Research On The Mechanism Of The Meiyu Front Rainstorm Over The Lower Reaches Of Yangtze River In 2009

Using the objective reanalysis data and observed rainfall, the large-scale circulation characteristics of Meiyu over the lower reaches of Yangtze River in 2009 are diagnosed. And the structures of Meiyu front of the two intense rainstorm processes are comparative analyzed. The results show as follows:(1) Meiyu-period in 2009 is shorter than that in average years. The date of Meiyu onset is later and the date of Meiyu ending is similar. The days of continuous rainfall are less. The totally three intense rainstorm programs occur during the whole Meiyu period. (2)On upper level of troposphere, the South Asian High is powerful. The lower reaches of Yangtze River located on southward of west jet and northward of south jet. The northly wind area has the strong divergence filed. The strong east wind both occur near 110E during the two rainfall processes, which maximum wind speed exceed 25m/s.The southwest wind from the Bay of Bengal and South China Sea and the eastward wind from West Pacific Ocean combine into one unit which provides plenty of water vapor for the rainfalls. Observed rainfalls areas located at the south side of 200hPa upper level jet stream, which is at the left side of low level jet stream and the right side of shear line at 850hPa.(3)In the Meiyu front filed, the temperature gradient is not clear. Meanwhile, the gradients of equivalent potential temperature and water vapor flux are very clear. (4) Comparative analysis of the structure of Meiyu front between the two intense rainfalls shows that the first rainstorm distributes in northeast-southwest and the second rainstorm distributes nearly in west-east which over the lower reaches of Yangtze River distributes in southeast-northwest. The two observed rainfalls take place on the south side of the Meiyu front. As for the thermal and dynamic structures, the first rainstorm program has more favorable conditions than the second. And then based on routine observed data, data on Temperature of Black Bold observed by satellite and objective reanalysis data, a Meiyu front heavy rain occurred over the lower reaches of Yangtze River from 6 to 7 July 2009 was simulated and analyzed. The occurrence and development mechanism are emphatically studied. It is found that a shear line is on the north side of the lower reaches of Yangtze River at low level of troposphere. It causes the occurrence of low-level jet in the troposphere lower level. The diagnostic analyses of conventional physical quantity show that the divergence filed composed of the west jet and east jet at upper level has the suction function to lower layer atmosphere which enhance the vertical ascend movement. The warm and wet flux brought by the lower jets provides sufficient water vapor and causes the structure of potential instability to form and maintain. The configuration of vorticity filed and divergence field in lower and upper troposphere is advantageous to upward motion. The environment on the south side of Meiyu front is high-powered and unstable where the lower instability energy is transported upward by ascend motion which supplies enough energy. It is found through the analysis of moist potential vorticity that the zero line movement of MPV1 at 950hPa has certain indication of the ovement of rainfall area. The extreme value center has certain corresponding relation with strong rainstorm. The convection instability has maintained during the whole rainstorm and baroclinic instability has taken place during the first half of rainstorm process. The simulation result served by WRF meso-scale numerical model shows that several meso-β-scale systems and meso-γ-scale systems on the south side of shear line. This paper analyzes one of several meso-β-scale systems. It shows that meso-scale low-level jet first occurs. And then meso-scale ascend movement occurs on the left front of lower-level jet core. The convergence center takes place on the lower level and the divergence center locates on the middle level. When the water vapor transports to condensation height, the latent heat of condensation releases and let the temperature of middle level atmosphere go up. The convergence enhances rapidly and extends upward. The coupling of intense divergence on upper levels, meso-scale jet on lower levels and meso-scale vortex let the system develop and eastward move continuously.