| Because of the influence of the complex topography of Tianshan Mountains and high temperature and drought in summer, heavy rain will help cool and ease the drought in the hot-dry season. On the other hand, there are the characteristics of dispersed, shorted duration and high intensity in Tianshan area. Once a heavy rain occurs, it will seriously threaten people’s lives and property. Therefore, no matter from which aspects to analyze, it is very necessary to strengthen the analysis and study of heavy rain occurred in Tianshan area.A heavy rainfall happened on 22-23 June 2010 in the eastern part of the north Tianshan Mountains is selected as a research case in this paper. Synoptic background, vapor and unstable conditions are analyzed by using 1°×1° NCEP/NCAR FNL data. Analysis results show that Upper-level jet stream and south asia high double size at 200 hPa,, synoptic situation of "two ridges a slot" at 500 hPa, wind field convergence occurring above rainfalls area at 700 hPa and cold air piling up and lifting in the middle Tianshan Mountains area provided favorable circulation background of storm rainfall. The source of water vapor transport is that the vapor comes from the Atlantic Ocean with anticyclonic flow direction which carrying the vapor from Arctic Ocean and Caspian Sea, passing the Balkhash Lake into Xinjiang region. Showalter index and the index of air mass reflect that the instability region is mainly located in 44°N and in the northeast region of Xinjiang, which is covered by not stability region basically.On this basis, Weather Research & Forecast (WRF) is used to study the kinetic process and microphysical mechanism of this storm. Analysis results show that when the northwest airflow encounters Bogda Mountain, vertical airflow is forced up in the windward slope, and forming convective cells. Convection cells maintain in front of the mountain owing to the obstruction. And multi-cells in front of the mountain approach toward the mountain. Meanwhile, evaporative cooling caused by precipitation and the friction drag of sinking airflow close to the windward slope drainage of the surface, and environment in northwest airflow hedge convergence give rise to convection area extension from the hillside to the piedmont. Graupel and cloud water are the main source of rain, and distribution of graupel is closely associated with cloud water. Accretion of cloud water by graupel is the main microphysical process of graupel, and accretion of rain and collection of snow by graupel are the secondary effect to form graupel.In order to discuss the influence of dynamics and microphysical mechanism is discussed in the middle and eastern part of the north Tianshan Mountains (Bogda Mountain), this paper made the sensitivity test of changing the terrain height, the results show that when the terrain height of the middle of the north Tianshan decreases, the precipitation area is mainly affected by the northwest airflow, giving rise to upward movement range increasing, but the intensity weaken. And heavy rain occurred with a delay. In aspects of rain distribution, rainfall in the eastern part of the north Tianshan Mountains area is reduced. In terms of microphysical structure, the main microphysical source of snow, graupel and rain is not changed, but mixing ratio of hydrometeor and percent conversion are reduced. When Bogda Mountain height decreases, there is a weak convection on the east side of Bogda Mountain, vertical upward movement weakening, which resulting in the precipitation in the eastern part of the north Tianshan Mountains is decreased. Through analyzing cloud microphysical process, mixing ratio of hydrometeor is significantly reduced, especially in the mixing ratio of graupel. The conversion rate of snow, graupel and rain is evidently decreased, the process of graupel melting into rain weakening, but the collection of cloud water by rain has no evident changes. |
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