Statistical Characteristics of Convection Initiation over Different Terrains in the Middle Reaches of the Yangtze River Basin Based on the Doppler Weather Radar Observations

Using a convection initiation (CI) identification method designed for isolated convection, 11,646 CI events (CIEs) were identified based on composite reflectivity (CR) in the middle reaches of the Yangtze River Basin from May to September of 2016–2020. CIEs occur more frequently in July and August (62.7%) and from 11 BJT to 16 BJT (62.2%) with a diurnal peak at 12 BJT. CI area (area of the connected region with CR  40 dBZ at CI time) has a diurnal peak at 16 BJT, which is related to the frequent occurrence of large-area CIEs (LACIEs, 1%) with a CI area larger than 62 km². Most LACIEs occur under stronger atmospheric instability and higher vertical wind shear with the rapid intensification and expansion of convective regions after CI. Regions with high-frequency CIEs correspond well with mountain terrains. Mountain CIEs, which are under the weak unstable stratification and low vertical wind shear, occur about twice (5,831) as frequently as foothill or plain CIEs. Compared to other terrains, the local heterogeneity of soil moisture (SM) near CI locations is the strongest over the mountains. Mountain CIEs occur at the center of a high SM region located in the transition zone between the positive and negative SM gradients, with the highest SM gradient magnitude located to the north side of CI locations.