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This study investigates the effectiveness of ensemble singular vectors (EnSVs) for a mesoscale convective system over the East China Sea, a challenge due to its strong nonlinearity. Employing breeding ensembles with varying horizontal resolutions, we compare linear EnSV predictions with nonlinear perturbed forecasts. The results indicate that EnSVs consistently capture sensitivity to synoptic‐scale features across all resolutions. However, the growth rate of the EnSVs decreases with increasing resolution. While nonlinear effects become more pronounced at higher resolutions, the nonlinearly developed perturbations from initial EnSVs by full models still demonstrate significant growth in the target region. The nonlinearity is likely associated with mesoscale error growth. These findings suggest that mesoscale EnSVs are capable of identifying key growing modes, but they have limitations in fully capturing the nonlinear amplification of errors inherent in mesoscale phenomena.

This study aims to establish process‐level benchmarks linking Mesoscale Convective Systems (MCSs) at various stages of their life cycle to their thermodynamic environment. The relationship between MCS precipitation and an empirical buoyancy measure (BL${\\mathrm{B}}_{L}$ ) is examined using collocated satellite‐observed MCS tracks and reanalysis data. A positive relationship is identified between the frequency of tropical MCSs and that of high BL${\\mathrm{B}}_{L}$conditions. The buoyancy measure, integrating instability and entrainment, helps elucidate thermodynamic characteristics throughout the MCS life cycle. Environments with high instability and moderate subsaturation are frequently linked to the initial stage, while environments with low instability and near saturation are frequently linked to the mature stage. Stable and highly subsaturated environments are more likely associated with the termination of the life cycle. These associations are qualitatively similar for oceanic and land MCSs. Overall, the MCS‐environment relationships can serve as observational benchmarks with which to diagnose MCS‐resolving models.

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.

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 event Return 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

Tibetan Plateau mesoscale convective systems (TP_MCSs) shape regional weather and precipitation, yet their thermodynamical and dynamical background characteristics, and microphysical structures remain poorly understood due to sparse observations. To address this research gap, we integrate a 7‐year TP_MCS database with ERA5 reanalysis and satellite observations to construct vertical profiles, enabling a systematic examination of TP_MCSs' structures. Results reveal distinct stage‐dependent characteristics, with peak convective intensity, the lowest 0°C level, the largest hydrometeor diameters, and maximum rainfall rates during the development stage. TP_MCSs' convection intensity is modulated through coupled thermodynamical (temperature, humidity), dynamical (vertical motion, moisture transport), and microphysical (hydrometeor characteristics) interactions that regulate buoyancy, latent heating, and entrainment processes. Moreover, TP_MCSs' precipitation is governed by competing enhancement (buoyancy, moisture transport) and suppression (dry, cold entrainment) mechanisms. These findings are helpful to improve understanding of TP_MCSs' vertical structure and aid convection parameterization in climate models.

Since the 1980s, Sahel rainfall totals, extreme rainfall, and the share of rainfall from extreme events have all trended upward. In observational and reanalysis data sets, these increases are linked to trends in mesoscale convective systems (MCSs) and extreme deep convection (cold clusters). Throughout this period, precipitation metrics have increased first via increases in MCS frequency and the relative increase in cold clusters, and later via an increase in storm precipitation intensity. Until the late 2000s, increases in the frequency of strong storms were supported by increased vertical shear of the zonal wind, as the African easterly jet intensified in response to the strengthening meridional temperature gradient over the Sahel. Afterward, the storm frequency and vertical wind shear stopped increasing. Yet, extreme precipitation continued to increase, as the storms' precipitation intensity increased. We link the higher precipitation intensity to an increase in atmospheric moisture in both the boundary layer and aloft.

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 set We produced such a data set using all‐sky satellite infrared radiances, MCS‐resolving regional simulations, and ensemble data assimilation Compared 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

Mesoscale convective systems (MCSs) contribute significantly to summer precipitation in the tropics and midlatitude. Although soil moisture (SM) effects on convection are globally recognized, its specific role on mature MCSs in East China remains unclear. Using convection‐permitting simulations spanning 22 summers, we find that convective cores within mature MCSs preferentially develop on the drier side of strong SM gradients (∼200 km). This is evidenced by a 2.5‐fold increase in core occurrences downstream of the steepest 10% of SM gradients versus a near‐uniform surface. SM gradients shape sensible heat flux gradients via evapotranspiration, while upstream pre‐storm rain‐producing clouds suppress surface available energy. These processes favor MCSs through enhancing near‐surface temperature gradients which strengthen moisture convergence and zonal wind shear. Our results highlight the critical role of SM gradients in favoring MCS propagation in East China. As climate change intensifies SM heterogeneity, improved land‐surface representation offers potential for advancing rainfall prediction and projection.

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.

A convection-permitting regional model simulation for August 2006 and observations are evaluated to better understand the diurnal cycle of precipitation over the Sahel. In particular, reasons for a nocturnal rainfall maximum over parts of the Sahel during the height of the West African monsoon are investigated. A relationship between mesoscale convective system (MCS) activity and inter-tropical front (ITF)/dryline dynamics is revealed. Over 90% of the Sahel nocturnal rainfall derives from propagating MCSs that have been associated with topography in earlier studies. In contrast, in this case study, 70–90% of the nocturnal rainfall over the southern Sahel (11°N–14°N) west of 15°E is associated with MCSs that originate less than 1000 km upstream (to the north and east) in the afternoon, in a region largely devoid of significant orography. This MCS development occurs in association with the Sahel ITF, combined with atmospheric pre-conditioning. Daytime surface heating generates turbulent mixing that promotes planetary boundary layer (PBL) growth accompanied by a low-level reversal in the meridional flow. This enhances wind convergence in the low-level moist layer within 2°–3° of latitude of the equatorward side of the ITF. MCSs tend to form when this vertical mixing extends to the level of free convection and is accompanied by a mid-tropospheric African easterly wave disturbance to the east. This synoptic disturbance enhances the vertical wind shear and atmospheric instability over the genesis location. These results are found to be robust across the region.