A Comparative Analysis of the Synoptic Conditions of Two Heavy Rainfall Cases West of the Pearl River Estuary

Abstract

This paper compares the synoptic-scale circulation and thermodynamic conditions of two representative heavy rainfall cases that were concentrated west of the Pearl River Estuary under different seasonal backgrounds. The first case occurred in the early rainy season, with strong precipitation lasting for several hours. The second case occurred in the late rainy season, with extreme flash downpour in just two hours. The analysis reveals that the main cause was different for the two rainfall cases. The main cause of the first rainfall case was converging polar cold air and marine warm air that formed a synoptic scale front that drove the strong ascent. The main cause of the second case was a cold backflow from the Taiwan Strait, resulting in a sharply contrasting equivalent potential temperature front and triggering very strong upward motion. The moist potential vorticity (MPV) showed a negative value before both rainstorms, indicative of the importance of convective instability. A sandwich-like configuration of the MPV led to the development of strong vertical motion for the second case.