (or, dimensionality reduction) and classification. This conceptual shift calls for methodological studies that would improve our understanding of how orthogonal modes of variability, typically used to describe teleconnections, relate to SOM outputs. Here, we define three idealized modes of variability and use their various combinations to generate synthetic data sets. Many variants of SOMs are generated for SOMs of various shapes and sizes. The results show that projection of modes on a SOM array is sensitive not only to data structure, but also to various SOM parameters. The leading mode of variability projects rather strongly on SOMs if its explained variance is markedly higher than that of the second‐order mode; the remaining modes project considerably more weakly, and all modes tend to blend when their explained variance is similar, which leads to underrepresentation of some phases of modes and/or combinations of modes among the SOM patterns. Furthermore, we show that some features of SOM topology that were previously considered a proof of data nonlinearity appear even if the underlying modes of variability are strictly linear. Plain Language Summary: One of the tasks of a climatologist is to group together days with similar weather over a region, and to find out how such types of regional weather are influenced by distant locations. These distant influences, also known as teleconnections, are intricate and studying how the methods we use to study them deal with these intricacies is one of the keys that helps us understand the climate. One way to start this task is to apply simple and well‐understood data sets to methods and examine how they interpret the data. In the paper, we studied the popular method of self‐organizing maps in such a way. We found that the method usually identifies the strongest teleconnection, but the images of weaker teleconnections tend to be unclear or even blend together. We use our results to revisit findings of some previous studies and to discuss how self‐organizing maps could be improved in the future to give us more accurate information on the atmosphere. Key Points: Synthetic fields are generated as linear combinations of three idealized functions of spatial co‐variability, or circulation modesSelf‐organizing maps have limited ability to represent the second and third modes, and modes tend to blend when their variance is similarPrincipal component analysis may detect data structures conducive to high sensitivity of self‐organizing maps to the choice of parameters [ABSTRACT FROM AUTHOR]