Characteristics and synoptic environment of torrential rain in the warm sector over South China: a composite study

The characteristics and synoptic environment of an extreme rainfall event (maximum 24-h accumulated rainfall, 464 mm) in the warm sector over South China on 20 May 2016 are investigated. This warm sector torrential rain (WSTR) was occurring in the warm zone over the trumpet-shaped topography of Xinyi in the west of Guangdong Province. Large-scale analysis shows that the WSTR was continuously influenced by strong warm horizontal temperature advection from the Beibu Gulf and South China Sea accompanying with a cold front in the northwest. Water vapor fluxes in the lower troposphere reached 500 kg m−1 s−1 and total column water vapor exceeded 20 g kg−1 in the early stage of the WSTR. The topographic analysis showed that the WSTR was caused by the mesoscale convective boundary between the down-valley winds and the outbreak of the south winds from the south plain regions. The convections were strengthened and maintained due to the new generations of the convections by cold outflows and the south winds, as well as blocking effects caused by the trumpet-shaped topography over Xinyi. The strong convections were alleviated due to the weakening of south wind from the plain regions. Climatology statistical analysis confirms that the torrential rains are mainly concentrated during the night and early morning at 1200–2100 UTC and reach the peak at 1700–1800 UTC with a consistent high frequency of the surface wind convergence over Xinyi.