Convection Initiation During the Meiyu Environment in the Yangtze-Huai River Basin of China

Convection is the main contributor to heavy rainfall over China's Yangtze-Huai River Basin (YHRB) during Meiyu season; however, the mechanisms of convection initiation (CI) associated with the Meiyu front are still not well understood. In this study, a large set of 86,099 CI events, identified from composite reflectivity data in YHRB over six Meiyu seasons, were used to investigate the characteristics of the spatiotemporal distribution of CI in YHRB. The result showed that the overwhelming majority of CI events (similar to 90%) occurred in the region of existing stratiform clouds. Meanwhile, CI tended to concentrate in mountainous areas and exhibited two triggering modes. By relating the CI events with an objective analysis of ERA5 reanalysis data, it was also revealed that the characteristics of CI occurrence varied with patterns of Meiyu circulation and their interactions with local topography, and the warm air advection pattern dominated the Meiyu CI. We further illustrated that CI on the plains occurred with a morning peak corresponding to environments of high 0-3 km shear (SHR3) and low most unstable convective available potential energy (MUCAPE), while the CI near or over mountains had an afternoon peak corresponding to low SHR3 and high MUCAPE environments. Plain Language Summary Convection occurs frequently in East Asian summer monsoon regions and brings a large amount of rainfall, especially over the Yangtze-Huai River Basin in China during Meiyu season. Understanding the convection initiation (CI) process is crucial for forecasting subsequent precipitation and therefore has been an important research topic in regional/global climate and hydrological studies. Here, for the first time, we present a statistical analysis of a large quantity of CI events in the Yangtze-Huai River Basin selected over six Meiyu seasons. This analysis revealed that about 90% of CI events during Meiyu were embedded in existing stratiform cloud bands, a distinct feature of the Meiyu CI from other types of CI. The hotspots, intensity, and diurnal cycle of CI events varied with large-scale patterns of the dynamic and thermodynamic environment. These findings provide key information for improving our understanding of how convection initiates in the summer monsoon environment, which may lead to an improved forecast of convection-related severe weather.

Automated summary

Convection is the main contributor to heavy rainfall in China's Yangtze-Huai River Basin during Meiyu season. This study investigates the characteristics of convection initiation (CI) associated with the Meiyu front using a large dataset of CI events. The analysis reveals that most CI events occur in the region of existing stratiform clouds and exhibit two triggering modes in mountainous areas. The occurrence of CI is influenced by the Meiyu circulation patterns and their interactions with local topography.