Cause analysis of persistent torrential rain associated with monsoon depression occurred in Guangdong on August 2018

Abstract

Using the NCEP finally reanalysis data (1°×1°),intensive AWS (auto weather station) data,Doppler radar data in Guangdong,the formation mechanism of a persistent torrential rain event associated with monsoon depression occurred in Guangdong from 27 August to 1 September 2018 was analyzed. The main conclusions are as follows. (1) Factors such as the atmospheric circulation environment of a strong stable SAH,the anomalous two-ridge of the western Pacific subtropical high and the strengthening northward southwest monsoon low-level jet are the reason why the monsoon depression has been strengthened considerably and moved slowly to the west. (2) The life cycle of the monsoon depression can be divided into two phases,which include the volatility increasing phase and the weakening phase. The evolution of the intensity of the monsoon depression coincides with the area of the heavy rainfall,but not with the maximum daily rainfall. When the monsoon depression weakened and moved away on August 30,the record breaking extreme torrential rain occurred in the eastern Guangdong is covered by the trough of the monsoon depression. (3) The most intensive boundary-layer wind convergence,the strongest water vapor convergence and the most instable convection stratification all occurred at the stage when the extreme torrential rain happened. Additionally,a surface convergence line is originally formed between the warm moist air southerly flow from the ocean and a cold dome generated by previous convection. The stagnation of the western part of the convergence line is blocked and uplifted by the Lotus Mountain,triggering and maintaining the echo of extreme torrential rain. (4) The evolution of radar echo can be divided into three stages:stage of weak scatter echoes moving westward,stage of belt-shaped echoes associated with strong and short rainbands moving northeast and stage of weakening echoes moving southeast. The echo of heavy precipitation has a typical structure of convective precipitation in the warm cloud with low mass center.

Keywords

• monsoon depression
• persistent torrential rain
• low-level jet
• surface convergence lines
• strong and short rainband