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北京地区一次平流雾过程的分析和数值模拟
The Analysis and Simulation of an Advection FogEvent in Beijing
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摘要: 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。Abstract: 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.
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Key words:
- advection fog;
- radiation cooling;
- inversion
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图 1 2007年2月20日19:30 FY-2C气象卫星雾监测图像 (a) 及21日02:30NOAA-17极轨气象卫星图像 (b)
Fig. 1 Fog image monitored by FY-2C at 19:30 20 Feb 2007 (a) and image of NOAA-17 at 02:30 21 Feb 2007(b)
图 2 2007年2月20日06:00-22日00:00首都机场能见度、气温和水汽压随时间演变
Fig. 2 Time evolution of visibility, temperature and vapor pressure from 06: 00 20 Feb 2007 to 00:00 22 Feb 2007 in Beijing Capital International Airport
图 3 2007年2月20日12: 00地面气压场 (粗黑色实线)、不低于2 g/ kg比湿 (阴影)、流场 (细实线) 和测站风 (a) 以及同时次北京地区自动气象站风场分布 (黑色粗虚线表示辐合线) (b)
Fig. 3 Surface pressure (thick solid lines), specific humidity (shadow) more than 2 g/ kg, stream field (thin solid lines) and station wind at 12: 00 20 Feb 2007 (a) and wind distribution of A WS around Beijing at the same time (black dotted line:convergent line)(b)
图 4 2007年2月20日18: 00(a) 和19:00(b) 模拟的雾区 (能见度小于1 k m, D03)
Fig. 4 The simulated foggy area (visibility less than 1 km, D03) at 18:00(a) and 19: 00(b)20 Feb 2007
图 5 2007年2月20日12: 00至21日06: 00 MM5(D03) 模拟的首都机场单站温度-露点廓线图
细实线为温度, 粗实线为露点
Fig. 5 The profiles of temperature versus dew point in Beijing Capital International Airport from 12:00 20 Feb 2007 to 06:00 21 Feb 2007 simulated by MM5(D03)
thin solid line:temperature; thick solidline:dew point
图 6 模拟 (D03) 的2007年2月20日12: 00(a, b), 20日18:00(c, d), 21日00:00(e, f), 21日06:00(g, h), 22日00:00(i, j) 沿40°N, 116°E至38°N, 120°E所做温度、风和液态水含量垂直剖面 (a, c, e, g, i; 其中低层密集等值线表示液态含水量, 单位: g/ kg) 以及地面层能见度、风场水平分布 (b, d, f, h, j; 其中粗实线表示能见度小于1 km的雾区; 直线表示垂直剖面的投影) (虚线表示温度; 箭头为风矢; 三角表示首都机场大致位置)
Fig. 6 Vertical profiles of temperature, wind, liquid water content along 40°N, 116°E to 38°N, 120°E (a, c, e, g, i; the denser contourat lower layer is for liquid water contert, unit:g/ kg) and distribution charts of surface layer visibility and wind field (b, d, f, h, j; the bold solid line is for the area with visibility less than 1 km) simulated at 12:00 20 Feb 2007 (a, b), 18: 00 20 Feb 2007 (c, d), 00: 00 21 Feb 2007(e, f), 06:00 21 Feb 2007 (g, h), 00:00 22 Feb 2007 (i, j)(the broken line is for temperature; arrow is for wind vector; the triangle is the approximate location of Beijing Capital International Airport)
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