北京地区一次平流雾过程的分析和数值模拟

2007年2月21日北京地区发生了一次严重的平流雾, 对广大群众的出行和交通影响甚大, 属高影响天气事件。该文利用首都机场地面观测、北京市自动气象站观测以及NCEP分析场等资料对该过程进行分析, 同时利用MM5模式对该过程进行数值模拟研究。分析表明:造成北京地区此次平流雾的主要天气形势是弱低压辐合型。平流雾发生前, 北京地区没有明显冷空气侵入, 大气层结相对稳定, 地面观测到中尺度辐合线, 其南侧的东南气流向北京地区输送了水汽, 为夜间雾的形成提供了良好的基础条件。模拟结果表明:模拟的雾区范围及其移动基本与实况吻合, 显示了中尺度模式预报平流雾的潜在能力。进一步分析表明:雾区的边缘具有明显的水平温度梯度; 在贴地面层东南气流被雾区阻挡偏向西, 在雾区前沿辐合; 雾区的逆温区前沿930 hPa以下存在一个明显的垂直热力环流, 雾区下沉, 雾区前沿上升。

The Analysis and Simulation of an Advection Fog Event in Beijing

A dense advection fog event occurs in Beijing on 21 February 2007. Since the fog occurs during the Chinese Spring Festival, this unexpected event makes a mess of the traffic. The surface observation data of the Beijing Capital International Airport, the auto-observations across Beijing area and NCEP 1°× 1° analysis are used to analyze this process. And a numerical simulation is made using the meso-scale model MM5. The analyses and simulation show that weak convergent low is the primary weather pattern of the dense fog event. There is no obvious cold air intruding and the atmospheric stratification is relatively stable prior to the event. Meanwhile there is a meso-scale surface convergent line, at the south of which moisture is transported to Beijing area by the southeast airflow. These weather conditions offer good basic conditions for the night-fog formation. The simulation of this advection fog event indicates that the simulated fog area and the motion are basically coincided with the actual situation, which show the potential ability of MM5 to forecast advection fog event. And further analyses shows that 6-7 h before the occurrence of the fog, inversion layer first occurs in the ground layer, and then the inversion layer top continuously rises and becomes thicker. Moreover, the coincidence or the separation of temperature curve and dew-point curve correlate with the occurrence or dissipation of fog. Besides, there is obvious horizontal temperature gradient at the front edge of the fog area, and at the surface layer southeast airflow is blocked by the fog and turns to west, then converges at the front edge of the fog. In addition, below 930 hPa, at the front edge of the vertical temperature inversion area, there is a vertical thermodynamic circulation with downdraft at the fog area and updraft at the front edge of the fog area. During the event, there is a complete warm center above the fog area, thick inversion layer and weak updraft. Such stable situation causes the long duration of the fog. And during the dissipation of fog, the large area of fog is separated into patches. In some areas where temperature rises faster, the stronger ascending motion destroys the inversion, so the fog area reduces as a result.