The South China coast has a high incidence of warm-sector heavy rainfall (WSHR) events. The ageostrophic winds in the boundary layer in most of these events associated with the southwesterly boundary layer jets (BLJs) mainly exhibit strong convergence at rainfall area. In this paper, we analyze two cases of WSHR in May 2013 and May 2015, which occurred in similar synoptic environments but varied in intensity, extent, and duration of rainfall, where the ageostrophic winds are the confrontational confluence and asymptotic confluence pattern, respectively. ERA-5 reanalysis data and the diagnostic equation of ageostrophic wind are used to examine the factors affecting the ageostrophic winds in the northern land region and the southern offshore region of the rainfall. The results suggest that land-sea contrast leads to the convergence of ageostrophic winds in the rainfall area. Boundary layer friction dominates the northeasterly ageostrophic winds on land. The diurnal variation of BLJs dominates the ageostrophic winds and their diurnal variation at sea, contributing southwesterly or southeasterly ageostrophic winds, so the phase difference between the land and sea forms confrontational or asymptotic confluence, respectively. BLJs with different intensities, extents, and diurnal variations can lead to different ageostrophic wind patterns and their confluence modes. The land-sea thermal contrast can directly affect ageostrophic winds, and it can also affect the diurnal variation of BLJs, thus affecting the ageostrophic winds and their confluence mode. It is further verified that the BLJs and thermal forcing are important in warm-sector heavy rainfall processes in South China.

The disparities in circulation backgrounds and divergence-vorticity configurations of locally developed warm-sector short- and long-term heavy precipitation events (SHPEs & LHPEs) are compared in this work. Results show that double LLJs accompany the evolution of both types of rainstorms but SHPEs are mainly located in the negative relative vorticity (RV) area to the right of the synoptic-related LLJ, while LHPEs are located in the positive RV area to the left side of LLJ. When triggering, both types of rainstorms maintain the structure of bottom convergence and lower-middle layer (875-700 hPa) divergence. Then, in the lower-middle layer of the SHPE, the negative RH encourages the transition from divergence to convergence, which prevents convection from continuing, but the LHPE always retains its prior structure, allowing it to endure longer. This work provides new insights into the evolution of rainstorms with different durations, which can aid in the accurate forecasting of warm-sector rainstorms.