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It is well known that stratospheric sudden warmings (SSWs) are a result of the interaction between planetary waves (PWs) and the stratospheric polar vortex. SSWs occur when breaking PWs slow down or even reverse this zonal wind jet and induce a sinking motion that adiabatically warms the stratosphere and lowers the stratopause. In this paper we characterize this downward progression of stratospheric temperature anomalies using 18 years (2003–2020) of Sounding of the Atmosphere using Broadband Radiometry (SABER) observations. SABER temperatures, derived zonal winds, PW activity and gravity wave (GW) activity during January and February of each year indicate a high‐degree of year‐to‐year variability. From 11 stratospheric warming events (9 major and 2 minor events), the descent rate of the stratopause altitude varies from 0.5 to 2 km/day and the lowest altitude the stratopause descends to varies from <20 to ∼50 km (i.e., no descent). A composite analysis of temperature and squared GW amplitude anomalies indicate that the downward descent of temperature anomalies from 50 to ∼25 km lags the downward progression of increased GW activity. This increased GW activity coincides with the weakening and reversal of the westward zonal winds in agreement with previous studies. Our study suggests that although PWs drive the onset of SSWs at 30 km, GWs also play a role in contributing to the descent of temperature anomalies from the stratopause to the middle and lower stratosphere. Key Points: Eighteen‐years of Sounding of the Atmosphere using Broadband Radiometry (SABER) temperatures indicate variable downward progression of stratospheric sudden warming (SSW) related temperature anomaliesDescent rate of temperature anomalies varies from ∼0.5 to 2 km/day with lower descent leading to warmer temperatures at 30 kmGravity waves contribute to the downward progression of temperature anomalies from the stratopause and the onset of SSWs at 30 km [ABSTRACT FROM AUTHOR]

Abstract: We introduce the Clouds Above the United States and Errors at the Surface (CAUSES) project with its aim of better understanding the physical processes leading to warm screen temperature biases over the American Midwest in many numerical models. In this first of four companion papers, 11 different models, from nine institutes, perform a series of 5 day hindcasts, each initialized from reanalyses. After describing the common experimental protocol and detailing each model configuration, a gridded temperature data set is derived from observations and used to show that all the models have a warm bias over parts of the Midwest. Additionally, a strong diurnal cycle in the screen temperature bias is found in most models. In some models the bias is largest around midday, while in others it is largest during the night. At the Department of Energy Atmospheric Radiation Measurement Southern Great Plains (SGP) site, the model biases are shown to extend several kilometers into the atmosphere. Finally, to provide context for the companion papers, in which observations from the SGP site are used to evaluate the different processes contributing to errors there, it is shown that there are numerous locations across the Midwest where the diurnal cycle of the error is highly correlated with the diurnal cycle of the error at SGP. This suggests that conclusions drawn from detailed evaluation of models using instruments located at SGP will be representative of errors that are prevalent over a larger spatial scale. [ABSTRACT FROM AUTHOR]

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

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

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

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

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

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

The Arctic has experienced the frequent and severe fires in recent years. Due to the unique geography and vegetation type in Arctic region, the fire trend and its driving mechanism are different from other regions and still unclear. This study investigated the fire trends across Arctic region based on Moderate‐resolution Imaging Spectroradiometer (MODIS) burned area product MCD64A1 during 2001–2020. In addition, the burned area change was attributed at spatial and temporal scales based on 15 natural and anthropogenic factors. Results show that the average burned area in Arctic region during 2001–2020 is 31.2 × 103 km2, with a maximum of 51.9 × 103 km2 in 2019. The change of burned area in Arctic region is spatially heterogeneous, with a significant increasing trend (Z > 1.96) in central Russia and a significant decreasing trend (Z < −1.96) in western Russia. There is a clear seasonality in the occurrence of fires in the Arctic, with a peak in the summer months and almost no fires in the winter. Relative humidity has the largest explanatory power for the spatial variation of burned area (q = 0.487), followed by vapor pressure deficit (q = 0.475) and land surface temperature (q = 0.367). The combined effect of precipitation and land surface temperature is the most powerful interaction (q = 0.587) to explain the spatial variation of burned area. The significant increase in time‐series burned area is primarily driven by the decrease of relative humidity. The results of this study are helpful to understand the fire trends and the contribution of climate change to Arctic fires. Plain Language Summary: In recent years, the Arctic has experienced frequent and severe fires. Due to the unique geographical location and vegetation type of the Arctic, it is necessary to understand its fire trends and driving mechanisms. This paper explores the spatial and temporal trends of fires in the Arctic from 2001 to 2020 based on the MODIS satellite‐based burned area product MCD64A1, and considers 15 natural and human influences to explore the driving mechanisms. The results show that the Arctic region burned an average of 31.2 × 103 km2 in 2001–2020, with the highest burned area of 51.9 × 103 km2 in 2019, and that burning events were mostly concentrated in summer. The burned area increases significantly in central Russia and decreases significantly in the west. Relative humidity is the most influential factor in the spatial and temporal variability of Arctic fires, and precipitation and land surface temperature have the greatest combined effect on Arctic fires. The results of this study contribute to the understanding of the process of Arctic fire change and the impact of climate change on Arctic fires. Key Points: The burned area significantly increased (decreased) in central (western) RussiaThe interaction between precipitation and land surface temperature has the greatest effect on Arctic firesThe change in burned area is mainly driven by the change in relative humidity [ABSTRACT FROM AUTHOR]

The South China coast has a high incidence of warm‐sector heavy rainfall (WSHR) events. The ageostrophic winds in the boundary layer in most of these events associated with the southwesterly boundary layer jets (BLJs) mainly exhibit strong convergence at rainfall area. In this paper, we analyze two case studies of WSHR in May 2013 and May 2015, which occurred in similar synoptic environments but varied in intensity, extent, and duration of rainfall, where the ageostrophic winds are the confrontational confluence and asymptotic confluence pattern, respectively. ERA‐5 reanalysis data and the diagnostic equation of ageostrophic wind are used to examine the factors affecting the ageostrophic winds in the northern land region and the southern offshore region of the rainfall. The results suggest that land‐sea contrast leads to the convergence of ageostrophic winds in the rainfall area. Boundary layer friction dominates the northeasterly ageostrophic winds on land. The diurnal variation of BLJs dominates the ageostrophic winds and their diurnal variation at sea. It can contribute southwesterly or southeasterly ageostrophic winds, so the phase difference between the land and sea forms confrontational or asymptotic confluence, respectively. BLJs with different intensities, extents, and diurnal variations can lead to different ageostrophic wind patterns and their confluence modes. The land‐sea thermal contrast can directly affect ageostrophic winds, and it can also affect the diurnal variation of BLJs, thus affecting the ageostrophic winds and their confluence mode. It is further verified that the BLJs and thermal forcing are important in WSHR processes in South China. Plain Language Summary: Extremely heavy rainfall frequently occurs in coastal Southern China, which causes great damage and is difficult to accurately forecast. Most of this kind of rainfall is associated with southwesterly wind in the lower atmosphere and strong convergence of the lower airflow. Our purpose is to better understand the factors that lead to strong convergence, which is an important factor for heavy rainfall. Since the geostrophic wind component has no divergence, the convergence is mostly contributed by the ageostrophic wind component, which doesn't conform to the geostrophic balance. We find that different intensities, ranges, and diurnal changes of the southwesterly wind in the lower atmosphere can lead to different distributions of ageostrophic winds in the South China Sea. The thermal difference between land and sea can influence the diurnal change of the southwesterly wind in the lower atmosphere, and it can also affect the ageostrophic winds and their convergence. The results will potentially provide a better understanding of heavy rainfall events in Southern China and may lead to better forecasts and issuing of warnings. Key Points: Land‐sea contrast leads to the strong convergence in the coastal warm‐sector extreme heavy rainfall area in South ChinaMarine boundary layer jets (BLJs) and their diurnal variation can influence the ageostrophic winds and then affect the convergenceThe land‐sea thermal contrast can affect the ageostrophic winds and the convergence by changing the diurnal variation of BLJs [ABSTRACT FROM AUTHOR]

Classical bow echoes (CBEs) normally develop due to rear‐inflow jets, while merger‐formation bow echoes (MFBEs) evolve from the merger between linear systems and preline convection. MFBEs are rarely documented before, and the key processes remain unclear because of the small‐scale and fast‐evolving merging processes. Using radar observations during 2015–2019, this study examines the characteristics of these two types of bow echoes over South China. Eighteen MFBEs and 11 CBEs are identified, accounting for 62% and 38% of the total 29 bow echoes, respectively. MFBEs commonly occur over coastal regions in the afternoon, where southerly sea breezes provide favorable thermodynamic conditions. About half CBEs develop over mountainous areas in the morning, and northerly winds enhance the local baroclinity and kinematic convergence. A new Merger‐Classical index is proposed to distinguish the environments for the development of two types of bow echoes, disclosing the more favorable near‐surface thermal conditions for MFBEs. In addition, more severe weather occurs in metropolitan area near the Pearl River Delta region during MFBEs, in which the accumulated precipitation is about 2.5 times higher than that during CBEs. This study reveals the discrepancies between MFBEs and CBEs over South China, emphasizing the MFBEs account for more than half of the total bow echoes and cause more high‐impact weather in the metropolitan area. This study proposes a new perspective on the environment in which bow echo forms by merger process over South China, urging for more research to explore the underlying processes. Plain Language Summary: Bow echoes produce damaging surface winds and heavy rainfall. Previous studies normally explained bow echoes using the classical theory that emphasized the importance of rear‐inflow jet, and such echoes are termed as the classical bow echoes (CBEs) in this study. Additionally, the warm moist environmental conditions over South China facilitate bow echo formation through merger process between linear systems and convective cells. Bow echoes formed in this manner are termed as the merger‐formation bow echoes (MFBEs). Due to the small‐scale and fast‐evolving process of merging, MFBEs are rarely documented in previous literature. This study is the first to investigate the spatiotemporal distributions of MFBEs and CBEs over South China using 5‐year radar observations, pointing out the MFBEs account for more than half of the total bow echoes and cause more high‐impact weather in metropolitan area than CBEs. In addition, we propose a new Merger‐Classical index to distinguish the environments for MFBEs and CBEs. The differences between MFBEs and CBEs are statistically significant at the 95% confidence level. This paper emphasizes a new perspective on the environment in which bow echo forms by merger process, underscoring the need for more in‐depth research in the future. Key Points: Merger‐formation bow echoes (MFBEs) and classical bow echoes (CBEs) account for 62% and 38% over South China during 2015–2019, respectivelyMFBEs mainly occur over the coastal regions in the afternoon, while about half CBEs develop over the mountainous areas in the morningThe proposed Merger‐Classical index distinguishes developing environments for MFBEs and CBEs, and MFBEs produce heavier rainfall near metropolitan area [ABSTRACT FROM AUTHOR]

Evaluating the impact of topography on stratospheric ozone is a crucial step toward achieving a deeper comprehension of the role of topography uplift in driving paleoclimatic evolution and stratospheric dynamics and chemistry. This paper quantitatively estimates the contribution of Asian topography to Arctic stratospheric ozone by topography experiments in the Whole Atmosphere Community Climate Model version 6. The results indicate that Asian topography exerts an especially robust influence on Arctic stratospheric ozone during winter. Compared to the flattened topography, Asian topography significantly increases the ozone by 15% in the lower Arctic stratosphere, accompanied by the highest accumulation of total ozone appearing in the polar region north to the North American continent. The intensified residual mean circulation transport is principally responsible for the increase in ozone, and Asian topography has a stronger impact on the vertical residual mean transport than on the meridional residual mean transport. Further examination demonstrates that the robust strengthening of planetary waves caused by Asian topography significantly contributes to the enhancement of the residual mean circulation. Specifically, the upper Arctic stratosphere experiences a 42.7% increase in the amplitude of planetary waves with wavenumber 1, which means the weakened polar vortex. The diagnosis of the mean Lorenz energy cycle further indicates that Asian topography decreases the zonal mean kinetic energy and increases the eddy kinetic energy in the stratosphere. Therefore, the enhancement of residual mean circulation transport and the weakening of the polar vortex both work together to promote the accumulation of Arctic ozone. Plain Language Summary: The stratosphere refers to the atmospheric layer located between 15 to 50 km above the surface, which contains approximately 90% of the global ozone. The stratospheric ozone layer serves as a protective shield for the Earth's ecosystem, safeguarding it against the damaging impacts of ultraviolet radiation. Through numerical simulations with and without the Asian topography (e.g., the Tibetan Plateau), this study evaluates how the Asian topography affects stratospheric ozone, which helps to understand the role of topography uplift in driving paleoclimatic evolution and stratospheric dynamics and chemistry. The results show that the Arctic experiences the strongest influence from Asian topography on stratospheric ozone. In comparison to the absence of the Asian topography, the total ozone over the Arctic can be increased up to 40 DU during winter, with the most significant average increase in the lower stratosphere at 15%. The Asian topography can promote the stratospheric low‐pressure system encircling the Arctic (i.e., polar vortex, the ozone inside it is usually lower) to shift toward Eurasia. Concomitantly, more planetary waves generated by the Asian topography travel poleward and upward, allowing the polar vortex to be weakened and ozone transport from the lower latitudes to be enhanced. Key Points: The Asian topography has a strong impact on Arctic stratospheric ozone during winterIncreasing in Arctic stratospheric ozone is induced mainly by the enhancement of residual mean circulation transport in winterThe more poleward and upward planetary waves generated by the Asian topography are responsible for the intensified residual mean circulation [ABSTRACT FROM AUTHOR]

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

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

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

For various problems in atmospheric electricity it is necessary to understand the behavior of corona discharge in wind. Prior work considers grounded electrode systems, of relevance for earthed towers, trees, or windmills subjected to thunderstorms fields. In this configuration, the effect of wind is to remove the shielding ions from the coronating electrode vicinity strengthening the corona and increasing its current. There are a number of cases, such as isolated wind turbine blades or airborne vehicles, that are not completely represented by the available models and experiments. This paper focuses on electrode systems that are electrically isolated from their environment and reports on a wind tunnel campaign and accompanying theoretical work. In this configuration, there are two competing effects: the removal of the shielding ions by the wind, strengthening the corona; and the electrode system charging negatively (for positive corona) with respect to its environment, weakening the corona. This leads to three different operating regimes; namely, for positions that favor ion recapture, charging is limited and current increases with wind as in the classical scaling; for positions that favor ion transport by the wind, the system charges negatively and the current decreases with wind; for the later configuration, as wind increases, the current can vanish and the system potential saturates. The results from this work demonstrate that classical scaling laws of corona discharge in wind do not necessarily apply for isolated electrodes and illustrate the feasibility of using a glow corona in wind for controlled charging of a floating body. Plain Language Summary: In the enhanced fields in thunderstorms, pointed objects such as towers, trees, wind turbine blades, or aircraft produce corona discharges. Corona discharges can also be produced in laboratory setups using sharp electrodes. In the presence of wind, the discharge is strengthened and the current increases as the wind disperses the ions that were locally reducing the magnitude of the electric field. This is a well‐accepted phenomenon that appears when the electrode setup is grounded. This paper explores what happens when the electrodes are electrically isolated rather than grounded, of relevance for aircraft and some wind turbine blades. It is shown, through a wind tunnel campaign and numerical simulation, that in this case the structure becomes electrically charged due to the evacuation of ions and this effect acts to weaken the corona discharge. The results from this work demonstrate that, for electrically isolated electrodes, the corona current can actually decrease with wind and illustrate the feasibility of using corona discharge in wind to charge a floating structure such as an aircraft. Key Points: Classical scaling laws of corona discharge in wind do not apply for electrically isolated electrodesFor electrically floating electrodes corona current can decrease in wind in response to chargingGlow corona discharge in wind can be used for controlled charging of an electrically isolated body [ABSTRACT FROM AUTHOR]

The Grid‐point Atmospheric Model of the IAP LASG version 3 (GAMIL3) has been developed by upgrading the horizontal resolution, methods of parallel computation, boundary layer scheme, aerosol parameterization, convective parameterization, stratocumulus cloud fraction scheme, land component, and coupler, as well as tuning some moist physical parameters with large uncertainties. Its performance is evaluated, and the results show significant improvements compared with the previous version, GAMIL2. The simulated performance of mean states is notably enhanced, including the energy budget terms at the top of the atmosphere (TOA) and surface, shortwave/longwave cloud radiative forcing (SWCF/LWCF), precipitation, zonal wind, low‐level temperature, 500‐hPa geopotential height, and snow cover fraction in the Northern Hemisphere. The characteristics of internal variability are captured well, such as the frequency band and active areas of quasi‐biweekly (QBW) oscillation, spectral power of convectively coupled equatorial waves (CCEWs), Madden‐Julian Oscillation (MJO) eastward propagation, and heat flux response to El Niño‐Southern Oscillation (ENSO), and these variabilities are generally strengthened in GAMIL3. In addition, the anthropogenic aerosol climate effects are weakened when using the forcings recommended by CMIP6. Plain Language Summary: The Coupled Model Intercomparison Project (CMIP) provides a multimodel platform for research in climate change originating from unforced variability or in response to changes in anthropogenic forcings. The Atmospheric Model Intercomparison Project (AMIP) experiment, which is part of the sixth phase of CMIP (CMIP6), is designed to evaluate the atmospheric component in the climate system. The GAMIL3 is the atmospheric component of Flexible Global‐Ocean‐Atmosphere‐Land System Model Grid‐point version 3 (FGOALS‐g3), one of the CMIP6 models, and it has been developed from GAMIL2. This paper presents a description of the GAMIL3 model and its upgrades, the performance of the simulated mean state and internal variability, and the anthropogenic aerosol climate effects. This description is useful for understanding climate variability and change. Key Points: This paper presents GAMIL3 and the results of its AMIP simulationsOverall performance is improved, and climate variabilities are strengthenedAnthropogenic aerosol effects using the CMIP6 protocol are weaker than those from the model's default approach [ABSTRACT FROM AUTHOR]

We present in this work the third catalog of terrestrial gamma‐ray flashes (TGFs) by the AGILE mission and the new search algorithm that was developed to produce it. We firstly introduce the new selection criteria, designed from the characteristics of WWLLN‐identified TGFs, and then applied on all data from March 2015 to September 2018. Association with sferics was performed by an independent search, described in a companion paper by Lindanger et al. (2020, https://doi.org/10.1029/2019JD031985). This search showed that many TGFs were not recognized by the existing selection algorithm, hence the need for this work. Several new selection criteria were tested and are compared in this paper. We then present the chosen selection criteria and the obtained sample, which includes 2,780 events and represents the most extensive TGF catalog available for the equatorial regions. Finally, we discuss the characteristics of this sample, including geographic distribution, intensity and duration, and seasonal variations. Key Points: A new set of selection criteria for AGILE TGFs is designed based on a lightning‐associated TGF sampleA sample of 2,780 events detected by AGILE over a 3‐year period is presented, the largest sample to date across the equatorGeographical, local time, and seasonal variability of the sample is discussed [ABSTRACT FROM AUTHOR]

This study proposes a method for direct assimilation of radar reflectivity in cycled deterministic forecast systems using a time‐lagged ensemble based ensemble Kalman filter with the "cloud‐dependent" background error covariances. With this method, forecasts from multiple deterministic forecast cycles that are initialized at different times but valid at the same time, are collected as a time‐lagged ensemble to compute the background error covariances. An algorithm to compute the specified "cloud‐dependent" background error covariances was devised, in which the vertical background error covariance is computed for each of the "cloud‐feature" bins which are divided according to the column maximum radar reflectivity and cloud top height. The single‐observation experiments and the observing system simulation experiments (OSSEs) indicate that the radar reflectivity data assimilation method was able to capture the main thermodynamic and microphysical features of convective clouds and was effective in reducing the spin‐up time of convection, and generating convective cells which were missed in the background. Meanwhile, assimilating "zero‐reflectivity" observations helped suppress spurious convection. The method provided overall more accurate thermodynamic analysis, radar reflectivity and precipitation analysis and forecast of the MCS, while did not significantly increase computational cost, compared to a cloud‐analysis based latent‐heat nudging approach. Plain Language Summary: With a cycled forecast system, for a given time, several forecasts are produced by the model initialized at different times. Such forecasts can serve as a time‐lagged ensemble, in which the differences in initial conditions among different forecast cycles are used in lieu of uncertainty estimates of the initial atmospheric state. In this study, we propose a method for direct assimilation of radar reflectivity data in cycled deterministic forecast systems, using a time‐lagged ensemble based ensemble Kalman filter. Forecasts from multiple deterministic forecast cycles that are initialized at different times but valid at the same time are collected as a time‐lagged ensemble to compute the background error covariances. We devised an algorithm to compute the specified "cloud‐dependent" background error covariances, in which the vertical background error covariance is computed for each of the "cloud‐feature" bins which are divided according to the column maximum radar reflectivity and cloud top height. The assimilation experiments indicate that the radar reflectivity data assimilation scheme can improve the analysis and short‐term forecast of the mesoscale convective system without significantly increasing computational cost compared to a cloud‐analysis based latent nudging method. Key Points: This paper describe a scheme to directly assimilate radar reflectivity data for cycled deterministic forecast systemsTime‐lagged ensemble is used to compute the "cloud‐dependent" background error covariancesThis new radar reflectivity data assimilation scheme can improve the analysis and short‐term forecast of mesoscale convective system [ABSTRACT FROM AUTHOR]

Long‐term efforts have sought to extend global model resolution to smaller scales enabling more accurate descriptions of gravity wave (GW) sources and responses, given their major roles in coupling and variability throughout the atmosphere. Such studies reveal significant improvements accompanying increasing resolution, but no guidance on what is sufficient to approximate reality. We take the opposite approach, using a finite‐volume model solving the Navier‐Stokes equations exactly. The reference simulation addresses mountain wave (MW) generation and responses over the Southern Andes described using isotropic 500 m, central resolution by Fritts et al. (2021), https://doi.org/10.1175/JAS-D-20-0207.1 and Lund et al. (2020), https://doi.org/10.1175/JAS-D-19-0356.1. Reductions of horizontal resolution to 1 and 2 km result in (a) systematic increases in initial MW breaking altitudes, (b) weaker, larger‐scale generation of secondary GWs and acoustic waves accompanying these dynamics, and (c) significantly weaker and less extended responses in the mesosphere in latitude and longitude. Horizontal resolution of 4 km largely suppresses instabilities, but allows weak, sustained mean‐flow interactions. Responses for 8 km resolution are very weak and fail to capture any aspects of the high‐resolution responses. The chosen mean winds allow efficient MW penetration into the mesosphere and lower thermosphere, hence only exhibit strong pseudo‐momentum deposition and mean wind decelerations at higher altitudes. A companion paper by Fritts et al. (2022), https://doi.org/10.1029/2021JD036035 explores the impacts of decreasing resolution on responses in the thermosphere. Plain Language Summary: Mountain waves play major roles in the large‐scale circulation and structure of the atmosphere extending well into the thermosphere. Their primary influences at large scales depend on energy and momentum transports from sources at lower altitudes and deposition at higher altitudes accompanying wave‐breaking dynamics and instabilities that cannot currently be described by global models. Responses to these small‐scale dynamics in the stratosphere and lower mesosphere include local mean flow decelerations that extend large distances downstream, upstream, and laterally. The ability to describe these dynamics degrades rapidly with decreasing resolution, causing current global models to exhibit systematic circulation biases that significantly limit and degrade weather and climate prediction. We assess these impacts with a suite of simulations of wintertime flow over the S. Andes at horizontal resolutions from 0.5 to 8 km. Our results reveal that spatial resolution of ∼2 km or better is required to adequately address these deficiencies extending into the mesosphere. Key Points: Simulations of mountain wave (MW) dynamics reveal decreasing fidelity in MW and large‐scale fields at degraded resolutionTwo‐km resolution or better yields reasonable fidelity to MW fields, momentum fluxes, and large‐scale fields into the mesosphereFour‐km resolution fails to capture realistic MWs, fluxes, and responses; 8 km resolution yields no resemblance to well‐resolved MW fields [ABSTRACT FROM AUTHOR]

Studying the spatiotemporal variations of the urban heat island (UHI) effect and its cause is important towards understanding urban climate change, planning and green development, and disaster mitigation. In this paper, by using surface observations and reanalysis data with objective classification of synoptic weather patterns (SWPs), we analyze the associations between canopy UHI intensity (CUHI) and SWPs in the planetary boundary layer (PBL) and their potential drivers during wintertime of the period 2012–2017. Six dominant types of SWP are identified as follows: In the case of Types 3, 4, and 6, weak high‐pressure systems exist to the south of Beijing, resulting in weak southerly winds with low PBL height, large cloud coverage and high relative humidity (RH). These conditions are generally conducive to a strengthening of the CUHII. In contrast, under Types 1, 2, and 5, high‐pressure systems are located to the northwest of Beijing, and the associated strong northwesterly flows of dry and cold air strengthen the boundary layer mixing process, resulting in large wind speed and low RH. This is conducive to a weakening of the CUHII. In general, our work reveals the impacts of SWPs on the strength of CUHII mainly via the modulation of local weather conditions at diurnal and interannual scales, while spatial pattern of CUHII is largely dominated by local climate zones. Our findings have implications for CUHII forecasts, as well as impact assessments and policymaking in the context of UHI‐related energy conservation in winter over high‐density urban areas on the synoptic scale. Key Points: The wintertime CUHII in Beijing is sensitive to Synoptic Weather Patterns (SWPs) at diurnal and interannual scaleTemporal variation in the strength of CUHII is closely linked with SWPs, while spatial pattern of CUHII is mainly sharped by LCZsDaily amplitudes and peaks of CUHII vary under different SWPs [ABSTRACT FROM AUTHOR]

Atmospheric hydroperoxides are a significant component of the atmosphere's oxidizing capacity. Two of the most abundant hydroperoxides, hydrogen peroxide (H2O2) and methyl hydroperoxide (MHP, CH3OOH), were measured in the remote atmosphere using chemical ionization mass spectrometry aboard the NASA DC‐8 aircraft during the Atmospheric Tomography Mission. These measurements present a seasonal investigation into the global distribution of these two hydroperoxides, with near pole‐to‐pole coverage across the Pacific and Atlantic Ocean basins and from the marine boundary layer to the upper troposphere and lower stratosphere. H2O2 mixing ratios are highest between 2 and 4 km altitude in the equatorial region of the Atlantic Ocean basin, where they reach global maximums of 3.6–6.5 ppbv depending on season. MHP mixing ratios reach global maximums of 4.3–8.6 ppbv and are highest between 1 and 3 km altitude, but peak in different regions depending on season. A major factor contributing to the global H2O2 distribution is the influence of biomass burning emissions in the Atlantic Ocean basin, encountered in all four seasons, where the highest H2O2 mixing ratios were found to correlate strongly with increased mixing ratios of the biomass burning tracers hydrogen cyanide (HCN) and carbon monoxide (CO). This biomass burning enhanced H2O2 by a factor of 1.3–2.2, on average, in the Atlantic compared with the Pacific Ocean basin. Plain Language Summary: Hydroperoxides, a large class of compounds that contain the R–OOH chemical structure, exist in the gas phase in the atmosphere. These compounds are key to the chemistry of the atmosphere because of the role they play in the atmosphere's ability to process and ultimately remove chemical species. Two of the most abundant atmospheric hydroperoxides were measured as part of the Atmospheric Tomography Mission, which collected samples of the atmosphere over the Pacific and Atlantic Ocean basins far from human influences. This paper presents a summary of the global distribution of these hydroperoxides across the four different seasons (winter, spring, summer, fall) and investigates the role that smoke from large‐scale fires on the continents plays in altering the amount of atmospheric hydroperoxides above the Atlantic Ocean. Key Points: The Atmospheric Tomography Mission provides an unprecedented investigation into the global seasonal distribution of hydroperoxidesChemical Ionization Mass Spectrometry is a sensitive technique for studying hydroperoxides in the remote atmosphereBiomass burning emissions increase H2O2 mixing ratios in the Atlantic Ocean compared to corresponding latitudes in the Pacific Ocean basin [ABSTRACT FROM AUTHOR]

In this paper, we simulate an observed mountain wave event over central Europe and investigate the subsequent generation, propagation, phase speeds and spatial scales, and momentum deposition of secondary waves under three different tidal wind conditions. We find the mountain wave breaks just below the lowest critical level in the mesosphere. As the mountain wave breaks, it extends outwards along the phases and fluid associated with the breaking flows downstream of its original location by 500–1,000 km. The breaking generates a broad range of secondary waves with horizontal scales ranging from the mountain wave instability scales (20–300 km), to multiples of the mountain wave packet scale (420 km+) and phase speeds from 40 to 150 m/s in the lower thermosphere. The secondary wave morphology consists of semi‐concentric patterns with wave propagation generally opposing the local tidal winds in the mesosphere. Shears in the tidal winds cause breaking of the secondary waves and local wave forcing which generates even more secondary waves. The tidal winds also influence the dominant wavelengths and phase speeds of secondary waves that reach the thermosphere. The secondary waves that reach the thermosphere deposit their energy and momentum over a broad area of the thermosphere, mostly eastward of the source and concentrated between 110 and 130 km altitude. The secondary wave forcing is significant and will likely be very important for the dynamics of the thermosphere. A large portion of this forcing comes from nonlinearly generated secondary waves at relatively small‐scales which arise from the wave breaking processes. Key Points: Mountain wave breaking generates a broad range of secondary wavesThe secondary wave parameters and morphology are significantly influenced by the tidal windsThe secondary waves contribute significant forcing in the tidal shears and over a large area in the thermosphere [ABSTRACT FROM AUTHOR]

The SMART‐s (Spectral Measurements for Atmospheric Radiative Transfer—spectroradiometer) acquires Sun/sky observations for retrieving optimal information on trace gases and aerosols with minimal assumptions. Overall, the algorithm of SMART‐s incorporates a series of retrievals, from fundamental quantities (i.e., column abundance of trace gases and aerosol loading) to higher‐order geophysical parameters (e.g., aerosol physicochemical properties and vertical profiles), utilizing Sun/sky spectral radiance measurements. This paper describes the theoretical basis for column retrievals of trace gases and aerosols. Associated profile retrievals will be presented in follow‐up papers. The current algorithm retrieves the fine/coarse mode of the particle size distribution and spectral complex index of refraction and, thereby, the spectral aerosol single‐scattering albedo ω0. SMART‐s retrieval is unique particularly in its high spectral resolution of the complex index of refraction and ω0 from near‐ultraviolet to near‐infrared wavelengths, which is pivotal information for atmospheric chemistry, climate and other inversions. We theoretically assessed information content and retrieval accuracy of the algorithm and compared different type of measurements including the Aerosol Robotic Network (AERONET) and standard Pandora. For the same levels of radiometric accuracy, SMART‐s measurements provide the most informative aerosol retrievals based on theoretical error analyses. Higher spectral resolution measurements are particularly beneficial for particle size distribution and fine‐mode refractive index retrievals. We applied this algorithm to the AERONET Sun/sky measurements at Kanpur, India, in 2016 to assess algorithm consistency. Even with different assumptions and numerical methods for the inversion, SMART‐s retrieved aerosol parameters agreed well with the AERONET operational products (e.g., absolute mean bias errors less than 0.01 for ω0). Key Points: The SMART‐s algorithm retrieves aerosol particle size distributions and spectral complex index of refraction from about 310 to 800 nmWe applied the SMART‐s algorithm to the AERONET Sun/sky measurements and the retrievals were consistent with the operational products of AERONETHigh spectral resolution of the SMART‐s retrievals can provide unique information for atmospheric chemistry, air quality, and climate studies [ABSTRACT FROM AUTHOR]

Much progress has been made in delineating and understanding the "tidal climate" of the mesosphere and lower thermosphere, that is, tidal variability on seasonal or longer timescales. Short‐term tidal variability on timescales of a few days is much less understood, mainly due to observational constraints imposed by satellite local solar time sampling. This paper presents a new approach to study the "tidal weather" in 16 years of SABER temperatures. Our focus is on the eastward‐propagating nonmigrating diurnal tide with zonal wave number 3 (DE3) that originates from tropical convection. The statistics of short‐term tidal variability derived from "tidal deconvolution" are analyzed using an information‐theoretic approach based on Bayesian statistics and time‐dependent probability density functions. The paper particularly discusses the statistical characteristics of interannual changes in short‐term DE3 variability on a quasi‐10‐day wave (Q10DW) timescale and relates it to various forcing and propagation conditions such as Quasi‐Biennial Oscillation (QBO), El Niño‐Southern Oscillation (ENSO), and solar cycle. Understanding the response to these drivers requires a separate treatment of symmetric and antisymmetric DE3 tidal modes. Symmetric DE3 mode variability is enhanced during the easterly phase of the QBO due to enhanced Q10DW activity and during the La Niña phase of ENSO due to enhanced convective forcing but does not show a solar cycle dependence because of stable polar vortex conditions in the southern hemisphere. Antisymmetric DE3 mode variability is enhanced in the westerly phase of the QBO during solar maximum due to Q10DW activity related to a more unstable polar vortex in the northern hemisphere. Key Points: An information‐theoretic approach is presented to statistically analyze short‐term tidal variability on various planetary wave timescalesDE3 short‐term tidal variability on a 10‐day planetary wave timescale depends on QBO, ENSO, and solar cycleStratospheric polar vortex conditions are important for transmitting solar cycle signals into DE3 short‐term variability [ABSTRACT FROM AUTHOR]

The editors of Journal of Geophysical Research (JGR): Atmospheres wish to sincerely thank the 2,573 outstanding scientists who reviewed manuscripts for the journal in 2019. Peer review is a crucial process to ensure the integrity and rigor of science. Your reviews have helped to improve the qualities of research by many authors, stimulated new ideas, and impacted the careers of many young scientists. Through these reviews, you contributed not only to the high standard of JGR‐Atmospheres but also to the standing of our scientific discipline. Of the 2,573 reviewers listed below for the 1,757 new papers and 1,358 revised papers submitted to the journal (amounting to a total of 3,115 papers), 491 people reviewed three or more papers. On behalf of the journal, the authors, and the community, we thank all reviewers for your selfless service and dedication to the scientific community. We look forward to your continuing support! Key Point: The editors thank the 2019 peer reviewers [ABSTRACT FROM AUTHOR]

This paper proposes a new algorithm to determine the wave turbopause based on numerical differentiation using temperature data collected from the Sounding of the Atmosphere using Broadband Emission Radiometry onboard the Thermosphere Ionosphere Mesosphere Energetics Dynamics satellite. The vertical gradient profile of the temperature standard deviation, which serves as a proxy of wave activity, is obtained based on Tikhonov regularization. In the vicinity of the altitude of wave turbopause, wave damping is much reduced, and upward wave propagation occurs with little damping effect. The corresponding vertical gradient profile should reach a maximum with values that are no longer negative. Thus, we determine the wave turbopause as the unique maximum that is the first after the last change in the derivative value from negative to positive. The wave turbopause altitudes determined by this new method agree well with those of previous studies, especially regarding the behavior of the monthly variation. The error of the wave turbopause altitude derived from this new method is 1.9 km, which is much improved from the error of 3.3 km estimated by the previous method. Additionally, latitudinal variations of the wave turbopause altitude for different seasons and their monthly variations for 0°, 30°N, and 70°N latitudes during 2016–2018 are presented. Similar to the mesopause, the wave turbopause also exhibits a two‐level structure with a summer minimum at high latitudes. Key Points: A numerical differentiation method based on the Tikhonov regularization is proposed to determine the wave turbopause altitudeThe error of the wave turbopause altitude in this paper is 1.9 km much improved upon the previous error of 3.3 kmThe latitudinal and seasonal distribution of the wave turbopause altitude is similar to those of previous studies [ABSTRACT FROM AUTHOR]

We use World Wide Lightning Location Network (WWLLN) data on the radiated radio frequency electromagnetic energy per stroke to identify the upper tip of the global lightning stroke energy distribution. The mean stroke energy is about 1,000 J per stroke in the very low frequency band between 5 and 18 kHz, while the distribution used in this paper is limited to strokes in that band above 1 MJ, about 3 orders of magnitude above the mean. It is shown that these energies are representative of the tip of the optical distribution, first identified by Turman (1977) above 10 GW per stroke, which he termed "superbolts." The distribution peaks globally in the Northern Hemisphere winter (November–February) with most superbolts being found in the North Atlantic west of Europe, the winter Mediterranean Sea, and a strong local maximum over the Andes in South America. We identify regions with somewhat fewer superbolts in the North Pacific east of Japan in winter, along the equator of the Atlantic and Indian Oceans and south of South Africa. We find very few superbolts during April–October each year. While superbolts are scattered around the globe, the local occurrence peaks do not coincide with the usual three main lightning "chimneys." Unlike the distribution of all normal global lightning, we find superbolts predominantly over the oceans and seas, with fewer over the continents, just the opposite of all global lightning. Key Points: Superbolts, have more than 106 J of radiated VLF energy, which is comparable or larger than Turman'ssuperbolts with >10 GW optical powerSuperbolts dominantly occur during Nov‐Feb over water with peaks in the N. Atlantic and N. Pacific, the Mediterranean and over the AndesOne‐to‐one stroke comparisons with the Earth Networks Total Lightning Network show that all superbolts have peak currents over 105 A [ABSTRACT FROM AUTHOR]

The melting layer of precipitation has a major impact on remote sensing and telecommunications. However, there is a shortage of observational studies to validate and constrain the melting layer models especially for high‐frequency radar bands. In this paper, we report how multifrequency radar Doppler spectra can be used to retrieve the melting layer attenuation at Ka‐ and W‐bands. The presented analysis is based on identifying Rayleigh scattering regions in radar Doppler spectra measurements where dual‐wavelength spectral ratios can be related to differential attenuation. We show that the estimated attenuation at Ka‐ and W‐bands agrees reasonably well with previously reported studies, but there are indications of differences at higher rain rates. We advocate that this technique can be applied to long‐term observations to advance our knowledge of the melting process. The parameterizations of melting layer attenuation as a function of rain rate and radar reflectivity are also presented. Plain Language Summary: While the melting layer is a relatively narrow layer in precipitation systems, it has a significant impact on telecommunication and remote sensing applications. For spaceborne and ground‐based radar measurements, unaccounted attenuation in the melting layer may cause significant errors in retrievals of rainfall rate and ice cloud properties, respectively. There are two approaches to quantify attenuation in the melting layer, namely, through modeling or observations. Modeling studies have been carried out and are reported in the scientific literature; however, it is acknowledged that there is a shortage of observational studies that could help us to constrain the models. In this paper, we present a study that utilizes radar Doppler spectra recorded at X‐, Ka‐, and W‐bands to retrieve melting layer attenuation. We show that the estimated attenuation at Ka‐ and W‐bands agrees reasonably well with previous studies, but there are indications of differences at higher rain rates. With the utilization of this technique, long‐term observations are expected to advance our knowledge of the melting process, as well as modeling of the melting layer attenuation for applications in millimeter‐wavelength radars, passive microwave remote sensing, 5G/6G commercial links, and space‐Earth telecommunications. Key Points: Ka‐/W‐band melting layer attenuation is derivedThe melting layer attenuation is derived by analyzing X‐, Ka‐, and W‐band radar Doppler spectraPresented results are in good agreement with previously reported melting layer modeling results for rain rates smaller than 3 mm/hr [ABSTRACT FROM AUTHOR]

Lightning channels are made of plasma. As a consequence, the driving electrical current changes the channel's resistance in a nonlinear fashion. The resistance has an intricate dependence on the history of Joule heating and various cooling processes, as well as on the various kinetic processes that dictate the population balance of electrons within the channel. Such dependence cannot be captured by an analytic function, as often attempted. In this paper, we introduce a minimal numerical model that can qualitatively capture the temporal dynamics of the key plasma properties of a lightning channel, including its electric field, temperature, plasma density, radius, and the resulting nonlinear resistance. Through a series of novel parameterizations, we introduce six zero‐dimensional equations that can capture both nonequilibrium/low‐temperature and local thermodynamic equilibrium/high‐temperature plasma regimes. In this manuscript, we go to great lengths to validate the model, showing that it can reproduce the finite time scale of streamer‐to‐leader transition, replicate the negative differential resistance behavior of steady‐state plasma arcs, and properly describe the temporal evolution of temperature in a return stroke channel. Finally, the model is applied to the simulation of optical emissions from rocket‐triggered lightning strikes, explaining the measured delay between the rise of current and visible light, as well as reproducing the direct relationship between peak current and peak radiated power and between charge transferred to ground and total radiated energy. Plain Language Summary: A number of unsolved puzzles in lightning physics are rooted in the plasma nature of lightning channels. One such example is why do negative cloud‐to‐ground lightning flashes transfer charge to ground in a series of multiple strikes that reuse the same channel, while positive flashes mostly have a single stroke? In this paper we introduce a computer simulation tool to capture the plasma nature of lightning and model its properties. One of the key results presented here is the model's ability to explain the experimentally derived relationship between optical and electrical properties in triggered lightning flashes. Key Points: A novel physics‐based yet computationally efficient model of the lightning channel nonlinear resistance is introducedThe model reproduces the finite time scale of streamer‐to‐leader transition and the steady‐state negative differential resistanceThe model captures the experimentally inferred direct relationship between electrical and optical properties of rocket‐triggered lightning [ABSTRACT FROM AUTHOR]

Clouds and shadows pose a significant barrier for land surface optical and infrared remote sensing image processing and their various applications. The detection and removal of clouds and shadows from satellite images have always been critical preprocessing steps. To date, a variety of methods have been designed to solve this problem. Some require particular channels, while others are heavily dependent on the availability of temporally adjacent images (reference images). Moreover, many methods are too complex to use by common users. For those reasons, in this paper an alternative scheme for detecting clouds and shadows is proposed based on simulated top‐of‐atmosphere radiance fields. At the same time, a simple approach to remove clouds and shadows is also provided. The results indicate that the new method can properly identify both clouds and shadows in satellite images. Especially, it shows obvious advantage over the Moderate Resolution Imaging Spectroradiometer cloud product (MOD35) for shadow detection. Although the proposed cloud removal method is simple, the radiances of a contaminated image can be reasonably reconstructed with root‐mean‐square error < 3.0 W/m2·sr·μm and mean bias < 1.0 W/m2·sr·μm for all seven Moderate Resolution Imaging Spectroradiometer reflective bands for our case studies. These results prove the effectiveness of the proposed scheme in identifying and removing clouds and shadows from remotely sensed images. Meanwhile, these findings provide some new ideas for the remote sensing community, especially in the fields of cloud detection and image processing. Key Points: A new scheme for detecting and removing clouds and shadows is proposed based on simulated TOA radiance fieldsA simple but effective approach to remove the clouds and shadows in the images is provided based on the simulated imageThe method works well for both cloud/shadow detection and image reconstruction; it shows obvious advantage over MOD35 in shadow detection [ABSTRACT FROM AUTHOR]

Particulate matter (PM) air pollution is recognized as a major cause of adverse health outcomes, yet much remains unknown about which fractions and compositions are the most harmful. Efforts in this direction can be improved by employing a cell‐free approach, dithiothreitol (DTT) assay, the so‐called oxidative potential (OP). Here, we provide a detailed examination of OP from fractionated PM by a combination extraction method of solvent and solid‐phase extractions (SPE). We estimate that the average OP of water‐soluble matter (WSM; 4.27 nmol/min/m3) is up to 1.2 times higher than methanol‐soluble matter (3.66 nmol/min/m3) in most seasons apart from summer. Of these WSM, OP is distributed almost equally between hydrophobic and hydrophilic fractions. OP of PM is mainly related to transition metals (Cu: 12.1%, Mn: 2.5%); apart from these, more than 80% of the total OP remains unidentified. Identification of this unexplained OP by a novel approach, integrating the chromophore characteristics with parallel factor analysis, has shown that chromophore1 (C1) accounted for 98% of OP, although this factor only made a small contribution to the total fluorescence. Our paper highlights the importance of a holistic approach by evaluating the chemical structure‐activity relationship between optical characteristics and OP. Information presented here draws a connection between light‐absorbing organic aerosols and a possible toxicological mechanism on the basis of their optical properties. [ABSTRACT FROM AUTHOR]

This study explains why observed storms with long concentric eyewall (CE) duration often have a large moat size. A series of slab boundary layer model experiments are conducted with the free atmosphere forcing from different CE structures. The experimental results indicate that the inner eyewall pumping (IEP), the maximum vertical motion at the inner eyewall region, increases with the increase in moat size d and vortex maximum wind vm, the decrease in the inner eyewall radius rm, and increase in inner eyewall vorticity. A scaling law is derived from hundreds of SBL experiments with four variables: IEP win, vortex inner radius rm, vortex maximum wind vm, and moat size d. The dimensionless scaling law is w∗∼d∗, ${w}^{\ast }\sim \sqrt{{d}^{\ast }},$ where w∗ = win/vm, d∗ = ζd/c, ζ is the vorticity 2vm/rm, and c is a constant gravity wave speed. The moat size divided by the Rossby length c/ζ, the dimensionless moat size d∗, combines the effect of the moat size and the vortex pressure gradient force to accelerate the inflow and to enlarge the IEP for inner eyewall convection maintenance. A phase diagram of IEP with vortex structure dimensionless moat ζd/c and vortex intensity vm is constructed based on the scaling law for the aircraft and satellite observations. The eyewall replacement cycle may be viewed as the process to reduce the IEP from the decrease of the vortex intensity and the vortex size of the dimensionless moat. This leads to the ultimate disappearance of the inner eyewall. Plain Language Summary: The paper explains the large moat size in the long‐lived concentric eyewalls tropical cyclones with boundary layer inner eyewall pumping (IEP) win. A scaling law from hundreds of experiments is found win∼vmζd/c∼vm1.5d0.5rm−0.5, ${w}_{in}\hspace*{.5em}\sim \hspace*{.5em}{v}_{m}\sqrt{\zeta d/c}\hspace*{.5em}\sim {v}_{m}^{1.5}{d}^{0.5}{{r}_{m}}^{-0.5},$ where vm is the vortex intensity, rm the inner eyewall radius, d the moat size, ζ the vorticity 2vm/rm and c is a constant gravity wave speed. The radial inflow is greatly reduced when it passed through the outer eyewall region. The inflow then reaccelerates by the pressure gradient force (PGF). A decrease of Rossby length implies a larger PGF. The dimensionless moat ζd/c, the moat d divided by the Rossby length c/ζ, combines the effect of the moat size that allows the inflow to be accelerated and the effect of the inflow acceleration enhanced by the PGF. Large dimensionless moat is with a large IEP for the maintenance of the inner eyewall convection. A phase diagram of IEP with ζd/c (structure) and vm (intensity) variables is constructed to for aircraft and satellite data diagnosis. The demise of the inner eyewall may be viewed as the reduction of IEP by the decrease of storm intensity and the dimensionless moat size. Key Points: Large moat size in long‐lived concentric eyewall storms is explained with the scaling law of boundary‐layer inner‐eyewall pumpingA large dimensionless moat implies a stronger boundary layer inflow due to a stronger pressure gradient force and a large moat distanceObservations show decreases of vortex intensity, dimensionless moat, and inner‐eyewall pumping in the replacement cycle on a phase diagram [ABSTRACT FROM AUTHOR]

Accurate and high‐resolution air temperature prediction is important in many different applications. Hourly air temperature forecasting in mountainous areas is necessary and important because mountainous areas are becoming increasingly important areas of human activities. At present, scientists successfully employ numerical weather prediction (NWP) models, such as the Weather Research and Forecasting (WRF) model, to achieve reliable forecasts. However, air temperature forecasting and modeling over complex geographical zones are still difficult tasks. The WRF model is a mesoscale model and does not adequately account for the influence of terrain on the air temperature. It is important to downscale larger‐scale models to a much finer scale. In this paper, a statistical temperature downscaling method based on geographically weighted regression (GWR) and diurnal temperature cycle (DTC) models is proposed. A statistical downscaling scheme of WRF simulation data is designed to forecast the hourly air temperature from 1‐km spatial resolution to 30 m, up to 24 hr in advance. The combined downscaling model's root‐mean‐square error (RMSE) decreased by 0.87°C at the automatic weather station (AWS) level and 0.62°C over the domain when compared to WRF simulations, and the mean absolute error (MAE) decreased by 0.71°C and 0.51°C, respectively, at these two levels. The results reveal that the combined downscaling model performs very well in correcting and downscaling the air temperature in WRF simulations in the study areas. Key Points: A statistical downscaling scheme was designed for weather research and forecasting (WRF) simulationsTo downscale the hourly air temperature from 1‐km spatial resolution to 30‐m, up to 24 hr in advance in a mountainous areaCompared to WRF simulation results, root‐mean‐square error (RMSE) and mean absolute error (MAE) of WRF‐downscaled results decreased by 0.87°C and 0.71°C, respectively, at the automatic weather station (AWS) level [ABSTRACT FROM AUTHOR]

The retrieval of the asymmetry parameter from nephelometer measurements can be challenging due to the inability to detect the whole angular range. Here, we present a new method for retrieving the asymmetry parameter of ice crystals with relatively large size parameters (>50) from polar nephelometer measurements. We propose to fit the angular scattering measurement with a series of Legendre polynomials and the best fitted coefficients give the asymmetry parameter. The accuracy of the retrieval is analyzed by accessing the smoothness of the phase function, which is closely linked to the complexity of ice particle. It is found that the uncertainty of retrieval could be smaller than 0.01, provided the measured intensity profile is smooth enough. As an application, we report an case study on Arctic cirrus, which shows a mean value for the asymmetry parameter of 0.72. Plain Language Summary: The asymmetry parameter of ice crystals is a parameter that can largely determine cirrus cloud's interaction with solar radiation energy, and therefore its magnitude is important for climate and weather prediction models. In‐situ measurements using nephelometers is a direct way to measure partial angular scattering functions, and the accuracy of these measurements is of upmost importance. In this paper, we report a novel method for retrieving the asymmetry parameter from polar nephelometer measurements. Depending on the smoothness of the measured angular scattering function, the accuracy of the retrieval could be very high. We report a case study over the Arctic region, showing a low asymmetry parameter around 0.72. Key Points: A new algorithm is developed for retrieving the asymmetry parameter of large and complex ice crystals from polar nephelometer measurementsAccuracy of retrieval could be better than 0.01, provided sufficient smoothness of the phase functionA case study of Arctic cirrus measurement during the CIRRUS‐HL campaign is reported [ABSTRACT FROM AUTHOR]

Information on decadal temperature variations over the northwestern Tibetan Plateau (TP) is still very limited due to a lack of long‐term instrumental data. In this paper, we present a 489‐year (1524–2012 A.D.) reconstruction of annual mean temperature for the northwestern TP using the δ18O record of a 58.82 meter‐long shallow ice core retrieved on the Chongce ice cap in the western Kunlun Mountains. The reconstructed temperature exhibits significant decadal variations, which were mainly driven by the Atlantic Multidecadal Variability. Decadal temperature variation during the Little Ice Age (LIA) is significantly larger than that during the post‐industrial warming period, possibly due to such external forcing as large fluctuations of solar activity and frequent major volcanic eruptions during the LIA. In addition, our reconstruction shows that the recent warming since the 1980s is moderate, comparable to an earlier warm interval in the mid‐20th century, which may partly explain the relatively stable status of glaciers in this region since 1970s. Key Points: The Atlantic Multidecadal Variability plays a critical role in the decadal temperature variations over the northwestern Tibetan PlateauLarge temperature variations before the 1850s were associated with the external forcings, including solar activities and volcanic eruptionsThe weak warming over the northwestern Tibetan Plateau since the 1980s have contributed to the "Karakoram Anomaly" [ABSTRACT FROM AUTHOR]

The physical and optical properties of aerosols and the change of planetary boundary layer height (PBLH) are of great scientific significance for the study of atmospheric environment and air quality. This paper intends to present integrated observations of aerosol physical‐optical characteristics and PBLH properties under different air pollution levels in Shouxian (National Climatology Observatory (32°26' N, 116°47' E)), China. The combined measurements of ground‐based Lidar and Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) are analyzed from December 2016 to November 2017 in this study. Results show that the extinction coefficients decrease significantly with height below 2 km. The extinction coefficients below 1 km, the aerosol optical depth (AOD), and the PBLH increase with the aggravation of pollution. The daily average aerosol extinction coefficient near the surface is about 0.78 km−1, the AOD is 0.87 ± 0.33 m (mean ± std, the same below), and the PBLH is 827 ± 132 m in polluted conditions (PM2.5 > 115 μg m−3). With the increase in pollution and the decrease in PBLH, more aerosols are accumulated at a lower height. The daily average ratios of AOD below PBLH (AODPBLH) to total AOD (AODtot) are about 64% in polluted conditions. According to the CALIPSO‐observed aerosol vertical profile, aerosols are identified for about 20.54% of the vertical space below 6 km altitude under polluted conditions. The spherical fine particles with lower particulate depolarization ratio (PDR, 79% of the PDR is <0.2), lower color ratio (CR, 64% of the CR is <0.75), and stronger scattering ability are dominant in 0∼2 km. Key Points: With the increase in pollution and the decrease in planetary boundary layer height, more aerosols are accumulated at the lower heightThe aerosol extinction coefficients decrease significantly with height below 2 km, and aerosol scattering ability is strong in 0∼2 kmThe regular fine particles with lower particulate depolarization ratio (PDR) (79% of the PDR is <0.2), lower lower color ratio (CR) (64% of the CR is <0.75) are dominant in 0∼2 km [ABSTRACT FROM AUTHOR]

We conducted a comprehensive observation of tower‐initiated upward lightning (UL) during the summer season from 2012 to 2020, using high‐speed video and the Beijing Broadband Lightning Network (BLNET). A total of 25 UL flashes were observed, and 21 of them were triggered by nearby positive cloud‐to‐ground (CG) lightning. It is interesting to note that eight (38%) of the parent +CG lightning flashes have more than one return stroke (RS). The distance between the UL and the grounding point of its parent +CG lightning was 4–38 km, where nine (43%) of the UL were triggered less than 2 ms after the positive RS, and 11 (52%) of them were triggered after a long time delay of 5–187 ms. Based on high‐speed video and mapping results, the short time delay was associated with the reactivation of the decayed established leader above the tower. The long time delay was associated with the negative leader extending to the region above the tower during the long continuing current. The positive E‐field change during RS or continuing current period favored the inception of an upward leader. Although both preliminary breakdown (PB) and RS in a +CG flash produced positive E‐field changes, an upward positive leader induced by PB aborted its sustained propagation after +RS. Plain Language Summary: In favorable conditions, an upward leader could initiate from the top of high constructions and develop toward the cloud base, named upward lightning (UL). Observation revealed that most UL transfer negative charge into the ground from the cloud and, quite often, such negative UL tends to be triggered by nearby positive cloud‐to‐ground (CG) lightning. So far, the in‐cloud channel structure that favors the initiation of UL remains unclear. This paper shows how nearby +CG flashes triggered upward leaders based on 21 cases from two tall towers in Beijing based on multi‐band detection of E‐field change, lightning mapping results, and high‐speed video observations. Nearly all UL was initiated after the occurrence of +RS, and UL can be initiated immediately (<2 ms) after +RS or after a long delay (5–187 ms) following +RS. There are two interesting findings: first, eight (38%) of the parent +CG have multiple strokes with different terminations; second, one attempted upward leader aborted its development after the occurrence of +RS, although lightning activities before or after +RS both produced positive electric field changes. Thus, we suggested two types of in‐cloud structures of +CG lightning and compared the effects of their leader propagation on UL initiation. Key Points: Eight (38%) of 21 positive cloud‐to‐ground lightning flashes that triggered upward lightning (UL) have more than one return stroke (RS)Propagating in‐cloud negative leaders after positive RS were the most efficient triggering component for ULApproaching positive leaders at lower altitudes above the tower prevented the sustained propagation of an upward positive leader [ABSTRACT FROM AUTHOR]

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

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

This study establishes a new blocking climatology in the Southern Hemisphere (SH) by using the improved detection method with the reanalysis data from 1979 to 2016. Furthermore, it investigates the dynamical linkage between SH blocking and ENSO phases. It is found that the ENSO warm phase is more conducive to the blocking formation than the cold phase while the blocking intensity is not affected by the ENSO phases. In addition, blocking in ENSO warm phase is characterized with upper‐level divergence anomaly and the cold phase with convergence anomaly equatorward of blocking onset region. Through the diagnostic analysis of tropopause divergence for the composite blocking events via an improved divergence tendency equation, the study shows that the ageostrophic effect exhibits dual effects, namely, it induces anticyclonic ageostrophic vorticity favoring the formation of blocking whereas it results in convergence anomaly close to the blocking onset region thus inhibiting blocking locally. In addition, the nonlinear term in the divergence tendency equation is found to be a dominant forcing for favoring blocking onset. Nevertheless, despite the primary contributions from the ageostrophic effect and nonlinear term to blocking formation, they could barely explain the SH blocking onset preference in ENSO warm phase. Finally, through the localized Eliassen‐Palm flux this study reveals that in ENSO warm phase the composite blocking onset region and the immediate upstream region are characterized by more localized EP flux convergence, thereby leading to more effective westerly wind weakening and favoring the blocking formation. Plain Language Summary: Previous research shows that in the Southern Hemisphere (SH), the ENSO warm phase is more conducive to the blocking formation than the cold phase, but the dynamic reasons are rarely studied. Starting from the tropopause divergence with the improved divergence tendency equation and tropical Rossby wave propagation associated with midlatitude wave‐flow interaction, this paper studies the connection between the tropics and extratropics from these two perspectives. Finally, this study reveals that the ENSO warm phase is more conducive to the blocking formation than the cold phase in the SH. Key Points: New blocking climatology is established in the Southern Hemisphere (SH) using an improved detection methodThe ageostrophic wind plays a dual effect in the SH blocking formationENSO warm phase is more conducive to blocking formation in the SH with more eddy convergence [ABSTRACT FROM AUTHOR]

The impact of the quasi‐biennial oscillation (QBO) on tropical convection and precipitation is investigated through nudging experiments using the UK Met Office Hadley Center Unified Model. The model control simulations show robust links between the internally generated QBO and tropical precipitation and circulation. The model zonal wind in the tropical stratosphere was nudged above 90 hPa in atmosphere‐only and coupled ocean‐atmosphere configurations. The convection and precipitation in the atmosphere‐only simulations do not differ between the experiments with and without nudging, which may indicate that SST‐convection coupling is needed for any QBO influence on the tropical lower troposphere and surface. In the coupled experiments, the precipitation and sea‐surface temperature relationships with the QBO phase disappear when nudging is applied. Imposing a realistic QBO‐driven static stability anomaly in the upper‐troposphere lower‐stratosphere is not sufficient to simulate tropical surface impacts. The nudging reduced the influence of the lower troposphere on the upper branch of the Walker circulation, irrespective of the QBO, indicating that the upper tropospheric zonal circulation has been decoupled from the surface by the nudging. These results suggest that grid‐point nudging mutes relevant feedback processes occurring at the tropopause level, including high cloud radiative effects and wave mean flow interactions, which may play a key role in stratospheric‐tropospheric coupling. Plain Language Summary: The interaction between the stratosphere and the troposphere is well known to produce surface impacts in the extratropics. However, whether stratosphere‐troposphere interactions affect the surface in the tropics associated with the variability of the stratospheric quasi‐biennial oscillation (QBO) is yet to be determined because the observational record is too short and tropical tropospheric variability masks any potential signal of the stratosphere. In this paper, we examine hypotheses that explain how the QBO could affect tropical deep convection and tropical precipitation variability through targeted model experiments which prescribe the equatorial stratosphere. Our results indicate that prescribing the zonal wind in the stratosphere remove links between surface precipitation and the QBO. Therefore, nudging the stratosphere decouples the stratospheric mean flow and anomalies from the tropospheric processes, which emphasizes the role of feedbacks. Key Points: Nudging the stratospheric quasi‐biennial oscillation (QBO) removes impacts to tropical precipitationThe QBO impact on upper‐level static stability is not sufficient to influence tropical precipitation in this modelFeedbacks, muted by nudging, may be key for stratospheric‐tropospheric coupling [ABSTRACT FROM AUTHOR]

A large portion of lakes on the Tibetan Plateau (TP) have experienced considerable expansion in the past decades. However, how the lake surface area expansion has influenced regional precipitation, especially its underlying mechanism, has not been investigated. By selecting the most dramatically expanded lake on the TP as a case study, this paper investigates the magnitude and extent of the precipitation response to lake expansion and its underlying mechanism using twin Weather Research and Forecasting (WRF) simulations with and without lake expansion. The results show that the precipitation response to the lake expansion exhibits pronounced spatial and seasonal variations and is generally more intense in the downwind direction than in the upwind direction. During the wet season (the South Asian summer monsoon, SASM), precipitation decreases in the downwind area by −25% ± 16% and −13% ± 15% in the 0–50 km and 50–100 km intervals (from the lake's center), respectively. Conversely, the precipitation increases in the downwind area during the dry season (before the freeze and after the SASM) by 27% ± 36% in the 0–50 km interval. The thermal and evaporative effects, which are generated by lake expansion and propagate in the downwind direction, are the key factors responsible for the precipitation changes. The cooling effect, which generally occurs in the daytime, stabilizes the atmosphere and triggers downward motion, consequently inhibiting precipitation. The warming effect has the opposite effect, and the additional water vapor strengthens this process. However, the effects vary with the seasons, causing seasonal variations in the precipitation response. Plain Language Summary: A large portion of lakes on the Tibetan Plateau have experienced considerable expansion in the past decades. This study aims to assess the regional precipitation response to the expansion of a large lake Seling Co on the Tibetan Plateau and its underlying mechanisms. The results show that the precipitation response to the lake expansion exhibits pronounced spatial and seasonal variations and is generally more intense in the downwind direction than in the upwind direction. The expanded lake tends to inhibit precipitation in the wet season and promote it in the dry season. These findings suggest more attention need to be paid to the areas located downwind of these expanded lakes and an urgent need to improve the understanding of the climate response to the imbalance of the Tibetan Plateau. Key Points: Precipitation response to the expansion of Seling Co on the Tibetan Plateau exhibits pronounced spatial and seasonal variationsThe precipitation response is generally more intense in the downwind directionThe thermal and evaporative effects of the expanded lake dominate the precipitation response [ABSTRACT FROM AUTHOR]

Single‐ and multi‐layer clouds are commonly observed over the Southern Ocean in varying synoptic settings, yet few studies have characterized and contrasted their properties. This study provides a statistical analysis of the microphysical properties of single‐ and multi‐layer clouds using in‐situ observations acquired during the Southern Ocean Cloud‐Radiation Aerosol Transport Experimental Study. The relative frequencies of ice‐containing samples (i.e., mixed and ice phase) for multi‐layer clouds are 0.05–0.25 greater than for single‐layer clouds, depending on cloud layer height. In multi‐layer clouds, the lowest cloud layers have the highest ice‐containing sample frequencies, which decrease with increasing cloud layer height up to the third highest cloud layer. This suggests a prominent seeder‐feeder mechanism over the region. Ice nucleating particle (cloud condensation nuclei) concentrations are positively (negatively) correlated with ice‐containing sample frequencies in select cases. Differences in microphysical properties are observed for single‐ and multi‐layer clouds. Drop concentrations (size distributions) are greater (narrower) for single‐layer clouds compared with the lowest multi‐layer clouds. When differentiating cloud layers by top (single‐ and highest multi‐layer clouds) and non‐top layers (underlying multi‐layer clouds), total particle size distributions (including liquid and ice) are similarly broader for non‐top cloud layers. Additionally, drop concentrations in coupled environments are approximately double those in decoupled environments. Plain Language Summary: Weather and climate models continue to struggle simulating cloud microphysical properties over the Southern Ocean, including cloud phase occurrence frequencies. High resolution observations of Southern Ocean clouds are crucial toward improving model simulations. This paper uses in situ observations to compare and contrast microphysical properties and phase frequencies of single‐ and multi‐layer clouds, as well as relates phase frequencies with ice nucleating particle and cloud condensation nuclei number concentrations. In situ observations used in this study were acquired during the Southern Ocean Cloud‐Radiation Aerosol Transport Experimental Study. A suite of cloud probe instrumentation is used to classify 1 Hz cloud samples as either liquid, ice or mixed phase (i.e., liquid and ice particles in the same sample volume). The lowest occurrence frequency of liquid phase samples is observed in the lowest cloud layers of multi‐layer clouds, whereas the highest frequency is observed in single‐layer clouds. Ice nucleating particle number concentrations are negatively correlated with liquid phase occurrence frequencies in select cases. In contrast, cloud condensation nuclei number concentrations are positively correlated with liquid phase frequencies, although this is only observed for clouds above the boundary layer. Key Points: The occurrence frequency of ice is greater in multi‐layer clouds than in single‐layer cloudsDrop number size distributions are broader in multi‐layer clouds compared to single‐layer cloudsLiquid drop number concentrations are approximately double in environments coupled with the surface compared to decoupled environments [ABSTRACT FROM AUTHOR]

A machine‐learning classifier for radiation waveforms of negative return strokes (RSs) is built and tested based on the Random Forest classifier using a large data set consisting of 14,898 negative RSs and 159,277 intracloud (IC) pulses with 3‐D location information. Eleven simple parameters including three parameters related with pulse characteristics and eight parameters related with the relative strength of pulses are defined to build the classifier. Two parameters for the evaluation of the classifier performance are also defined, including the classification accuracy, which is the percentage of true RSs in all classified RSs, and the identification efficiency, which is the percentage of correctly classified RSs in all true RSs. The tradeoff between the accuracy and the efficiency is examined and simple methods to tune the tradeoff are developed. The classifier achieved the best overall performance with an accuracy of 98.84% and an efficiency of 98.81%. With the same technique, the classifier for positive RSs is also built and tested using a data set consisting of 8,700 positive RSs. The classifier has an accuracy of 99.04% and an efficiency of 98.37%. By examining misclassified waveforms, we show evidence that some RSs and IC discharges produce special radiation waveforms that are almost impossible to correctly classify without 3‐D location information, resulting in a fundamental difficulty to achieve very high accuracy and efficiency in the classification of lightning radiation waveforms. Plain Language Summary: Lightning location systems are required to classify return strokes (RSs) from intracloud (IC) discharges accurately and efficiently because the RS is the main discharge component that poses direct threats to the human society. In this paper, we report a machine‐learning classifier for negative RSs built using a large data set with accurate 3‐D location information. The classifier has an accuracy of 98.84% (98.84% of classified RSs are correct classifications) and an efficiency of 98.81% (98.81% of RSs can be correctly classified). With the same technique, we also built a classifier for positive RSs with similarly high accuracy and efficiency. Our classifiers only require some simple waveform parameters so the same technique can be used by various lightning location systems relatively easily. A sample Python script to use the classifier is provided and readers are encouraged to test the classifier using their own data set. We also demonstrate that some RSs and IC discharges produce abnormal waveforms, so 100% accuracy or efficiency is fundamentally difficult to realize using only waveform information. Key Points: A machine‐learning classifier for negative return strokes (RSs) is built using a large data set with 3‐D location informationBoth an accuracy and an efficiency of about 98.8% are achieved and the accuracy‐efficiency tradeoff can be easily controlledSome RSs and intracloud discharges produce special waveforms that are fundamentally difficult to classify without 3‐D location results [ABSTRACT FROM AUTHOR]

Long‐range transport and vertical distribution of aerosols are important factors for assessing the uncertainty in aerosol radiative forcing. This paper reveals the vertical distribution and trends of aerosol optical properties in China using 15 years of Cloud‐Aerosol Lidar with Orthogonal Polarization (CALIOP) data. The Hybrid Single‐Particle Lagrangian integrated trajectory model was used to analyze the transport and trends of aerosols in the layer with the highest occurrence frequencies of dust, polluted dust, polluted continental and elevated smoke aerosols. The results indicated that (a) there were significant regional and seasonal differences in the vertical distribution of aerosols. The aerosol optical depth (AOD) trend in a given region depends on the changes in the aerosol type with the highest frequency and the layer corresponding to the largest AOD in the vertical profile. The frequency of polluted dust aerosols was the highest in Beijing‐Tianjin‐Hebei (BTH) and Central China. The considerable decrease in the 0–2 km AOD led to a significant trend of the column AOD. (b) The changes of AOD and main aerosol types in a region are also affected by the changes of aerosol sources and long‐range transport pathways. In the BTH, dust aerosols originated from the Mongolian Plateau, accounting for 57.88% of the total trajectories. The Pearl River Delta was dominated by elevated smoke aerosols, with trajectories mainly originating from the Myanmar and Vietnam, accounting for 27.38% and 29.59%, respectively. The trend of 15‐year backward trajectories of dust aerosols on the Tibetan Plateau indicated that the trajectory from India is increasing. Plain Language Summary: Aerosols are one of the main factors affecting global climate change, influencing global radiation through direct and indirect effects. Long‐range transport aerosols and their vertical distribution are important factors for assessing the uncertainty in aerosol radiative forcing. At present, there is limited research on the vertical distribution and transport pathways of different types of aerosols in multiple regions over China. Considering that aerosols in different regions of China exhibit large vertical distribution and transport differences, we revealed the vertical distribution and trends of aerosol optical properties in China using 15 years of Cloud‐Aerosol Lidar with Orthogonal Polarization data. The Hybrid Single‐Particle Lagrangian Integrated Trajectory model was used to analyze the transport and trends of different aerosols. We found significant regional and seasonal differences in the vertical distributions of aerosols. The changes in the aerosol optical depth and main aerosol types in a given region were also significantly affected by the changes in aerosol transport sources and transport pathways. This study provides comprehensive insight into the vertical distribution and transport characteristics of different types of atmospheric aerosols in the Chinese region over the last 15 years, which is very important to further evaluation of the impact of different aerosol types on the radiative effects and the environment. Key Points: The vertical distribution of aerosol optical properties exhibits notable seasonal and regional differences using 15 years of Cloud‐Aerosol Lidar with Orthogonal Polarization (CALIOP) dataThe trend of total aerosol optical depth (AOD) mainly depends on the joint changes of the highest frequency aerosol and the maximum AOD layer in a vertical profileThe changes of AOD and main aerosol types in a region are also affected by the changes of aerosol transport sources and transport pathways [ABSTRACT FROM AUTHOR]

A heavy nocturnal warm sector rainfall occurred over the western coastal region of South China in June 2020, of which the precipitation magnitude was seriously underestimated by the operational TRAMS (Tropical Regional Atmosphere Model System) model. In this study, by improving the configuration of initial conditions (ICs) and lateral boundary conditions (LBCs) in offline nesting, the simulation of the convection initiation (CI) process can be well improved. The CI simulation is found to be sensitive to the vertical resolution in ICs in this case. When the low‐level vertical resolution of ICs is increased, the nocturnal near‐surface cold layer caused by inland mountains can be resolved better, which is necessary to form the convergence line along the coastline. Another important impact of increasing the vertical resolution of ICs is the successful simulation of horizontal convective rolls (HCRs) over the northern South China Sea. The HCRs apparently increase the depth of the warm‐moist marine boundary layer jet (MBLJ), and finally lead to the CI along the coastline. The increased vertical resolutions of LBCs can reduce the discontinuity of simulated moist tongue across the southern lateral boundary of the TRAMS model, which provides more moisture for the warm sector rainfall through MBLJ. LBCs with Higher temporal frequency help to alleviate the western position bias of rain belt by reducing the temporal interpolation error. Our study highlights the importance of finer ICs and LBCs in offline nesting for regional operational NWP systems in the South China region. Plain Language Summary: Warm‐sector heavy rainfall in South China usually develops under weakly forced weather condition, numerical forecasting of its initiation and intensification process is a difficult problem. To investigate its sensitivity to the configuration of initial conditions (ICs) and lateral boundary conditions (LBCs), a simulation study is carried out for a nocturnal warm sector rainfall event happened in western Guangdong. When the low‐level vertical resolution of ICs is increased, the nocturnal near surface cold layer from inland mountains can be resolved better. Another important impact of increasing vertical resolution of ICs is the successful simulation of horizontal convective rolls (HCRs) over the northern South China Sea. The HCRs apparently increase the depth of warm‐moist marine boundary layer jet (MBLJ), and finally lead to the CI along coast line. Additionally, the vertical resolution and the updating frequency of the lateral boundary conditions also have important effects on the simulation of the rainstorm intensification process. This paper emphasizes the importance of accurate ICs and LBCs to improve the predictability of warm‐sector rainfall in South China, which can provide practical reference for the design of operational systems. Key Points: Increasing low‐level vertical resolution of initial conditions is critical to the convection initiation (CI) simulation of warm‐sector rainfall caseThe increased vertical resolutions of lateral boundary conditions (LBCs) can provide more moisture for the CI and upscale convective growth process through marine boundary layer jetHigher temporal frequency LBCs is help to alleviate the western position bias of rain belt by reducing the temporal interpolation error [ABSTRACT FROM AUTHOR]