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The National Science Foundation–sponsored Lake-Effect Electrification (LEE) field campaign intensive observation periods occurred between November and early February 2022–23 across the eastern Lake Ontario region. Project LEE documented, for the first time, the total lightning and electrical charge structures of lake-effect storms and the associated storm environment using a lightning mapping array (LMA), a mobile dual-polarization X-band radar, and balloon-based soundings that measured vertical profiles of temperature, humidity, wind, electric field, and hydrometeor types. LEE also observed abundant wind turbine-initiated lightning, which is climatologically more likely during the winter. The frequent occurrence of intense lake-effect storms and the proximity of a wind farm with nearly 300 turbines each more than 100 m tall to the lee of Lake Ontario provided an ideal laboratory for this study. The field project involved many undergraduate (>20) and graduate students. Some foreseen and unforeseen challenges included clearing the LMA solar panels of snow and continuous operation in low-sunlight conditions, large sonde balloons prematurely popping due to extremely cold conditions, sonde line breaking, recovering probes in deep snow in heavily forested areas, vehicles getting stuck in the snowpack, and an abnormally dry season for parts of the LEE domain. In spite of these difficulties, a dataset was collected in multiple lake-effect snowstorms (11 observation periods) and one extratropical cyclone snowstorm that clarifies the electrical structure of these systems. A key finding was the existence of a near-surface substantial positive charge layer (1 nC m−3) near the shoreline during lake-effect thunderstorms. SIGNIFICANCE STATEMENT: Winter lightning is an exciting phenomenon and important to study as man-made objects (e.g., wind turbines) can be more likely to be struck this time of year. We involved over 20 undergraduate students in collecting total lightning, mobile radar, and sounding (e.g., electric field) data in several lake-effect storms. These storms have been known to have substantial electrification and occur many times each winter in a small, consistent region east of Lake Ontario. We analyzed 246 lightning flashes from the main data collection period, and over a third of them were associated with towers. The results from this study will have impacts on our general understanding of electrification in winter and summer storms. [ABSTRACT FROM AUTHOR]

Previous observational studies have indicated that mesoscale convective systems (MCSs) contribute the majority of precipitation over the Bay of Bengal (BoB) during the summer monsoon season, yet their initiation and propagation remain incompletely understood. To fill this knowledge gap, we conducted a comprehensive study using a combination of 20-yr satellite observations, MCS tracking, reanalysis data, and a theoretical linear model. Satellite observations reveal clear diurnal propagation signals of MCS initiation frequency and rainfall from the west coast of the BoB toward the central BoB, with the MCS rainfall propagating slightly slower than the MCS initiation frequency. Global reanalysis data indicate a strong association between the offshore-propagating MCS initiation frequency/rainfall and diurnal low-level wind perturbations, implying the potential role of gravity waves. To verify the hypothesis, we developed a 2D linear model that can be driven by realistic meteorological fields from reanalysis. The linear model realistically reproduces the characteristics of offshore-propagating diurnal wind perturbations. The wind perturbations, as well as the offshore propagation signals of MCS initiation frequency and rainfall, are associated with diurnal gravity waves emitted from the coastal regions, which in turn are caused by the diurnal land–sea thermal contrast. The ambient wind speed and vertical wind shear play crucial roles in modulating the timing, propagation, and amplitude of diurnal gravity waves. Using the linear model and satellite observations, we further show that the stronger monsoonal flows lead to faster offshore propagation of diurnal gravity waves, which subsequently control the offshore propagation signals of MCS initiation and rainfall. Significance Statement: Rainfall over the Bay of Bengal (BoB) is primarily contributed by large and organized rainfall systems in the summer monsoon season. During this season, these systems are commonly observed over the east coast of India around midnight, the western BoB in the morning, and the central BoB in the afternoon. This eastward rainfall propagation is confirmed by observations, reanalysis data, and a theoretical model to have a strong association with atmospheric diurnal gravity waves. These waves are caused by land–sea thermal contrast and can trigger rainfall systems over the offshore regions. We also found that the diurnal gravity waves, as well as their triggered rainfall systems, can be greatly modulated by the large-scale monsoonal flows. All the above findings improve our understanding of diurnal rainfall cycle over the BoB and may contribute to the future improvement of rainfall forecast over the region. [ABSTRACT FROM AUTHOR]

This study examines the behaviors of coastal mesoscale convective systems (MCSs) to understand why the maximum climatological rainfall appears offshore of West Africa using satellite measurements. West Africa has a higher amount of rainfall offshore than inland during the monsoon season, but the reason for this offshore rainfall maximum remains unclear. MCSs that propagate offshore from coastal West Africa can develop into high-impact weather events in the eastern Atlantic, yet we know little about the MCS transitions across coastlines. Therefore, better understanding of how MCSs evolve during the coastal-to-maritime transition and contribute to the offshore climatological rainfall maximum over West Africa is key to advancing the knowledge of high-impact weather in this coastal region. The offshore rainfall can be contributed by either offshore-propagating MCSs generated over land or the MCSs initiated over the ocean. While continental MCSs are known to have more intense deep convection, their frequency of propagating offshore is found to be small. Instead, we find that the offshore environment supports the frequent formation of maritime MCSs, leading to their dominant contribution to offshore rainfall. However, an exception is found for the 8°–10°N offshore area, where the presence of high terrain generates MCSs with intense rainfall that more frequently propagate offshore and enhance offshore rainfall. These MCSs also often intensify their rainfall and expand in size as they move offshore. These results suggest that understanding the frequency, intensity, and transition of MCS generated over land and the ocean is key to better comprehending and simulating the rainfall activity over the coastal-to-marine transition zone. [ABSTRACT FROM AUTHOR]

On 10 August 2020, a derecho caused widespread damage across Iowa and Illinois. Des Moines station data show that the arrival of the gust front was characterized by an abrupt shift to northerly flow, exceeding 22 m s−1 for ∼20 min. To test the hypothesis that this northerly jet is associated with a horizontal potential vorticity (PV) dipole in the lower troposphere, we investigated the structure of PV in the University of Wisconsin Nonhydrostatic Modeling System (UWNMS) and of absolute vorticity in High-Resolution Rapid Refresh (HRRR) forecast analyses. This structure is described here for the first time. The negative PV member coincides with the downdraft, while the positive PV member coincides with the updraft, with a northerly jet between. The westerly inflow jet descends anticyclonically in the downdraft, joining with northerly flow from the surface anticyclone. The resulting northerly outflow jet creates the trailing comma-shaped radar echo. The speed of propagation of the derecho is similar to the westerly wind maximum in the 3–5-km layer associated with the approaching synoptic cyclone, which acts as a steering level for resonant amplification. Idealized diagrams and 3D isosurfaces illustrate the commonality of the PV dipole/northerly jet structure. Differences in this structure among the three model states are related to low-level wind shear theory. The PV dipole coincides with the pattern of diabatic stretching tendency, which shifts westward and downward relative to the updraft/downdraft with increasing tilt. The PV dipole can contribute toward dynamical stability in a derecho. Significance Statement: The purpose of this work is to investigate the structure of potential vorticity (PV) in the lower troposphere in a derecho. It is found that a northerly outflow jet occurs between an east–west-oriented horizontal PV dipole, which is described here for the first time. The negative PV member coincides with the downdraft and is inertially unstable, while the positive PV member coincides with the updraft. This work contributes toward the theory of resonant structures and longevity. The 3–5-km westerly inflow layer constitutes a steering level, which controls propagation speed despite differences in structure. The degree of westward tilt with height is related to the pattern of forcing by diabatic stretching in producing the PV dipole. [ABSTRACT FROM AUTHOR]

Ten bow echo events were simulated using the Weather Research and Forecasting (WRF) Model with 3- and 1-km horizontal grid spacing with both the Morrison and Thompson microphysics schemes to determine the impact of refined grid spacing on this often poorly simulated mode of convection. Simulated and observed composite reflectivities were used to classify convective mode. Skill scores were computed to quantify model performance at predicting all modes, and a new bow echo score was created to evaluate specifically the accuracy of bow echo forecasts. The full morphology score for runs using the Thompson scheme was noticeably improved by refined grid spacing, while the skill of Morrison runs did not change appreciably. However, bow echo scores for runs using both schemes improved when grid spacing was refined, with Thompson runs improving most significantly. Additionally, near storm environments were analyzed to understand why the simulated bow echoes changed as grid spacing was changed. A relationship existed between bow echo production and cold pool strength, as well as with the magnitude of microphysical cooling rates. More numerous updrafts were present in 1-km runs, leading to longer intense lines of convection which were more likely to evolve into longer-lived bow echoes in more cases. Large-scale features, such as a low-level jet orientation more perpendicular to the convective line and surface boundaries, often had to be present for bow echoes to occur in the 3-km runs. [ABSTRACT FROM AUTHOR]

The importance of mesoscale convective systems (MCSs) and their precipitation is well-established, and any future spatiotemporal shifts in their frequency or intensity could have far-reaching societal impacts. This work describes how MCS activity in the conterminous United States east of the continental divide (ECONUS) is modified by two future climate change scenarios. For this study, MCSs are identified in output from a convection-permitting regional climate model (CP-RCM) for three 15-year periods—namely, a retrospective baseline (1990–2005) and two end-of-century (2085–2100) climate change scenarios based on RCP 4.5 (EoC 4.5) and RCP 8.5 (EoC 8.5). The data reveal an eastward shift in regional MCS activity. Annually, days with MCSs largely remain the same or decrease west of the Mississippi River, whereas areas east of the Mississippi River experience more MCS days and MCS precipitation. The largest seasonal increases in MCS days and precipitation occur during the spring in parts of the Midwest and Northeast, whereas the largest decreases occur in parts of the Southern Plains during the summer. Overall, EoC 8.5 produced larger regional changes compared to EoC 4.5, suggesting that future CP-RCM experiments could benefit from considering multiple climate change scenarios. [ABSTRACT FROM AUTHOR]

An investigation of lake effect (LE) and the associated synoptic environment is presented for days when all five lakes in the Great Lakes (GL) region had LE bands [five-lake days (5LDs)]. The study utilized an expanded database of observed LE clouds over the GL during 25 cold seasons (October–March) from 1997/98 to 2021/22. LE bands occurred on 2870 days (64% of all cold-season days). Nearly a third of all LE bands occurred during 5LDs, although 5LDs consisted of just 17.1% of LE days. A majority of 5LDs (56.5%) had lake-to-lake (L2L) bands, and these days comprised 43.5% of all L2L occurrences. 5LDs occurred with a mean of 26.1 (SD = 6.2) days per cold season until 2008/09 and then decreased to a mean of 13.8 (SD = 5.5) days during subsequent cold seasons. January and February had the largest number of consecutive LE days in the GL with a mean of 5.7 and 5.4 days, respectively. As the number of consecutive LE days increases, both the number of 5LDs and the occurrence of consecutive 5LD increase. This translates to an increased potential of heavy snowfall impacts in multiple, localized areas of the GL for extended time periods. The mean composite synoptic pattern of 5LDs exhibited characteristics consistent with lake-aggregate disturbances and showed similarity to synoptic patterns favorable for LE over one or two of the GL found by previous studies. The results demonstrate that several additional areas of the GL are often experiencing LE bands when a localized area has active LE bands occurring. [ABSTRACT FROM AUTHOR]

This paper investigates the impact of the environmental conditions during the first half of the 2020 mei-yu season (Y20) and the southwest vortex (SWV), as well as their interaction, on heavy precipitation in southern Taiwan during late May 2020, based on a quantitative approach through ensemble simulations. The control experiment successfully replicates observed heavy precipitation in southern and central Taiwan and reveals a positive spatial correlation between precipitation occurrence probabilities and mean accumulated precipitation, emphasizing continuous rainfall accumulation over intermittent extreme events. Comparative analyses with sensitivity experiments elucidate that the Y20, featuring an extended western North Pacific subtropical high, intensify pressure gradients and southwesterly flow near Taiwan, favoring precipitation in windward regions but hindering it in the east. The SWV creates a moist and vortical environment near Taiwan, amplifying moisture supply and westerly winds, promoting precipitation in southern Taiwan, and enhancing frontal activity. The interaction between the SWV and the Y20, though limited in its impact on providing favorable wind and moisture conditions for precipitation southwest of Taiwan, significantly contributes to precipitation in southern Taiwan. The reason is that although the SWV primarily enhances moisture and the Y20 predominantly boost southwesterly flow, creating favorable conditions for rainfall, substantial precipitation occurs only when both factors converge in a nonlinear interaction. The interaction increases frontal activity over the Taiwan Strait and influences the movement and strength of the SWV, enhancing southwesterly flow and moisture flux in southwestern Taiwan. [ABSTRACT FROM AUTHOR]

An unprecedented heat wave occurred over the Pacific Northwest and southwest Canada on 25–30 June 2021, resulting in all-time temperature records that greatly exceeded previous record maximum temperatures. The impacts were substantial, including several hundred deaths, thousands of hospitalizations, a major wildfire in Lytton, British Columbia, Canada, and severe damage to regional vegetation. Several factors came together to produce this extreme event: a record-breaking midtropospheric ridge over British Columbia in the optimal location, record-breaking midtropospheric temperatures, strong subsidence in the lower atmosphere, low-level easterly flow that produced downslope warming on regional terrain and the removal of cooler marine air, an approaching low-level trough that enhanced downslope flow, the occurrence at a time of maximum insolation, and drier-than-normal soil moisture. It is shown that all-time-record temperatures have not become more frequent and that annual high temperatures only increased at the rate of baseline global warming. Although anthropogenic warming may have contributed as much as 1°C to the event, there is little evidence of further amplification from increasing greenhouse gases. Weather forecasts were excellent for this event, with highly accurate predictions of the extreme temperatures. Significance Statement: This paper describes the atmospheric evolution that produced an extreme heat wave over the Pacific Northwest during June 2021 and puts this event into historical perspective. [ABSTRACT FROM AUTHOR]

Severe convection occurring in high-shear, low-CAPE (HSLC) environments is a common cool-season threat in the southeastern United States. Previous studies of HSLC convection document the increased operational challenges that these environments present compared to their high-CAPE counterparts, corresponding to higher false-alarm ratios and lower probability of detection for severe watches and warnings. These environments can exhibit rapid destabilization in the hours prior to convection, sometimes associated with the release of potential instability. Here, we use self-organizing maps (SOMs) to objectively identify environmental patterns accompanying HSLC cool-season severe events and associate them with variations in severe weather frequency and distribution. Large-scale patterns exhibit modest variation within the HSLC subclass, featuring strong surface cyclones accompanied by vigorous upper-tropospheric troughs and northward-extending regions of instability, consistent with prior studies. In most patterns, severe weather occurs immediately ahead of a cold front. Other convective ingredients, such as lower-tropospheric vertical wind shear, near-surface equivalent potential temperature (θe) advection, and the release of potential instability, varied more significantly across patterns. No single variable used to train SOMs consistently demonstrated differences in the distribution of severe weather occurrence across patterns. Comparison of SOMs based on upper and lower quartiles of severe occurrence demonstrated that the release of potential instability was most consistently associated with higher-impact events in comparison to other convective ingredients. Overall, we find that previously developed HSLC composite parameters reasonably identify high-impact HSLC events. Significance Statement: Even when atmospheric instability is not optimal for severe convective storms, in some situations they can still occur, presenting increased challenges to forecasters. These marginal environments may occur at night or during the cool season, when people are less attuned to severe weather threats. Here, we use a sorting algorithm to classify different weather patterns accompanying such storms, and we distinguish which specific patterns and weather system features are most strongly associated with severe storms. Our goals are to increase situational awareness for forecasters and to improve understanding of the processes leading to severe convection in marginal environments. [ABSTRACT FROM AUTHOR]

The scientific community has long acknowledged the importance of high-temporal-resolution radar observations to advance science research and improve high-impact weather prediction. Development of innovative rapid-scan radar technologies over the past two decades has enabled radar volume scans of 10–60 s compared to 3–5 min with traditional parabolic dish research radars and the WSR-88D radar network. This review examines the impact of rapid-scan radar technology, defined as radars collecting volume scans in 1 min or less, on atmospheric science research spanning different subdisciplines and evaluates the strengths and weaknesses of the use of rapid-scan radars. In particular, a significant body of literature has accumulated for tornado and severe thunderstorm research and forecasting applications, in addition to a growing number of studies of convection. Convection research has benefited substantially from more synchronous vertical views, but could benefit more substantially by leveraging multi-Doppler wind retrievals and complementary in situ and remote sensors. In addition, several years of forecast evaluation studies are synthesized from radar testbed experiments, and the benefits of assimilating rapid-scan radar observations are analyzed. Although the current body of literature reflects the considerable utility of rapid-scan radars to science research, a weakness is that limited advancements in understanding of the physical mechanisms behind observed features have been enabled. There is considerable opportunity to bridge the gap in physical understanding with the current technology using coordinated efforts to include rapid-scan radars in field campaigns and expanding the breadth of meteorological phenomena studied. Significance Statement: Recently developed rapid-scan radar technologies, capable of collecting volumetric (i.e., three-dimensional) measurements in 10–60 s, have improved temporal sampling of weather phenomena. This review examines the impact of these radar observations from the past two decades on science research and emerging operational capabilities. Substantial breadth and impact of research is evident for tornado research and forecasting applications, in addition to documentation of other rapidly evolving phenomena associated with deep convection, such as tornadoes, hail, lightning, and tropical cyclones. This review identifies the strengths and weaknesses of how these radars have been used in scientific research to inform future studies, emerging from the increasing availability and capability of rapid-scan radars. In addition, this review synthesizes research that can benefit future operational radar decisions. [ABSTRACT FROM AUTHOR]

There is an increasing need to improve the accuracy of extreme weather forecasts for life-saving applications and in support of various socioeconomic sectors in East Africa, a region with remarkable mesoscale systems due to its complex topography defined by sharp gradients in elevation, inland water bodies, and landuse conversions. This study sought to investigate the impacts of the Weather Research and Forecasting (WRF) model spatial resolution on resolving rainfall variability with topography utilizing nested domains at 12 and 2.4 km resolutions. The model was driven by the National Centers for Environmental Prediction (NCEP)-Global Data Assimilation System (GDAS) Global Forecast System (GFS) final (FNL) reanalysis to simulate the weather patterns over East Africa from 3rd April 2018 to 30th April 2018, which were evaluated against several freely available gridded weather datasets alongside rainfall data from the Kenya Meteorological Department (KMD) stations. The reference datasets and the model outputs revealed that the highlands had more rainfall events and higher maximum daily rainfall intensity compared to the surrounding lowlands, attributed to orographic lifting enhancing convection. Rainfall was inversely proportional to altitude from 500 m to 1,100 m above sea level (ASL) for both coarse and fine resolutions. The convection-permitting setup was superior in three aspects: resolving the inverse altitude-rainfall relationship for altitudes beyond 3000 m ASL, simulating heavy rainfall events over the lowlands, and resolution of the diurnal cycle of low-level wind. Although the coarse resolution setup reasonably simulated rainfall over large mountains, only the convection-permitting configuration could accurately resolve rainfall variability over contrasting topographical features. The study notes that high-resolution modeling systems and topography-sensitive bias correction techniques are critical for improving the quality of operational weather forecasts in East Africa.

Derechos are extensive swaths of damaging winds produced by some long-lived, widespread mesoscale convective systems. Little research has been conducted concerning how derecho mechanisms might change in a future, warmer climate. In this study, the pseudo–global warming method is utilized to evaluate how the 10 August 2020 midwestern U.S. derecho, the costliest thunderstorm event in U.S. history to date, might differ if it instead occurred in a warmer climate at the end of this century. The 10 August derecho event is first simulated in its observed environment, and then resimulated in environments altered according to projections from different climate models using a high-emissions climate change scenario. Results suggest that near the end of this century, a similar derecho event may not necessarily have more intense winds but could possibly impact a geographical area 50% to 100% larger. The physical chain of events leading to this greater geographical impact result from the derecho winds beginning earlier in the storm lifetime, due to increased precipitation combined with decreased relative humidity right above the ground, and derecho winds extending northward due to a strengthening of the parent storm from increased instability there. All these factors enhance the area of evaporative cooling and thus the cold pool, which in turn extends the area covered by the rear-inflow jet within the storm, the likely main mechanism for most of the damaging winds at the ground in the historical event. More study of other cases is required to evaluate the generality of this result. [ABSTRACT FROM AUTHOR]

Over the course of the century, much research has been done to understand how derechos form, what environments support derecho development, and how forecasts of derechos can be improved. Following the definition, climatology, and societal impacts of derechos in Part I of this manuscript, Part II offers a thorough review on the parent MCS structures that produce derechos, which synoptic setups support derecho-producing MCSs, and the successes and failures of forecasting derechos. This manuscript reviews the 3D structure of MCSs that support derecho development, as well as both the strongly and weakly forced synoptic environments where derechos frequently occur. In addition, successes and failures common among most derecho numerical forecast studies are discussed to suggest where the greatest improvements in derecho forecasting may be made. [ABSTRACT FROM AUTHOR]

Mesoscale convective systems (MCSs) bring large amounts of rainfall and strong wind gusts to the midlatitude land regions, with significant impacts on local weather and hydrologic cycle. However, weather and climate models face a huge challenge in accurately modeling the MCS life cycle and the associated precipitation, highlighting an urgent need for a better understanding of the underlying mechanisms of MCS initiation and propagation. From a theoretical perspective, a suitable model to capture the realistic properties of MCSs and isolate the bare-bones mechanisms for their initiation, intensification, and eastward propagation is still lacking. To simulate midlatitude MCSs over land, we develop a simple moist potential vorticity (PV) model that readily describes the interactions among PV perturbations, air moisture, and soil moisture. Multiple experiments with or without various environmental factors and external forcing are used to investigate their impacts on MCS dynamics and mesoscale circulation vertical structures. The result shows that mechanical forcing can induce lower-level updraft and cooling, providing favorable conditions for MCS initiation. A positive feedback among surface winds, evaporation rate, and air moisture similar to the wind-induced surface heat exchange over tropical ocean is found to support MCS intensification. Both background surface westerlies and vertical westerly wind shear are shown to provide favorable conditions for the eastward propagation of MCSs. Last, our result highlights the crucial role of stratiform heating in shaping mesoscale circulation response. The model should serve as a useful tool for understanding the fundamental mechanisms of MCS dynamics. [ABSTRACT FROM AUTHOR]

Accurate representation of mesoscale scale convective systems (MCSs) in climate models is of vital importance to understanding global energy, water cycles, and extreme weather. In this study, we evaluate the simulated MCS features over the United States from the newly developed GFDL global high-resolution (∼50 km) AM4 model by comparing them with the observations during spring to early summer (April–June) and late summer (July–August). The results show that the spatial distribution and seasonality of occurrence and genesis frequency of MCSs are reasonably simulated over the central United States in both seasons. The model reliably reproduces the observed features of MCS duration, translation speed, and size over the central United States, as well as the favorable large-scale circulation pattern associated with MCS development over the central United States during spring and early summer. However, the model misrepresents the amplitude and the phase of the diurnal cycle of MCSs during both seasons. In addition, the spatial distribution of occurrence and genesis frequency of MCSs over the eastern United States is substantially overestimated, with larger biases in early spring and summer. Furthermore, while large-scale circulation patterns are reasonably simulated in spring and early summer, they are misrepresented in the model during summer. Finally, we examine MCS-related precipitation, finding that the model overestimates MCS-related precipitation during spring and early summer, but this bias is insufficient to explain the significant dry bias observed in total precipitation over the central United States. Nonetheless, the dry biases in MCS-associated precipitation during late summer likely contribute to the overall precipitation deficit in the model. [ABSTRACT FROM AUTHOR]

The realism of convective organization in operational convection-permitting model simulations is objectively assessed, with a particular focus on the mesoscale aspects, such as convective mode. A tracking and classification algorithm is applied to observed radar reflectivity and simulated radar reflectivity from the operational ACCESS-C convection-permitting forecast domain over northern Australia between October 2020 and May 2022, and the characteristics of real and simulated convective organization are compared. Mesoscale convective systems from the operational forecast model are approximately twice as likely to be oriented parallel to the ambient wind and ambient wind shear than those observed by radar, indicating a bias toward the "training line" systems typically associated with more extreme rainfall. During highly humid active monsoon conditions, simulated convective systems have larger ground-relative speeds than systems observed in radar. Although there is less than 5% difference between the ratios of simulated and observed trailing, leading and parallel stratiform system observations, significant differences exist in other wind shear–based classifications. For instance, in absolute terms, simulated systems are 10%–35% less likely to be upshear tilted, and 15%–30% less likely to be downshear propagating than observed systems, suggesting errors in simulated cold pool characteristics. Significance Statement: Remarkable progress has been made simulating thunderstorms in operational weather forecasting computer models. While some details of individual storm clouds may be unrealistic, how these storm clouds self-organize, i.e., cluster and regenerate, can be explicitly simulated, with this organization often appearing realistic. However, assessing the realism of this organization in an objective, systematic way has proven challenging. Here we assess organized convection in Australia's current high-resolution weather prediction model. In some respects, simulated storm clouds organize realistically. High-altitude icy cloud mostly trails behind groups of storm clouds in both simulations and reality. In other respects, organization is unrealistic. Simulated storm clouds are twice as likely to orient along the mean wind direction than in reality, likely contributing to extreme rainfall biases. [ABSTRACT FROM AUTHOR]

Multiple parallel rainbands (MPRBs) involve the organization of mesoscale convective systems (MCSs) characterized by multiple parallel convective rainbands, which may produce high rainfall accumulation. A total of 178 MPRBs were identified from 2016 to 2020 in China, which were classified into the initiation type (∼40%), where rainbands initiate individually, and differentiation type (∼60%), where rainbands form through the splitting of large rainbands or merging of smaller cells. Results showed that the occurrence frequency of MPRBs peaks in July with a midnight major peak and a morning minor peak. The highest occurrence frequency is observed in the northern Beibu Gulf and its coastal areas, with minor high frequencies in Guangdong, northern Jiangxi, and southern Shandong provinces, typically in a southwesterly low-level jet to the west of the subtropical high. MPRBs mainly contain 3–4 rainbands with a spacing distance of 30–50 km and an orientation generally consistent with the direction of 850-hPa winds and 0–1-km vertical wind shear. MPRBs generally move slower than that of squall lines in East China ranging from 4 to 8 m s−1 with 16% being quasi-stationary, which is mainly due to the occurrence of band back building mainly associated with cold pool. Most MPRBs have training effects with band training as the dominant mode. Because of the band training effect and slower movement of MPRBs mainly due to band back building, 71% of MPRBs are associated with enhanced maximum hourly rainfall. Rainfall severity may be alleviated somewhat by the generally short duration of MPRBs with 78% being shorter than 2 h. Significance Statement: The purpose of this study is to document the general features of mesoscale convective systems (MCSs) with a specific organization of multiple parallel rainbands (MPRBs). MCSs with this unique organization tend to produce extremely heavy rainfall partly due to the training of multiple rainbands as well as their slow movement because of back building. The organization pattern of MPRBs was previously found in a case study. The possible formation mechanism was also previously examined based on case studies. As a complement to these studies, this work aims to reveal the temporal and spatial distributions, movement and duration, morphology, precipitation patterns, and environmental features of MPRBs in China based on statistics using 5-yr radar reflectivity data. [ABSTRACT FROM AUTHOR]

Coastal convection is often organized into multiple mesoscale systems that propagate in either direction across the coastline (i.e., landward and oceanward). These systems interact non-trivially with synoptic and intraseasonal disturbances such as convectively coupled waves and the Madden–Julian oscillation. Despite numerous theoretical and observational efforts to understand coastal convection, global climate models still fail to represent it adequately, mainly because of limitations in spatial resolution and shortcomings in the underlying cumulus parameterization schemes. Here, we use a simplified climate model of intermediate complexity to simulate coastal convection under the influence of the diurnal cycle of solar heating. Convection is parameterized via a stochastic multicloud model (SMCM), which mimics the subgrid dynamics of organized convection due to interactions (through the environment) between the cloud types that characterize organized tropical convection. Numerical results demonstrate that the model is able to capture the key modes of coastal convection variability, such as the diurnal cycle of convection and the accompanying sea and land breeze reversals, the slowly propagating mesoscale convective systems that move from land to ocean and vice-versa, and numerous moisture-coupled gravity wave modes. The physical features of the simulated modes, such as their propagation speeds, the timing of rainfall peaks, the penetration of the sea and land breezes, and how they are affected by the latitudinal variation in the Coriolis force, are generally consistent with existing theoretical and observational studies. [ABSTRACT FROM AUTHOR]

Based on the hourly merged precipitation product, the performance of the European Centre for Medium-Range Weather Forecasts (ECMWF) Integrated Forecasting System (IFS) in simulating the diurnal variations of precipitation during warm season over the western periphery of the Sichuan basin (SCB) has been evaluated, and the underlying physical causes associated with the wet biases have also been investigated. The results show that the IFS well reproduces the spatial distributions of precipitation amount, frequency, and intensity over the SCB, as well as their diurnal variations, but the simulated precipitation peaks earlier than the observation with notable wet biases over the western periphery of the SCB. In addition, the strong wet biases exhibit notable regional differences over the western periphery of the SCB. The simulated wet biases over the southwestern periphery of the SCB expand westward to higher altitudes along the windward slope, with the maximum wet biases occurring at night. The westward expansion of the simulated stronger upward motions results in a westward shift of precipitation. However, the simulated precipitation over the northwestern periphery of the SCB has small difference in terms of the location; hence, the overestimated precipitation is associated with the stronger atmospheric instability, resulting from the higher potential temperature and the larger specific humidity near the surface. The findings revealed in this study indicate that the ECMWF forecast shows distinct uncertainties over the different complex terrain, and thus offers a promising way forward for improvements of model physical processes. [ABSTRACT FROM AUTHOR]

The purpose of this study is to diagnose mesoscale factors responsible for the formation and development of an extreme rainstorm that occurred on 20 July 2021 in Zhengzhou, China. The rainstorm produced 201.9 mm of rainfall in 1 h, breaking the record of mainland China for 1-h rainfall accumulation in the past 73 years. Using 2-km continuously cycled analyses with 6-min updates that were produced by assimilating observations from radar and dense surface networks with a four-dimensional variational (4DVar) data assimilation system, we illustrate that the modification of environmental easterlies by three mesoscale disturbances played a critical role in the development of the rainstorm. Among the three systems, a mesobeta-scale low pressure system (mesolow) that developed from an inverted trough southwest of Zhengzhou was key to the formation and intensification of the rainstorm. We show that the rainstorm formed via sequential merging of three convective cells, which initiated along the convergence bands in the mesolow. Further, we present evidence to suggest that the mesolow and two terrain-influenced flows near the Taihang Mountains north of Zhengzhou, including a barrier jet and a downslope flow, contributed to the local intensification of the rainstorm and the intense 1-h rainfall. The three mesoscale features coexisted near Zhengzhou in the several hours before the extreme 1-h rainfall and enhanced local wind convergence and moisture transport synergistically. Our analysis also indicated that the strong midlevel south/southwesterly winds from the mesolow along with the gravity-current-modified low-level northeasterly barrier jet enhanced the vertical wind shear, which provided favorable local environment supporting the severe rainstorm. [ABSTRACT FROM AUTHOR]

Mesoscale convective systems (MCSs) are the dominant rainfall producer over the United States during the warm season, causing natural disasters and severe weather every year. Global climate models have large uncertainty in projecting precipitation changes in the future climate. Here, a simple Lagrangian parcel model is used to investigate the impact of global warming on MCS initiation and growth. The single-column parcel model projects a mean precipitation decrease over the central United States and an increase to its east, in agreement with the CMIP5 model projection. It also highlights the crucial role of current climate mean-state model bias in exaggerating the change in future mean precipitation projection by 25%. As for convective population, the model captures the decreased occurrence frequency of weak to moderate convection and increased frequency of strong convection due to the increased CAPE and CIN, in agreement with convection-permitting regional simulations. Novel parameterizations of gust-front propagation speed and subsidence strength are developed as guided by cloud-resolving simulations. The multicolumn parcel model employing those parameterizations captures readily the cold pool–induced upscale growth feature. It simulates smaller mesoscale clusters over the central United States under global warming due to gust-front slowdown and subsidence strength enhancement, which are further attributed to land aridity–induced weakening of initial accumulated precipitation and strengthening of updraft speed, respectively. That said, mesoscale clusters could become bigger under more favorable conditions in future climate, including boundary layer moistening, convection lifetime lengthening, and cold pool mechanical-lifting enhancement, which require further investigations to improve mechanistic understanding of future MCS changes. [ABSTRACT FROM AUTHOR]

A study of the vertical structure of postfrontal shallow clouds in the marine boundary layer over the Southern Ocean is presented. The central question of this two-part study regards cloud phase (liquid/ice) of precipitation, and the associated growth mechanisms. In this first part, data from the Measurements of Aerosols, Radiation, and Clouds over the Southern Ocean (MARCUS) field campaign are analyzed, starting with a 75-h case with continuous sea surface-based thermal instability, modest surface heat fluxes, an open-cellular mesoscale organization, and very few ice nucleating particles (INPs). The clouds are mostly precipitating and shallow (tops mostly around 2 km above sea level), with weak up- and downdrafts, and with cloud-top temperatures generally around 2188 to 2108C. The case study is extended to three other periods of postfrontal shallow clouds in MARCUS. While abundant supercooled liquid water is commonly present, an experimental cloudphase algorithm classifies nearly two-thirds of clouds in the 08 to 258C layer as containing ice (cloud ice, snow, or mixed phase), implying that much of the precipitation grows through cold-cloud processes. The best predictors of ice presence are cloud-top temperature, cloud depth, and INP concentration. Measures of convective activity and turbulence are found to be poor indicators of ice presence in the studied environment. The water-phase distribution in this cloud regime is explored through numerical simulations in Part II. [ABSTRACT FROM AUTHOR]

The processes underlying heavy rainfall in the higher elevations of the Himalayas are still not well known despite their importance. Here, we examine the detailed process causing a heavy rainfall event, observed by our rain gauge network in the Rolwaling valley, eastern Nepal Himalayas, using ERA5 and a regional cloud-resolving numerical simulation. Heavy precipitation (112 mm day−1) was observed on 8 July 2019 at Dongang (2790 m above sea level). Most of the precipitation (81 mm) occurred during 1900–2300 local time (LT). The synoptic-scale environment is characterized by a monsoon low pressure system (LPS) over northeastern India. The LPS lifted moisture upward from the lower troposphere and then horizontally transported it into the eastern Nepal Himalayas within the middle troposphere, increasing the content of the water vapor around Dongang. A mesoscale convective system passed over Dongang around the time of the intense precipitation. The numerical simulation showed that surface heat fluxes prevailed under the middle tropospheric (∼500 hPa) southeasterly flow associated with the LPS around a mountain ridge on the upwind side of Dongang until 1900 LT, enhancing convective instability. Topographic lifting led to the release of the enhanced instability, which triggered the development of a mesoscale precipitation system. The southeasterly flow pushed the precipitation system northward, which then passed over Dongang during 2000–2200 LT, resulting in heavy precipitation. Thus, we conclude that the heavy precipitation came from the multiscale processes such as three-dimensional moisture transport driven by the LPS and the diurnal variation in heat fluxes from the land surface. SIGNIFICANCE STATEMENT: Precipitation in the Himalayas is closely related to the hydrological cycle, floods, and landslide disasters in South Asia. Thus, elucidating the features of precipitation in the Himalayas is important. This study explored multiscale processes leading to a heavy precipitation event that was observed on 8 July 2019 at Dongang in the Rolwaling valley of the eastern Nepal Himalayas. We identified new processes producing heavy precipitation in the Himalayas: the three-dimensional synoptic-scale moisture transport driven by a monsoon low pressure system and the effect of the diurnal variation in heat fluxes from the land surface on the development and movement of a mesoscale precipitation system causing heavy precipitation. These findings broaden our understanding of heavy precipitation in the Himalayas. [ABSTRACT FROM AUTHOR]

Based on the Doppler weather radar and surface observations, the key mesoscale systems and features of the rainstorm structure during the period of the extreme precipitation in Henan province on 20 July 2021 are investigated. The results show that a nearly meso-α-scale West Henan Low Vortex (WHLV) near the Songshan Mountain, a surface mesoscale front, a horizontal shear convergence line in the lower troposphere and two strong low-level jets (LLJs) were the main mesoscale systems that triggered the extreme precipitation process. Many mesoscale vortices including meso-β-γ-scale vortices (i.e., meso-vortices) were found within the WHLV. Hourly precipitation over 50 mm was mostly caused by the storms with meso-vortices. In the heaviest precipitation stage of the Zhengzhou Storm (ZZS), a clear meso-vortex above 2 km AGL was identified with the diameter of 15–20 km and the vorticity of 1.0–2.0 × 10−3 s−1, while its lifetime was about 2 h. The low-level ambient airflows converged into the storm from the north, east and south, forming a strong low-level convergence that promoted the development of the storm. Strong convergence and uplift occurred along the east edge of the storm, while the strong easterly LLJ converged with the shallow outflow of the storm. The strongest updraft under 2 km AGL occurred at the northeast end of the storm when a short-lived meso-γ-scale vortex formed at that area. Both the strong low-level convergence and the merge of the convective cells from the east resulted in the eastward propagation of the ZZS. [ABSTRACT FROM AUTHOR]

In the classical model of mesoscale convective systems (MCSs), a system generates new convective cells on the downshear side of its cold pool, with the cells fed at low levels from the front, and the stratiform cloud trailing behind the system in the upshear direction, where "front" and "behind" typically refer to the system's ground-relative velocity. In this study we present an algorithm for identifying and tracking MCSs in radar reflectivity data, and objectively diagnosing organizational characteristics related to the classical model, namely, the offset of stratiform cloud from convective cloud relative to system velocity, the low-level inflow direction, and the shear-relative tilt and propagation directions. When applied to the 15-yr radar record covering the Darwin region of northern Australia, the algorithm indicates that 65%–80% of MCS observations are consistent with the classical model, at least when the four classifications can be made unambiguously. However, these observed characteristics occur almost entirely in the drier phases of the Australian monsoon. During the humid, active monsoon phase, observed characteristics consistent with the classical model are rare, and most systems exhibit nonclassical upshear propagation. Significance Statement: In this study we developed a computer program for tracking large storms in radar data. The program tracks storms through time and space, recording their three-dimensional structure. Knowledge of this structure allows our program to automatically put storms into different, commonly used categories. These categories provide clues about the processes that can make storms grow, persist, or decay. Our computer program is useful because the classification process is very time consuming when done by humans. Also, when applied to radar datasets from northern Australia, our program suggests most large storms are consistent with standard theories of what make storms grow and persist. However, when the air is very humid, large storms inconsistent with standard theories become common. [ABSTRACT FROM AUTHOR]

A constrained data assimilation (CDA) system based on the ensemble variational (EnVar) method and physical constraints of mass and water conservations is evaluated through three convective cases during the Midlatitude Continental Convective Clouds Experiment (MC3E) of the Atmospheric Radiation Measurement (ARM) program. Compared to the original data assimilation (ODA), the CDA is shown to perform better in the forecasted state variables and simulated precipitation. The CDA is also shown to greatly mitigate the loss of forecast skills in observation denial experiments when radar radial winds are withheld in the assimilation. Modifications to the algorithm and sensitivities of the CDA to the calculation of the time tendencies in the constraints are described. [ABSTRACT FROM AUTHOR]

A West Sumatra squall line occurred on 10 January 2016, with a clear offshore propagation of convection. Satellite-derived products from Himawari-8 Advanced Himawari Imager and the Geostationary Cloud Algorithm Testbed Geocat are used to investigate the westward propagation of cloudiness from Sumatra to the Indian Ocean with a lifetime of 1.5 days. A convective mask based on deep convective cell detection and a cell-tracking algorithm are used to estimate the propagation speed of the cloud system. Two distinct mesoscale convective responses are identified: 1) a rapid development in South Sumatra is influenced by the convective environment over the Indian Ocean. The propagation speed is estimated to be ∼5 m s−1 within the first 200 km from the coast. This speed is consistent with density currents. In contrast, 2) the coupling to the inertia–gravity wave is only evident for the northwest of Sumatra with speeds of ∼12 m s−1. The analysis of brightness temperature from the 10.4-μm spectral band and cloud-top temperature showed that the lifetime of the squall line is approximately 30 h with a propagating distance of ∼1000 km. Retrieved cloud properties and tracking of the offshore propagation indicated that the cloud structure consisted of multiple types of cells, propagating as envelopes of convection, and revealed the influence of large-scale variability of the Indian Ocean. Filtered OLR anomalies, satellite-derived rainfall, moisture flux convergence, and background winds flow around Sumatra are used to explore the effects of Kelvin wave activity that likely influenced the lifetime of the squall line. [ABSTRACT FROM AUTHOR]

A new method that automatically determines the modality of an observed particle size distribution (PSD) and the representation of each mode as a gamma function was used to characterize data obtained during the High Altitude Ice Crystals and High Ice Water Content (HAIC-HIWC) project based out of Cayenne, French Guiana, in 2015. PSDs measured by a 2D stereo probe and a precipitation imaging probe for particles with maximum dimension (Dmax) > 55 μm were used to show how the gamma parameters varied with environmental conditions, including temperature (T) and convective properties such as cloud type, mesoscale convective system (MCS) age, distance away from the nearest convective peak, and underlying surface characteristics. Four kinds of modality PSDs were observed: unimodal PSDs and three types of multimodal PSDs (Bimodal1 with breakpoints 100 ± 20 μm between modes, Bimodal2 with breakpoints 1000 ± 300 μm, and Trimodal PSDs with two breakpoints). The T and ice water content (IWC) are the most important factors influencing the modality of PSDs, with the frequency of multimodal PSDs increasing with increasing T and IWC. An ellipsoid of equally plausible solutions in (No–λ–μ) phase space is defined for each mode of the observed PSDs for different environmental conditions. The percentage overlap between ellipsoids was used to quantify the differences between overlapping ellipsoids for varying conditions. The volumes of the ellipsoid decrease with increasing IWC for most cases, and (No–λ–μ) vary with environmental conditions related to distribution of IWC. HIWC regions are dominated by small irregular ice crystals and columns. The parameters (No–λ–μ) in each mode exhibit mutual dependence. [ABSTRACT FROM AUTHOR]

Using coastal radar and surface observations of Taiwan, an investigation of intensity and structure variations in the inner core of Typhoon Haitang (0505) is conducted. Within a 3-h period (1933–2233 UTC 17 July 2005), Haitang experienced intensity vacillation with merging of the eyewall with a rainband induced by the coastal barrier jet (CBJ); concentric eyewall breakdown and weakening; and eyewall recovery, contraction, and re-intensification. The northerly flow of the CBJ converged with the southerly flow of a leeside meso-low to form a west–east line of convection south of the storm when the storm was still 100 km offshore. The rainband propagated radially inward and triggered eyewall–rainband interaction. The interaction resulted in approximately 30% amplification of precipitation and 20% decrease in the axisymmetric tangential wind. Barotropic instability is speculated to be the underlying dynamic process. The recovery of the eyewall, following nearshore eyewall axisymmetrization and contraction, resulted in a 40% intensity increase before landfall. Significance Statement: The behaviors and underlying physical processes of a landfalling tropical cyclone (TC) under the influence of complex terrain are studied by using coastal radars. The TC inner core structure and intensity change, including concentric eyewall breakdown, weakening, eyewall recovery, eyewall contraction, and re-intensification that occurred 3 h before TC made landfall are documented. The terrain-induced coastal barrier jet and leeside meso-low helped to form an intense line convection near the southern fringe of the eyewall and triggered the rainband–eyewall interaction. [ABSTRACT FROM AUTHOR]

This study investigates the organizational modes of convective storms and associated severe weather in spring and summer (March–August) of 2015–19 over southern China. These storms are classified into three major organizational structures (cellular, linear, and nonlinear), including 10 dominant morphologies. In general, cellular systems are most frequent, followed by linear systems. Convective storms are common in spring, increasing markedly from April to June, and peak in June. Convective storm cases are usually longer lived in spring, while shorter lived in summer. They also present pronounced diurnal variations, with a primary peak in the afternoon and several secondary peaks during the night to the morning. Approximately 79.7% of initial convection clearly exhibits a dominant eastward movement, with a faster moving speed in spring. Convective storms frequently evolve among organizational modes during their life spans. Linear systems produce the most severe weather observations, in which convective lines with trailing stratiform rain are most prolific. Bow echoes are most efficient in producing severe weather events among all systems, despite their rare occurrences. In spring, lines with parallel stratiform rain are abundant producers of severe wind events, ranking the second highest probability. In summer, embedded lines produce the second largest proportion of intense rainfall events, whereas lines with leading stratiform rain are most efficient in generating extremely intense rainfall and thus pose a distinct flooding threat. Broken lines produce the largest proportion of severe weather events among cellular storms. In contrast, nonlinear systems possess the weakest capability to produce severe weather events. Significance Statement: Under the influence of the East Asian summer monsoon, severe weather events produced by convective storms occur frequently in China, leading to serious natural disasters. Numerous studies have demonstrated that the morphologies of convective storms are helpful to improve our understanding and prediction of convective storms. However, fewer attempts have been made to examine the convective morphologies over southern China. We aim to reveal the general features of convective organizational modes (e.g., frequencies, durations, variations, etc.) and determine which particular types of severe weather are more or less likely to be associated with particular convective morphologies. These results are of benefit to local forecasters for better anticipating the storm types and issuing warnings for related hazardous weather. [ABSTRACT FROM AUTHOR]

A local long-lived convective system developed at midnight over inland South China, producing record-breaking rainfall in Guangzhou on 7 May 2017. This study examines the physical processes responsible for nocturnal convection initiation (CI) and growth. Observational analyses show that the CI occurs in the warm sector under weakly forced synoptic conditions at 500 hPa, while moderate but nocturnally enhanced low-level southeasterlies with a mesoscale moist tongue at 925 hPa intrude inland from the northern South China Sea. Convection-permitting model results show that mesoscale low-level convergence and increased moisture at the leading edge of the southeasterlies are favorable for CI dynamically and thermodynamically. Local ascent and potential instability are further enhanced by orographic lifting and warm moist air from the urban surface, respectively, which trigger convection in northern Guangzhou. The mesoscale moist tongue of southeasterly flows then meets convectively generated outflows, thereby maintaining strong updrafts and continuously triggering back-building convective cells in eastern Guangzhou. Sensitivity tests are conducted to estimate the relative roles of ambient southeasterly moist tongue and urban thermal effects. The southeasterly moist tongue provides moisture that is crucial for CI, while warm moist air from the urban surface is lifted at the leading edge of the southeasterlies and locally facilitates convection. Therefore, the mesoscale processes of lifting and moistening due to nocturnal southeasterlies and their strong interaction with the local factors (orographic lifting, urban heating, and cold-pool-related ascent) provide the sustained lifting and instability crucial for triggering the local long-lived convective systems. The multiscale processes shed light on the understanding of the nocturnal warm-sector heavy rainfall inland. [ABSTRACT FROM AUTHOR]

A climatology of synoptic drylines on the subtropical southern African interior plateau (SAP) is developed using ERA5 reanalysis specific humidity and surface temperature gradients and an objective detection algorithm. Drylines are found to occur regularly during spring and summer (September–March), and almost daily during December of that period, but rarely in winter. A westward shift in peak dryline frequency takes place through the summer. Drylines peak first over the eastern parts of the SAP during November with a mean of 10 drylines and then over the central (mean of 12) and western SAP (mean of 20) in December. During midsummer, drylines over the eastern SAP are negatively correlated with drylines in the west. Between 1980 and 2020, a significant correlation exists between ENSO and dryline days over the eastern (r = 0.44; p value = 0.004) and central (r = 0.41; p value = 0.008) SAP with fewer drylines (up to 10) occurring during years with increased surface moisture and more drylines (up to 45) occurring during years with decreased surface moisture. Drylines forming over the eastern parts of the SAP were more likely to move westward than drylines over the central and western parts. Onset times across the SAP show that drylines have a tendency to form during either the late morning to early afternoon (1100 and 1400 LST) or during the early evening hours (1700 and 2000 LST), suggesting that the surface heat trough (Kalahari heat low) and westward moisture transport mechanisms, such as the Limpopo low-level jet and ridging highs, are responsible for the formation of most drylines across the SAP. Significance Statement: "Drylines" are used to describe boundaries separating regions of very dry air from those with much higher moisture content. The importance of these drylines is that they tend to act as a trigger for thunderstorms, which can produce severe weather. In this study, we build a long-term climatological description of drylines in subtropical southern Africa. We find that drylines are most frequent over eastern South Africa during the early summer, a time when storms with large hail and damaging winds are most likely to occur. Drylines are sensitive to moisture circulation patterns and respond differently during El Niño and La Niña years, with generally more drylines during El Niño over eastern South Africa and fewer during La Niña. [ABSTRACT FROM AUTHOR]

The degree of improvement in convective representation in NWP with horizontal grid spacings finer than 3 km remains debatable. While some research suggests subkilometer horizontal grid spacing is needed to resolve details of convective structures, other studies have shown that decreasing grid spacing from 3–4 to 1–2 km offers little additional value for forecasts of deep convection. In addition, few studies exist to show how changes in vertical grid spacing impact thunderstorm forecasts, especially when horizontal grid spacing is simultaneously decreased. The present research investigates how warm-season central U.S. simulated MCS cold pools for 11 observed cases are impacted by decreasing horizontal grid spacing from 3 to 1 km, while increasing the vertical levels from 50 to 100 in WRF runs. The 3-km runs with 100 levels produced the deepest and most negatively buoyant cold pools compared to all other grid spacings since updrafts were more poorly resolved, resulting in a higher flux of rearward-advected frozen hydrometeors, whose melting processes were augmented by the finer vertical grid spacing, which better resolved the melting layer. However, the more predominant signal among all 11 cases was for more expansive cold pools in 1-km runs, where the stronger and more abundant updrafts focused along the MCS leading line supported a larger volume of concentrated rearward hydrometeor advection and resultant latent cooling at lower levels. [ABSTRACT FROM AUTHOR]

This study investigates the characteristics of mesoscale convective systems (MCSs) as a function of MCS organizational mode over China, using long-term precipitation radar observations from the Tropical Rainfall Measuring Mission (TRMM) and Global Precipitation Measurement Mission (GPM). The spaceborne radar-based MCS climatology shows maximum population over low-elevation regions, marked decrease over the foothills, and minimum frequency over the Tibetan Plateau. Linear and nonlinear MCSs account for 17% and 83% of the MCSs over China, respectively. Linear MCSs have much stronger convective intensity and heavier precipitation than nonlinear MCSs, as indicated by TRMM convective proxies. Interestingly, though broad-stratiform MCSs have the weakest convection, they produce the heaviest (maximum) rain rate and the largest amount of heavy rainfall among nonlinear MCSs. Among various types of linear MCSs, bow echoes (BEs) and no-stratiform (NS) systems exhibit the strongest convective intensity, embedded lines exhibit the weakest, and convective lines with trailing/leading stratiform in between. BEs and NSs share the most vertically extended structure, strongest microwave ice scattering, and highest lightning flash rates, but NS systems have a much lower surface rain rate likely due to a drier environment. Vertical radar reflectivity profiles suggest that both ice-based and warm-rain processes play an important role in the precipitation processes of linear MCSs over China, including the most intense BE storms. In short, this study helps to better understand the convective organization, precipitation structure, and ensemble microphysical properties of MCSs over China, and potentially provides guidelines for evaluating high-resolution model simulations and satellite rainfall retrievals for monsoonal MCSs. [ABSTRACT FROM AUTHOR]

Intense deep convection and large mesoscale convective systems (MCSs) are known to occur downstream of the Andes in subtropical South America. Deep convection is often focused along the Sierras de Córdoba (SDC) in the afternoon and then rapidly grows upscale and moves to the east overnight. However, how the Andes and SDC impact the life cycle of MCSs under varying synoptic conditions is not well understood. Two sets of terrain-modification experiments using WRF are used to investigate the impact of topography in different synoptic regimes. The first set is run on the 13–14 December 2018 MCS case from RELAMPAGO, which featured a deep synoptic trough, strong lee cyclogenesis near the SDC, an enhanced low-level jet, and rapid upscale growth of an MCS. When the Andes are reduced by 50%, the lee cyclone and low-level jet that develop are weaker than with the full Andes, and the resulting MCS is weak and moves faster to the east. When the SDC are removed, few differences between the environment and resulting MCS relative to the control run are seen. A second set of experiments are run on the 25–26 January 2019 case in which a large MCS developed over the SDC and remained tied there for an extended period under weak synoptic forcing. The experiment that produces the most similar MCS to the control is when the Andes are reduced by 50% while maintaining the height of the SDC, suggesting the SDC may play a more important role in the MCS life cycle under quiescent synoptic conditions. [ABSTRACT FROM AUTHOR]

A theoretical, numerical-modeling-based examination of the sensitivity of vortex sheets along airmass boundaries to the following three characteristics is presented: 1) boundary-normal component of the vertical wind shear, 2) boundary-parallel component of the vertical wind shear, and 3) temperature perturbation within the parent air mass of the boundary. The overall aim of this work is to advance understanding of the sensitivity of micro-α- to meso-γ-scale vortex generation along airmass boundaries to the ambient environment. Density currents are simulated in a 2D domain that does not allow baroclinic generation of near-surface vertical vorticity (ζns) with parameterized latent heating for convection initiated at the associated airmass boundary and Coriolis turned on. Despite the absence of baroclinically generated ζns, with Coriolis turned on and without any boundary-parallel shear, ζns more than two orders of magnitude larger than planetary vorticity is generated along the boundary and located within the cold air. The magnitude of ζns is found to increase with increasing boundary-normal shear with statistically significant intra-experiment separations. Near-surface vertical vorticity ζns is found to scale inversely with boundary-parallel shear with a transition to negative leading-edge ζns in several of the larger boundary-normal shear simulations. An inverse and statistically significant relationship is found between ζns and the temperature perturbation within the parent air mass of the boundary (Δθ), and is a direct consequence of the dependence of boundary propagation speed on Δθ. Significance Statement: Research presented in this article aims to contribute to an improved understanding of the environmental controls on the generation of small-scale vortices along airmass boundaries. Vertical shear, both along and across an airmass boundary, as well as temperature of the air mass on the cool side of an airmass boundary are found to regulate the magnitude of near-surface vertical vorticity available to small-scale vortices. [ABSTRACT FROM AUTHOR]

During the early morning hours of 5 November 2018, a mature mesoscale convective system (MCS) propagated discretely over the second-most populous province of Argentina, Córdoba Province, during the Remote Sensing of Electrification, Lightning, and Mesoscale/Microscale Processes with Adaptive Ground Observations–Cloud, Aerosol, and Complex Terrain Interactions (RELAMPAGO–CACTI) joint field campaigns. Storm behavior was modified by the Sierras de Córdoba, a north–south-oriented regional mountain chain located in the western side of the province. Here, we present observational evidence of the discrete propagation event and the impact of the mountains on the associated physical processes. As the mature MCS moved northeastward and approached the windward side of the mountains, isolated convective cells developed downstream in the mountain lee, 20–50 km ahead of the main convective line. Cells were initiated by an undular bore, which formed as the MCS cold pool moved over the mountain ridge and perturbed the leeside nocturnal, low-level stable layer. The field of isolated cells organized into a new MCS, which continued to move northeastward, while the parent storm decayed as it traversed the mountains. Only the southern portion of the storm propagated discretely, due to variability in mountain height along the chain. In the north, taller mountain peaks prevented the MCS cold pool from moving over the terrain and perturbing the stable layer. Consequently, no bore was generated, and no discrete propagation occurred in this region. To the south, the MCS cold pool was able to traverse the lower-relief mountains, and the discrete propagation was successful. [ABSTRACT FROM AUTHOR]

ow squalls are sudden snow events that last less than 1 h, are characterized by low visibility and gusty winds, and can result in notable societal impacts. This analysis develops a climatology of non-lake-effect snow squall events in southern New England for 1994–2018 and investigates the synoptic environment and mesoscale factors conducive to their formation. National Weather Service surface observations were used to identify events; sea level pressure maps, composite radar charts, and a cell-tracking algorithm were used to determine their organization and movement; and ERA5 hourly reanalysis data were used to analyze the associated synoptic and infer mesoscale features, as well as convective and symmetric instability. A total of 100 events were identified and categorized into four distinct types on the basis of the direction of movement of the associated radar echoes, which is closely linked to characteristic synoptic structures and mesoscale factors. The four types are Classic (squall movement from the northwest; 72 events), Atlantic (from the southwest; 15 events), Northern (from the north; 9 events), and Special (varying; 4 events). All types have a 500-hPa trough over the Northeast but differ in the structure of the trough and its relation to lower-level flow, which accounts for the differences in movement of the squalls. The snow events occur in shallow, convective squall lines, and the ingredients for convection were present in all cases. Both upright and symmetric instability are typically present, all cases had at least one lower-tropospheric layer with cyclonic differential vorticity advection, and many cases were also associated with frontogenesis. [ABSTRACT FROM AUTHOR]

To examine the utility of smartphone pressure observations (SPOs), a climatology of mesoscale pressure features was developed to evaluate whether SPOs could better resolve mesoscale phenomena than existing surface pressure networks (MADIS). A comparison between MADIS and smartphone pressure analyses was performed by tracking and characterizing bandpass-filtered, mesoscale pressure features. Over the year 2018, nearly 3000 pressure features were tracked across the central and eastern United States. Pressure features identified by smartphone observations lasted, on average, 25 min longer, traveled 25 km farther, and exhibited larger amplitudes than features observed by MADIS. An examination of smartphone pressure features tracks by season and location found that almost all pressure features propagated eastward. With over 87% of observed pressure features associated with convection, the climatology of surface pressure features largely reflects the geographic and seasonal variation of mesoscale convection. Phase relationships between pressure features and other surface variables were consistent with those expected for mesohighs and wake lows. These results suggest that SPOs could enhance convective analyses and forecasts compared to existing surface networks like MADIS by better resolving mesoscale structures and features, such as wake lows and mesohighs. Significance Statement: While smartphone pressure networks provide unprecedented observation coverage and density, it was unclear whether they can add value to existing surface pressure networks. This study addresses this question by developing a yearlong record of mesoscale pressure features over the eastern and central United States. Analysis of this record revealed that smartphone analyses better resolved mesoscale pressure features, especially across the central United States where existing surface pressure networks are sparser. Nearly all observed pressure features were observed near precipitation, with five in six associated with convection. Relationships between mesoscale pressure features and other surface state variables were consistent with those expected for mesohighs and wake lows. [ABSTRACT FROM AUTHOR]

Spatial aggregation of deep convection and its possible role in larger-scale atmospheric behavior have received growing attention. Here we seek aggregation-correlated statistical properties of convective events in 5° × 5° boxes over the tropical Indian Ocean. Events are identified by box-averaged rainfall exceeding 5 mm day−1 at the center of a 4-day time window, and aggregation is estimated by an index [simple convective aggregation index (SCAI)] based on contiguous cold cloud areas and their geometrical distances in infrared imagery. A physical framework using gross moist stability (GMS) helps to interpret relationships between aggregation, box-scale ascent profiles, moist static energy budgets, and time evolution both within composite events and on longer time scales. For a given precipitation rate, more-aggregated events (with fewer and larger cloud objects on average) exhibit a drier area mean, greater horizontal gradient of moisture, more bottom-heavy ascent profile, and a greater prevalence of low-altitude cloud tops, especially for lower rain rates. In the GMS budget, this bottom-heavy ascent implies net energy import into the atmospheric column during the 4-day event composite. Consistently, net energy variations filtered to reveal longer time scales do indeed exhibit more-aggregated rain events in their growth phase than in their flat and decaying phases. More-aggregated scenes also have more drying by analysis than less-aggregated scenes in MERRA-2's assimilation budgets. This suggests that parameterized convection (lacking any organization effect) is raining out less water than nature's real, aggregated convection in such scenes. [ABSTRACT FROM AUTHOR]

The mesoscale vortex (MV) is an important rain-producing system. In this study, the reanalysis data and satellite precipitation products are used to classify MVs into three categories: mesoscale convective vortex (MCV), mesoscale stratiform vortex (MSV), and mesoscale dry vortex (MDV). Then, these three categories of midlevel MVs in China from 2007 to 2016 are investigated. A total of 21 053 MVs are obtained. Most MVs form in the northwest of parent convection, and 45% of MVs generate secondary convection. The Tibetan Plateau is the main MV source region. Steered by the westerlies, MVs mainly move eastward. MCV is active in summer, MDV in winter, and MSV in spring and autumn. MCV diurnal variations are closely related to local topography, and MDVs mainly form around midnight. Composite analyses show that MCVs form near the high-value center of convective available potential energy at the development stage of parent convection. The composite MCV forms near the low pressure trough and the thermal ridge at 500 hPa, and a low-level jet exists to the south of the MCV center. At the initiation and maturity stages of MCV, strong convergence and divergence respectively exist at low levels and 400 hPa. The vortex circulation mainly locates near 500 hPa. Above the vortex is a warm core associated with the latent heat release, and below is a cold anomaly related to the cold pool. In the downshear region, there is strong low-level convergence and ascending motion, higher humidity, and greater latent heat release, which favor the formation of secondary convection. [ABSTRACT FROM AUTHOR]

The ability to predict heavy rain and floods in Africa is urgently needed to reduce the socioeconomic costs of these events and increase resilience as climate changes. Numerical weather prediction in this region is challenging, and attention is being drawn to observationally based methods of providing short-term nowcasts (up to ∼6-h lead time). In this paper a freely available nowcasting package, pySTEPS, is used to assess the potential to provide nowcasts of satellite-derived convective rain rate for West Africa. By analyzing a large number of nowcasts, we demonstrate that a simple approach of "optical flow" can have useful skill at 2-h lead time on a 10-km scale and 4-h lead time at larger scales (200 km). A diurnal variation in nowcast skill is observed, with the worst-performing nowcasts being those that are initialized at 1500 UTC. Comparison with existing nowcasts is presented. Such nowcasts, if implemented operationally, would be expected to have significant benefits. Significance Statement: A freely available, easy-to-use nowcasting package has been applied to satellite-retrieved rainfall rates for West Africa, and extrapolations have useful skill at up to 4 h of lead time. [ABSTRACT FROM AUTHOR]

We present a scaling analysis for the stratified turbulent and small-scale turbulent regimes of atmospheric flow with emphasis on the mesosphere. We distinguish rotating-stratified macroturbulence turbulence (SMT), stratified turbulence (ST), and small-scale isotropic Kolmogorov turbulence (KT), and we specify the length and time scales and the characteristic velocities for these regimes. It is shown that the buoyancy scale (Lb) and the Ozmidov scale (Lo) are the main parameters that describe the transition from SMT to KT. We employ the buoyancy Reynolds number and horizontal Froude number to characterize ST and KT in the mesosphere. This theory is applied to simulation results from a high-resolution general circulation model with a Smagorinsky-type turbulent diffusion scheme for the subgrid-scale parameterization. The model allows us to derive the turbulent root-mean-square (rms) velocity in the KT regime. It is found that the turbulent RMS velocity has a single maximum in summer and a double maximum in winter months. The secondary maximum in the winter MLT we associate with a secondary gravity wave–breaking phenomenon. The turbulent rms velocity results from the model agree well with full correlation analyses based on MF-radar measurements. A new scaling for the mesoscale horizontal velocity based on the idea of direct energy cascade in mesoscales is proposed. The latter findings for mesoscale and small-scale characteristic velocities support the idea proposed in this research that mesoscale and small-scale dynamics in the mesosphere are governed by SMT, ST, and KT in the statistical average. Significance Statement: Mesoscale dynamics in the middle atmosphere, which consists of atmospheric turbulence and gravity waves, remains a complex problem for atmospheric physics and climate studies. Due to its high nonlinearity, the mesoscale dynamics together with the small-scale turbulence is the primary source of uncertainties and biases in high-altitude general circulation models (GCM) in the middle atmosphere. We use the stratified turbulence theory and the gravity wave–resolving GCM to characterize different scaling regimes and to define various length, time, and velocity scales, that are relevant for the mesoscale and small-scale dynamical regimes. Our results highlight the importance of stratified turbulence in the mesosphere and lower-thermosphere region. [ABSTRACT FROM AUTHOR]

Parallel stratiform-type mesoscale convective systems (MCS) have been found to comprise less than 20% of the central U.S. MCSs, but parallel stratiform MCSs have been reviewed by few studies and only a handful have been sampled using airborne platforms. This study conducts a detailed review of the 11May 2011 MCS that was observed during the Midlatitude Continental Convective Clouds Experiment. In situ data from the University of North Dakota Cessna Citation II Weather Research Aircraft and the Oklahoma Mesonet are used in conjunction with radar reflectivity data and a multiple-Doppler wind retrieval to present an in-depth analysis of the kinematic and microphysical processes within this hybrid parallel-trailing stratiform MCS. Results suggest the MCS started with parallel stratiform characteristics and then developed trailing stratiform features over the course of a couple of hours such as leading line convection and the presence of rear inflow, which is a feature not observed in previous studies of parallel stratiform MCSs. Based on surface observations and multi-Doppler wind field retrievals, the cold pool influenced the 11 May 2011 MCS and may have helped it to transition toward the trailing stratiform mode. Within the stratiform region, in situ measurements show hydrometeor aggregation and sublimation occurring above the melting layer via a faster rate of decline in total hydrometer concentration compared to the total water content, the presence of a bimodal droplet size spectra, and increasing dispersion and decreasing slope, with respect to increasing temperature in both the unimodal and bimodal gamma distributions. [ABSTRACT FROM AUTHOR]

An aerosol indirect effect on deep convective cores (DCCs), by which increasing aerosol concentration increases cloud-top height via enhanced latent heating and updraft velocity, has been proposed in many studies. However, the magnitude of this effect remains uncertain due to aerosol measurement limitations, modulation of the effect by meteorological conditions, and difficulties untangling meteorological and aerosol effects on DCCs. The Cloud, Aerosol, and Complex Terrain Interactions (CACTI) campaign in 2018–19 produced concentrated aerosol and cloud observations in a location with frequent DCCs, providing an opportunity to examine the proposed aerosol indirect effect on DCC depth in a rigorous and robust manner. For periods throughout the campaign with well-mixed boundary layers, we analyze relationships that exist between aerosol variables (condensation nuclei concentration > 10 nm, 0.4% cloud condensation nuclei concentration, 55–1000-nm aerosol concentration, and aerosol optical depth) and meteorological variables [level of neutral buoyancy (LNB), convective available potential energy, midlevel relative humidity, and deep-layer vertical wind shear] with the maximum radar-echo-top height and cloud-top temperature (CTT) of DCCs. Meteorological variables such as LNB and deep-layer shear are strongly correlated with DCC depth. LNB is also highly correlated with three of the aerosol variables. After accounting for meteorological correlations, increasing values of the aerosol variables [with the exception of one formulation of aerosol optical depth (AOD)] are generally correlated at a statistically significant level with a warmer CTT of DCCs. Therefore, for the study region and period considered, increasing aerosol concentration is mostly associated with a decrease in DCC depth. [ABSTRACT FROM AUTHOR]

A valid time shifting (VTS) method is explored for the GSI-based ensemble variational (EnVar) system modified to directly assimilate radar reflectivity at convective scales. VTS is a cost-efficient method to increase ensemble size by including subensembles before and after the central analysis time. Additionally, VTS addresses common time and phase model error uncertainties within the ensemble. VTS is examined here for assimilating radar reflectivity in a continuous hourly analysis system for a case study of 1–2 May 2019. The VTS implementation is compared against a 36-member control experiment (ENS-36), to increase ensemble size (3 × 36 VTS), and as a cost-savings method (3 × 12 VTS), with time-shifting intervals τ between 15 and 120 min. The 3 × 36 VTS experiments increased the ensemble spread, with largest subjective benefits in early cycle analyses during convective development. The 3 × 12 VTS experiments captured analysis with similar accuracy as ENS-36 by the third hourly analysis. Control forecasts launched from hourly EnVar analyses show significant skill increases in 1-h precipitation over ENS-36 out to hour 12 for 3 × 36 VTS experiments, subjectively attributable to more accurate placement of the convective line. For 3 × 12 VTS, experiments with τ ≥ 60 min met and exceeded the skill of ENS-36 out to forecast hour 15, with VTS-3 × 12τ90 maximizing skill. Sensitivity results demonstrate preference to τ = 30–60 min for 3 × 36 VTS and 60–120 min for 3 × 12 VTS. The best 3 × 36 VTS experiments add a computational cost of 45%–67%, compared to the near tripling of costs when directly increasing ensemble size, while best 3 × 12 VTS experiments save about 24%–41% costs over ENS-36. Significance Statement: The purpose of this work is to study a valid time shifting method to improve the prediction of severe convective storm systems over the continental United States. This method improves ensemble-based radar reflectivity analyses by including ensemble member information at times before and after the analysis time, thereby increasing the ensemble size at just a fractional added computational cost. The results show the method can boost the accuracy of high-resolution convection prediction out to at least 12 h. This case study motivates future systematic testing in a real-time setting and potential implementation to enhance the U.S. operational ensemble-based convection-allowing forecast model and data assimilation system. [ABSTRACT FROM AUTHOR]

Atmospheric rivers (ARs), tropical storms (TSs), and mesoscale convective systems (MCSs) are important weather phenomena that often threaten society through heavy precipitation and strong winds. Despite their potentially vital role in global and regional hydrological cycles, their contributions to long-term mean and extreme precipitation have not been systematically explored at the global scale. Using observational and reanalysis data, and NOAA's Geophysical Fluid Dynamics Laboratory's new high-resolution global climate model, we quantify that despite their occasional (13%) occurrence globally, AR, TS, and MCS days together account for ∼55% of global mean precipitation and ∼75% of extreme precipitation with daily rates exceeding its local 99th percentile. The model reproduces well the observed percentage of mean and extreme precipitation associated with AR, TS, and MCS days. In an idealized global warming simulation with a homogeneous SST increase of 4 K, the modeled changes in global mean and regional distribution of precipitation correspond well with changes in AR/TS/MCS precipitation. Globally, the frequency of AR days increases and migrates toward higher latitudes while the frequency of TS days increases over the central Pacific and part of the south Indian Ocean with a decrease elsewhere. The frequency of MCS days tends to increase over parts of the equatorial western and eastern Pacific warm pools and high latitudes and decreases over most part of the tropics and subtropics. The AR/TS/MCS mean precipitation intensity increases by ∼5% K−1 due primarily to precipitation increases in the top 25% of AR/TS/MCS days with the heaviest precipitation, which are dominated by the thermodynamic component with the dynamic and microphysical components playing a secondary role. [ABSTRACT FROM AUTHOR]

One of the most prolific tornado outbreaks ever documented occurred on 26–27 April 2011 and comprised three successive episodes of tornadic convection that culminated with the development of numerous long-track, violent tornadoes over the southeastern United States during the afternoon of 27 April. This notorious afternoon supercell outbreak was preceded by two quasi-linear convective systems (hereinafter QLCS1 and QLCS2), the first of which was an anomalously severe nocturnal system that rapidly grew upscale during the previous evening. Here in Part II, we use a series of RUC 1-h forecasts and output from convection-permitting WRF-ARW simulations configured both with and without latent heat release to investigate how environmental modifications and upscale feedbacks produced by the two QLCSs contributed to the evolution and exceptional severity of this multiepisode outbreak. QLCS1 was primarily responsible for amplifying the large-scale flow pattern, inducing two upper-level jet streaks, and promoting secondary surface cyclogenesis downstream from the primary baroclinic system. Upper-level divergence markedly increased after QLCS1 developed, which yielded strong isallobaric forcing that rapidly strengthened the low-level jet (LLJ) and vertical wind shear over the warm sector and contributed to the system's upscale growth and notable severity. Moreover, QLCS2 modified the mesoscale environment prior to the supercell outbreak by promoting the downstream formation of a pronounced upper-level jet streak, altering the midlevel jet structure, and furthering the development of a highly ageostrophic LLJ over the Southeast. Collectively, the flow modifications produced by both QLCSs contributed to the notably favorable shear profiles present during the afternoon supercell outbreak. Significance Statement: The tornado outbreak that impacted the United States on 26–27 April 2011 was part of an extended outbreak that produced 343 tornadoes and numerous fatalities. This paper is Part II of a study that describes the meteorological factors supporting such a prolific event. Herein we investigate the convectively forced environmental modifications that occurred during a 36-h period encompassing three successive convective episodes. The first two episodes collectively altered the upper-level flow pattern and markedly enhanced low-level winds throughout the warm sector. These modifications served as upscale feedbacks that contributed to the first episode's exceptional severity and to the remarkable vertical shear profiles that supported numerous long-track and violent tornadoes during the final episode on the afternoon of 27 April. [ABSTRACT FROM AUTHOR]