Thermodynamic Structures in the Low-Level Atmosphere During Extreme Rainfall Events over the South China Coast

This study aims to analyze the synoptic patterns conducive to extreme rainfall (ER) over the South China Coast (SCC) and the low-level thermodynamic structure over the sea. Using observational and reanalysis data, a clustering analysis of the 700 hPa synoptic conditions is conducted during the pre-summer rainy seasons of South China from 1988 to 2017. The conditions are classified into three patterns: Type-L (Low-pressure type), Type-I (Intermediate type), and Type-H (High-pressure type). The circulation characteristics and diurnal variations associated with ER in each pattern are examined. Type-L is characterized by a low-level southwest airflow from the South China Sea (SCS), which strengthens coastal convergence, providing abundant moisture and favorable thermodynamic conditions. Therefore, Type-L is associated with more ER events than the other patterns. In Type-I, the SCC lies between a shallow trough and a ridge, influenced by southerly airflow. Type-H is dominated by a high-pressure system over the eastern SCS, where moisture conditions are weaker than in the first two patterns, leading to fewer ER events. During ER events, a significant warm belt below 850 hPa forms on the lee side of the Indochina Peninsula due to descending motion. In all synoptic patterns, a profound low-level warm advection occurs from the southwest. However, moisture transfer paths notably differ. In Type-L, low-level moisture primarily originates from the western SCS, driven by strong southwesterly winds in the boundary layer. In contrast, moisture in Type-I and Type-H is sourced from the central and eastern SCS, respectively. The diurnal variation of low-level winds is a key factor in controlling the diurnal cycles of moisture transport and rainfall. The enhancement of ER at night is related to the land-sea thermal contrast over the SCC, coupled with an increase in moisture and meridional winds. Among the three patterns, Type-I is primarily driven by local thermal convection, which plays an important role in ER.