下垫面物理过程在一次北京地区强冰雹天气中的作用

Effects of Underlying Surface Physical Process on a Severe Hail Event Occurred in Beijing Area

北京地区下垫面具有地形和城市边界层的双重性造成了天气变化的多样性。结合2005年5月31日发生的强冰雹天气个例,分析了下垫面物理过程在北京强对流天气中的作用。结果表明,山区的地形、城市边界层对雹云发生发展的不同阶段以至冰雹的落区、强度等都有相当大的影响。其中,此次降雹的初始回波是在西部山区地形的热力环流和动力强迫下局地触发的;城市下垫面热力作用形成的地面中尺度风场辐合线以及所造成的上升运动和能量、水汽的聚集,是对流回波在城市中心上空明显增强的主要原因;北部山区的喇叭口地形、西部山区的迎风坡抬升作用对局地强对流的发生、入境积云的发展亦有重要影响。

The dual characteristics of its topography and urban boundary layer in Beijing area result in weather change diversity. The effects of such underlying surface physical process on the strong convective weather evolution are analyzed by combining a severe hail event occurred on May 31 2005 in Beijing, Results indicate that the mountainous topography and urban boundary layer have great effects on the hailstorm evolvement, even on the hail falling area and intensity. The initial radar echoes of this case are sprung locally by the thermodynamic circulation and dynamical forcing induced by the mountainous topography in the west. The primary factors that intensify the convective radar echoes when it moves over the centre city downtown include the surface mesoscale wind field convergence line engendered by urban underlying surface thermodynamics effects, as well as the air ascending and the congregating energy and water vapor. Also, the trum-pet-like topography in the north mountainous area and the windward slope uplifting in the west have significant influence on the occurrence of local strong convective process and the development of those moving-in cumulus.