A Diagnostic and Numerical Study on a Rainstorm in South China Induced by a Northward-Propagating Tropical System

A strong cyclonic wind perturbation generated in the northern South China Sea (SCS) moved northward quickly and developed into a mesoscale vortex in southwest Guangdong Province, and then merged with a southward-moving shear line from mid latitudes in the period of 21-22 May 2006, during which three strong mesoscale convective systems (MCSs) formed and brought about torrential rain or even cloudburst in South China. With the 1°× 1°NCEP (National Centers for Environment Prediction) reanalysis data and the Weather and Research Forecast (WRF) mesoscale model, a numerical simulation, a potential vorticity inversion analysis, and some sensitivity experiments are carried out to reveal the formation mechanism of this rainfall event. In the meantime, conventional observations, satellite images, and the WRF model outputs are also utilized to perform a preliminary dynamic and thermodynamic diagnostic analysis of the rainstorm systems. It is found that the torrential rain occurred in favorable synoptic conditions such as warm and moist environment, low lifting condensation level, and high convective instability. The moisture transport by strong southerly winds associated with the rapid northward advance of the cyclonic wind perturbation over the northern SCS provided the warm and moist condition for the formation of the excessive rain. Under the dynamic steering of a southwesterly flow ahead of a north trough and that on the southwest side of the West Pacific subtropical high, the mesoscale vortex (or the cyclonic wind perturbation), after its genesis,moved northward and brought about enormous rain in most parts of Guangdong Province through providing certain lifting forcing for the triggering of mesoscale convection. During the development of the mesoscale vortex, heavy rainfall was to a certain extent enhanced by the mesoscale topography of the Yunwu Mountain in Guangdong. The effect of the Yunwu Mountain is found to vary under di erent prevailing wind directions and intensities. The location of the heavy rainfall was in a degree determined by the trumpet-shaped topography of the Zhujiang Delta. It is identified that the topographic effect on precipitation depends on the relative position between the terrain and the mesoscale storm systems. The short distance from the SCS to South China facilitates the moisture transport, which offers ease for the heavy rain to form in South China. Finally, the role played by land-sea contrast in the fast intensification of the MCSs in South China is not yet clear, and the interaction between the MCSs and the mesoscale vortex needs to be clarified as well.