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The onset, evolution, and propagation processes of convective cells can be reflected by the organizational morphology of mesoscale convective systems (MCSs), which are key factors in determining the potential for heavy precipitation. This paper proposed a method for objectively classifying and segmenting MCSs using geosynchronous satellite observations. Validation of the product relative to the classification in radar composite reflectivity imagery indicates that the algorithm offers skill for discriminating between convective and stratiform areas and matched 65% of convective area identifications in radar imagery with a false alarm rate of 39% and an accuracy of 94%. A quantitative evaluation of the similarity between the structures of 50 MCSs randomly obtained from satellite and radar observations shows that the similarity was as high as 60%. For further testing, the organizational modes of the MCS that caused the heavy precipitation in Northwest China on August 21, 2016 (hereinafter known as the "0821" rainstorm) were identified. It was found that the MCS, accompanied by the "0821" rainstorm, successively exhibited modes of the isolated cell, squall line with parallel stratiform (PS) rain, and non-linear system during its life cycle. Among them, the PS mode might have played a key role in causing this flooding. These findings are in line with previous studies. [ABSTRACT FROM AUTHOR]

The formation mechanisms of the record-breaking rainfall event during the Dragon Boat Rainy Season (DBRS) of 2022 are comprehensively analyzed from the synoptic scale and the mesoscale perspectives. The extreme rainfall event is characterized by the highest rainfall amount since 1981, and an abnormal spatial distribution with much higher (lower) rainfall amount in the northern (southern) part of South China. The abnormal circulation and thermodynamic conditions are mainly responsible for the extreme rainfall. The favorite synoptic condition for rainfall is the combination of warm advection, frontal forcing, orographic lifting and low-level jet favor the convection development. The similar configurations repeatedly impact South China during the DBRS of 2022, causing multiple heavy rainfall events, leading to the extreme rainfall of the whole period. The abnormal moisture convergence together with the frontal zone, which is stronger than the climatology, results in the rainfall centers over the northern part of South China. 54.35% of the rainfall amount is related to mesoscale convective systems (MCSs) which mainly originate from four regions. The MCSs from the four regions are characterized by different formation peaks, spatial scales, lifetimes and propagations. The large-scale warm and moist air mass, the moistening caused by synoptic advection and the local diabatic heating are responsible for the increasing instability for the MCSs. The low-level jets play an important role in the formation of MCSs by providing moisture. The thermodynamic (dynamic) environmental conditions control the formation of MCSs in the afternoon (night). [ABSTRACT FROM AUTHOR]

The limited capabilities of global climate models in simulating mesoscale convective systems (MCSs) restrict our understanding of how global warming impacts MCSs. This study uses a high‐resolution numerical model with large‐ensemble experiments to simulate MCSs during the record‐breaking extreme rainfall event in Henan Province, China, in July 2021. We compare the changes in the MCS's strength, size, and structure in a real‐world simulation (RW) and a 0.8°C colder simulation (analog to no‐anthropogenic‐warming‐world simulation, short for NAWW) to assess the response of MCSs to global warming. Our results show that the total rainfall from the MCS increased by 10.0% in RW compared to NAWW, with a 3.1% increase in area and a 6.7% increase in rainfall intensity. The development of MCS becomes more rapid in response to warming since the pre‐industrial era. The warmer and wetter climate results in higher convective available potential energy, and accelerates the MCS growth, but then the narrower low‐convective inhibition regions suppress the continuous growth of MCS. During the mature phase, the maximum hourly rainfall intensity (Pmax) can increase by up to 26.5%/K, while Pmax locations can either remain unchanged or shift depending on the interaction of flows and terrains. These results highlight varying responses of MCSs to global warming during its different stages and provide valuable insights into the changing characteristics of extreme rainfall events under global warming. Plain Language Summary: Mesoscale convective systems (MCSs) play a crucial role in generating heavy rainfall during severe rainfall events, such as the record‐breaking rainfall occurred in Henan Province, China, in July 2021. Here, we investigate whether the MCSs during this extreme event are intensified due to global warming. We use a numerical model to simulate the MCSs under two different scenarios—the current climate background and a 0.8°C colder climate, which analog to the pre‐industrial climate. Our results indicate that the total rainfall from MCSs increased by 10.0% due to global warming, with a 3.1% larger MCSs area and a 6.7% stronger hourly rainfall intensity. Global warming also leads to a faster development of MCSs by providing favorable warmer and wetter atmosphere. When the MCSs are mature, the maximum hourly rainfall intensity can increase by up to 26.5% per °C. The center of MCSs could remain unmoved or shift, depending on the interaction between low‐level flow and terrain. This study illustrates that MCSs can produce much stronger rainfall under global warming with varying changes during the developing and mature phases. These results provide deeper insight into the impacts of global warming to extreme rainfall events. Key Points: Global warming intensifies the mesoscale convective system (MCS) during the record‐breaking extreme rainfall event in July 2021 in Henan, ChinaGlobal warming leads to a faster growth of MCS during its developing stageThe interaction between terrains and moist flow may lead to different responses of rainfall centers to warming [ABSTRACT FROM AUTHOR]

The historic 22–26 May 2015 flood event in Texas and Oklahoma was caused by anomalous clustered mesoscale convective systems (MCSs) that produced record‐breaking rainfall and $3 billion of damage in the region. A month‐long regional convection‐permitting simulation is conducted to reconstruct multiple clustered MCSs that lead to this flood event. We further use the pseudo global warming approach to examine how a similar event may unfold in a warmer climate and the driving physical factors for the changes. Tracking of MCSs in observations and simulations shows that the historical simulation reproduces the salient characteristics of the observed MCSs. In a warmer climate under a high‐emission (SSP5‐8.5) scenario, the Southern Great Plains is projected to experience a near surface warming of 4–6 K, accompanied by enhanced moisture transport by the strengthened Great Plains low‐level jet. A warmer and moister lower troposphere leads to 36%–59% larger convective available potential energy, supporting wider and more intense convective updrafts and rainfall production. Consistently, MCSs have wider convective areas and stronger rainfall intensities, producing 50% larger rain volumes during the mature stage. Extreme (99.5%) MCS rainfall frequency and amount will increase by threefold. However, MCS stratiform rain area decreases as a result of elevated stratiform cloud bases that lead to stronger sublimation and evaporation of precipitation in response to warming, resulting in reduced weak‐to‐moderate surface precipitation. Results suggest that global warming greatly increases precipitation intensity of clustered MCS events under strong synoptic influence, with much higher potential to produce serious floods without additional climate adaptation. Plain Language Summary: Changes in extreme rainfall produced by thunderstorms in a warmer climate have significant implications to society. However, their projections are highly uncertain due to the inability of the coarse‐resolution global climate models to simulate key processes in thunderstorms. Here, we use a kilometer‐scale regional model to examine how the record‐breaking May 2015 flood event in Texas and Oklahoma may unfold in a warmer climate. By tracking the organized thunderstorms that produced extreme rainfall in observations and simulations, we found that the model reproduced many key characteristics of the observed storms. In a warmer climate where the Great Plains warm by 4–6 K (a low‐climate‐mitigation scenario), these storms produce 50% more total rainfall and the extreme precipitation increase by threefold as a result of the warmer atmosphere that contain more moisture and larger convective instability. A sharpening of the storm is found where wider and stronger convective vertical motions produce more heavy rainfall while areas with weaker rainfall decrease. These results suggest if similar events were to occur in a warmer climate, the probability of more severe flash flooding may increase. Current flood control and mitigation strategies may need to adapt to the possible strengthening of organized thunderstorms in future climate. Key Points: The historic flood‐producing mesoscale convective systems are reproduced in simulations to examine their changes in a warmer climateWarming strengthens the Great Plains low‐level jet to transport warmer and more unstable air that fuels stronger convective stormsStorm systems have wider and stronger updrafts, producing a threefold increase in extreme precipitation frequency with warming [ABSTRACT FROM AUTHOR]

This study investigates the importance of convective systems for extreme rainfall along the northern coast of the Gulf of Guinea (GG) and their relationship with atmospheric and oceanic conditions. Convective system data (MCSs), daily precipitation, sea surface temperature (SST) and moisture flux anomalies from June to September 2007–2016 are used. The results show that 2/3 of MCSs crossing Abidjan are produced in June, which is the core of the major rainy season. Likewise, 2/3 of MCSs originate from continental areas, while 1/3 come from the ocean. Oceanic MCSs are mostly initiated close to the coast, which also corresponds to the Marine Heat Waves region. Continental MCSs are mostly initiated inland. The results also highlight the moisture flux contribution of three zones which have an impact on the onset and the sustaining of MCSs: (i) the seasonal migration of the intertropical convergence zone (ITCZ), (ii) the GG across the northern coastline, and finally (iii) the continent. These contributions of moisture fluxes coincide with oceanic warming off Northeast Brazil and the northern coast of the GG both two days before and the day of extreme rainfall events. The ocean contributes to moisten the atmosphere, and therefore to supply and sustain the MCSs during their lifecycle. [ABSTRACT FROM AUTHOR]

Mesoscale convective systems (MCSs) are crucial in modifying the water cycle and frequently induce high‐impact weather events over eastern China. Radar and Climate Prediction Center (CPC)‐4 km satellite‐derived infrared cloud top temperature (Tb) data were used to thoroughly analyze the long‐term climatology of MCSs over eastern China, particularly in the Yangtze–Huaihe River Basin (YHR) in the warm season from 2013 to 2018. For the first time, we contrasted the effects of data set selection and threshold setting on research outcomes. The large‐scale environments of MCSs initiation were also investigated using the latest global reanalysis data ERA5. It is found that striction of thresholds, including duration, reflectivity/Tb, area, and linearity, would lead to a greater proportion of early‐morning MCSs. Satellite‐identified MCSs differed from radar‐derived ones, exhibiting afternoon diurnal peaks, faster movement speeds, longer travel distances, and expansive impact areas. The center of MCS and related precipitation shifted northward from Pre‐Meiyu to Post‐Meiyu seasons, contributing to up to 20% of total rainfall, with most MCSs moving along eastward trajectories. MCSs typically had the most substantial impact in the Meiyu season because of the most prolonged duration, largest convective core area, and strongest precipitation intensity. Warm‐season MCSs initiated ahead of midlevel troughs and were related to strong anomalous low‐level convergence and midlevel upward. The circulation anomalies were the strongest in the Pre‐Meiyu season among the three subseasons, with most moisture sourced from the southwest. Plain Language Summary: This work used radar and satellite data to analyze the climate characteristics of a kind of large thunderstorms known as mesoscale convective systems (MCSs) in the Yangtze–Huaihe River Basin (YHR) in eastern China. These systems cover large areas, lasting from a few hours to a few days. The analysis revealed that stricter tracking thresholds would lead to more early‐morning MCSs. Satellite‐identified MCSs differed from those identified by radar. They tended to have more thunderstorms in the afternoon, move faster, travel longer distances, and cover larger areas. MCSs also contributed to up to 20% of the total warm‐season rainfall in the YHR. MCSs typically had the most substantial impact in the rainy season of eastern China from mid‐June to mid‐July. Warm‐season MCSs initiated ahead of low‐pressure systems and were related to strong anomalous low‐level convergence and midlevel upward motion. During the time before the rainy season, the large‐scale forcing needed to be anomaly enough to trigger MCS formation. Most of the moisture that caused MCSs came from southwest direction instead of stemming locally during this period. Key Points: Six‐year climatology of warm‐season mesoscale convective systems (MCSs) in Yangtze–Huaihe River Basin is investigated using satellite, radar data, and ERA5Criteria setting for tracking MCSs noticeably influences the derived featuresMeiyu season MCSs featured longest duration, largest areas, and heaviest precipitation, while the background circulation anomalies are the weakest [ABSTRACT FROM AUTHOR]

During the night of March 31st, 2017, a severe precipitation event affected the city of Mocoa, in the tropical Andes. Total 24-h accumulated precipitation during that day was the fourth largest on record. Satellite data shows that the event was associated with a Mesoscale Convective System (MCS) that formed over the Amazon and moved westward, reaching the tropical Andes. Reanalysis data suggests that a rapid intensification of the Orinoco Low-Level Jet (OLLJ) traveling southwestward parallel to the Andes was a precursor that favored the zones of convergence for MCS formation. Upstream intensification of the OLLJ was evident 8 h prior to the Mocoa precipitation event. Given the lack of a dense network of observations in this understudied region, we use the Weather Research and Forecasting model (WRF) to explore the plausible mesoscale structures in the OLLJ region associated with the initiation and development of the MCS. We study an ensemble of simulations with different grid spacings (12, 4 and 1.3 km) and Planetary Boundary Layer (PBL) schemes (YSU, MYNN and QNSE). The more realistic MCSs were obtained with the QNSE and YSU schemes, given that the corresponding simulations included a density current in the lowest levels moving parallel to the Andes, with a sharp line of convergence and large vertical velocities over the leading edge of the mesoscale disturbance. In contrast, the MYNN scheme produced a weaker OLLJ and no density current. It is suggested that the stronger vertical mixing in the MYNN scheme was associated with the vertical dilution of the OLLJ, and with a much weaker low-level traveling perturbation via the upward radiation of energy by gravity waves. Our results help to better understand flood-producing extreme events over the poorly studied Andes-Amazon region and provide the groundwork for improved predictability of such storms. [ABSTRACT FROM AUTHOR]

When the negatively buoyant air in the cloud downdrafts reaches the surface, it spreads out horizontally, producing cold pools. A cold pool can trigger new convective cells. However, when combined with the ambient vertical wind shear, it can also connect and upscale them into large mesoscale convective systems (MCS). Given the broad spectrum of scales of the atmospheric phenomenon involving the interaction between cold pools and the MCS, a parameterization was designed here. Then, it is coupled with a classical convection parameterization to be applied in an atmospheric model with an insufficient spatial resolution to explicitly resolve convection and the sub‐cloud layer. A new scalar quantity related to the deficit of moist static energy detrained by the downdrafts mass flux is proposed. This quantity is subject to grid‐scale advection, mixing, and a sink term representing dissipation processes. The model is then applied to simulate moist convection development over a large portion of tropical land in the Amazon Basin in a wet and dry‐to‐wet 10‐days period. Our results show that the cold pool edge parameterization improves the organization, longevity, propagation, and severity of simulated MCS over the Amazon and other different continental areas. Plain Language Summary: In nature, cold pools are formed by cold air masses descending from the low to mid‐troposphere in thunderstorms. When these drafts reach the surface, they spread out horizontally. A manifestation of cold pools is the relatively high speed at the gust front, which can lift environmental air producing new convective cells. Moreover, depending on ambient conditions, the cold pools may help organize the new convective cells, increasing their aggregation and forming the so‐called mesoscale convective systems (MCSs). MCSs, which cover hundreds to thousands of km2, significantly impact the global scale circulation, energy budget, hydrological cycle, and population safety. Forecasting MCSs is challenging for global circulation models (GCM) due to the broad spectrum of scales of the involved atmospheric phenomenon. The computational limitations, at present and for some time to come, do not allow running in real‐time GCMs, which explicitly solves all relevant scales of motion. This paper describes a methodology to account for essential interplays between cold pools edges and moist convection to be applied in the GCMs of weather and climate forecasting. We show that the method improves the model simulation of the main types of MCSs over the Amazon Basin and other continental areas. Key Points: A model scheme for including effect of cold pools edges in triggering new convective cells and storm propagation is presentedThe scheme is coupled with a convection parameterization and applied in the modeling of moist convective systemsThe method improves the organization, longevity, propagation, and severity of the simulated mesoscale convective systems [ABSTRACT FROM AUTHOR]

Twenty years of IMERG precipitation estimates are used to evaluate the contributions of mesoscale convective system (MCS) rainfall to total rainfall in the Congo Basin. Studying these systems advances our basic understanding of Congo Basin rainfall on all time scales. The seasonality of MCS rainfall in the Congo Basin follows the seasonality of total rainfall with high rainfall in spring, summer, and fall and a winter dry season in each hemisphere. In the equinoctial seasons, MCS rainfall accounts for ≥ 80% of total rainfall within 5° of the equator with the highest rainfall rates occurring along the eastern and western boundaries of the basin. In boreal summer, MCS rainfall maxima occur near the Cameroon Highlands (9°–18°E) and in boreal winter, they occur along the eastern orography (22°–28°E). The 80% percent contribution is sustained in the continental interior (15°–25°E, 5°S–5°N) throughout the year. The diurnal cycle of MCS rainfall is similar to that of total rainfall. Diurnal cycles are unimodal in the equinoctial seasons but are regionally and seasonally inhomogeneous in the solstitial seasons. Regardless of modality, MCS rainfall is highest at 15Z (1600/1700 LT) and lowest at 10Z. MCS percent contribution changes little throughout the diurnal cycle but is highest (≥ 90%) at 04Z close to the continental interior. Larger MCSs contribute their greatest percentage of MCS rainfall (83–92%) between 04Z and 07Z, while more-intensely precipitating MCSs have no seasonally or regionally consistent diurnal cycle. Seasonal and diurnal MCS rainfall maxima are associated with unstable MSE profiles in the lower troposphere. Changes in moisture drive the seasonal cycle of MSE while changes in temperature drive its diurnal cycle. [ABSTRACT FROM AUTHOR]

In December 2021, Super Typhoon Rai caused significant devastation to the South Philippines and East Malaysia. In the meantime, an unprecedented flood event occurred in Peninsular Malaysia at 2,000 km west of the typhoon's path, causing comparable socioeconomic impacts as Rai. Record‐breaking 3‐day precipitation was received by Peninsular Malaysia during 16–18 December. Based on the storm tracking results, this study identified two mesoscale convective systems (MCSs) that were directly responsible for the flooding. The two MCSs were directly initiated by a tropical depression and sustained by an elongated easterly water vapor corridor originating from the Super Typhoon Rai. The return period and joint frequency analysis of key drivers indicate that the 3‐day downpour was more severe than a "once‐in‐a‐century" event. Historical records suggest such anomalous moisture channel has become more frequent in Southeast Asia, which alarms heightened attention in forecasting winter flood. Plain Language Summary: On 16–18 December, Peninsular Malaysia received a record‐shattering 3‐day precipitation, resulting in catastrophic socioeconomic impacts. Due to the temporal coincidence with Super Typhoon Rai but far away in space, there were speculations that there might be a teleconnection between the two events. Our results reveal that their relationship could be more straightforward. Based on the analyses of storm tracking database and synoptic data records, we found that two consecutive mesoscale convective systems were responsible for the heavy precipitation, which were produced by a tropical depression that hovered over the peninsula. Meanwhile, Super Typhoon Rai provided a long‐range water vapor transport, akin to adding fuel (i.e., moisture) to the engine (i.e., the tropical depression), and therefore, the precipitation over the peninsula was significantly enhanced. Such long‐range moisture transport has become more frequent during the boreal winter season, posing an increasing risk of flooding in Southeast Asia. Key Points: A stretched moisture channel from Typhoon Rai and a strong tropical depression are key synoptic drivers for the flooding eventReturn period and joint probability of key drivers indicate that the 2021 Peninsular Malaysia flood was more severe than "once‐in‐a‐century"There is an increasing trend in such anomalous moisture channel, suggesting a rising risk of severe flooding in Southeast Asia [ABSTRACT FROM AUTHOR]

The monitoring of mesoscale convective systems (MCS) is typically based on satellite infrared data. Currently, there is limited research on the identification of MCS using true color composite cloud imagery. In this study, an MCS dataset was created based on the true color composite cloud imagery from the Fengyun-4B geostationary meteorological satellite. An MCS true color composite cloud imagery identification model was developed based on the Swin-Unet network. The MCS dataset was categorized into continental MCS and oceanic MCS, and the model's performance in identifying these two different types of MCS was examined. Experimental results indicated that the model achieved a recall rate of 83.3% in identifying continental MCS and 86.1% in identifying oceanic MCS, with a better performance in monitoring oceanic MCS. These results suggest that using true color composite cloud imagery for MCS monitoring is feasible, and the Swin-Unet network outperforms traditional convolutional neural networks. Meanwhile, we find that the frequency and distribution range of oceanic MCS is larger than that of continental MCS, and the area is larger and some parts of it are stronger. This study provides a novel approach for satellite remote-sensing-based MCS monitoring. [ABSTRACT FROM AUTHOR]

In the early morning of May 22, 2020, an extremely heavy rainfall process occurred in the Guangdong-Hong Kong-Macao Greater Bay Area. The maximum hourly and three-hour rainfall broke the historical record during the "Dragon Boat Water" period in Guangdong province. This paper uses multi-source observation data and ERA5 reanalysis data to study the evolution process and mesoscale characteristics of the mesoscale convective system (MCS) causing the local extreme precipitation. The results are shown as follows. (1) The heavy precipitation process mainly concentrated in the early morning hours, with the characteristics of short duration, extreme local rainfall intensity, and large accumulated rainfall. Two precipitation peaks appeared in the three-hour precipitation process. (2) This process is a torrential rain process in the warm sector. A low-level southwest monsoon provides sufficient warm and humid water vapor, and a low vortex shear line provides good uplifting conditions. The process of meso-α-scale monsoon cloud clusters was quasi-stationary and grew by upscaling into MCC, triggering the local nighttime extreme precipitation in the Greater Bay Area. (3) Two stages of peak precipitation in the process is closely related to the propagation of meso-γ-scale convective cells in the life cycle and the change of the motion vector. Several meso-γ-scale convective cells form a multi-cell storm pattern, which is arranged in the northwest-southeast lateral direction. These cells are triggered to develop successively and have their own ground convergence lines. The first stage of peak precipitation is caused by the convective cell moving to the southeast under the guidance of the ambient wind. Subsequently, the convergence lines of different cells are connected, and the propagation of convection to the southwest is significantly accelerated, which changes the movement vector of the convective system, thus causing the peak precipitation in the second stage. (4) There are obvious differences in the structure of the meso-γ-scale convective cells that cause the two periods of peak precipitation. The strong echoes of the peak precipitation in the first stage have a higher extension height with supercell-like structural characteristics such as inclination, inflow gap, and echo overhang, and the echoes of the second stage of peak precipitation are characterized by convective, low mass cores, causing more extreme and localized short-duration heavy precipitation. [ABSTRACT FROM AUTHOR]

Abstract When the negatively buoyant air in the cloud downdrafts reaches the surface, it spreads out horizontally, producing cold pools. A cold pool can trigger new convective cells. However, when combined with the ambient vertical wind shear, it can also connect and upscale them into large mesoscale convective systems (MCS). Given the broad spectrum of scales of the atmospheric phenomenon involving the interaction between cold pools and the MCS, a parameterization was designed here. Then, it is coupled with a classical convection parameterization to be applied in an atmospheric model with an insufficient spatial resolution to explicitly resolve convection and the sub‐cloud layer. A new scalar quantity related to the deficit of moist static energy detrained by the downdrafts mass flux is proposed. This quantity is subject to grid‐scale advection, mixing, and a sink term representing dissipation processes. The model is then applied to simulate moist convection development over a large portion of tropical land in the Amazon Basin in a wet and dry‐to‐wet 10‐days period. Our results show that the cold pool edge parameterization improves the organization, longevity, propagation, and severity of simulated MCS over the Amazon and other different continental areas.

During the night of March 31st, 2017, a severe precipitation event affected the city of Mocoa, in the tropical Andes. Total 24-h accumulated precipitation during that day was the fourth largest on record. Satellite data shows that the event was associated with a Mesoscale Convective System (MCS) that formed over the Amazon and moved westward, reaching the tropical Andes. Reanalysis data suggests that a rapid intensification of the Orinoco Low-Level Jet (OLLJ) traveling southwestward parallel to the Andes was a precursor that favored the zones of convergence for MCS formation. Upstream intensification of the OLLJ was evident 8 h prior to the Mocoa precipitation event. Given the lack of a dense network of observations in this understudied region, we use the Weather Research and Forecasting model (WRF) to explore the plausible mesoscale structures in the OLLJ region associated with the initiation and development of the MCS. We study an ensemble of simulations with different grid spacings (12, 4 and 1.3 km) and Planetary Boundary Layer (PBL) schemes (YSU, MYNN and QNSE). The more realistic MCSs were obtained with the QNSE and YSU schemes, given that the corresponding simulations included a density current in the lowest levels moving parallel to the Andes, with a sharp line of convergence and large vertical velocities over the leading edge of the mesoscale disturbance. In contrast, the MYNN scheme produced a weaker OLLJ and no density current. It is suggested that the stronger vertical mixing in the MYNN scheme was associated with the vertical dilution of the OLLJ, and with a much weaker low-level traveling perturbation via the upward radiation of energy by gravity waves. Our results help to better understand flood-producing extreme events over the poorly studied Andes-Amazon region and provide the groundwork for improved predictability of such storms.

Abstract In December 2021, Super Typhoon Rai caused significant devastation to the South Philippines and East Malaysia. In the meantime, an unprecedented flood event occurred in Peninsular Malaysia at 2,000 km west of the typhoon's path, causing comparable socioeconomic impacts as Rai. Record‐breaking 3‐day precipitation was received by Peninsular Malaysia during 16–18 December. Based on the storm tracking results, this study identified two mesoscale convective systems (MCSs) that were directly responsible for the flooding. The two MCSs were directly initiated by a tropical depression and sustained by an elongated easterly water vapor corridor originating from the Super Typhoon Rai. The return period and joint frequency analysis of key drivers indicate that the 3‐day downpour was more severe than a “once‐in‐a‐century” event. Historical records suggest such anomalous moisture channel has become more frequent in Southeast Asia, which alarms heightened attention in forecasting winter flood.

Heavy rainfall events in the warm season (May–September) over the Tibetan Plateau (TP) region and its downstream areas are often closely related to eastward‐propagating Tibetan Plateau Vortices (TPVs). Hence, improving the prediction of TPVs and their associated convective activity is of paramount importance, given the significant potential impacts they can have on densely populated downstream regions, including but not limited to flooding and damages. In this study, a typical long‐lived TPV that occurred in July 2008 was used for the first time to explore the benefit of assimilating satellite all‐sky infrared radiances on the cloud and precipitation prediction of the TPV‐induced eastward‐propagating mesoscale convective system (MCS). The all‐sky infrared radiances from the water vapor (WV) channel of the geostationary Meteosat‐7 and other conventional observations were assimilated into a 4‐km grid spacing regional model using the ensemble Kalman filter. The results revealed that the all‐sky infrared data assimilation improved the cloud, precipitation, dynamical, and thermodynamical analyses as well as 0–12‐hr deterministic and ensemble forecasts. Compared with the experiment in which the all‐sky infrared radiances were not assimilated (non‐radiance experiment), the experiment with assimilated all‐sky infrared radiances yielded clearly improved initial wind and cloud fields, 1–12‐hr cloud forecasts, and 1–6‐hr precipitation forecasts. This study indicates that assimilation of all‐sky satellite radiances has the potential for improving the operational cloud and precipitation forecasts over the TP and its downstream areas. [ABSTRACT FROM AUTHOR]

Mesoscale convective systems (MCSs) frequently occur over Asia during the warm season, often producing intense precipitation with associated socioeconomic impacts. Here we reveal significant trends in MCS occurrence frequency and related precipitation in Asia during the warm season (March–September) in 2001–2020, using a tracking method that combines cloud and precipitation criteria with high‐resolution satellite data from the Global Precipitation Measurement mission. To examine whether there are differences between MCSs of different scales, both meso‐α scales (MαCSs) and meso‐β scales (MβCSs), with horizontal scales of 200–2,000 km and 20–200 km, are tracked. The distribution pattern of frequency and related precipitation of both MαCSs and MβCSs are quite similar and manifest positive trends over East Asia (EA) and Northeast Asia, and negative trend over Southeast Asia (SEA). The MCS precipitation trend contributes significantly to total precipitation trend, with MαCSs contributing the most. Our analysis indicates the trend in lower‐tropospheric water vapor flux convergence has a similar spatial pattern to the MCS frequency and related precipitation trend. Based on an atmospheric moisture flux decomposition analysis, the water vapor flux convergence trend can largely be explained by the change in horizontal wind convergence, while the specific humidity trend driven largely by temperature change plays a minor role. The trend in wind convergence in EA and SEA is possibly related to the evident trend in the lower‐tropospheric anticyclone over the western North Pacific and SEA, which might be due to the relatively stronger warming in the Indian Ocean during the past two decades. Plain Language Summary: Mesoscale convective systems (MCSs) occur frequently in Asia during the warm season and usually bring hazardous weather events and a large proportion of rainfall. In this study, we investigate the changing trends in MCSs over Asia during the warm season (March–September) in 2001–2020, using the data derived by high‐resolution satellites. The results show that the MCS frequency and related rainfall increase in East Asia (EA) and Northeast Asia and decrease in Southeast Asia (SEA). These changes are related to a trend in water vapor flux convergence–the amount of water vapor being transported by the winds into a given region. This trend can either be caused by winds converging more into a region, or higher amounts of moisture upwind of the region. The trend is mainly explained by higher wind convergence and this largely explains MCS trends. The trend in water vapor, which itself is mainly driven by higher temperatures, also plays a role. The horizontal wind convergence trend in EA and SEA is related to the lower‐tropospheric anticyclone change over the western North Pacific and SEA, which might be a result of the stronger warming in the Indian Ocean during the past two decades. Key Points: Frequency and rainfall of Mesoscale convective systems (MCSs) show positive trends over East Asia (EA) and Northeast Asia and negative trend over Southeast Asia (SEA) in 2001–2020MCS trends are related to trends in water vapor flux convergence which are mainly explained by horizontal wind convergence trendsHorizontal wind convergence trends over EA and SEA may be related to the relatively stronger warming in the Indian Ocean [ABSTRACT FROM AUTHOR]

Torrential rain with the greatest social impact in Chongqing in the year of 2021 occurred in the central and eastern parts of Sichuan basin from August 7 to August 8. Using multi-source observation and ERA5 reanalysis data, the characteristics of mesoscale convective systems during the heavy rain process were analyzed. The heavy rainfall occurred under the background of the warm Southwest Vortex induced by the trough moving into the Sichuan basin, and had evident characteristics of stages, bounds, and extremes. The heavy rainfall formed successively in the southeast of the Southwest Vortex center and near the boundary layer convergence line in the east and south of the Southwest Vortex. The heavy rainfall of each stage was produced by mesoscale-β convective systems which moved slowly and maintained for 3~6 h. Warm and humid instability and weak vertical wind shear provided favorable environment for the development of mesoscale convective systems. Vorticity analysis showed that the development of the Southwest Vortex was mainly due to the low-level convergence and vertical vorticity transport. However, the development of positive vorticity in the rainstorm area was not exactly the same as that of the Southwest Vortex, and the horizontal vorticity tilting effect was relatively significant. The positive vorticity of the heavy rainstorm area in the first stage was mainly due to the significant low-level convergence, vertical vorticity transport, and horizontal vorticity tilting effect near the Southwest Vortex center in the middle and low level troposphere. The positive vorticity of the heavy rainstorm area in the second and third stages were mainly due to the horizontal vorticity tilting effect above the boundary layer and the convergence in the boundary layer. The boundary convergence triggered mesoscale convective activities in the warm and humid atmosphere, which promoted the heavy rain in the second and the third stages.

This study investigates the importance of convective systems for extreme rainfall along the northern coast of the Gulf of Guinea (GG) and their relationship with atmospheric and oceanic conditions. Convective system data (MCSs), daily precipitation, sea surface temperature (SST) and moisture flux anomalies from June to September 2007–2016 are used. The results show that 2/3 of MCSs crossing Abidjan are produced in June, which is the core of the major rainy season. Likewise, 2/3 of MCSs originate from continental areas, while 1/3 come from the ocean. Oceanic MCSs are mostly initiated close to the coast, which also corresponds to the Marine Heat Waves region. Continental MCSs are mostly initiated inland. The results also highlight the moisture flux contribution of three zones which have an impact on the onset and the sustaining of MCSs: (i) the seasonal migration of the intertropical convergence zone (ITCZ), (ii) the GG across the northern coastline, and finally (iii) the continent. These contributions of moisture fluxes coincide with oceanic warming off Northeast Brazil and the northern coast of the GG both two days before and the day of extreme rainfall events. The ocean contributes to moisten the atmosphere, and therefore to supply and sustain the MCSs during their lifecycle.

This study investigates the role of the land surface on the precipitation produced by an elevated mesoscale convective system (MCS) in Iowa between 24–25 June 2015 during the Plains Elevated Convection at Night (PECAN) field campaign. Previous studies have shown a strong effect of low‐level atmospheric moisture on the location of this MCS. A series of semi‐idealized and realistic simulations with irrigation are conducted to understand the effect of moisture perturbations on the MCS precipitation displacement. In general, numerical simulations place the MCS east of the observed location. Adding moisture directly in the low‐level atmosphere in the semi‐idealized experiments reduces this displacement error. However, experiments with perturbed soil moisture result in drying over Iowa induced by moisture flux divergence from cooler low‐level temperatures and higher surface pressure, causing the MCS to move further to the east. The irrigation impact on low‐level moisture is highly dependent on the length of simulation period. Shorter simulations on the order of days generate similar drying over Iowa but the opposite is found for month‐long simulations. Despite the lack of low‐level moistening in the perturbed soil moisture and short‐term irrigation experiments, the sensitivity to low‐level moisture is similar in all runs. More low‐level moisture generates a more convectively unstable environment with less inhibition and a lower level of free convection that leads to more rapid MCS development and a change in MCS location. Plain Language Summary: This study looks at how the land surface affects the precipitation produced by a cluster of thunderstorms (called an elevated mesoscale convective system, or MCS) that happened in Iowa between June 24–25, 2015. We use semi‐idealized simulations to study the effect of moisture on the location and amount of precipitation from the storm. We find that adding moisture directly to the air close to the ground makes the storm form closer to its observed location, while changes to the moisture in the soil can make the storm move further away. The impact of irrigation on the moisture near the ground depends on how long the simulation is run for. Overall, the study finds that more moisture in the air close to the ground leads to a more unstable atmosphere that causes the storm to form more quickly that matches better with observations. Key Points: The mesoscale convective system (MCS) timing and location are sensitive to low‐level atmospheric moistureAdding soil moisture is not equivalent to adding atmospheric moisture because it cools temperature and induces low‐level divergenceShort‐term irrigation simulations dry atmospheric moisture and move the MCS to the east, but long‐term irrigation does the opposite [ABSTRACT FROM AUTHOR]

The monitoring of mesoscale convective systems (MCS) is typically based on satellite infrared data. Currently, there is limited research on the identification of MCS using true color composite cloud imagery. In this study, an MCS dataset was created based on the true color composite cloud imagery from the Fengyun-4B geostationary meteorological satellite. An MCS true color composite cloud imagery identification model was developed based on the Swin-Unet network. The MCS dataset was categorized into continental MCS and oceanic MCS, and the model’s performance in identifying these two different types of MCS was examined. Experimental results indicated that the model achieved a recall rate of 83.3% in identifying continental MCS and 86.1% in identifying oceanic MCS, with a better performance in monitoring oceanic MCS. These results suggest that using true color composite cloud imagery for MCS monitoring is feasible, and the Swin-Unet network outperforms traditional convolutional neural networks. Meanwhile, we find that the frequency and distribution range of oceanic MCS is larger than that of continental MCS, and the area is larger and some parts of it are stronger. This study provides a novel approach for satellite remote-sensing-based MCS monitoring.

An extreme torrential rain event occurred in the Jianshi Canyon in the mountainous area of Southwest Hubei on 26 July 2020, which was mainly caused by quasi-stationary mesoscale convection system (MCS). Using conventional observations, the data from regional automatic weather stations, radar data and ERA5 reanalysis data, we have performed analysis on the atmospheric circulation and the evolution and formation mechanism of quasi-static MCS in this event. Results are as follows. (1) Affected by the continental high and the Mongolian cold vortex moving eastward and southward and the stable subtropical high, the MCS forms in the north of the subtropical high and near the low-level convergence line. The mid-and low-level convergence line and the southerly jet in front of the low vortex provide favorable environmental conditions for the development and maintenance of the MCS. (2) The MCS in the Jianshi Canyon shows the characteristics of quasi-stationary with backward propagation, whose direction is consistent with the canyon trend, and it experienced two stages: quasi-stationary backward propagation and quasi-stationary merging. (3) Under the complex terrain of Jianshi Canyon and Enshi "bell mouth", the interaction between cold outflow and continuously strengthened and sinking low-level southerly jet has a significant effect on the formation and development of the MCS. The low-level jet is forced to rise on the windward slope for sustained weak cold outflow, forming a stable lifting condition in front of the mountain, which is the main reason for the maintenance of quasi-stationary backward propagation. The blocking effect of the low-level jet and the terrain in the south of Jianshi on the cold pool, and the merging of convective cells moving northward, are the reasons for the formation of quasi-stationary merging of the MCS. (4) The complex terrain plays an important role in the evolution of the MCS. The canyon wind effect and the topographic lifting effect of windward slope result in the enhanced development of the MCS. The cold outflow guided by the slope terrain of Jianshi Canyon quickly descends the mountain to form the convergence zone with the southerly lifted by the windward slope at front of the mountain, which speeds up the backward propagation of the MCS. The γ-mesoscale topography in the south of Jianshi Canyon blocks the downhill cold outflow and makes it move slowly southward, resulting in the propagation speed of the MCS to slow down and show a quasi-stationary state.

The climate system of the Third Pole region, including the (TP) and its surroundings, is highly sensitive to global warming. Mesoscale convective systems (MCSs) are understood to be a vital component of this climate system. Driven by the monsoon circulation, surface heating, and large-scale and local moisture supply, they frequently occur during summer and mostly over the central and eastern TP as well as in the downstream regions. Further, MCSs have been highlighted as important contributors to total precipitation as they are efficient rain producers affecting water availability (seasonal precipitation) and potential flood risk (extreme precipitation) in the densely populated downstream regions. The availability of multi-decadal satellite observations and high-resolution climate model datasets has made it possible to study the role of MCSs in the under-observed TP water balance. However, the usage of different methods for MCS identification and the different focuses on specific subregions currently hamper a systematic and consistent assessment of the role played by MCSs and their impact on precipitation over the TP headwaters and its downstream regions. Here, we review observational and model studies of MCSs in the TP region within a common framework to elucidate their main characteristics, underlying mechanisms, and impact on seasonal and extreme precipitation. We also identify major knowledge gaps and provide suggestions on how these can be addressed using recently published high-resolution model datasets. Three important identified knowledge gaps are 1) the feedback of MCSs to other components of the TP climate system, 2) the impact of the changing climate on future MCS characteristics, and 3) the basin-scale assessment of flood and drought risks associated with changes in MCS frequency and intensity. A particularly promising tool to address these knowledge gaps are convection-permitting climate simulations. Therefore, the systematic evaluation of existing historical convection-permitting climate simulations over the TP is an urgent requirement for reliable future climate change assessments.

Extreme rainfall occurred over the South African east coast during 10–12 April 2022, leading to over 500 deaths and massive damage. This study seeks to understand a key feature, namely the progression of the rainfall maxima from the northern KwaZulu-Natal (KZN) coast during the night of 10–11 April by ~550 km to the Eastern Cape coast about 48 h later. The large-scale circulation was dominated by a cut-off low over the South African interior with a strong ridging anticyclone southeast of the country in the South Indian Ocean. Satellite, rain gauge, and ERA5 reanalysis are used to show that the timing and location of the heaviest rainfall are closely tied to, firstly, the formation of a Mesoscale Convective System (MCS) at ~0300UTC April 11 on the northern KZN coast which tracked south and then offshore, and secondly, a meso-trough which formed a coastal meso-low by ~2100 UTC April 11. The meso-low tracked south over the warm waters of the Agulhas Current before strengthening into Subtropical Depression Issa. Mesoscale interaction between the MCS and coastal meso-low determined the positioning and timing of the strongest onshore low-level jet, moisture convergence and uplift, and hence the progression of the heavy rainfall down the coast. Such mesoscale interaction has not previously been documented in southern Africa or the Southern Hemisphere. [ABSTRACT FROM AUTHOR]

An event‐based assessment of the sea surface temperature (SST) threshold at the genesis of tropical mesoscale convective systems (MCSs) is performed in this study. We show that this threshold (SSTG) has undergone a significant warming trend at a rate of ∼0.2°C per decade. The SSTG shows a remarkable correspondence with the tropical mean SST and upper‐tropospheric temperature on interannual and longer timescales. Using a high‐resolution global climate model that permits realistic simulations of tropical MCSs, we find that the observed features of SSTG are well simulated. Both observation and model simulations demonstrate that the upward tendency in SSTG primarily results from the environmental SST warming over MCS genesis regions rather than the changes in MCS genesis location. A continuous increase in SSTG is projected in a warming simulation, but the relationship between SSTG and upper‐tropospheric temperature remains unchanged, suggesting that the tropical tropospheric temperature generally follows a moist‐adiabatic adjustment. Plain Language Summary: The development of tropical ocean showers and thunderstorms requires a warm ocean surface as an energy source fueling the storm. As a good example, a sea surface temperature (SST) threshold of 26.5°C has long been recognized as a necessary condition for tropical cyclone genesis. Here we examine the SST threshold at the genesis of tropical mesoscale convective systems, which occur so frequently throughout the tropics that they play an important role in shaping the general circulation of the atmosphere as they redistribute moisture and energy in the atmosphere. Using satellite observations and a high‐resolution global climate model, we show that this threshold has undergone a significant warming trend during the past several decades and will continue to increase in the future. This warming trend is closely related to the changes in the tropical mean SST. A constant relationship between the SST threshold and upper‐tropospheric temperature is found in both the current climate and a warming climate, which corroborates the argument that tropospheric temperatures in the tropics approximately follow a moist adiabatic adjustment. Key Points: A significant warming trend in sea surface temperature at the genesis of tropical mesoscale convective systems (MCSs) sea surface temperature threshold (SSTG) has been observedA global climate model that can explicitly simulate MCSs captures the observed features of SSTG and projects a continuous increaseA constant relationship between SSTG and upper‐tropospheric temperature indicates the existence of a moist‐adiabatic adjustment [ABSTRACT FROM AUTHOR]

The characteristics of mesoscale convective systems (MCSs) over the Yunnan–Guizhou Plateau (YGP) during the warm seasons (April–August) are investigated using an automatic tracking algorithm based on long‐term (2000–2018) hourly geostationary satellites data of temperature of black body. A total of 1,845 MCSs generated over the YGP are identified and further classified into the eastward moving type (EMT; ~13.1%) and noneastward moving/dissipating type (NDT; ~86.9%). The two types of MCSs exhibit varying characteristics. The EMTs are mainly active in the eastern flank of the YGP with a longer mean lifespan (~16.5 hr), while the NDTs occur anywhere over the YGP with a preference for the central YGP with a shorter mean lifespan (~7.6 hr). The MCSs are observed most frequently during June–July, while the ratio of EMTs reaches the highest in April due to the strongest steering flows. The abundant moisture supply plays a vital role in the generation and development of MCSs in June, whereas the dynamic forcing and high CAPE are favourable to MCSs in July. In terms of the diurnal cycle, the NDTs are generally initiated in the afternoon, reach mature in the late afternoon, and dissipate at night. By contrast, the mature stage of EMTs shows double diurnal peaks in the late afternoon and early morning. The MCSs are usually generated in an instable environment accompanied by strong vertical wind shear and intense low‐level water vapour flux over the YGP, and the MCSs tend to vacate the YGP with strong mid‐level westerlies and instable environment in the downstream regions. Compared to MCSs in Tibetan Plateau, the ratio of the EMTs is higher with longer lifespans though fewer MCSs are generated over the YGP. [ABSTRACT FROM AUTHOR]

Based on Himawari-8 satellite observations, the mesoscale convective system (MCS) behaviors of two successive but distinct warm-sector rainfall episodes (EP1 and EP2) on 6–7 May 2018 over southeastern China were compared, with the latter episode being a record-breaking rainfall event. Results showed that MCSs played a dominant role in EP2, but not in EP1, by contributing over 80% of the extreme rainfall total and all the 10-min rainfalls over 20 mm. MCS occurrences were more frequent in EP2 than EP1, especially in the coastal rainfall hotspots, along with more frequent merging processes. Overall, the MCS samples in EP2 were larger in size, more intense, and moved slower and more in parallel to their orientation, which facilitated local rainfall accumulation. Two new indices are proposed—the overlap index (OLI) and merging potential index (MPI)—to evaluate two MCS processes vital for rainfall production: the repeated passage of an individual MCS over given areas and the merging between MCSs, respectively. Both OLI and MPI in EP2 were significantly larger than in EP1, which tended to produce larger maximum rainfall amount and stronger 10-min rain rates in the following hour. These results demonstrate the potential value of satellite-based MCS information for heavy rainfall nowcasting, which is particularly significant for warm-sector rainfall with its limited predictability. [ABSTRACT FROM AUTHOR]

Abstract Mesoscale convective systems (MCS) contribute about half of the world's precipitation and create flash flooding as well as other extreme weather events. Despite steady progress in research of these systems in the last several decades, better theoretical understanding is still needed to understand their dynamical organizations and to improve their forecasts in numerical models. By using the Moncrieff‐Green horizontal vorticity equation, this paper presents an analytical model of one type of MCS, the steady‐state squall lines. It describes the organization, propagation, and properties of mesoscale circulations of two‐dimensional steady‐state convective systems under sheared environment. Far‐side solutions are formulated to illustrate the underlying physical processes of squall line flows. Numerical procedures are described to solve the model under general environmental conditions. The model leads to the following prediction of squall line properties: Given the environmental profiles of wind and convective available potential energy (CAPE), the propagation speed, the depths and mass fluxes of the tilted ascending front‐to‐rear flow, the overturning updraft, and the descending rear inflow of the squall line can be all determined from the cold pool buoyancy or CAPE. The squall lines are therefore self‐organized dynamical systems that have limited degrees of freedom in their properties. The model advances the theoretical understanding of how mesoscale flow components interact to sustain organized convection. It provides a new tool to interpret squall line systems in high resolution models and to parameterize them in climate models.

Transient luminous events (TLEs) are electrical discharges in the upper atmosphere caused by vigorous thunderstorms. Six sprites, which are part of TLEs, were observed on 22 July 2013 from Mt. Fuji (3776 m above sea level), Japan. All the six sprites were associated with intense positive cloud-to-ground strikes (+CGs), whose causative positive charges can reside in the stratiform region. Consequently, we assumed that the main sprites causative charges could generate an in situ charging mechanism, accompanied by precipitation growth in the extensive stratiform region. Thus, we supposed that there can be a relationship between the time sequence of surface precipitation intensity and the sprite emissions. In this study, we conclude that time sequences and horizontal evolution of Mesoscale Convective Systems (MCSs) precipitation are associated with sprites. As the result, prior to sprites 1–5, the areal amount of strong precipitation (≥8 mm/h) increased considerably, and only a small increase occurred during sprite 6. Analyzing the time sequence of the percentage of strong and weak precipitation with respect to the total precipitation, it was found that sprites 1–6 occurred within 20 min after the local peaks with respect to strong precipitation compared to total precipitation. In particular, sprites 2–5 occurred very close to local peaks. The rise time to the first peak of the strong precipitation rate associated with the first sprite was 80 min, while the rise time to the last peak associated with sprite 6 was 30 min. The temporal differences until the peaks suggest that the charging speeds, or mechanisms, related to precipitation differ between sprites 1–5 and sprite 6 in parent MCSs. [ABSTRACT FROM AUTHOR]

Using NCEP FNL analysis data, Nanjing Doppler radar echo data, and the WRF model, the present study simulated and analyzed a mesoscale convective system (MCS) that occurred on August 19, 2017 in the middle and lower reaches of the Yangtze River. Among the MCS’ organizational modes, the convective lines with parallel stratiform (PS) and with trailing stratiform (TS) modes coexisted. It began with a ribbon structure, and finally evolved into a strong bow-shaped squall line. Cyclone shear and low-level jet were two important systems influencing the occurrence and development of the strong convective weather, and the strong developing ground front in the afternoon was a key triggering mechanism. Under the combined influence of vertical wind shear and cold pool, the initial convection on the west side developed into PS mode, and that on the east side developed into TS mode. As the vertical wind shear in the middle and low layers of the PS mode turned, the PS mode dissipated. The cold pool near the TS mode cooperated with the vertical wind shear, and the TS mode continued to develop into a strong bow-shaped squall line under the influence of strong rear inflow jets. The rear inflow jet strengthened under the influence of the mid-level and low-level vortices. The cyclonic vortex was mainly determined by the stretching term in the vorticity equation, while the anticyclonic vortex was mainly determined by the titling term. [ABSTRACT FROM AUTHOR]

A 62‐day "extremely violent Meiyu" occurred in the Yangtze River basin from June to July in 2020, and its abnormal precipitation caused serious losses to the social economy. In this paper, the characteristics of newly formed mesoscale convective systems (MCSs) and the environmental conditions affecting the MCS formation during this abnormal precipitation event are investigated based on hourly datasets, including the temperature of brightness blackbody (TBB) data derived from the FY‐2F geostationary meteorological satellite, and the three‐source merged precipitation products from China Meteorological Administration. The results show that there were 31,778 times of short‐duration heavy precipitation (hourly precipitation exceeding 20 mm) occurred in the Yangtze River basin from June to July in 2020. The 69.3% of short‐duration heavy precipitation (hourly precipitation exceeding 20 mm) are closely related to the MCSs. The increase of newly formed MCSs leaded to more short‐duration heavy precipitation. The abnormal precipitation in the Yangtze River basin from June to July is mainly caused by elongated MCSs with an average lifetime of 6 h. The number of the newly formed MCSs presents obvious diurnal variations, with the most frequent at 20:00 (Beijing Time, the same below). The newly formed MCSs are prone to appear near the Yangtze–Huaihe shear line, and the magnitude of southerly wind at 500 hPa determines the distance between the MCSs and the shear line. The enhancement of vorticity advection at 500 hPa and water vapor convergence are conducive to the MCS formation. The weakening of low‐level jet corresponds to the increase of newborn MCS. [ABSTRACT FROM AUTHOR]

Modern global reanalysis products have greatly accelerated meteorological research in synoptic‐to‐planetary‐scale phenomena. However, their use in studying tropical mesoscale convective systems (MCSs) and their regional‐to‐global impact has mostly been limited to supplying initial and boundary conditions for MCS‐resolving simulations and providing information about the large‐scale environments of MCSs. These limitations are due to difficulties in resolving tropical MCS dynamics in the relatively low‐resolution global models and that tropical MCSs often occur over poorly observed regions. In this work, a Tropical MCS‐resolving Reanalysis product (TMeCSR) was created over a region with frequent tropical MCSs. This region spans the tropical Indian Ocean, tropical continental Asia, Maritime Continent, and Western Pacific. TMeCSR is produced by assimilating all‐sky infrared radiances from geostationary satellites and other conventional observations into an MCS‐resolving regional model using the Ensemble Kalman Filter. The resulting observation‐constrained high‐resolution (9‐km grid spacing) data set is available hourly during the boreal summer (June‐August) of 2017, during which widespread severe flooding occurred. Comparisons of TMeCSR and European Center for Medium Range Weather Forecast Reanalysis version 5 (ERA5) against independent satellite retrievals indicate that TMeCSR's cloud and multiscale rain fields are better than those of ERA5. Furthermore, TMeCSR better captured the diurnal variability of rainfall and the statistical characteristics of MCSs. Forecasts initialized from TMeCSR also have more accurate rain and clouds than those initialized from ERA5. The TMeCSR and ERA5 forecasts have similar performances with respect to sounding and surface observations. These results indicate that TMeCSR is a promising MCS‐resolving data set for tropical MCS studies. Plain Language Summary: Thunderstorms provide much of the rainfall over the Tropics and have important impacts on global weather and climate. However, these important systems often occur over regions with sparse in‐situ observations. Hence, it is difficult to use in‐situ observations to study the detailed dynamics and thermodynamics of these thunderstorm systems. While combining observations with computer simulation data can produce three‐dimensional data sets over the Tropics, the currently available combination data sets have difficulty resolving these thunderstorm systems. In this study, we combined high‐resolution satellite measurements with high‐resolution weather simulations to produce a high‐resolution four‐dimensional data set. This new data set can capture tropical thunderstorm systems over an area spanning the tropical Indian Ocean to the western edge of Pacific Ocean. We compared the accuracy of our new data set against a gold standard global data set. Using independent satellite‐derived radiation and rainfall data, we found that our new data set has more accurate storm characteristics compared to the gold standard. These characteristics include clouds and rainfall. Furthermore, simulations initialized from our new data set had a similar advantage over simulations initialized from the gold standard. These promising results suggest that our new data set might be better at capturing tropical thunderstorm systems than the gold standard. Key Points: Tropical mesoscale convective systems (MCSs) research can benefit from an observation‐constrained MCS‐resolving reanalysis data setWe produced such a data set using all‐sky satellite infrared radiances, MCS‐resolving regional simulations, and ensemble data assimilationCompared to European Center for Medium Range Weather Forecast Reanalysis version 5, the new data set better captured cloud, rainfall, and frequency of tropical MCSs and produced better short‐term forecasts [ABSTRACT FROM AUTHOR]

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Vertical wind shear is known to play a key role in the organization and intensity of mesoscale convective systems (MCSs) in West and Central Africa. A decadal increase in vertical wind shear has recently been linked to a decadal increase in intense MCSs over the Sahel. Here, the effects of vertical wind shear on MCSs over West and Central Africa have been investigated using a 10‐year (1998–2007) MCS dataset. Strong vertical shear is associated with long‐lived, moderate speed, moderate size and cold (deep) storms with high rain rates. The observed cloud top heights of storms over the oceans are closer to their level of neutral buoyancies (LNBs) compared to their land counterparts on the same latitudes. We hypothesize that this is due to greater entrainment dilution over land compared to storms over the ocean. Vertical shear allows storm anvils to reach higher altitudes relative to their LNB, this is consistent with the colder top storms over the Sahel (a region with a high vertical shear) compared to the Congo, despite a higher LNB in the Congo. It is not possible to diagnose the exact mechanisms for this impact of vertical shear from the data, but it is consistent with recent work showing that shear reduces entrainment dilution of squall‐line updrafts. We conclude that modelling impacts of vertical shear, which are normally missed in convection parameterizations, are not only important for predictions of high impact weather, but also for modelling the mean distribution of storm heights across Africa. [ABSTRACT FROM AUTHOR]

Using routine meteorological observations, intensive observations from the automatic weather stations, satellite images, radar products, NCEP reanalysis data, fine grid model data and regional mesoscale model data, we have conducted an examination on the reasons for the miss of forecast and the predictability of a short-term extreme precipitation event occurred in the Shaying River Basin at the night on 18 August 2017. The results indicate that this event occurs under the circulation of a high in the west and a low in the east. The stability and continuance of shear lines in the low-level and boundary layer and the establishment of the ultra-low level southwest jet provide the water vapor transport and convergence conditions for the occurrence of severe precipitation. The weak cold air spreads southwards along the south of North China and gradually intrudes into the inverted trough forming a mesoscale convergence line (convergence center) in the Shaying River Basin and maintaining such condition for a long time to trigger the release of unstable energy, which is a good indicator in predicting the occurrence of rainstorm. The two mesoscale convective cloud clusters reinforced and developed jointly into a Meso-β scale Convective System (MβCS) with a quasistatic state. It maintains for 6 hours in the lower reaches of the Shaying River Basin, which is the direct reason that leads to the enhancement of precipitation. The mesoscale convective system is triggered by the cold pool and its front surface mesoscale convergence line, and the strong echo continuously develops and maintains in the high value zone of temperature gradient in front of the cold pool, resulting in short-term extreme precipitation. The weak dynamic condition, the water vapor being mainly concentrated in the boundary layer and the extremely favorable thermal instability condition during this event, as well as the limited capture ability of the numerical model in forecasting this kind of convective rainstorm, all these factors are the causes of the miss of forecast of the rainstorm. The insufficient estimation of numerical models and operational forecasts in the precipitation intensity and its extreme, and the lack of forecasters'experience in forecasting extreme weather, are also some of the reasons for the forecast errors.

Based on conventional meteorological observations, observational data from the regional automatic weather stations, NCEP reanalysis data with spatial resolution of 1°×1° and temporal resolution of 6 h, satellite images, Doppler weather radar and wind profiler data, as well as the retrieval results from BeijingVariational Doppler Radar Analysis System (VDRAS), we have conducted analysis of the precipitation characteristics, atmospheric circulation, mesoscale system evolution features and the causes of a regional warm-sector torrential rain event in the Beijing-Tianjin-Hebei region on 12 August 2020. The results show that the event occurs in the warm air mass below 500 hPa at the edge of the subtropical high, and the main influencing systems are the low-level jet and the warm shear line at the 850 hPa. The whole severe precipitation event presented clearly the phased characteristics; it can be divided into two stages corresponding to the two different rainstorm areas, i.e., the southern Hebei and the north part of the Beijing-Tianjin-Hebei region with different formation mechanisms. The severe precipitation in the first stage is caused by a meso-α-scale convective system (MαCS), which is triggered by the surface convergence line. Before the precipitation starts, the local water vapor and unstable energy is rich. When the surface convergence line triggers the release of unstable energy, a strong linear echo is formed. The severe precipitation is mainly caused by the continuous development of convective cells in the spiral echo corresponding to the surface cyclonic circulation. The severe precipitation in the second stage is generated by several β-scale cloud clusters, and the southerly low-level jet strengthened northward provides sufficient water vapor and better dynamic conditions for the occurrence of torrential rain. The low-level jet in the boundary layer and the mesoscale low vortex system are the crucial influence systems at the second stage of this event. The hourly extremely severe precipitation of 125.9 mm at a station in Xiongan New Area is caused jointly by the warm cloud precipitation at the tail of comma-shaped radar echo and the "train effect".

Abstract A 62‐day “extremely violent Meiyu” occurred in the Yangtze River basin from June to July in 2020, and its abnormal precipitation caused serious losses to the social economy. In this paper, the characteristics of newly formed mesoscale convective systems (MCSs) and the environmental conditions affecting the MCS formation during this abnormal precipitation event are investigated based on hourly datasets, including the temperature of brightness blackbody (TBB) data derived from the FY‐2F geostationary meteorological satellite, and the three‐source merged precipitation products from China Meteorological Administration. The results show that there were 31,778 times of short‐duration heavy precipitation (hourly precipitation exceeding 20 mm) occurred in the Yangtze River basin from June to July in 2020. The 69.3% of short‐duration heavy precipitation (hourly precipitation exceeding 20 mm) are closely related to the MCSs. The increase of newly formed MCSs leaded to more short‐duration heavy precipitation. The abnormal precipitation in the Yangtze River basin from June to July is mainly caused by elongated MCSs with an average lifetime of 6 h. The number of the newly formed MCSs presents obvious diurnal variations, with the most frequent at 20:00 (Beijing Time, the same below). The newly formed MCSs are prone to appear near the Yangtze–Huaihe shear line, and the magnitude of southerly wind at 500 hPa determines the distance between the MCSs and the shear line. The enhancement of vorticity advection at 500 hPa and water vapor convergence are conducive to the MCS formation. The weakening of low‐level jet corresponds to the increase of newborn MCS.

Abstract Vertical wind shear is known to play a key role in the organization and intensity of mesoscale convective systems (MCSs) in West and Central Africa. A decadal increase in vertical wind shear has recently been linked to a decadal increase in intense MCSs over the Sahel. Here, the effects of vertical wind shear on MCSs over West and Central Africa have been investigated using a 10‐year (1998–2007) MCS dataset. Strong vertical shear is associated with long‐lived, moderate speed, moderate size and cold (deep) storms with high rain rates. The observed cloud top heights of storms over the oceans are closer to their level of neutral buoyancies (LNBs) compared to their land counterparts on the same latitudes. We hypothesize that this is due to greater entrainment dilution over land compared to storms over the ocean. Vertical shear allows storm anvils to reach higher altitudes relative to their LNB, this is consistent with the colder top storms over the Sahel (a region with a high vertical shear) compared to the Congo, despite a higher LNB in the Congo. It is not possible to diagnose the exact mechanisms for this impact of vertical shear from the data, but it is consistent with recent work showing that shear reduces entrainment dilution of squall‐line updrafts. We conclude that modelling impacts of vertical shear, which are normally missed in convection parameterizations, are not only important for predictions of high impact weather, but also for modelling the mean distribution of storm heights across Africa.

A mesoscale convective systems (MCSs) event occurring in northwest China is investigated to discuss the influences of interaction between different couplings of low‐ and mid‐level winds and orography on the evolution of MCSs, using high‐resolution reanalysis data. The formation of a low‐level anticyclonic circulation related to the plateau high caused the sequential occurrence of reverse and perpendicular low‐ and mid‐level winds couplings and the stagnation and expansion of a high‐humidity zone originating from the mountains. In the case of reverse coupling of low‐ and mid‐level winds, mid‐level winds were constantly lifted by low‐level winds ascending the mountains, resulting in strong updrafts and relevant inertia–gravity waves occurring continuously in the upstream direction of mid‐level winds. The unstable stratification generated by the high‐humidity zone prevented the inertia–gravity waves from propagating into the updrafts, leading to the maintenance and connection of the updrafts. Meanwhile, a strong downdraft occurred and was maintained on the leeward side of the updrafts due to the coupling of the inertia–gravity waves associated with the uplift of mid‐level winds with the inertia–gravity wave triggered by low‐level winds ascending the mountains. Consequently, MCSs developed as they propagated into the updrafts, and perished abruptly when they reached the leeward‐side downdraft. In the case of perpendicular coupling of low‐ and mid‐level winds, a broad and strong low‐level updraft band formed and stayed on the windward side of the mountain ridge due to terrain‐induced uplift. Significant mid‐level updraft and an associated inertia–gravity wave were triggered on the side of the low‐level updraft band blown by mid‐level winds. The unstable stratification generated by the high‐humidity zone hindered the propagation of the gravity wave into the low‐level updraft band, leading to the maintenance of strength and continuity of the low‐level updraft band. Consequently, MCSs were maintained during their entire propagation along the low‐level updraft band. [ABSTRACT FROM AUTHOR]

From 17–22 July 2021, Henan Province experienced the most severe torrential rainfall event since 1975 with a maximum hourly precipitation of 201.9 mm appeared in Zhengzhou, which was the largest hourly rainfall thus far observed by meteorological observation stations over the Chinese Mainland. The appearance of a long‐lived (21‐hr) northwestward‐moving mesoscale convective vortex (MCV) and its interaction with its parent mesoscale convective system (MCS) was crucial to produce the extremely strong heavy rainfall in Zhengzhou. The backward trajectory analysis indicates that air particles in the lower troposphere beneath the MCS over Henan contributed mostly to the MCV's formation. These air particles experienced notable ascending motions and condensation with their strong cyclonic vorticity mostly produced 1‐hr before the MCV's formation. Vorticity budget denotes that strong upward transport of cyclonic vorticity and convergence‐related vertical stretching, both of which were mainly due to convection associated with the parent MCS, acted as dominant factors for the MCV's formation. After formation, the MCV first coupled with its parent MCS, during which its intensity, thickness, and precipitation were all maximized; then, it moved northwestward and decoupled from the MCS, during which it weakened rapidly and finally dissipated. Convection‐related upward cyclonic vorticity transport and inward horizontal advection of cyclonic vorticity associated with an inverted trough over the Henan Province dominated the vortex's development/maintenance in the coupling stage; whereas outward horizontal advection of cyclonic vorticity dominated the MCV's dissipation after it completely decoupled from its parent MCS. These differ notably from the findings documented in previous MCV‐related literature. Key Points: A long‐lived mesoscale vortex was crucial to produce the hourly precipitation of 201.9 mm that appeared in Zhengzhou on 20 July 2021The extremely strong hourly precipitation appeared in the stage when the vortex interacted with its parent mesoscale convective systemConvection‐related upward and net‐import horizontal transport of cyclonic vorticity dominated the vortex's development [ABSTRACT FROM AUTHOR]

Córdoba Province in Argentina is a global hotspot for deep hail-producing storms. Previous studies of hail formation and detection largely relied on satellite snapshots or modeling studies, but lacked hail validation, relying instead on proxy metrics. To address this limitation, this study used hail collected in the mountainous Córdoba region in collaboration with the citizen science program "Cosecheros de Granizo 2018–2020" including from a record-breaking hail event and from the 2018–2019 RELAMPAGO field campaign. Three cases including a MCS and two supercells, which have verified hail in different environment locations relative to the Sierras de Córdoba, were analyzed for multi-spectral signatures in GOES-16 satellite data. Brightness temperatures decreased over time after convective initiation, reaching values cooler than the tropopause with variations around those values of different magnitudes. Overall, all cases exhibited a slight weakening of the updraft and strong presence of smaller ice crystal sizes just prior to the hail report, especially for the larger hailstones. The results demonstrate promise in using satellite proxies for hail detection in multiple environments for different storm modes. The long-term goal is to better understand hail-producing storms and unique challenges of forecasting hail in this region. [ABSTRACT FROM AUTHOR]

An object‐based technique was utilized to identify hydrometeor size‐sorting signatures at lower levels in the convective regions of 10 mesoscale convective systems (MCSs) during the 2015 Plains Elevated Convection at Night (PECAN) field campaign. Composite statistical analysis indicates that the magnitudes of size‐sorting signatures (the separation distances between rain diameter maxima and concentration maxima) are nonlinearly correlated to the echo‐top height, rain mass beneath the melting level, and precipitation rates at higher percentiles. To explore this correlation, the weather forecasting and research model was used to simulate the 20 June 2015 MCS during PECAN. Statistical analysis of the model outputs indicates more active riming growth and quicker graupel fallout at warmer temperatures near areas with larger separation distances. While updraft intensity above the melting level was also positively correlated to separation distances, this correlation was only statistically significant within certain temperature ranges. Additional analyses reveal that the higher intense precipitation potential near signatures with large separation distances could be attributed to precipitation production from the melted graupel. Finally, spatial correspondence between graupel distribution at the melting level and rain distribution at the lowest model level illustrates the critical role of graupel sedimentation and sorting in creating size‐sorting signatures in MCSs during the PECAN field experiment. Plain Language Summary: Recent upgrades in the US radar networks to polarimetric capabilities enabled routine measurements of how the sizes and concentration of raindrops vary in weather systems. Statistical analysis of radar data and output from a numerical model shows that the spatial distributions of raindrop sizes and concentration in the low levels could be exploited to elucidate physical processes in rainy weather systems. Specifically, low‐level raindrop size‐concentration separation may provide extra information on how much latent heating is released or absorbed when water transitions between vapor, liquid, and solid phases. This new information could be important because heating aloft can affect the intensity of the weather systems and the amount of precipitation produced by weather systems. Key Points: A positive correlation exists between the magnitudes of size sorting signatures and convective depthLarger size sorting magnitudes are associated with more riming growth and quicker graupel falloutIntense convective precipitation in the PECAN MCSs seems to be dominated more by cold rain processes than warm rain processes [ABSTRACT FROM AUTHOR]