Summer Mesoscale Convective Systems over East China under Current and Future Climate

Mesoscale convective systems (MCSs) are large, organized deep convective storms producing intense, widespread rainfall. East China, strongly influenced by the summer monsoon, is a key hotspot where lives of citizens and economy are threatened by MCS related rainstorm, winds, and lightning. Understanding summer MCSs in current and future climate is therefore essential for risk assessment. Convection-permitting (CP) regional Numerical Weather Prediction (NWP) models have been widely used to study MCSs, yet their skill remains uncertain, especially in terms of capturing storm responses to background circulation. This study investigates climatological characteristics and life cycles of summer MCSs over eastern China under current and future climate, and examines the accompanying (thermo-)dynamic atmospheric environment and soil moisture (SM) conditions. The ability of a CP Weather Research and Forecasting (WRF) model to represent MCSs and their impacting factors has been systematically evaluated.
MCSs are mostly found in regions with abundant warm-moist air or complex topography. MCSs contribute approximately 20% of total rainfall. They typically initiate in the afternoon and intensify at night over low-elevation eastern and coastal regions, whereas initiating at midnight on the leeward sides of complex terrain. MCSs initially show rapid expansion of convective cloud with weakening, then peak and transition to stratiform-dominated cloud, after which rainfall decays quickly while system area shrinks more slowly.
CP WRF model shows reliable skill in reproducing climatology of MCSs. It shows added value over ERA5 in simulating MCS diurnal cycle. Large uncertainties remain in the diurnal peak time over complex terrain and the distribution of rainfall intensities. Using CP WRF model and ERA5 data, this study identifies total column water vapor (TCWV) and convective available potential energy (CAPE) as key thermodynamic factors. The product of storm-time TCWV and maximum vertical motion indicated by square root of CAPE scales MCS maximum precipitation. Storms are also found to be related to low-level zonal wind shear. Intense MCSs are associated with strong wind shear, and propagation of storms are correlated with shear. This is the first study to demonstrate that WRF model reproduces these effects, confirming its skill in simulating the organized thunderstorm response to background circulation.
A mechanism of SM favoring intensification of mature MCSs was further revealed with CP WRF simulations. Convective cores of mature MCSs tend to occur on the drier side of mesoscale (∼200 km) SM gradients. These gradients generate strong boundary-layer thermal structures, which enhance local moisture convergence and vertical wind shear. Precipitation and MCS cloud cover feed back positively on soil moisture and surface energy heterogeneity. This result highlights the importance of land-atmospheric interactions and suggests that improved land-surface representation could enhance MCS prediction.
Pseudo-Global Warming experiments under the high-end warming scenario Shared Socioeconomic Pathway (SSP)5-8.5 project a future shift toward fewer but more intense, convectively dominated MCSs with larger precipitation areas and longer lifespans. Projected extreme rainfall increases with super Clausius–Clapeyron scaling, reflecting enhanced updrafts and mesoscale circulation besides the thermodynamic profile. Stronger storm-time shear combined with higher TCWV and CAPE leads to greater peak convective precipitation and expanded convective areas. The increases in intensity, spatial coverage and life duration produce higher total rainfall per event, together with reduced frequency, amplifying flood hazards and climatic vulnerability.