Southern Rainstorm Climatic Characteristics, As Well As Trigger And Sustaining Mechanisms

The primary datasets used in this study include the NCAR/NCEP 1°×1°reanalysis data, satellite remote sensing data, daily observational precipitation at 740 stations in China, daily precipitation at south of Yangtze River, hourly precipitation from mesoscale automatic station and TRMM datasets. We analyzed the climate characteristics, vapor transportation and distribution of double rainbands form heavy rainfall systems in southern china by using the statistic method, potential vorticity analysis and numerical simulation. This paper also investigated the mechanisms related to the heavy rainfall in the first wet spell of South China rainy season, and the follows are the results:We found that there were two rain belts in the regions to the south of the Yangtze River and south China by analyzing the distribution of continuous rainfall in 1994,1998,2005. The first one usually locates to the north of low-level jet, while the other one to the south or around the center of low-level jet.The characteristics and mechanisms are very different for the three cases in 2001. The first case is characterized by a cold front with significant effect of cold air masses. The second is characterized by strong convection resulted from the interaction between the warm and wet stream form south and the southwestern low-level jet stream. The third case is characterized by synoptic scale convection associated with the landing of typhoons or tropical cyclones.We have discussed the climate characteristics of vapor transport in the first wet spell of South China rainy season in drought and flood years. The results showed that two vapor transport processes are observed. One persists form April to the onset of monsoon and the other time lasts form the onset of monsoon to June.We simulated one meiyu case occurred in June 1991 using the fifth generation Penn State/NCAR mesoscale model (MM5). The results showed that an unstable disturbance of thermal convection excited an inertial gravity wave at the early stage of rainfall. Then the low-level inertial gravity wave spread southward. The high-level inertial gravity wave which spread southward was beneficial to the generation of several rain belts. At the middle stage of rainfall, the high-level gravity wave propagated counterclockwise around the center of the vortex, and the rainfall enhanced. The high-level inertial gravity wave which spread northward resulted in the development of low level vortex and rainfall.Statistical analysis based on daily precipitation data at south of Yangtze River in the region to the south of Yangtze River from 1958 to 2000 shows that heavy rainfall frequently occur in South China. Consecutive rainfall could happen in South China and in the region to the south of Yangtze River at the same time due to the existence of double rainbands. The northern one was associated with cold front, while the southern one with warn sector.There was always a middle-level front which was associated with a dry sector and horizontal southward temperature gradient in the level from 400hPa to 600hPa over the rainband with a strong warm center at 300hPa over the rain area. A southern rainband was related to the horizontal southward temperature gradient in the level from 300hPa to 600hPa. Besides, there existed an obvious dry core with relative humidity less then 50 percent in the downward stream located in the south of warm sector rainstorm in the level from 200hPa to 600hPa level. The existence of middle-level front and low-level neutral convection could result in the development of slantwise vorticity, the enhancement of low-level positive vorticity and rainfall. When the middle-level front over the southern rain belt approched the front and the northern rain belt, the two dry regions merged and the low-level wet region became narrow which was unfavorable for the development of double rainbands.There are two rain bands to the South of the Yangtze River during consecutive raining processes. The northern one was related to the mass adjustment caused by ageostrophic flow in a westery jet stream. The southern band was associated with eastery jet stream and south Asian high to the south Asian high, the negative du/dt could result in ageostrophic northerly. The westery jet moving southward could result in ageostrophic southerly. The mass adjustment led to a strong up-level divergence and heavy warm rain.A mesoscale numerical simulation shows that the up-level warm rain was associated with middle-level dry invasion. So we need to pay attention the invasion of relatively dry and cold air in the region to the south of rain belt at the middle and upper level when we predict the rainfall in the warm sector.