Characteristics and Environment of Convection Initiation Associated with Warm-Sector Heavy Rainfall in South China

Understanding the characteristics of convection initiation (CI) associated with warm-sector heavy rainfall (WSHR) in South China is vital for improving quantitative precipitation forecasts. Using high-resolution radar mosaics, this study investigates the spatial and temporal characteristics of CI associated with WSHR in South China from 2016 to 2023, as well as the related environmental conditions, particularly comparing CI in the presence and absence of low-level jets (LLJs). The results show that CI associated with WSHR occurs more frequently over the coastal region than inland. More than 75% of CI cases occur at night. Most convection moves northeastward after initiation. Notably, 84.4% of CI cases occur in conjunction with LLJs, which provide favorable low-level onshore winds, vertical wind shear, and upward motion for CI. CI typically occurs downstream or on the left side of southerly–southwesterly boundary layer jets, and upstream or on the right side of southwesterly synoptic low-level jets. Prior to CI, the presence of LLJs supplies robust dynamic forcing and gradual moisture accumulation to counter an environment with greater convective inhibition than that in conditions without LLJs. In the absence of LLJs, CI depends on a more stringent pre-convective environment, featuring lower convective inhibition, lifting condensation level, and level of free convection. Additionally, significant enhancements in dynamic and moisture conditions throughout a deep near-surface to 700-hPa layer are required prior to initiation. CI without LLJs may alternatively occur through complex interactions. For instance, changes in ambient vertical wind shear induced by downward-propagating midlevel disturbances, combined with orographic lifting and land–sea contrast effects, can generate localized convergence capable of triggering convection.