A Comparison between Warm-Sector and Frontal Heavy Rainfalls during the First Rainy Season in South China

Warm-sector and frontal heavy rains are two main heavy rain types during the first rainy season (April-June) in South China. Warm-sector heavy rainfall in South China refers to the heavy rainfall that occurs within the weakly forced synoptic environment far from the frontal system or in the uniform southwesterly without fronts. It has long been difficult to forecast beforehand or simulate afterwards. Using hourly surface observations, NCEP FNL analysis, and TRMM rainfall data during April-June of 2003-2014, we analyzed the differences in the spatial and temporal distributions of warm-sector and frontal heavy rains in the first rainy season of south China, as well as their different environmental features.

Results show that warm-sector type has larger extreme precipitation, higher frequency of flash heavy rainfall, and larger contribution (over 40%) to total precipitation compared to frontal type. 71% of warm-sector heavy rains occur in June with a diurnal peak in the morning. The warm-sector rainfall was found to concentrate in the southwest and southeast coasts. Frontal heavy rains are more evenly distributed by month with the rainfall maxima appearing in the afternoon and early evening. The frontal rainfall concentrates more inland. In comparison with gauge observations, TRMM precipitation estimate significantly underestimates the heavy rainfall, particularly of the warm-sector type.

Environmental analyses show that most warm-sector rainfalls are associated with deep low-level jet (LLJ). The spatial distribution of warm-sector rainfall centers is related with the converging and uplifting effect of the mountains and trumpet-shaped terrains in South China coast in the prevailing southwesterly at 925 hPa; The monthly variation of warm-sector rainfall is closely associated with the onset of southwest monsoon, while its diurnal variation (i.e., peak in the early morning) is tightly connected to the land/sea breeze and diurnal variation of LLJ. For the frontal rains, however, only less than 50% of the events are associated with LLJ and the LLJs are generally shallow. The diurnal variation of frontal rainfall has no obvious relationship with the land/sea breeze.