2008年1月甘肃省连阴雪特征及成因分析

利用2008年1月甘肃省连阴雪实况资料及同期欧洲中期天气预报中心(ECWMF)逐日客观分析场资料、 1951_2000年NCEP/NCAR500 hPa高度再分析资料， 分析了甘肃省持续20天之久的低温连阴雪天气的特征， 以及这次过程形成和发展的天气气候原因， 并针对过程中强降雪发生时段进行了完全矢量散度诊断及数值模拟分析。结果表明： 2008年1月甘肃省的低温连阴雪天气具有降雪异常偏多、 气温异常偏低、 持续时间长等特点, 属60年不遇。大气环流的异常及稳定维持为连阴雪天气的发生提供了气候背景， 具体表现在阻塞形势建立并长时间异常维持， 西太平洋副热带高压的异常加强西伸、 维持。阻高崩溃及副热带高压减弱东退， 促使这次连阴雪过程减弱、 结束。诊断分析和MM5数值模拟结果表明， 高低空形势场以及中尺度的动力、 湿度因素的有效配合为较强降雪的出现提供了必要条件。在甘肃南部到中部出现较强降雪时， 青藏高原南支槽前有一支明显的偏南风向北输送， 并在高原东南侧形成辐合， 同时在高原中部也有明显的高原切变线发展东移。强降雪对应区域上空有明显的低层辐合、 高层辐散， 而且垂直上升运动发展旺盛。

Analyses on Characteristics and Formation Cause for Durative Snowfall of Gansu Province in January 2008

Using the real sustained snowfall data and objective analysis ECWMF data in January 2008, and the NCEP/NCAR 500 hPa reanalysis data from 1951 to 2000, the characteristics and cause of formation for the sustained snowfall process occurred in Gansu Province of January 2008 are investigated in this paper.(Q)-vector diagnose and numerical simulation and analysis of heavy snowfall are studied. The results show that the durative snowfall has some characteristices such as more snowfall, lower temperature and longer duration, etc. There is not a same one in the past 60 years. Such atmospheric circulation anomalies as abnormal durative blocking high and strengthened subtropical high-pressure form the climatic background of this sustained snow weather process. Blocking high collapsing and subtropical high-pressure weakening and moving eastward bring about the end of this process. Dignose and numerical simulation results from MM5 illustrate that the coordinate between upper-air and lower-air and favorable mesoscale thermodynamic and dynamic conditions supply the necessary requirement to the formation of heavy snowfall during this process. When heavy snowfall appears from south to middle parts of Gansu Province, strong south wind extends northward along east verge of Qinghai-Xizang Plateau to middle Gansu Province and forms convergence there. Meanwhile, shear line grows rapidly and moves eastward from middle Plateau. Prominent convergence in lower-air and divergence in upper-air exist and vertical wind develops intensely over the area where heavy snowfall appears.