Characteristics of Warm Season Convection over Pearl River Delta, China

This study examines, for the first time, the temporal and spatial distribution of convection over Pearl River Delta during warm season using three years of Ground-based Doppler radar data at Guangzhou, China. Results show the convection occurs most frequently over the southern coast and upstream of the eastern mountainous area of Guangdong Province, with the highest frequency in June and the lowest in September. This spatial distribution of convection corresponds roughly to the climatologically rainfall observed by gauges and satellites, suggesting that the heavy rain is mostly contributed by the convection in this area. The occurrence of the convection also exhibits a strong diurnal cycle in the warm season. During the May and June (Mei-Yu season), the occurrence of the convection over Pearl River Delta has a bimodal distribution with the frequency maximum in the early afternoon in association with solar heating, and a second peak between midnight and early morning. However, the midnight-to-morning peak in the occurrence of convection is much weaker in middle and late summer. Based on the analysis of the sounding and radar VAD wind data, the convection over the southern coast is found to occur preferentially on the days when the LLJs exist, especially in Mei-yu season. The warm, moist and unstable air is transported from ocean to land by LLJs, and lifted upward in the coastal area by the convergence induced by differential surface friction between land and ocean, together with the coastal front. Thus, the convection mostly occurs in the coast area. In contrast, the high frequency of the convection over upstream of mountainous area is more likely resulted from the orographic lifting effect.