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Abstract: Many weather forecast and climate models simulate warm surface air temperature (T2m) biases over midlatitude continents during the summertime, especially over the Great Plains. We present here one of a series of papers from a multimodel intercomparison project (CAUSES: Cloud Above the United States and Errors at the Surface), which aims to evaluate the role of cloud, radiation, and precipitation biases in contributing to the T2m bias using a short‐term hindcast approach during the spring and summer of 2011. Observations are mainly from the Atmospheric Radiation Measurement Southern Great Plains sites. The present study examines the contributions of surface energy budget errors. All participating models simulate too much net shortwave and longwave fluxes at the surface but with no consistent mean bias sign in turbulent fluxes over the Central United States and Southern Great Plains. Nevertheless, biases in the net shortwave and downward longwave fluxes as well as surface evaporative fraction (EF) are contributors to T2m bias. Radiation biases are largely affected by cloud simulations, while EF bias is largely affected by soil moisture modulated by seasonal accumulated precipitation and evaporation. An approximate equation based upon the surface energy budget is derived to further quantify the magnitudes of radiation and EF contributions to T2m bias. Our analysis ascribes that a large EF underestimate is the dominant source of error in all models with a large positive temperature bias, whereas an EF overestimate compensates for an excess of absorbed shortwave radiation in nearly all the models with the smallest temperature bias. [ABSTRACT FROM AUTHOR]

Abstract: We propose in this paper a statistical framework to study the evolution of the co‐occurrence of extreme daily rainfall in West Africa since 1950. We consider two regions subject to contrasted rainfall regimes: Senegal and the central Sahel. We study the likelihood of the 3% largest daily rainfall (considering all days) in each region to occur simultaneously and, in a 20 year moving window approach, how this likelihood has evolved with time. Our method uses an anisotropic max‐stable process allowing us to properly represent the co‐occurrence of daily extremes and including the possibility of a preferred direction of co‐occurrence. In Senegal, a change is found in the 1980s, with preferred co‐occurrence along the E‐50‐N direction (i.e., along azimuth 50°) before the 1980s and weaker isotropic co‐occurrence afterward. In central Sahel, a change is also found in the 1980s but surprisingly with contrasting results. Anisotropy along the E‐W direction is found over the whole period, with greater extension after the 1980s. The paper discusses how the co‐occurrence of extremes can provide a qualitative indicator on change in size and propagation of the strongest storms. This calls for further research to identify the atmospheric processes responsible for such contrasted changes in storm properties. [ABSTRACT FROM AUTHOR]

Abstract: A companion paper by Fritts, Laughman, et al. (2017) employed an anelastic numerical model to explore the dynamics of gravity waves (GWs) encountering a mesospheric inversion layer (MIL) having a moderate static stability enhancement and a layer of weaker static stability above. That study revealed that MIL responses, including GW transmission, reflection, and instabilities, are sensitive functions of GW parameters. This paper expands on two of the Fritts, Laughman, et al. (2017) simulations to examine GW instability dynamics and turbulence in the MIL; forcing of the mean wind and stability environments by GW, instability, and turbulence fluxes; and associated heat and momentum transports. These direct numerical simulations resolve turbulence inertial‐range scales and yield the following results: GW breaking and turbulence in the MIL occur below where they would otherwise, due to enhancements of GW amplitudes and shears in the MIL. 2‐D GW and instability heat and momentum fluxes are ~20–30 times larger than 3‐D instability and turbulence fluxes. Mean fields are driven largely by 2‐D GW and instability dynamics rather than 3‐D instabilities and turbulence. 2‐D and 3‐D heat fluxes in regions of strong turbulence yield small departures from initial T(z) and N2(z) profiles, hence do not yield nearly adiabatic “mixed” layers. Our MIL results are consistent with the relation between the turbulent vertical velocity variance and energy dissipation rate proposed by Weinstock (1981) for the limited intervals evaluated. [ABSTRACT FROM AUTHOR]

Abstract: The time resolved three‐dimensional (3‐D) spatial reconstruction of lightning channels using high‐speed video (HSV) images and VHF broadband interferometer (BITF) data is first presented in this paper. Because VHF and optical radiations in step formation process occur with time separation no more than 1 μs, the observation data of BITF and HSV at two different sites provide the possibility of reconstructing the time resolved 3‐D channel of lightning. With the proposed procedures for 3‐D reconstruction of leader channels, dart leaders as well as stepped leaders with complex multiple branches can be well reconstructed. The differences between 2‐D speeds and 3‐D speeds of leader channels are analyzed by comparing the development of leader channels in 2‐D and 3‐D space. Since return stroke (RS) usually follows the path of previous leader channels, the 3‐D speeds of the return strokes are first estimated by combination with the 3‐D structure of the preceding leaders and HSV image sequences. For the fourth RS, the ratios of the 3‐D to 2‐D RS speeds increase with height, and the largest ratio of the 3‐D to 2‐D return stroke speeds can reach 2.03, which is larger than the result of triggered lightning reported by Idone. Since BITF can detect lightning radiation in a 360° view, correlated BITF and HSV observations increase the 3‐D detection probability than dual‐station HSV observations, which is helpful to obtain more events and deeper understanding of the lightning process. [ABSTRACT FROM AUTHOR]

In this paper, we analyze the location error of time of arrival (ToA)-based lightning location systems (LLSs) caused by propagation effects over mountainous terrain around the Säntis tower located in the Swiss Alps. The study is based on a full-wave three-dimensional (3-D) finite difference time domain approach using the topographic map including the Säntis tower and the nearby sensors belonging to LLSs. It is found that the vertical electric fields are strongly affected by the presence of the mountainous terrain and the finite ground conductivity and that the location error associated with the ToA technique depends strongly on the used onset time estimation technique. The evaluated location errors associated with amplitude thresholds of 10% and 20% and the time of the linear extrapolation of the tangent at maximum field derivative are found to be smallest (about 300 m or less). Finally, we assess the accuracy of two simplified methods (terrain envelope method and tight-terrain-fit method) to account for the location error due to propagation over mountainous terrain. These two methods might represent an efficient alternative to estimate the additional time delay due to propagation over a nonflat terrain by using available topographic data. In addition, a possible real-time location error compensation algorithm using the elongated propagation path method to improve the location error of the LLSs in mountainous regions is presented and discussed. [ABSTRACT FROM AUTHOR]

Daily radiosonde observations between 2003 and 2013 from Halley research station, Antarctica (75°S, 26°W), are used to determine climatologies of gravity wave properties in the lower stratosphere (between 15 km and 22 km altitude). Individual waves are extracted from the radiosonde profile using wavelet analysis and separated into upward and downward propagating waves. An increase in the percentage of downward propagating waves (~30% of the waves) is seen during the winter months. For the upward and downward propagating waves, their horizontal and vertical wavelength, intrinsic frequency, energy density, pseudomomentum flux, and direction of propagation are determined. The upward propagating wavefield is found to be dominated by waves with short vertical wavelength (~1 km) and low intrinsic frequency ( ω ~ f). The downward propagating wavefield is composed of a wider distribution of vertical wavelength waves and has a larger proportion of higher-frequency waves present. The upward propagating waves show an increase in total energy density in autumn and spring; the larger increase occurs during spring (up to 1.7 J kg−1 in September). The downward propagating waves increase in total energy density occurs during wintertime (up to 0.7 J kg−1 in June). During winter the contributions of the upward and downward propagating waves to the total energy density and pseudomomentum flux are almost equal. This paper details the first study of individual gravity wave properties combined into upward and downward propagating wave climatologies in the lower stratosphere above Halley. [ABSTRACT FROM AUTHOR]

The Western North Atlantic Ocean (WNAO) and adjoining East Coast of North America are of great importance for atmospheric research and have been extensively studied for several decades. This broad region exhibits complex meteorological features and a wide range of conditions associated with gas and particulate species from many sources regionally and other continents. As Part 1 of a 2‐part paper series, this work characterizes quantities associated with atmospheric chemistry, including gases, aerosols, and wet deposition, by analyzing available satellite observations, ground‐based data, model simulations, and reanalysis products. Part 2 provides insight into the atmospheric circulation, boundary layer variability, three‐dimensional cloud structure, properties, and precipitation over the WNAO domain. Key results include spatial and seasonal differences in composition along the North American East Coast and over the WNAO associated with varying sources of smoke and dust and meteorological drivers such as temperature, moisture, and precipitation. Spatial and seasonal variations of tropospheric carbon monoxide and ozone highlight different pathways toward the accumulation of these species in the troposphere. Spatial distributions of speciated aerosol optical depth and vertical profiles of aerosol mass mixing ratios show a clear seasonal cycle highlighting the influence of different sources in addition to the impact of intercontinental transport. Analysis of long‐term climate model simulations of aerosol species and satellite observations of carbon monoxide confirm that there has been a significant decline in recent decades among anthropogenic constituents owing to regulatory activities. Key Points: Significant reductions in CO and particulate SO42−, black carbon, and primary organic matter are observed over multiple decadesHigh aerosol extinctions in free troposphere during March‐April‐May and June‐July‐August, whereas aerosols mostly confined to boundary layer at other timesOrganic matter and ammonium sulfate usually dominated PM2.5 composition, whereas sea salt was most abundant in wet deposition [ABSTRACT FROM AUTHOR]

A terrestrial gamma‐ray flash (TGF) associated with an Elve was recently reported by the Atmosphere‐Space Interactions Monitor (ASIM) (Neubert et al., 2020, https://doi.org/10.1126/science.aax3872). An extremely low frequency electromagnetic field pulse coinciding with this event was detected by two stations of our World Extremely Low Frequency (ELF) Radiolocation Array (WERA) project from a distance of more than 10,000 km. The recorded signal allowed us to radiolocate the source, determine its polarity, and calculate the charge moment of the associated atmospheric discharge. Our analysis shows that this TGF was associated with a powerful discharge that had a positive polarity and the charge moment p ≈ 125 C·km. Plain Language Summary: Observation of a terrestrial gamma‐ray flash (TGF, a flash of high‐energy X‐rays and gamma rays from a thunderstorm region into space) and an associated Elve phenomenon in the lower ionosphere was reported by the Atmosphere‐Space Interactions Monitor onboard the International Space Station. An extremely low frequency (below 300 Hz) electromagnetic field pulse from the TGF‐associated lightning was detected with magnetic antennas from a distance of more than 10,000 km by the Hylaty and Patagonia extremely low frequency (ELF) stations of our World ELF Radiolocation Array (WERA) project. In this paper we use these measurements to determine the charge moment of the TGF source lightning discharge, not provided by higher frequency measurements. Key Points: WERA radiolocation system uses magnetic antennas to detect ELF radio waves from powerful lightning discharges occurring all over the worldASIM registered the first Terrestrial Gamma‐ray Flash associated with an Elve. We provide fundamental physical parameters of the sourceELF impulses can be used to radiolocate lightning discharges associated with TGFs and estimate their polarity and charge moment [ABSTRACT FROM AUTHOR]

After a tropical storm makes landfall, its vortex interacts with the surrounding environment and the underlying surface. It is expected that diurnal variation over land will affect storm structures. However, this has not yet been explored in previous studies. In this paper, numerical simulation of postlandfall Tropical Storm Bill (2015) is conducted using a research version of the NCEP Hurricane Weather Research and Forecasting (HWRF) model. Results indicate that during the storm's interaction with midlatitude westerlies over the Great Plains, the simulated storm with the SLAB land‐surface scheme is stronger, with faster eastward movement and attenuation, and more asymmetric structures than that with the NOAH land‐surface scheme. More symmetric structures correspond with a slower weakening and slower eastward movement of the storm over land. Further diagnoses suggest an obvious response of the storm's asymmetric structures to diurnal effects over land. Surface diabatic heating in the storm environment is important for the storm's symmetric structures and intensity over land. Specifically, during the transition from nighttime to daytime, the evident strengthening of convective instability, atmospheric baroclinicity, and the lateral advection of high θe air in the storm environment, associated with the rapid increase in surface diabatic heating, are conducive to the development of vertical vorticity and storm‐relative helicity, thus contributing to the maintenance of the storm's symmetric structures and intensity after landfall. Key Points: An evident response of the storm's asymmetric structures to diurnal effects over land are revealedSurface diabatic heating in the storm environment is important for the evolution of the storm's symmetric structures and intensity over landEffects of surface diabatic heating storm's symmetric structure and intensity over land are mainly manifested in storm's upshear quadrants [ABSTRACT FROM AUTHOR]

A very high spatial resolution (∼25 m pixel at 90 km altitude) OH airglow imager was installed at the Andes Lidar Observatory on Cerro Pachón, Chile, in February 2016. This instrument was collocated with a Na wind‐temperature lidar. On 1 March 2016, the lidar data showed that the atmosphere was dynamically unstable before 0100 UT and thus conducive to the formation of Kelvin‐Helmholtz instabilities (KHIs). The imager revealed the presence of a KHI and an apparent atmospheric gravity wave (AGW) propagating approximately perpendicular to the plane of primary KHI motions. The AGW appears to have induced modulations of the shear layer leading to misalignments of the emerging KHI billows. These enabled strong KHI billow interactions, as they achieved large amplitudes and a rapid transition to turbulence thereafter. The interactions manifested themselves as vortex tube and knot features that were earlier identified in laboratory studies, as discussed in Thorpe (1987, https://doi.org/10.1029/JC092iC05p05231; 2002, https://doi.org/10.1002/qj.200212858307) and inferred to be widespread in the atmosphere based on features seen in tropospheric clouds but which have never been identified in previous upper atmospheric observations. This study presents the first high‐resolution airglow imaging observation of these KHI interaction dynamics that drive rapid transitions to turbulence and suggest the potential importance of these dynamics in the mesosphere and at other altitudes. A companion paper (Fritts et al., 2020, https://doi.org/10.1029/2020JD033412) modeling these dynamics confirms that the vortex tubes and knots yield more rapid and significantly enhanced turbulence relative to the internal instabilities of individual KHI billows. Key Points: High‐spatial resolution observations captured the breakdown of a Kelvin‐Helmholtz instability (KHI) in the mesosphereObservations reveal strong interactions among adjacent KHI billows via the formation of vortex tubes and knotsKHI billow interactions and their rapid cascade to turbulence were initiated by atmospheric gravity wave perturbations of the shear layer [ABSTRACT FROM AUTHOR]

A companion paper by Hecht et al. (2020, https://doi.org/10.1002/2014JD021833) describes high‐resolution observations in the hydroxyl (OH) airglow layer of interactions among adjacent Kelvin‐Helmholtz instabilities (KHI). The interactions in this case were apparently induced by gravity waves propagating nearly orthogonally to the KHI orientations, became strong as Kelvin‐Helmholtz (KH) billows achieved large amplitudes, and included features named "tubes" and "knots" in early laboratory KHI studies. A numerical modeling study approximating the KHI environment and revealing the dynamics of knots and tubes is described here. These features arise where KH billows are misaligned along their axes or where two billows must merge with one. They bear a close resemblance to the observed instability dynamics and suggest that they are likely to occur wherever KHI formation is modulated by variable wind shears, stability, or larger‐scale motions. Small‐scale features typical of those in turbulence develop in association with the formation of the knots and tubes earlier and more rapidly than those accompanying individual billows, supporting an earlier conjecture that tubes and knots are commonly major sources of intense turbulent dissipation accompanying KHI events in the atmosphere. Key Points: Variations in the thickness of an unstable shear layer cause KH billows to have different scales and alignments at adjacent locationsInteractions among misaligned KH billows cause "tubes" and "knots" that are distinct from secondary instabilities of individual billowsInstability dynamics of interacting KH billows can be much more intense and energetic than those of individual billows [ABSTRACT FROM AUTHOR]

The Southern Annular Mode (SAM) is the predominant atmospheric variability mode in the Southern Hemisphere. In this paper, we present the spatial variability results of the SAM pattern for the period 1979–2018. The SAM‐intrinsic pattern variability analysis is based on the principal component analysis (PCA), which is carried out for the ERA‐Interim 500 hPa geopotential height (GPH) data set. A spatiotemporally resolved data set of SAM pattern maps (PCA loadings) is derived by projecting monthly shifted sub‐sequences of SAM index values (PCA scores) on the corresponding GPH anomalies. The dominant SAM structure within single pattern fields is mapped automatically and can be interpreted as the Southern Hemisphere polar front. This data set allows an analysis of the geographical positions of the characteristic circumpolar SAM structure over four decades and shows considerable variability over space and time. Five different states of SAM patterns, which are associated with characteristic circulation anomalies during different phases of the study period, are identified. Station‐based Antarctic temperature anomalies can be synoptically explained by these circulation anomalies. The overall latitudinal trend of the SAM pattern indicates an intensification of the meridional structure, especially over the East Antarctic Southern Ocean. Furthermore, we show that the SAM pattern variability is significantly correlated with the Pacific Decadal Oscillation and the Atlantic Multidecadal Oscillation. Composites of 500 hPa GPH anomalies during the positive and negative phases of the respective indices indicate teleconnections with Pacific Decadal Oscillation and Atlantic Multidecadal Oscillation, and this can explain latitudinal trends of the SAM pattern. Key Points: We present a new approach to examine the spatiotemporal Southern Annular Mode pattern variabilityStation‐based Antarctic temperature anomalies are related to different structures of the Southern Annular ModeThe trend pattern shows an increasing meridional structure and correlations with Pacific and Atlantic multidecadal oscillations [ABSTRACT FROM AUTHOR]

Changes in concentrations of greenhouse gases lead to changes in radiative fluxes throughout the atmosphere. The value of this change, the instantaneous radiative forcing, varies across climate models, due partly to differences in the distribution of clouds, humidity, and temperature across models and partly due to errors introduced by approximate treatments of radiative transfer. This paper describes an experiment within the Radiative Forcing Model Intercomparision Project that uses benchmark calculations made with line‐by‐line models to identify parameterization error in the representation of absorption and emission by greenhouse gases. Clear‐sky instantaneous forcing by greenhouse gases is computed using a set of 100 profiles, selected from a reanalysis of present‐day conditions, that represent the global annual mean forcing from preindustrial times to the present day with sampling errors of less than 0.01 W m−2. Six contributing line‐by‐line models agree in their estimate of this forcing to within 0.025 W m−2 while even recently developed parameterizations have typical errors 4 or more times larger, suggesting both that the samples reveal true differences among line‐by‐line models and that parameterization error will be readily identifiable. Agreement among line‐by‐line models is better in the longwave than in the shortwave where differing treatments of the water vapor continuum affect estimates of forcing by carbon dioxide and methane. The impacts of clouds on instantaneous radiative forcing are estimated from climate model simulations, and the adjustment due to stratospheric temperature changes estimated by assuming fixed dynamical heating. Adjustments are large only for ozone and for carbon dioxide, for which stratospheric cooling introduces modest nonlinearity. Key Points: Mean clear‐sky instantaneous radiative forcing by greenhouse gases is computed with six benchmark models using 100 atmospheric profilesSampling error is several times smaller than the level of disagreement among models, which is itself smaller than parameterization errorThe impacts of clouds and stratospheric adjustment are roughly estimated; adjustments are large only for carbon dioxide and ozone [ABSTRACT FROM AUTHOR]

In this paper we report the first close, high‐resolution observations of downward‐directed terrestrial gamma‐ray flashes (TGFs) detected by the large‐area Telescope Array cosmic ray observatory, obtained in conjunction with broadband VHF interferometer and fast electric field change measurements of the parent discharge. The results show that the TGFs occur during strong initial breakdown pulses (IBPs) in the first few milliseconds of negative cloud‐to‐ground and low‐altitude intracloud flashes and that the IBPs are produced by a newly identified streamer‐based discharge process called fast negative breakdown. The observations indicate the relativistic runaway electron avalanches (RREAs) responsible for producing the TGFs are initiated by embedded spark‐like transient conducting events (TCEs) within the fast streamer system and potentially also by individual fast streamers themselves. The TCEs are inferred to be the cause of impulsive sub‐pulses that are characteristic features of classic IBP sferics. Additional development of the avalanches would be facilitated by the enhanced electric field ahead of the advancing front of the fast negative breakdown. In addition to showing the nature of IBPs and their enigmatic sub‐pulses, the observations also provide a possible explanation for the unsolved question of how the streamer to leader transition occurs during the initial negative breakdown, namely, as a result of strong currents flowing in the final stage of successive IBPs, extending backward through both the IBP itself and the negative streamer breakdown preceding the IBP. Key Points: Downward Terrestrial Gamma‐ray Flashes occur during strong initial breakdown pulses of negative cloud‐to‐ground and cloud lightningThe initial breakdown pulses consist of streamer‐based fast negative breakdown having transient sub‐pulse conducting events or "sparks"The streamer to leader transition of negative stepping occurs during strong currents in the final stage of initial breakdown pulses [ABSTRACT FROM AUTHOR]

Surface wind speed has great impacts on the economy, environment, and society around the world. Based on 24 global climate models (GCMs) from Coupled Model Intercomparison Project Phase 6 (CMIP6), this paper assesses the historical surface wind speed over China, and quantifies the advancements of CMIP6 over CMIP5. In addition, future changes of surface wind speed under the Shared Socioeconomic Pathway scenarios of 1‐2.6, 2‐4.5, and 5‐8.5 (SSP1‐2.6, SSP2‐4.5, and SSP5‐8.5) are also provided, by using 18 out of the 24 models with all three scenarios. Results show that multimodel ensemble mean of CMIP6 can well capture the spatial distributions of the annual, summer, and winter surface wind speed and generally performs better than both the median of the individual CMIP6 GCMs and multimodel ensemble mean of CMIP5 in monthly, seasonal, and annual wind climatology. However, CMIP6 GCMs fail to reproduce the observed decreasing trends, in terms of magnitude and/or sign. Meanwhile, the annual cycle of the surface wind speed simulated by CMIP6 GCMs falsely demonstrates the maximum in winter instead of spring. In the context of global warming, surface wind speed is projected to decrease in most parts of China in the middle and late 21st century under the three scenarios. Moreover, the trends of wind speed averaged over China will decrease significantly in both annual and winter under all three scenarios, but it will increase significantly in summer under SSP5‐8.5. Key Points: CMIP6 can well capture the spatial distribution and climatology of surface wind speed over China and outperforms CMIP5 in generalAmong the individual CMIP6 models, CAMS‐CSM1‐0 performs best in reproducing the present‐day surface wind speed over ChinaSurface wind speed is projected to decrease in the mid‐ and late‐21st century under SSP1‐2.6, SSP2‐4.5, and SSP5‐8.5 scenarios [ABSTRACT FROM AUTHOR]

The mechanism of the generation of reaction‐diffusion waves in the mesopause region (80–90 km) has been studied analytically. These waves are the propagating phase fronts arising in the oscillations of O, O3, H, OH, and HO2 concentrations. They appear in the presence of horizontal eddy diffusion in zonal direction when mesospheric photochemistry responds subharmonically (with a period of 2 days) to diurnal variations of solar radiation. The photochemical system in the mesopause region is a nonlinear oscillator which can be roughly described by a system of two differential nonautonomous equations with power law nonlinearity which was derived in our earlier papers. To model the wave propagation, we have considered a continuous chain of oscillators with diffusion coupling and with the phase of external periodic forcing depending linearly on spatial coordinates. It was found that the reaction‐diffusion waves are caused by specific "wind" type transport appearing in the equations for the amplitudes of 2‐day photochemical oscillations of O and H concentrations due to the zonal inhomogeneity of the external forcing. The obtained expression for the wave propagation velocity fully confirmed the earlier numerical results that the magnitude of the velocity is proportional to the diffusion coefficient and the gradient of the external forcing phase. The wave propagation direction is determined by the definite phase relations specified by internal parameters of the system only. Key Points: We study the 2‐day photochemical oscillations and their phase fronts zonally traveling under the action of eddy diffusionWaves are caused by "wind" type transport appearing in equations for oscillation amplitudes due to zonal inhomogeneity of solar radiationThe wave propagation direction is determined by definite phase relations of 2‐day photochemical oscillations [ABSTRACT FROM AUTHOR]

This study assesses the benefit of convective‐scale data assimilation (DA) for model initialization using well‐known functional relationships between lightning flash rate and vertical velocity. Based on the relationships, a lightning DA scheme to update model kinematic states was implemented in the Weather Research and Forecasting Data Assimilation (WRFDA) three‐dimensional variational (3DVar) system. This scheme combines total lightning observations with model‐based prescribed vertical velocity profiles to retrieve kinematic information through a DA scheme. With the availability of space‐borne lightning imagers in recent years, total lightning observations from the Lightning Mapping Imager (LMI) on board the FY‐4A geostationary satellite were assimilated in combination with radar DA. A detailed analysis of the impact of the lightning DA scheme on convective precipitation forecasting was conducted using a squall line case over Beijing on 13 July 2017. The assimilation of LMI data provides added benefits to the assimilation of radar radial winds by reducing wind errors and strengthening convergence along the squall line in the analysis. Although the microphysical states are identical due to the assimilation of reflectivity, the lightning DA scheme helps in promoting updraft developments at lightning observation locations, which improves the representation of mixed‐phase convection. The quantitative verification of short‐term convective forecasts indicated that the lightning DA adds value to the radar DA by improving the precipitation forecast skill. The lightning DA scheme was evaluated further for a heavy rainfall case in 2018 over the Beijing metropolitan area and revealed overall similar forecast improvements for composite reflectivity and accumulated rainfall. Plain Language Summary: Lightning flashes are closely related to the upward air motions in thunderstorms and hence are indicative of strong wind convergence. Currently, lightning imagers on board the geostationary satellites provide increased availability of lightning data over broad regions and can improve weather forecasting accuracy. This paper describes how the space‐borne lightning observations could be employed to update model kinematic states and improve convective precipitation forecasting. Key Points: A lightning data assimilation (DA) scheme to update model kinematic states was developed using a three‐dimensional variational (3DVar) systemThe event data from Lightning Mapping Imager (LMI) aboard the FY‐4A geostationary satellite were assimilated to reflect lightning horizontal dimensionThe new lightning DA scheme improves the convective analysis and storm forecasting in two severe convective cases [ABSTRACT FROM AUTHOR]

It is challenging to develop observationally based spatial estimates of meteorology in Alaska and the Yukon. Complex topography, frozen precipitation undercatch, and extremely sparse in situ observations all limit our capability to produce accurate spatial estimates of meteorological conditions. In this Arctic environment, it is necessary to develop probabilistic estimates of precipitation and temperature that explicitly incorporate spatiotemporally varying uncertainty and bias corrections. In this paper we exploit the recently developed ensemble Climatologically Aided Interpolation (eCAI) system to produce daily historical estimates of precipitation and temperature across Alaska and the Yukon Territory at a 2 km grid spacing for the time period 1980–2013. We extend the previous eCAI method to address precipitation gauge undercatch and wetting loss, which is of high importance for this high‐latitude region where much of the precipitation falls as snow. Leave‐one‐out cross‐validation shows our ensemble has little bias in daily precipitation and mean temperature at the station locations, with an overestimate in the daily standard deviation of precipitation. The ensemble is statistically reliable compared to climatology and can discriminate precipitation events across different precipitation thresholds. Long‐term mean loss adjusted precipitation is up to 36% greater than the unadjusted estimate in windy areas that receive a large fraction of frozen precipitation, primarily due to wind induced undercatch. Comparing the ensemble mean climatology of precipitation and temperature to PRISM and Daymet v3 shows large interproduct differences, particularly in precipitation across the complex terrain of southeast and northern Alaska. Plain Language Summary: It is challenging to create spatial maps of precipitation and temperature in Alaska and the Yukon. The rugged terrain and extreme conditions, particularly snow and wind, along with the extremely sparse station networks, limit our ability to estimate historical conditions. Thus, it is critical to understand the uncertainty in our spatial estimates of meteorological conditions. Here we develop a new estimate of uncertainty for historical meteorology and include corrections to precipitation measurements for errors due to snowfall and wind. We show that there are large differences between several independent mapping efforts. We then demonstrate that our ensemble product skillfully predicts the observed probability of occurrence of multiple precipitation events sizes (the ensemble is statistically reliable). Finally, our precipitation corrections increase precipitation estimates by more than 30% in some portions of Alaska. Key Points: It is difficult to estimate the spatial variability in meteorology across the Arctic due to complex topography, sparse observations, and frozen precipitationWe developed a probabilistic approach to estimate precipitation and temperature, including precipitation undercatchThere are large uncertainties in our precipitation and temperature estimates over much of Alaska [ABSTRACT FROM AUTHOR]

We report on estimates of the vertical flux of horizontal momentum associated with an extensive and bright mesospheric gravity wave event that occurred over the El Leoncito Observatory, Argentina (31.8°S, 69.3°W), during the night of 17–18 March 2016. Using a combination of ground‐based and space‐based data sources, we propose that the event was generated by a large and distant thunderstorm complex located to the southeast of the observation site. The nightglow, including the wave structures, was easily visible to naked eye observers, a phenomenon known as a Bright Night. Such "extreme events" illuminate the important role of upward coupling in space weather research. Plain Language Summary: In this paper, we report on a large and extensive gravity wave event that occurred in the upper mesosphere, 90 km over the El Leoncito Observatory, Argentina (31.8°S, 69.3°W), during the night of 17–18 March 2016. The wave event was clearly visible to the naked eye—an event known as a Bright Night. Using a combination of ground‐based and satellite data sources, we present evidence that the wave event was generated by a large thunderstorm complex located several hundred kilometers away from El Leoncito. We estimated the amount of momentum flux associated with the waves was over an order of magnitude larger than typically observed. Events such as these illustrate that large amounts of wave energy and momentum can be transported from thunderstorms and other weather‐related events, up into the space environment near 100 km. They also illuminate the important role of upward coupling in space weather research. Key Points: Characteristics and evolution of mesospheric gravity waves generated by the thunderstormMomentum flux estimates were obtained on timescales of minutesMomentum flux values were found to be much larger than for typical mesospheric gravity waves [ABSTRACT FROM AUTHOR]

During a winter thunderstorm on 6 February 2017 in Japan, photonuclear reactions such as 14N(γ, n)13N were triggered by a downward terrestrial gamma‐ray flash (TGF), as reported by Enoto et al. (2017, https://doi.org/10.1038/nature24630). In the present paper, we compare the observation with a simulation model of downward TGFs and subsequent photonuclear reactions constructed by the first paper of the series and Wada, Enoto, Nakazawa, et al. (2019, https://doi.org/10.1103/physrevlett.123.061103). The observation and model consist of three components: annihilation gamma rays from positrons produced by β+‐decay nuclei, deexcitation gamma rays originating from neutron captures, and radiation doses by TGF photons. Each component of the observation is reproduced by the simulation model, and we constrain a relation between the number of avalanche electrons and their production altitude of the downward TGF. The constraints by three components match within an order of magnitude. The downward TGF is estimated to comprise (0.5–2.5) × 1019 avalanche electrons above 1 MeV produced at an altitude of 1.4–2.7 km. Despite differences in altitude, direction, and season, downward TGFs in winter thunderstorms are thought to have the same mechanism of electron acceleration and multiplication in lightning as TGFs observed by in‐orbit satellites. Plain Language Summary: Lightning flashes are sometimes accompanied by strong high‐energy emission called "terrestrial gamma‐ray flashes" (TGFs). The energy of gamma rays in TGFs is high enough to trigger nuclear reactions with atmospheric nuclei such as nitrogen and oxygen. The nuclear reactions produce neutrons and unstable isotopes that emit positrons. In the companion paper of this series, we calculated the nuclear reactions and propagation processes of their products such as positrons and neutrons and modeled their characteristics at ground level by simulations. In the present paper, the simulation model is compared with the actual observation of a TGF and nuclear reactions during winter thunderstorms in Japan. The model successfully reproduces the observation and estimate the production altitude the number of energetic electrons involved in the TGF. Key Points: A simulation model of photonuclear reactions in lightning is compared with an observation during a winter thunderstorm reported by Enoto et al. (2017, https://doi.org/10.1038/nature24630)Three detected signatures originating from positrons, neutrons, and TGF photons are reproduced by the simulation modelA downward TGF triggering photonuclear reactions comprised 1018 to 1019 avalanche electrons produced at 1.4–2.7 km above sea level [ABSTRACT FROM AUTHOR]

Evaporation of water bodies is significantly controlled by the available energy (net radiation Rn minus heat storage G). Compared to deep lakes or lakes in the high‐latitude or high‐altitude regions, the heat storage effects of shallow lakes in subtropical regions are less well studied. In this paper, we tested the heat storage effect on the latent heat fluxes estimation using the Priestley‐Taylor (PT) equation and numerical water temperature simulations in an ephemeral and shallow lake, Poyang Lake, in China. Results showed that the estimation bias was significantly correlated with heat storage changes (r = 0.97). Overlooking the impacts of the heat storage changes will induce an increase in the RMSE (root‐mean‐square error) of the evaporation estimation from 10.1 to 54.4 W m−2. Evaporation was overestimated in June and underestimated in August because the daytime Rn tended to be stored in water in June, whereas the heat storage during daytime tended to be released as nighttime evaporation in August, when the vapor gradient between the saturated surface and the air was generally larger than in other months. Water temperature simulations showed that the diurnal changes in the heat storage in the layer that were less than 2 m deep govern G of the total water column. PT estimation of a shallow lake can be improved if the temperature of the top 2 m of water is known. Also, from the modeling perspective, because G was linearly correlated with Rn (r = 0.77), a simple equation (a*Rn + b) that approximates the available energy can improve the evaporation estimate. Key Points: We study the heat storage effect on the PT estimation in an ephemeral and shallow lake, Poyang LakeThe overlook of heat storage induced the RMSE of evaporation estimation increased over three timesThe conversion of heat storage in the water into nighttime LE was the key for evaporation estimation [ABSTRACT FROM AUTHOR]

Momentum and mass exchanges between the atmosphere and forests situated on complex terrain continue to draw significant research attention primarily because of their significance to a plethora of applications. In this paper, we investigated flows behavior on the leeward side of a two‐dimensional forested ridge under neutrally stratified conditions using large‐eddy simulations (LESs). The goal is to understand how variations in leaf area index (LAI), vertical canopy foliage distributions, and forest edge positions affect mean/turbulent flow statistics, momentum fluxes, and onset of recirculation patterns. Although pressure perturbations are dominated by the hill shape, it is demonstrated here that changes in canopy foliage distribution modulate intensities and patterns of the leeward adverse pressure gradients. Such changes in the adverse pressure gradients alter the mean velocity streamlines including the patterns and magnitudes of the leeward downward mean vertical velocity and the velocity variances and momentum flux in the wake region. While a downwind recirculation zone develops in all cases, the details regarding the incipient location and recirculation zone size vary including positions of the separation and reattachment points. Furthermore, changes in the strength and depth of the zone occur due to canopy‐induced changes in adverse pressure gradients, advection, and canopy drag. Because the recirculation zone impacts the local mean advective terms in momentum and scalar exchanges, the simulations here indicate that canopy morphology‐induced changes in the leeward flows have significant implications to both measurements and models of biosphere‐atmosphere exchange over complex terrain. Key Points: Flows over the leeward side of a partially and fully forested hill are studied using a multiple‐layer canopy module in the WRF‐LESCanopy morphologies and partial forest cover lead to pressure perturbations that deviate from hydrostatic approximationThe leeward recirculation is altered by canopy morphological changes and partily modulated the pressure‐velocity interaction [ABSTRACT FROM AUTHOR]

An atmospheric river (AR) impacting Tasmania, Australia, and the Southern Ocean during the austral summer on 28–29 January 2018 during the Southern Ocean Clouds, Radiation, Aerosol Transport Experimental Study campaign is analyzed using a modeling and observational approach. Gulfstream‐V dropsonde measurements and Global Precipitation Measurement radar analyses were used in conjunction with Weather Research and Forecasting model simulations with water vapor tracers to investigate the relative contributions of tropical and midlatitude moisture sources to the AR. Moisture associated with a monsoonal tropical depression became entrained into a midlatitude frontal system that extended to 60°S, reaching the associated low‐pressure system 850 km off the coast of Antarctica—effectively connecting the tropics and the polar region. Tropical moisture contributed to about 50% of the precipitable water within the AR as the flow moved over the Southern Ocean near Tasmania. The tropical contribution to precipitation decreased with latitude, from >70% over Australia, to ~50% off the Australian coast, to less than 5% poleward of 55°S. The integrated vapor transport (IVT) through the core of the AR reached above 500 kg m−1 s−1 during 1200 UTC 28 January to 0600 UTC 29 January, 1.29 times the average amount of water carried by the world's largest terrestrial river, the Amazon. The high IVT strength might be attributed to the higher water vapor content associated with the warmer temperatures across Australia and the Southern Ocean in austral summer. Plain Language Summary: Atmospheric rivers (ARs) are long, narrow corridors of strong horizontal water vapor transport, normally associated with a low‐level jet stream ahead of a cold front. They play a critical role in the global hydrologic cycle and have been studied extensively over the northeast Pacific. This and a companion paper are the first to analyze an AR event impacting Tasmania, Australia, and the Southern Ocean using a modeling and observational approach. The AR extended from the tropics over northern Australia southeastward to the southern Australian coast and over the Southern Ocean to 60°S. The flow at higher altitudes in the AR was primarily composed of moisture originating in the tropics, while at lower altitudes and warmer temperatures, the moisture was primarily sourced from middle latitudes as a result of the circulation associated with a cold front. The tropical contribution to precipitation decreased with latitude, from >70% over Australia, to ~50% off the Australian coast, to less than 5% poleward of 55°S. Water vapor transported through the core of the AR was more than the average amount of water carried by the world's largest river, the Amazon, during the same time interval. Key Points: The contribution of moisture from the tropics to precipitation within an Australian summer atmospheric river is documentedOver 50% of the moisture arriving at the Southern Ocean at altitudes above the 0°C isotherm was sourced from tropical latitudesThe tropical contribution to precipitation decreased with latitude, >70% over Australia, ~50% off Australia, and <10% poleward of 60°S [ABSTRACT FROM AUTHOR]

A rare disturbance of the stratospheric Antarctic polar vortex in September 2019 led to a significantly higher than usual polar total ozone column. We use assimilation of ozone, HCl, and N2O data from the Microwave Limb Sounder with the Global Earth Observing System (GEOS) Constituent Data Assimilation System driven by reanalysis meteorology to study the evolution of the 2019 Antarctic polar ozone. We find that the maximum 2019 ozone hole area was near 10 × 106 km2, and as little as 20% of that in 2018 in mid‐September. However, the magnitude of vortex‐averaged chemical ozone depletion was not significantly different between the 2 years despite earlier chlorine deactivation in 2019. The assimilation results show that most of the differences between 2018 and 2019 Antarctic ozone resulted from two factors: (1) the geometry of the 2019 vortex, with ozone‐rich middle‐stratospheric air masses overlying the lower portion of the vortex and leading to a significant reduction of the total column, and (2) significantly reduced vortex volume. The anomalously small ozone hole of 2019 was comparable in size to the record breaking 2002 case and the mechanisms responsible were similar in the two cases. While the 2019 sudden stratospheric warming is classified as minor, its impact on ozone was very significant. Plain Language Summary: A significant dynamical disturbance of the stratospheric polar vortex occurred over Antarctica in September 2019. Named sudden stratospheric warmings (SSWs), such events are relatively common in the Northern Hemisphere but exceedingly rare over the southern high latitudes. SSWs are known to disturb the chemical composition of the polar vortex, which slows down the reactions that lead to ozone depletion. In this paper we use observations of chemical composition of the 2019 Antarctic polar vortex from National Aeronautics and Space Administration (NASA)'s satellite sensor, the Microwave Limb Sounder, combined with a Global Earth Observing System model simulation to study the chemistry and dynamics of this unusual polar winter/spring season. We show that the 2019 SSW, although not classified as "major," led to more than a 50% reduction of the ozone hole area at its peak extent, compared to other years since 2004. Furthermore, we demonstrate that most of this reduction was caused by an unusual geometry and small size of the polar vortex, while the chemistry of the 2019 ozone hole was comparable to that in the undisturbed year 2018. The 2019 event can be regarded as an example of a minor sudden stratospheric warming with a major impact. Key Points: The 2019 ozone hole area was about 10 ×106 km2 or less, compared to over 20×106 km2 typical for Septembers 2005–2018The anomalously high Antarctic total ozone resulted from an unusual polar vortex size and geometry rather than from chemistryEven a minor sudden stratospheric warming in the Southern Hemisphere can have a big impact on polar total ozone [ABSTRACT FROM AUTHOR]

In situ observations of reduced stratospheric water vapor combined with those of ice particle formation are rarely conducted. On the one hand, they are essential to broaden our knowledge about the formation of polar stratospheric clouds (PSCs). On the other hand, the observed profiles allow the comparison with global circulation models. Here we report about a balloon‐borne observation above Sodankylä, Finland on 26 January 2005. The frostpoint hygrometer detected layers of reduced water vapor by up to 2 ppmv from 18.5 to 23 km. Beneath, a 1‐km‐deep layer of increased water vapor was identified. An aerosol backscatter sonde measured the presence of stratospheric ice clouds. According to meteorological analysis the PSCs were formed upstream above the east coast of Greenland due to mountain wave‐induced cooling. The inertia‐gravity waves generated a large and persistent stratospheric wake far downstream of Greenland and led to the observed dehydration. Comparing the most recent ERA5 data with operational analyses from 2005, we find an improved representation of mesoscale internal gravity waves, dehydration and PSC formation for this particular event. Plain Language Summary: This paper investigates unique in situ observations of stratospheric water vapor and combines them with nearly simultaneous optical property measurements of ice particles. The presented results reveal Antarctic‐like stratospheric dehydration and rehydration inside the Arctic polar vortex. Balloon‐borne observations of this kind are extremely rare and constitute a valuable data set for extending our knowledge about both the cloud formation processes and the representation of water vapor in global circulation models. Here, we make use of state‐of‐the art numerical weather prediction models with high spatial resolution. They allow to resolve dynamically processes that were not accessible when the observations were conducted in 2005. Besides the large‐scale cooling inside the Arctic polar vortex, it is the far‐ranging impact of the numerically resolved stratospheric mountain waves that was responsible for maintaining the thermodynamic conditions leading to the unique observations over Sodankylä, Finland. Key Points: We present unique balloon‐borne observations of stratospheric water vapor and aerosol backscatter above northern ScandinaviaThe balloon‐borne measurements reveal Antarctic‐like dehydration and rehydration and the existence of solid ice particlesThe far‐ranging impact of Greenland's mountain wave‐induced temperature fluctuations is a key process [ABSTRACT FROM AUTHOR]

Creating synthetic ice cloud profiles that are distributed according to realistic probability density functions is fundamentally essential for submillimeter wave radiometer studies. In this paper, we develop an algorithm to create synthetic ice cloud profiles by combing in situ microphysics measurements and spaceborne radar reflectivity observations using Bayesian Monte Carlo Integration (MCI). We first conduct comprehensive studies of the in situ data from the Tropical Composition, Cloud and Climate Coupling (TC4) experiment based on a bimodal particle size distribution (PSD) scheme and the realistic particle scattering properties for different crystal habits. Ice water content and radar reflectivity simulations from the in situ PSD are compared with the in situ measurements qualitatively and quantitatively. We then evaluate the Bayesian MCI by applying the algorithm to the Cloud Radar System (CRS) during the TC4 campaign and compare the brightness temperature simulations for the retrievals with the Compact Scanning Submillimeter Imaging Radiometer (CoSSIR) measurements. Finally, we apply the Bayesian MCI to the Cloud Profiling Radar (CPR) measurements on CloudSat, and we use the cumulative distribution functions (CDFs) and empirical orthogonal functions (EOFs) to preserve the one‐point statistics and the two‐point statistics that allow for the creation of any number of synthetic ice cloud profiles that are statistically consistent with the Bayesian retrieval results. The synthesized profiles are a useful characterization of the ice cloud vertical structure that combines two disparate data sources, and the synthesis algorithm constitutes an essential initial step for producing critical information in submillimeter radiometry studies. Key Points: We propose an algorithm to synthesize ice cloud profiles that are distributed according to realistic probability density functionsThe algorithm combines the in situ microphysics with the radar measurements using Bayesian Monte Carlo IntegrationWe evaluate the synthesis algorithm with the data from the TC4 campaign and the CloudSat radar observations [ABSTRACT FROM AUTHOR]

Stratospheric water vapor (SWV) is recognized as a potentially important positive feedback in global warming. The SWV change induces significant downward radiative flux perturbation at the tropopause and therefore is hypothesized to substantially amplify the surface warming. To test this hypothesis, we use a global climate model to quantify the surface warming contributed by the SWV change in the context of the quadrupled CO2. By prescribing the SWV increase as an external forcing, we find that SWV only accounts for 0.42 K surface warming, making up merely 5.4% of the total CO2‐caused surface warming (7.7 K). The efficacy of the stratosphere‐adjusted SWV forcing is small (38%), where the efficacy is defined as the ratio of the global temperature response per unit radiative forcing relative to that of the CO2 forcing. With the aid of a series of auxiliary experiments, we find that although the stratosphere‐adjusted SWV forcing at the top of atmosphere (TOA) is significant (1.13 W m−2), more than half of the forcing is offset by a high‐cloud decrease and an upper tropospheric warming in the tropospheric adjustment. The direct radiative impact of the SWV increase on surface temperature is negligible, and the SWV‐induced surface temperature change is a result of interactions between the radiative and nonradiative processes. Plain Language Summary: What role does the stratospheric water vapor play in global warming? Previous works demonstrated that stratospheric water vapor increase exerts significant downward radiative flux perturbation at the tropopause. This paper investigates whether this SWV radiative perturbation can substantially amplify the surface warming. Our simulation shows a negative answer to this question. The surface warming attributed to the stratospheric water vapor change is small, due to the effect of the tropospheric adjustment. The stratospheric water vapor increase causes a decrease in the high clouds and an increase in the upper tropospheric temperature, which offsets more than half of the stratospheric water vapor‐induced radiative perturbation. Key Points: The surface warming efficacy of the stratosphere‐adjusted SWV forcing is low (38%)SWV increase accounts for about 5% of total warming in the context of CO2‐driven global warmingMore than half of the stratosphere‐adjusted SWV forcing is offset by the high cloud decrease and the upper tropospheric warming in the tropospheric forcing adjustment [ABSTRACT FROM AUTHOR]

Quantifying the underground water exchange between adjacent regions is a common question in field investigations and groundwater flow models. This paper proposes an innovative approach to assess underground water exchange patterns based on Gravity Recovery and Climate Experiment (GRACE) observations, the water balance method, and Darcy's law. The dynamic patterns of flux changes are investigated over four boundaries of an arid plain. Moreover, the causes of anthropogenic and climate factors are evaluated using the entropy method and the partial least squares regression (PLSR) model. The results indicate that terrestrial water storage (TWS) in the study area shows a decreasing trend at a rate of −1.77 ± 0.21 mm/yr from 2003 to 2015. The water resources recharging the study area from its southern boundary show a noticeable increasing trend, which is mainly attributed to the warming climate and increasing precipitation on the Qilian Mountains. The Ecological Water Diversion Project and the imbalance between precipitation and evapotranspiration are the main factors inducing the decline of flux changes for the northern, western, and eastern boundaries. Under the constrains of empirical values of hydrogeological parameters, GRACE‐based methods find that increment of the flux changes in the southern boundary may account for approximately 0.43% of the investigated flux in 2000, and the decrement of flux changes in the eastern boundary may be about 0.01% to the previous measured results. Anthropogenic activities are the dominant driving force for the changes in water resources, with the weight of 88.70%. The presented method can provide insights into the patterns of flux change in data‐poor areas. Key Points: An innovative approach is proposed to assess underground water exchange between regions using GRACE dataWater resources recharged from the southern boundary show a noticeable uptrend owing to the warming climate and increasing rainfallThe influences of anthropogenic and climate factors are evaluated using the entropy method and the PLSR mode [ABSTRACT FROM AUTHOR]

This paper examines the accuracy of Weather Research and Forecasting model coupled with Chemistry (WRF‐Chem) generated 72 hr fine particulate matter (PM2.5) forecasts in Delhi during the crop residue burning season of October‐November 2017 with respect to assimilation of the Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol optical depth (AOD) retrievals, persistent fire emission assumption, and aerosol‐radiation interactions. The assimilation significantly pushes the model AOD and PM2.5 toward the observations with the largest changes below 5 km altitude in the fire source regions (northeastern Pakistan, Punjab, and Haryana) as well as the receptor New Delhi. WRF‐Chem forecast with MODIS AOD assimilation, aerosol‐radiation feedback turned on, and real‐time fire emissions reduce the mean bias by 88–195 μg/m3 (70–86%) with the largest improvement during the peak air pollution episode of 6–13 November 2017. Aerosol‐radiation feedback contributes ~21%, ~25%, and ~24% to reduction in mean bias of the first, second, and third days of PM2.5 forecast. Persistence fire emission assumption is found to work really well, as the accuracy of PM2.5 forecasts driven by persistent fire emissions was only 6% lower compared to those driven by real fire emissions. Aerosol‐radiation feedback extends the benefits of assimilating satellite AOD beyond PM2.5 forecasts to surface temperature forecast with a reduction in the mean bias of 0.9–1.5°C (17–30%). These results demonstrate that air quality forecasting can benefit substantially from satellite AOD observations particularly in developing countries that lack resources to rapidly build dense air quality monitoring networks. Key Points: MODIS AOD assimilation and aerosol‐radiation feedback improved accuracy of 72 hr PM2.5 forecasts in New Delhi by 70–86%Persistent fire emission assumption works really well for Delhi air quality forecasting system and introduces uncertainty of less than 6%Improved air quality forecasts also lead to improved surface temperature forecast through aerosol‐radiation feedback [ABSTRACT FROM AUTHOR]

Iceland contains some of the largest and most active dust sources in the Arctic, with the frequency of major emission events increasing in the wake of catastrophic volcanic eruptions, such as the 2010 Eyjafjallajökull eruption. Relatively juvenile Icelandic volcaniclastic dust particles are porous and highly angular, and they frequently bear ultrafine nanoscale dustcoats. Their low density is speculated to affect deposition rates in the atmosphere, while substantial amounts of water adsorbed onto their expansive surface area may affect nucleation, aggregation, and settling. Using a 2‐D laser Doppler anemometer, the present paper reports on a set of highly controlled fall column experiments designed to evaluate the dependency of the settling velocity upon particle morphology (e.g., size, shape, density, and surface area) and relative humidity (RH). Measurements for samples obtained from four Icelandic field sites are compared to those for solid glass spheres of similar diameter. The results confirm that the amount of adsorbed water increases with RH and the particle specific surface area. Higher dust concentrations are found to favor particle interaction and the formation of aggregates, especially at higher RH. Under dry conditions (20 % < RH < 30 %), the horizontal drift velocity measured for the angular, porous Icelandic particles exceeds that observed for solid glass spheres, although these measurements are an order of magnitude smaller than the vertical velocity. As compared to particle diameter, shape does not appear to influence either the settling rate or drag coefficient at Reynolds numbers ≤10−2, owing to the dominance of viscous effects over form drag. Key Points: Settling velocities measured in still, dry air for volcaniclastic dust are well approximated by those for glass spheres of equivalent diameterParticle angularity increases horizontal drift (dispersion) during dust depositionDue to particle aggregation, the settling rate of volcanoclastic particles with high specific surface area increases with atmospheric humidity [ABSTRACT FROM AUTHOR]

Chu, P. C. True gravity in atmospheric Ekman layer dynamics. Journal of Geophysical Research: Atmospheres, 126, e2021JD035293. https://doi.org/10.1029/2021JD035293. The above article from Journal of Geophysical Research Atmospheres, published online on 29 September 2021, has been retracted by agreement between the journal Editor‐in‐Chief, Minghua Zhang; AGU; and Wiley Periodicals LLC. The retraction has been agreed because the conclusions of the paper were found to be incorrect, since they depend on the choice of the coordinate system that does not apply to practical application of the theory of atmospheric Ekman boundary layer. The author disagrees with this assessment. [ABSTRACT FROM AUTHOR]

This work presents the first lidar observations of a Quasi‐Biennial Oscillation (QBO) in the interannual variations of stratospheric gravity wave potential energy density (Epm in 30–50 km) at McMurdo (77.84°S, 166.67°E), Antarctica. This paper also reports the first identification of QBO signals in the distance between McMurdo and the polar vortex edge. Midwinter stratospheric gravity wave activity is stronger during the QBO easterly phase when the June polar vortex expands and the polar night jet shifts equatorward. During the QBO westerly phase, gravity wave activity is weaker when the polar vortex contracts and the polar night jet moves poleward. Nine years of lidar data (2011–2019) exhibit the mean Epm winter maxima being ~43% higher during QBO easterly than westerly. The June polar vortex edge at 45 km altitude moves equatorward/poleward during QBO easterly/westerly phases with ~8° latitude differences (39.7°S vs. 47.7°S) as revealed in 21 years of MERRA‐2 data (1999–2019). We hypothesize that an equatorward shifted polar vortex corresponds to less critical level filtering of gravity waves and thus higher Epm at McMurdo. The critical level filtering is characterized by wind rotation angle (WRA), and we find a linear correlation between the WRA and Epm interannual variations. The results suggest that the QBO is likely controlling the interannual variations of the Epm winter maxima over McMurdo via the critical level filtering. This observationally based study lays the groundwork for a rigorous numerical study that will provide robust statistics to better understand the mechanisms that link the tropical QBO to extratropical waves. Key Points: We report a Quasi‐Biennial Oscillation in Antarctic gravity wave activity—potential energy density is enhanced during QBO easterly wintersAntarctic polar vortex edge moves equatorward/poleward during QBO easterly/westerly phases with a maximum latitude difference of ~8° in JuneAn equatorward shifted polar vortex corresponds to less critical level filtering of gravity waves and thus higher Epm in QBO easterly winters [ABSTRACT FROM AUTHOR]

Biomass burning releases a vast amount of aerosols into the atmosphere, often leading to severe air quality and health problems. Prediction of the air quality effects from biomass burning emissions is challenging due to uncertainties in fire emission, plume rise calculation, and other model inputs/processes. Ensemble forecasting is increasingly used to represent model uncertainties. In this paper, an ensemble forecast was conducted to predict surface PM2.5 during the 2018 California Camp Fire event using the National Oceanic and Atmospheric Administration (NOAA) HYSPLIT dispersion model at 0.1° horizontal resolution. Different combinations of four satellite‐based fire emission data sets (FEER, FLAMBE, GBBEPx, and GFAS), two plume rise schemes (Briggs and Sofiev), various meteorology inputs, and model setup options were used to create the forecast ensemble, for a total of 112 experiments. The performance of each ensemble member and the ensemble mean were evaluated using ground‐based observations, with four statistical metrics and an overall rank. The ensemble spread of the 112 members reached 1,000 μg/m3, highlighting the large uncertainty in wildfire forecast. The ensemble mean displayed the best performance. Each fire emission product contributed to one or more members among the top 10 performers, revealing the forecasting dependence on both the quality of fire emissions data and model representation of emission, transport, and removal processes. In addition, an ensemble size reduction technique was introduced. With the help of this technique, the ensemble size was reduced from 112 to 28 members and still produced an ensemble mean that yielded comparable or even better performance to that of the full ensemble. Key Points: The difference between satellite fire emission data sets is large, which reaches a factor of 10Ensemble forecast with different emission data sets, meteorology, and plume rise schemes can reduce forecast uncertaintyThe new ensemble size reduction technique can save calculation time while improving the performance of the ensemble forecast [ABSTRACT FROM AUTHOR]

This paper for the first time numerically characterizes the water vapor spatial and temporal variation during the life cycle of super typhoons, using the 3D tomographic water vapor modeling method. Global Positioning System (GPS) data sets collected in Hong Kong were used to reconstruct the water vapor density field with a temporal resolution of 30 min during three super typhoon events in 2013 and 2014. The spatiotemporal variabilities of water vapor at five altitude layers (<0.8, 0.8–1.6, 1.6–3.0, 3.0–5.2, and 5.2–8.5 km) were examined. It was found that in the lower troposphere (<1.6 km), the precipitable water vapor (PWV) had only minor fluctuation during typhoon events. But significant PWV increase was observed at altitude above 1.6 km, and PWV kept increasing with the altitude, leading to the increase of the total water vapor. Hong Kong's PWV was observed to increase as the typhoons were about 795 to 821 km away from Hong Kong. The PWV spatial (distance) increase rate was 0.031–0.037 mm/km in the upper layers (1.6–8.5 km) and temporal increase rate was 12.710–24.272 mm/day in the upper layers (1.6–8.5 km). The typhoons had the largest impact on the PWV variation in the top layer (layer 5, 5.2–8.5 km). Prior to typhoon impact, PWV in layer 5 accounts for only 12.77% to 13.68% in the upper layers (1.6–8.5 km). With the typhoon impact, the PWV in layer 5 accounts for 21.28% to 26.38%. This work has demonstrated the potential to use water vapor tomographic technique to study water vapor evolution during super typhoons. Key Points: The 3D tomographic water vapor model allows detailed monitoring of water vapor change during three super typhoon eventsPrecipitable water vapor (PWV) in the lower troposphere (0–1.6 km) has almost no change during the whole life cycle of super typhoonsPWV temporal and spatial increase rates in layers (1.6–8.5 km) are about 12.710 to 24.272 mm/day and about 0.031 to 0.037 mm/km, respectively [ABSTRACT FROM AUTHOR]

Microbursts are meteorological phenomena in the lower troposphere which can produce damaging surface winds and pose a severe risk to aircraft flying close to the ground. As these events usually span less than 4 km and 15 min, the spatiotemporal resolution is a challenge for numerical simulations. Although research of microburst using operative mesoscale models is scarce, the Weather Research and Forecasting (WRF) model has been used in the diagnosis of this phenomenon. In this paper, such model is used to simulate several microburst conducive days using two different boundary conditions. The energy spectra of the simulations are computed to evaluate the effective resolution of the model. The results are in line with previous studies and produce no notable differences among the boundary conditions. Nonetheless, the energy spectra show an overenergetic troposphere at microscale resolutions, rendering the effective resolution inadequate for microburst forecasting using the simulated physics variables. Thus, mesoscale indices are analyzed as a prognostic tool. The wind index, the wet microburst severity index and the microburst windspeed potential index do not show high forecasting performances, even though improving the results of climatology. Also, notable differences among the boundary conditions can be seen. The most consistent results are achieved by the wet microburst severity index. Key Points: The WRF model shows an unrealistic energy spectrum when microscale spatial resolutions are usedThe effective resolution of the simulations casts doubts over the ability to forecast microburstsWINDEX, WMSI, and MWPI microburst indices show better forecasting performance than climatology, despite being suboptimal in general terms [ABSTRACT FROM AUTHOR]

This work is the first of a two‐part study that aims to develop a computationally efficient bias correction framework to improve surface PM2.5 forecasts in the United States. Here, an ensemble‐based Kalman filter (KF) technique is developed primarily for nonrural areas with approximately 500 surface observation sites for PM2.5 and applied to three (GEOS‐Chem, WRF‐Chem, and WRF‐CMAQ) chemical transport model (CTM) hindcast outputs for June 2012. While all CTMs underestimate daily surface PM2.5 mass concentration by 20–50%, KF correction is effective for improving each CTM forecast. Subsequently, two ensemble methods are formulated: (1) the arithmetic mean ensemble (AME) that equally weights each model and (2) the optimized ensemble (OPE) that calculates the individual model weights by minimizing the least‐square errors. While the OPE shows superior performance than the AME, the combination of either the AME or the OPE with a KF performs better than the OPE alone, indicating the effectiveness of the KF technique. Overall, the combination of a KF with the OPE shows the best results. Lastly, the Successive Correction Method (SCM) was applied to spread the bias correction from model grids with surface PM2.5 observations to the grids lacking ground observations by using a radius of influence of 125 km derived from surface observations, which further improves the forecast of surface PM2.5 at the national scale. Our findings provide the foundation for the second part of this study that uses satellite‐based aerosol optical depth (AOD) products to further improve the forecast of surface PM2.5 in rural areas by performing statistical analysis of model output. Plain Language Summary: Air quality forecasting plays an important role in informing the general public and decision‐makers on reducing exposure to air pollution. Air quality models simulating atmospheric constituents such as particulate matter with a diameter less than 2.5 μm (PM2.5) are often used to provide daily forecasts. However, these models are subject to large error and uncertainty as a result of the incomplete representation of the real atmosphere. Here, we develop a computationally efficient framework to improve model forecasts by performing bias correction on model outputs. We focus on nonrural areas in the continental United States and show that our technique improves model forecasts of surface PM2.5. In a companion paper, we focus on the application of satellite data to improve PM2.5 forecasting in rural areas. Key Points: The chemical transport models (GEOS‐Chem, WRF‐Chem, and WRF‐CMAQ) show systematically low bias of PM2.5Model output postprocessing with surface data and the ensemble Kalman filter technique improves the PM2.5 forecast at both local and urban scaleThe Successive Correction Method extends the PM2.5 forecast improvement from the local to regional scale [ABSTRACT FROM AUTHOR]

Aerosol‐cloud interactions remain a major source of uncertainty in our understanding and modeling of the Earth's hydrological cycle. Based upon a diagnostic and modeling analysis utilizing the latest field measurements from the Integrative Monsoon Frontal Rainfall Experiment (IMFRE) 2018, this paper reports the effects of aerosols on the cloud properties along the Mei‐yu front over the Middle Reaches of Yangtze River in China. Numerical experiments with the Weather Research and Forecasting (WRF) model suggest that increasing aerosol number concentration reduces surface precipitation by ~8.8% and delays the onset of rainfall by ~30 min. Furthermore, warm clouds are suppressed but the convective cores are slightly intensified. This corresponds to an overall aerosol effect of "localization and invigoration" of the Mei‐yu rainfall and thus an elevated probability of short‐term heavy rainfall. The signals of "convective invigoration" with a bulk scheme in this study are relatively weak compared to those simulated by bin microphysics. The increased aerosol concentration strengthens Mei‐yu front and changes local morphology of the front, consistent with earlier studies demonstrating positive effects of convective heating on the genesis and maintenance of Mei‐yu front via conditional instability of the second kind (CISK) and diabatic generation of potential vorticity. Also discussed are the uncertainties of bulk microphysics in simulating aerosol‐cloud interactions, which may shed light on the design of future field campaigns to further understand the impact of aerosol‐cloud interactions on weather and climate over China in boreal summer. Key Points: Increased aerosol concentration suppresses warm clouds and slightly strengthens convection,leading to an overall effect of "localization and invigoration" of the Mei‐yu rainfallEnhanced latent heating in polluted conditions intensifies updrafts of convective coresIncreased aerosol concentration strengthens the Mei‐yu front and changes the local morphology of the front [ABSTRACT FROM AUTHOR]

Recent increases in computing power mean that atmospheric models for numerical weather prediction are now able to operate at grid spacings of the order of a few hundred meters, comparable to the dominant turbulence length scales in the atmospheric boundary layer. As a result, models are starting to partially resolve the coherent overturning structures in the boundary layer. In this resolution regime, the so‐called boundary layer "gray zone," neither the techniques of high‐resolution atmospheric modeling (a few tens of meters resolution) nor those of traditional meteorological models (a few kilometers resolution) are appropriate because fundamental assumptions behind the parameterizations are violated. Nonetheless, model simulations in this regime may remain highly useful. In this paper, a newly formed gray zone boundary layer community lays the basis for parameterizing gray zone turbulence, identifies the challenges in high‐resolution atmospheric modeling and presents different gray zone boundary layer models. We discuss both the successful applications and the limitations of current parameterization approaches, and consider various issues in extending promising research approaches into use for numerical weather prediction. The ultimate goal of the research is the development of unified boundary layer parameterizations valid across all scales. Key Points: The grid resolution of atmospheric models has become fine enough that are able to partially resolve turbulent motions in the boundary layerThis resolution regime comprises the turbulence "gray zone" where the assumptions behind traditional parameterizations are no longer validWe review the current efforts by modelers to overcome the gray zone problem in order to provide useful simulations at high resolutions [ABSTRACT FROM AUTHOR]

The Atlantic multidecadal variability (AMV) acts as a key source of multidecadal variability and plays an important role in climate change in many regions of the Northern Hemisphere, resulting in great social and economic impacts. This paper shows that, since the late 19th century, the AMV has experienced a significant increasing trend. The AMV intensified at a rate of 0.0040°C per decade from 1870 to 2014, which is statistically significant at the 95% confidence level. The AMV teleconnects the amplified signal from the North Atlantic across the Eurasian continent to North America through a circumglobal teleconnection pattern, which further intensifies the regional climate variability and climate extremes in North Africa, Europe, East Asia, and North America. Results from the single‐forcing and all‐but‐one‐forcing experiments of the Community Earth System Model, version 1, indicate that anthropogenic aerosol emissions have played a more important role in the AMV intensification compared to other external forcings, especially during the period from the 1900s to 1990s. Key Points: The AMV has experienced a significant increasing trendThe intensified AMV has amplified the regional climate variability and climate extremes in the Northern HemisphereAnthropogenic aerosol emissions have played an important role in the AMV intensification [ABSTRACT FROM AUTHOR]

The Soberanes Fire burned 53,470 ha (132,127 acres) along the central California coast between 22 July and 12 October 2016, generating dense smoke and a variety of gaseous compounds that drifted eastward into the San Joaquin Valley Air Basin (SJVAB), an "extreme" nonattainment area for ozone (O3). These gases included nitrogen oxides (NOx) and volatile organic compounds, the photochemical precursors of O3. The fire started during the California Baseline Ozone Transport Study, a field campaign that brought aircraft, surface, and remote sensing measurements of O3 and related species to central California. In this paper, we use the California Baseline Ozone Transport Study measurements to assess the impact of the Soberanes Fire on ozone and particulate air quality in the SJVAB. We focus our analysis on 27 July to 2 August when the smoke haze was heaviest and the highest O3 concentrations in the SJVAB during 2016 were recorded. Our analyses suggest that while 40 to 60 ppbv of fire‐generated O3 was transported to the eastern SJVAB in the 1‐ to 3‐km‐altitude range, relatively little smoke or fire‐generated O3 reached the surface in the Visalia area. Key Points: Aircraft and remote sensing measurements are used to investigate impacts of the Soberanes Fire on air quality in the San Joaquin ValleyOzone enhancements in excess of 50 ppbv were measured by both lidar and aircraft between 1 and 3 km above the surfaceNo significant enhancements in surface ozone were observed [ABSTRACT FROM AUTHOR]

We present a complete and systematic search for terrestrial gamma‐ray flashes (TGFs), detected by AGILE, that are associated with radio sferics detected by the World Wide Lightning Location Network (WWLLN) in the period February 2009 to September 2018. The search algorithms and characteristics of these new TGFs will be presented and discussed. The number of WWLLN identified (WI) TGFs shows that previous TGF selection criteria needs to be reviewed as they do not identify all the WI TGFs in the data set. In this analysis we confirm with an independent data set that WI TGFs tend to have shorter time duration than TGFs without a WWLLN match. TGFs occurs more often on coastal and ocean regions compared to the distribution of lightning activity. Several multipulse TGFs were identified and their WWLLN match are always associated with the last gamma‐ray pulse. We also present the first Terrestrial Electron Beam detected by AGILE. This data set together with the TGF sample identified by selection criteria (companion paper Maiorana et al., 2020) constitute the 3rd AGILE TGF catalog. Key Points: A sample of 592 AGILE detected TGFs associated to lightning sferics is presentedTGF duration, geographic dependence of the TGF/lightning ratio, and multipulse TGFs are discussedThe first Terrestrial Electron Beam detected by AGILE is presented [ABSTRACT FROM AUTHOR]

The location of the Earth Polychromatic Imaging Camera (EPIC) aboard the Deep Space Climate Observatory (DSCOVR) offers a global view of the Earth and captures its atmosphere at different locations and time of the day. In this paper, we take advantage of that unique feature to study the daytime variability of cloud fraction in a seasonal and zonal context. We observe that the ensemble behavior over ocean has a distinctive convex shape, with higher cloud fractions at early morning and late afternoon, while no such pattern is seen over land. Unique perspectives are obtained by analyzing the cloud fraction of the globe as a whole and separately for each hemisphere, and by studying the effect of the viewing zenith angle on cloud fraction retrievals. Plain Language Summary: Images from the Earth Polychromatic Imaging Camera instrument aboard the Deep Space Climate Observatory satellite are unique compared to those from other satellites since the left edge of the image corresponds to sunrise, the right edge to sunset, and the middle to noon. That capability of capturing different parts of the day in a single picture allows us to study the daytime behavior of cloud fraction in new ways. When conducting separate analysis over ocean and land, we find that cloud fraction over ocean is generally higher in the early morning and late afternoon and minimum during the central hours of the day. However, the behavior over land is generally flat with only a very slight increase in the afternoon. Key Points: EPIC observes the sunlit side of the Earth with a frequency between 1 and 2 hr providing information on cloudiness at different local timesFour years of EPIC global cloud fraction was analyzed as a function of season and latitudeThe daytime evolution of cloudiness over ocean exhibits a clear convex shape, while it has less pronounced features over land [ABSTRACT FROM AUTHOR]

A carbonaceous aerosol plume associated with wildfires in British Columbia in August 2017 reached the stratosphere a few days following the fires onset. The Earth Polychromatic Imaging Camera (EPIC) sensor onboard the Deep Space Climate Observatory (DSCOVR) satellite observed the spatial and temporal evolution of the plume for about 6 weeks. EPIC's near‐hourly observations of the Earth's sunlit disk allowed for monitoring the smoke plume several times per day. High ultraviolet aerosol index values pointed to the presence of an aerosol layer in the upper troposphere and lower stratosphere, confirmed with Cloud‐Aerosol Lidar with Orthogonal Polarization (CALIOP) lidar observations. EPIC aerosol optical depth retrievals and CALIOP information on layer height were used to estimate the stratospheric portion of the aerosol mass, reaching a maximum of 268 ± 86 Kt 3 days after the onset of the wildfires. About 10% of this aerosol mass reached the stratosphere on 13 August, whereas the remaining 90% did so over the following 2 days. Observed daytime and inferred nighttime rates of stratospheric aerosol mass increase showed a more rapid daytime increase, consistent with variability in diurnal insolation, suggesting that solar radiation driven processes may have contributed to the observed stratospheric aerosol mass increase. Global modeling results support the possible role of diabatic aerosol self‐lofting. Ozone Mapping and Profiler Suite Limb Profiler (OMPS‐LP) aerosol retrievals showed the spread of stratospheric aerosols in the Northern Hemisphere, reaching peak concentration between October 2017 and January 2018, when about half the Northern Hemisphere experienced a detectable enhancement of stratospheric aerosols with respect to preinjection conditions. The lifetime and aerosol mass of this smoke event were comparable to those of a moderate volcanic eruption. Plain Language Summary: The 2017 wildfire season in western North America was unusually intense. As documented by several media outlets (https://globalnews.ca/news/3921710/b‐c‐year‐in‐review‐2017‐wildfires/), over 1,300 fires and more than 1.2 million hectares burned were reported between April and November in the Canadian British Columbia province. In mid‐August, the combination of very intense boreal fires and typical summer meteorological conditions led to the rapid formation fire‐triggered convective clouds of considerable vertical extent. Because of the pyroconvective activity of these clouds, thousands of tons of carbon‐containing smoke particles, also known as aerosols, were injected above 10 km in a few hours, some of which may have been directly injected in the stratosphere. Heating of the aerosol layer resulting from absorption of solar radiation induced lofting of the smoke plume into the stratosphere (generally above 12 km in this area), a region of the atmosphere where aerosol removal processes are slow. Because of their long residence time, stratospheric aerosols are very important as they affect Earth's radiative budget and climate. In this paper, we use satellite observations and models to describe the extended three‐dimensional spatial distribution and long lifetime of the resulting aerosol plume that made this an unusual aerosol event. Key Points: Carbonaceous aerosols generated by the 2017 Canadian wildfires reached the upper troposphere and lower stratosphereDiabatic heating from aerosol absorption of solar radiation may have triggered the lofting of the aerosol layer from the upper troposphere to the stratosphereThe aerosol layer covered the northern hemisphere from about 30°N to the North Pole for a 10‐month period between August 2017 to June 2018 [ABSTRACT FROM AUTHOR]

The variations of the horizontal phase velocity of an internal gravity wave, generated by wave "blocking" or "reflection" due to an inhomogeneous wind field, have been predicted theoretically and numerically investigated but had yet to be captured experimentally. In this paper, through a collaborative observation campaign using a sodium (Na) Temperature/Wind lidar and a collocated Advanced Mesospheric Temperature Mapper (AMTM) at Utah State University (USU), we report the first potential evidence of such a unique gravity wave process. The study shows that a small‐scale wave, captured by the AMTM, with initial observed horizontal phase velocity of 37 ± 5 m/s toward the northwest direction, experienced a large and increasing headwind as it was propagating in the AMTM field of view. This resulted in significant deceleration along its initial traveling direction, and it became quasi‐stationary before it was "reflected" to the opposite direction at later time. The USU Na lidar measured the horizontal wind and temperature during the event, when the wave was found traveling within a temperature inversion layer and experiencing an increasing headwind relative to the wave. The wind agrees well with the expected value for wave blocking suggested by the wave tracing theory, implying the existence of a large horizontal wind gradient that night near the OH layer altitudes. The study indicates the critical role of horizontal winds and their horizontal gradients in determining propagation in vertical and horizontal directions. Key Points: We observe blocking and reflection of a small‐scale gravity waveThe wave blocking event occurred when the wave was experiencing strong and increasing headwindThe wave was propagating in an upper mesosphere temperature inversion layer formed within the OH layer [ABSTRACT FROM AUTHOR]

The tropical Asian regions are particularly prone to catastrophic droughts due to high temperature and evaporation and their sensitivity to variable oceanic‐atmospheric circulation. The extent to which future global warming may intensify droughts in this populous region is a matter of utmost concern. Palaeoclimatic data can help inform scientists learn more about the temporal patterns and drivers of monsoon change over geological timescales. In this paper, we present a new 8,200 year biomarker record preserved in the annually laminated sediments of Maar Lake Twintaung, Myanmar. The δ13C27‐35 sedimentary record is mainly derived from leaf wax lipids in the lake catchment, which is presently dominated by C3 vegetation in the modern lake catchment. The δ13C27‐35 is mainly regulated by plant physiological and biochemical responses to drought stress and, as such, can be used as a proxy of past changes in moisture. The δ13C27–35 record reveals distinct decadal‐to‐centennial‐scale droughts superimposed on a trend of gradually decreasing summer monsoon intensity from the mid‐Holocene to late Holocene. Within the limits of the dating uncertainties, these decadal‐to‐centennial‐scale droughts are found to be well correlated with the southward shift of the Intertropical Convergence Zone. It highlights thepimportance of the Intertropical Convergence Zone shift (tropical monsoon trough) in regulating monsoon rainfall in the region. In addition, most of the droughts are likely to be linked with active volcanism and solar minima and suggest a coupled process between external drivers and internal climate dynamics. Key Points: Leaf wax n‐alkanes record in varved sediment of Maar lake Twintaung, MyanmarThe result indicates distinct decadal‐to‐centennial‐scale droughts over the past 8.2 kaThe drought cycles link with the ITCZ shift, solar and volcanic eruption [ABSTRACT FROM AUTHOR]

We revisit the findings from the FORTE satellite program (1997–2004), which collected optical imaging of lightning as well as optical and radio frequency time series waveforms globally from low‐earth‐orbit. These include surveys of the earth's radio frequency anthropogenic noise environment; earth surface reflectivity at radio frequencies; a scheme for classifying lightning discharge types on the basis of their very high frequency time domain power envelope; insights into the polarization and radiation pattern characteristics of different lightning types, with implications for the underlying discharge processes; and estimates of cloud optical properties based on the statistics of scattered light. Most significantly, FORTE was uniquely suited to capture large samples of data from the rare discharge known as "narrow bipolar events," enabling detailed examination of their basic characteristics and confirming that they appear to result from fundamentally distinct physical processes compared to other lightning. In particular, despite representing huge charge transfer they evidently produce little‐to‐no light output. Plain Language Summary: Because lightning occurs in or below clouds and clouds scatter visible light, it is difficult to ascertain details about the physical processes that result in lightning through examination of the light output alone. Radio frequency data can provide greatly improved insights, but measurements carried out on the ground can only sample small portions of the globe. The FORTE satellite carried sensors to measure both optical light and radio waves from lightning and therefore offered detailed insights into lightning processes on a global scale. One particular type of lightning event, the "narrow bipolar event," is the most powerful emitter of radio waves among all types of lightning, but it is quite rare in any given storm. These events make up about half of the FORTE data, however, and have therefore allowed study of them on unprecedented, global scale. This paper reviews many key findings from the FORTE program, including such surprises as the fact that narrow bipolar events are dark (giving off little‐to‐no visible light, despite their radio power), which is just one of their physical characteristics that has yet to be explained. Key Points: Key findings from the FORTE satellite mission (ca. 1997–2004) are summarizedParticular findings confirm the physically unique nature of narrow bipolar eventsThe FORTE data provided one of the largest joint lightning optical/radio frequency data sets studied to date. [ABSTRACT FROM AUTHOR]

Does lake evaporation increase or decrease under the scenario of climate warming? This paper aims to answer this question by investigating the controlling mechanism of interannual variations in lake evaporation at a subtropical lake. The research methodology is based on continuous eddy covariance measurement over >7 years and a diagnostic analysis using the surface energy balance principle. The results indicate that lake evaporation was enhanced mainly by increasing energy inputs including solar radiation and incoming longwave radiation and was weakened by surface feedback through outgoing longwave radiation. The incoming longwave radiation was positively correlated with cloud cover. Bowen ratio and surface albedo had slight effect on the change of lake evaporation. The annual lake evaporation can be predicted by the Priestly‐Taylor model using a larger coefficient of 1.39 than the original value of 1.26, suggesting that advection or entrainment in the atmospheric boundary layer may play a role in lake evaporation. Key Points: Incoming shortwave and incoming longwave radiation controls the interannual change of lake evaporationAnnual Bowen ratio and incoming and outgoing longwave radiation are predictable functions of temperature changeThe Brutsaert formula with a constant relative humidity predicts well the interannual variability in the incoming longwave radiation [ABSTRACT FROM AUTHOR]

This paper is the first to document the development of a type of bow echo, termed merger‐formation bow echo (MFBE), in southeast China evolving from a subtropical squall line (SL) merging with a preline convective cell (CC). Although this MFBE did not produce damaging surface winds, its intense rain rate resulted in local flooding. The evolution of the kinematic, thermodynamic, and microphysical structures is investigated using the variational Doppler radar analysis system (VDRAS) analysis and polarimetric radar observations. Key factors of this MFBE event including the rear‐inflow jet (RIJ) and cold pool exhibited different characteristics from those in classical bow echoes and limited MFBE cases. As the SL propagated towards the coast, a CC was triggered along a sea breeze front. The SL did possess a RIJ, but a bow‐shaped reflectivity did not appear until the SL‐CC merger. The RIJ weakened and became elevated during the merger, while the leading edge of SL cold pool accompanied by a weak diverging outflow did not advance with the reflectivity field. The updraft was strengthened due to the merger, resulting in enhanced precipitation falling ahead of the original SL. The subcloud evaporation locally cooled the air ahead of the SL and merged with the original SL cold pool. This combined cold pool advanced forward rapidly and caught up with the bowing radar reflectivity to form this MFBE. This study illustrates the processes of a SL‐CC merger leading to the formation of a different type of MFBE that did not produce damaging surface winds. Key Points: The first VDRAS analysis combines polarimetric radar to document microphysical and dynamic processes of a merger‐formation bow echo in ChinaRear‐inflow jet weakens during the formation of this nonclassical bow echoEvaporative cooling from intensified rainfall leads to the surge of cold pool [ABSTRACT FROM AUTHOR]

Following the description of the basic climatic characteristics of Meiyu in East Asia, we present a comprehensive review of multi–time scale variabilities of the Meiyu together with the long‐term change of extreme precipitation during the Meiyu in this paper. The dynamic prediction skills of the Meiyu on subseasonal to seasonal time scales are further described and illustrated by using present numerical prediction models. For the multi–time scale variabilities of the East Asian Meiyu, they are closely related with the impacts of both quasi‐biweekly and 30–60 day oscillation from both the tropics and the middle and high latitudes on intraseasonal time scale, the influences of El Niño–Southern Oscillation and the Tropospheric Biennial Oscillation on interannual time scale, the variations of East Asian summer monsoon on decadal time scale, and the continued impacts of global warming, Pacific Decadal Oscillation and the Atlantic Multidecadal Oscillation on interdecadal scale and long‐term trend. Under the background of the global warming and the influences of urbanization and aerosol effects, the Meiyu shows some specific characteristics including significantly weakened continuous rain, uneven spatial distribution of precipitation, and increased (decreased) number of strong (weak) precipitation days. Improvements of prediction on rain belt location, total precipitation amount, persistent heavy precipitation events, and low‐frequency variability oscillation during Meiyu season have been achieved with the improved dynamic models of the Beijing Climate Center since 2010. Key Points: Meiyu's variabilities are related with intraseasonal oscillation, interannual to multidecadal oscillation of ocean, and global warmingMeiyu impacted by global warming, urbanization, and aerosol effects exhibits stronger rainfall eventsImprovements of prediction of East Asian Meiyu on multi‐time scale have been achieved with the present operational climate models [ABSTRACT FROM AUTHOR]

Decadal‐scale trends in aerosol and cloud properties provide important ways for understanding aerosol‐cloud interactions. In this paper, by using MODIS products (2003–2017), we analyze synergetic trends in aerosol properties and warm cloud properties over global ocean. Cloud droplet number concentration (CDNC) and aerosol parameters (aerosol optical depth, angstrom exponent, and aerosol index) show consistent decreasing trend over East Coast of the United States (EUS), west coast of Europe (WEU), and east coast of China (EC), and no significant trend in liquid water path is found over these regions during the period 2003–2017. Over regions with significant long‐term trends of aerosol loading and CDNC (e.g., EUS and WEU), the sensitivity of CDNC to aerosol loading based on the long‐term trend is closer to those derived from ground and aircraft observations and larger than those derived from instantaneous satellite observations, providing an alternative way for quantifying aerosol‐cloud interactions. A clear shift in the normalized probability density function of CDNC between the first 5 years (2003–2007) and the last 5 years (2013–2017) is found, with a decrease of around 50% in the occurrence frequency of high CDNC (>400 cm−3) over EUS and WEU. The relative variances of cloud droplet effective radius generally decrease with decreasing aerosol loading, providing large‐scale evidence for the effects of anthropogenic aerosols on the dispersion of cloud droplet size distribution. The long‐term satellite data sets provide great opportunities for quantifying aerosol‐cloud interactions and further confronting these interactions in climate models in the future. Key Points: Three major pollutant outflow regions show consistent decrease of CDNC, AOD, angstrom exponent, and aerosol indexThe sensitivity of CDNC to aerosol loading based on the long‐term trend is closer to those derived from ground and aircraft observationsThe relative variances of cloud droplet effective radius generally decrease with decreasing aerosol loading [ABSTRACT FROM AUTHOR]