| A heavy rain storm which happened in Sichuan province from July 31 to August 1 2009 is analyzed by using regular meteorological data, NCEP/NCAR 1°×1°reanalysis data and daily ground station precipitation data. By using the NCEP reanalysis data and mesoscale filter technology and the model output with high spatial and temporal resolution, the mechanism and characteristic of the process of this heavy rain can be studied in detail. Synoptic analysis showed that during this heavy rain 500hPa circumpolar latitude circulation took the pattern of typical two-trough and two-ridge. There was a closure trough high up in the air of Tibet. The southeast winds in the subtropical high pressure belt brought a lot of water vapor. The eastern Sichuan was controlled by the stronger southwestern airflow. Through the diagnostic analysis and model simulation of this heavy rainfall can be found that Divergence field, water vapor flux and water vapor flux divergence had good indication on the rainfall. Rich water vapor provided the condition of the occurrence and development of the rain storm. The convergence of water vapor in the lower level was the main source of the rain storm. Divergence in the upper level while convergence in the lower level was conducive to the development of the development of convection, additionally, the convergence of water vapor was mainly due to the convergence of flow in the lower level. The barotropic term and baroclinic term of MPV comprehensively reflected the convective instability and baroclinic instability of rain storm zone. High value PV in the high level downloaded and the dry cold air of high value PV enhanced the disturbance in the lower level, forcing the elevation of the warm air in the lower level, promoting the release of convective unstable energy and latent heat and the development of heavy rain. Vertical wind shear made the mean flow kinetic energy into eddy kinetic energy, stimulating the generation of gravity waves. When gravity waves passed through developmental cumulonimbus, increasing the ascent speed in the cloud, making the instability released, thereby produced precipitation. There were distribution chains of convergence and divergence existing over the region of heavy rain which indicated the existence of gravity waves. The convergence and divergence zone began to form in the slope at west Sichuan plateau and gradually moved eastward, whose value was gradually decreased. The affect of gravity waves to this precipitation process was proportional. The convergence and divergence zone corresponded to the strong vertical movement region and with the development of gravity waves the increasing of flow was more strongly, which made to the development of convection. Then gravity waves continue moved eastward and reduced gradually until the final demise. The geostrophic adjustment process inducing by unbalanced flow near the exit area of the jet stream in the upper level also played a very important role in the occurrence and development of mesoscale gravity waves. The gravity waves near the exit area of the jet stream in the upper level were induced by the process of geostrophic and shear instability. Form the generation order of geostrophic adjustment and shear instability, geostrophic adjustment was advanced to shear instability. Therefore, we can consider that nonlinear balance equation could predict the movement of precipitation while the Richardson number had the simultaneous characteristic with precipitation. Additionally, during this heavy rainfall, fluctuations basically met the waveguide conditions of Lindzen, etc. however, in the analysis of the previous text the gravity waves continue moved eastward and reduced gradually until the final demise. The existence of gravity waves in July 31 and the subsequent development had significant affect on rain storm but when it came to August 1 convection played a major role in the development of rain storm and gravity waves were no longer affecting on rain storm.
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