Contrasting frontal and warm-sector heavy rainfalls over South China during the early-summer rainy season

Abstract

Heavy rainfalls occur frequently during early summer (April–June) over South China, causing severe floods. Using 12 years of hourly rain-gauge data, we identify a large number of regional heavy rainfall events and categorize them into two major types based on the strength of synoptic forcing. It is shown that frontal heavy rainfalls associated with fronts or shear lines mainly occurs over inland regions, whereas warm-sector heavy rainfall under weakly forced synoptic environment is observed in coastal areas. The rainfall maxima of both types tend to form over the low-lying plains or sea surfaces adjacent to windward mountains. Frontal heavy rainfall usually propagates southwards with a cold front, while warm-sector events move relatively slowly and produce coherent patterns of rainfall. The occurrence of warm-sector rainfall increases markedly from April to June in a close association with the onset of summer monsoon, in contrast to frontal rainfall with less monthly variation. Warm-sector rainfall also exhibits pronounced diurnal variation, with a peak in the early morning, as a result of intensified convergence between nocturnal low-level jets in southerly monsoon and land breezes. In contrast, frontal rainfall has an afternoon peak, due to the arrival of eastward-propagating rain systems and daytime heating on land. As frontal (warm-sector) heavy rainfall has relatively high (low) predictability in numerical models, understanding their occurrence and formation is of benefit to operational forecasting and decision-making for disaster prevention.