The Simulation And Analysis Of The Effect Of Topographic Forcing On Two Typical Precipitation In Hunan Province

There are two factors, which determine the rainfall and the area location for rainfall, is atmosphere circulation and topography. However, in the meso-and-micro scale weather system, the effect of topography on rainfall could be strong under the even distribution of water vapor. Therefore, topography's effectiveness has always been considered in the analysis of weather events.Hunan province is in the monsoon climate area. Its topography is very complicate, and then Hunan precipitation distribution is not even. Based on mean rainfall amount, the whole province can be classified 6 climate areas. Statistical data suggests that the rainfall in mountain area is more than in basin and plain. For weather systems, frontal precipitation is distinct in its middle and north, and in southeast and southwest, typhoon's influence is stronger than others. In the thesis, the topography influence on the precipitation distribution of Hunan province is studied by analyzing a heavy precipitation happened during of 10/28/2008 to 11/07/2008 and the No.0604 "Bilis" typhoon landed in July of 2006. The numerical model has been employed to simulate the weather processes. Moreover, topographic sensitivity experiments have been preformed to study the terrain height influence on the precipitation distribution and the topographic forcing role in the mechanism.The Results indicated that the topographic forcing can block the flow and make atmospheric stratification unstable during the progress of frontal precipitation. The airflow directions and the convection among the different layers may be influenced because of mountains. According to the sensitivity experiments, the velocity of convection and the extreme value of vorticity and divergence are changed after the topographic height adjustment. The deep convection and the release of unstable energy can act as a trigger. As a result, the precipitation is triggered by the dynamic forcing, and also the atmospheric stability of leeward slope is affected. That is, mountain effects on the trigger mechanisms of water vapor transportation and precipitation can be considered as a factor, which caused the drought in the middle of Hunan.For the investigation of typhoon, the typhoon wind field would be distorted by topography forcing. Topographic sensitivity experiments suggested that, due to the resistance in low layers, the wind speed will slow down and the strong convergence location will shift, so that the range and amplitude of high-level jet and the path of cyclone move will be changed. Thus, the distribution of rainfall can be affected through the jet introducing by topographic forcing. In addition, rainfall is almost distributed in mountain area, and performs the precipitation in the windward slope. The trend of Nanling is perpendicular to the direction of the typhoon wind field which moved west forward. Consequently, rainfall in southeast is more than other areas in Hunan. The reason also can be used to reveal the strong influence of westward moving typhoon on Hunan weather.