CHARACTERISTICS OF WESTERN SOUTH CHINA PRE-FLOOD SEASON PERSISTENT RAINSTORM IN JUNE 2020 AND ITS MECHANISM.

From the end of May to the first ten days of June in 2020, a 12 day heavy rain process occurred in Guangxi. Due to the extreme complexity of its formation, it is necessary to analysis deeply and reveal its characteristics and causes. Based on hourly EC ERA5 0.25° x 0.25° reanalysis and real data, a multiscale comprehensive analysis of the process is carried out. The results show that:(1) This process has the characteristics of long duration of heavy rainfall, wide range, strong intensity, large accumulated rainfall, breaking the historical record of 24 h rainfall in many places and particularly serious disaster. (2) The process occurs under the circulation background of favorable configuration of mid-and-high-latitudes circulation, subtropical high, Bay of Bengal trough, low-level jet and South Asia high. There is no stable confrontation between the South Asian high transition layer and the subtropical high. there is a Transition from the meridional circulation to zonal circulation in mid-and-high-latitudes. The influence of cold air on South China is fluctuating, it causes alternating effects of cold front, stationary front and warm-sector. Thus, the local, regional and provincial rainstorms occur alternately, which is obviously different from the previous continuous rainstorms. (3) The atmosphere accumulates large energy, and huge energy is accumulated before the rainstorm in the fronts and warm-sector. Energy accumulation and effective release are positively correlated with the maximum hourly rainfall. The high temperature and humidity of the lower atmosphere and the deep wet layer of the whole atmosphere and the smaller CIN and lower TCL_P are conducive to energy accumulation and convection triggering. Warm sector and static Front Rainstorm have easier environmental conditions for triggering. The extreme convergence of energy, power and water vapor leads to the extreme rainfall. (4) The frontal rainstorm is triggered by the surface mesoscale frontal area, shear line and terrain. The combination of systems and the enhancement of low-level environmental wind field organize the development of convection. The warm-sector rainstorm is triggered by the cyclonic shear on the left side of the boundary layer jet axis, the convergence of low-level jet outlet and topographic uplift. The pulsation of jet stream enhances the development of convection. (5) The frontal rainstorm is a mobile cloud belt, and the warm sector rainstorm is a less mobile cloud cluster. The lower the cloud top brightness temperature, the greater the maximum hourly rainfall. The cloud top brightness temperature lower than 200 K can be used as the criterion for the maximum hourly rainfall greater than 50 mm/h. The maximum hourly rainfall occurs after the minimum cloud top brightness temperature reaches the minimum value. The heavy rainfall begins with the maximum radar RCS ≥ 45 dBZ and lasts for the maximum radar RCS ≥ 50 dBZ. The front echo is cold cloud echo with high centroid, and the warm-sector echo is warm cloud echo with low centroid. (6) Topography plays an important role in triggering and maintaining rainstorm, and it is more obvious for warm-sector rainstorm.(7) There are differences between the observation and reanalysis data in the penetration analysis of weak cold air, and the former is easier to capture the convective trigger. [ABSTRACT FROM AUTHOR]