Mechanisms of Ageostrophic Wind Convergence in the Boundary Layer of Coastal Warm-Sector Extreme Heavy Rainfall in South China

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 case studies 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. It can contribute 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 WSHR processes in South China. Extremely heavy rainfall frequently occurs in coastal Southern China, which causes great damage and is difficult to accurately forecast. Most of this kind of rainfall is associated with southwesterly wind in the lower atmosphere and strong convergence of the lower airflow. Our purpose is to better understand the factors that lead to strong convergence, which is an important factor for heavy rainfall. Since the geostrophic wind component has no divergence, the convergence is mostly contributed by the ageostrophic wind component, which doesn't conform to the geostrophic balance. We find that different intensities, ranges, and diurnal changes of the southwesterly wind in the lower atmosphere can lead to different distributions of ageostrophic winds in the South China Sea. The thermal difference between land and sea can influence the diurnal change of the southwesterly wind in the lower atmosphere, and it can also affect the ageostrophic winds and their convergence. The results will potentially provide a better understanding of heavy rainfall events in Southern China and may lead to better forecasts and issuing of warnings. Land-sea contrast leads to the strong convergence in the coastal warm-sector extreme heavy rainfall area in South ChinaMarine boundary layer jets (BLJs) and their diurnal variation can influence the ageostrophic winds and then affect the convergenceThe land-sea thermal contrast can affect the ageostrophic winds and the convergence by changing the diurnal variation of BLJs