Offshore Transition of Mesoscale Convective Systems over the Yangtze-Huai River Basin: Intensity Change and Environmental Controls

Mesoscale convective systems (MCSs) experience complex changes in intensity as they move from land to the ocean. Predicting the offshore evolution of these systems poses a significant forecasting challenge. Using radar observations and ERA5 reanalysis data from the warm seasons of 2016–2018, this study investigates the characteristics and environmental controlling factors of land-to-sea (LS) MCSs over the Yangtze-Huai River Basin (YHRB). These systems are classified into offshore-intensifying and offshore-weakening types, accounting for approximately 40% and 60% of the total cases, respectively. Spatiotemporally, intensifying MCSs predominantly occur in spring (April–May; 56%) at lower latitudes, whereas weakening MCSs peak in summer (June–July; 45%) at higher latitudes. Composite analysis reveals that intensifying MCSs occur in environments with weaker thermodynamic instability but stronger dynamic forcing. Specifically, most unstable convective available potential energy (MUCAPE) is ~43% lower with reduced total column water (TCW), yet they are characterized by enhanced upper- and lower-level jets and greater vertical wind shear (VWS). A critical factor facilitating intensification is the cooler marine atmospheric boundary layer (MABL), which promotes enhanced isentropic lifting and lowers the lifting condensation level (LCL) via increased relative humidity. Crucially, offshore intensity evolution is governed by environmental change rather than absolute coastal conditions. Weakening MCSs originate in thermodynamically and dynamically superior environments over land but undergo considerable deterioration in thermodynamic and moisture conditions (MUCAPE drops ~47%, TCW decreases) upon moving offshore. Conversely, intensifying MCSs experience modest MUCAPE reduction (~26%) but benefit from significant enhancement of dynamic conditions (low-level VWS strengthening) and moisture enhancement (TCW increases in 60% of cases) during transit, enabling their intensification over the ocean despite lower absolute instability at the coast. These findings provide crucial insights into how coastal environments alter storm intensity, and offer practical insights to improve offshore weather forecasting.