The Numerical Simulation Of A Severe Rainfall In The Southeast Of Hu-Nan Province Caused By TC BILIS

After the tropical storm of BILIS (200604) landing, the Low-pressure circulation stayed in the China east coast area for a quite long time , interacted with the southwest monsoon, and led to a recorded rainfall in the southeast part of Hunan province. In order to reveal the key impact mechanisms of the rainstorm process based on its synoptic and climate background, using the condensed observation data analysis, numerical simulation and numerical model experiments, the following aspects are studied in this article:(1) The characteristics of climatic spatial and temporal distribution of typhoon rainstorms in Hunan province from 1949 to 2006;(2) The synoptic analysis on the rainstorm occurred in southeast of Hunan province with unconventional intensive observational data, such as satellite imageries and Doppler radar data;(3) The diagnostic and quantitative analysis of the heavy rain by using the moist potential vorticity;(4) Model simulation of the rainstorm process. During the simulation, a group of physical and sensitivity experiments have been designed to check the impacts of quantitative moisture flux, surface heat flux and the terrain effects on the location and intensity of the rainstorm for the purpose to clarify the mechanism of the rainstorm process and its evolution characteristics;.(5) Develop a physical conceptual model of the severe rainfall in the southeast of Hunan province.Through the studies mentioned above, the main results are shown as following:(1) After BILIS' landing, it's weakened to a low pressure circulation. It moved slowly since it was surrounded by continent high pressure in the north, subtropical high pressure in the east and equatorial high pressure in the south. Simultaneously the Pacific high pressure strengthened and moved from east to west, the increased potential gradient between subtropical high and the BILIS low pressure system led to strong southwest wind, which compensated the momentum loss attenuated by the surface friction. Furthermore, the water vapor transportation channel in the east part of BILIS maintained and strengthened. In this case, BILIS weakened slowly and its low pressure circulation lasted for a longer time. The water vapor supply through this channel from South China Sea summer monsoon was sufficient to develop a strong meso-scale convection system and the torrential rainfall over southeast part of Hunan province.(2) There were two strong moisture channels; one is the southwest wind from subtropical anticyclone to the north of South China Sea mentioned above, the other and more important is the northeast wind channel from the northwest part of low pressure circulation of BILIS. Both two channels transported water vapor together to the southwest part of Hunan for a long time and lead to form the deep-water vapor layer and vapor flux convergence. The numerical simulation experiments shown that although the southwest monsoon and south channel acted as an important role in the water vapor transportation, but it did not transport the water vapor directly to the southeastern part of Hunan. However, the water vapor went first to the north and northwest counter-clockwise around the low pressure circulation of the BILLS, and then poured to the southeastern of Hunan. Therefore, as for this torrential rainfall, the northeast wind at the north of low pressure circulation of BILIS played a more direct and significant role to the moisture transportation responsible for the intensive rain over southeast part of Hunan. Furthermore, the northward trumpet-shaped terrain of southeast part of Hunan has a positive contribution to the intensity of this rainfall process.(3) The mountain morphology and uneven land surface in southeast of Hunan caused different surface heat flux and affected the location and intensity of precipitation in BILIS case and the climate in the period of 1949-2006. The simulation results show that the surface heat flux can change the location of rainstorm and move the center of storm rainfall to the bigger surface heat flux area. The increasing convergence can enhance the organized strong meso-scale convection system On the other hand, The upward heat flux can change the lower level temperature and humidity led to develop more unstable stratification. and provide more favorable thermodynamic conditions for the rainstorm weather.(4) The diagnostic analysis of moist potential vorticity indicates that the strong convection instability developed during the relative colder air invaded into the trumpet-shaped area of southeast Hunan. With BILIS moving westward, both the areas of strong convection instability of and the vertical shear line of strong wind in low layer appeared on the southwest part of low pressure circulation of BILIS.The precipitation center and the positive center of MPV1 or MPV2 is highly consistent.As a result,the evolvement of MPV should have good indication to the rainstorm.