陕西冬季一次大雾天气生消机制的数值模拟

Numerical Simulation of Mechanisms of Formation and Dissipation of a Dense Fog of Shaanxi Province in Winter of 2005

利用非静力平衡中尺度模式WRF、NCEP 1°×1°再分析资料及常规观测资料,对2005年12月30～31日发生在陕西的大雾天气过程进行了数值模拟,分析了大雾天气过程形成的主要原因及雾的生消机制。结果表明,WRF模式能较好地模拟出雾的水平分布特征、强度和生消过程,反映出实际雾的生消变化规律。适当提高模式水平分辨率能较明显地改进模拟效果。这次大雾为平流辐射雾,长波辐射冷却是大雾形成和发展的主要原因。逆温层的发展、维持和近地面层较高的相对湿度对雾的产生和发展起着重要作用。近地面层有弱的水汽辐合是大雾发展和维持的主要原因之一。大雾形成和发展阶段,900 hPa以下的辐合上升运动和900 hPa以上的辐散下沉运动有利于在上升和下沉运动区的界面层中形成逆温层,逆温层的形成有利于低层水汽的积累。随着高空转为辐合上升运动,900 hPa以下为辐散下沉运动,接着日出后,太阳短波辐射增温等的共同作用,使逆温减弱直至被破坏。中高云的存在影响了近地面层逆温的形成和加强,推迟了雾的形成和消散。暖平流的输入有利于逆温层的形成发展。

Using the WRF model,NCEP reanalysis data and the routine meteorological observation data, a dense fog process in Shaanxi Province during December 30～31,2005 is numerically simulated.The results show that the WRF could well simulate the occurrence and development of the fog as well as the horizontal distribution feature and intensity.The higher horizontal resolution could improve the simulation results about horizontal range, formation and dissipation of fog.It is an advection-radiation fog.Long wave radiation cooling is very important for the formation and maintenance of fog,in addition to the development and maintenance of inversin layer and high relative humidity.Weak water vapour convergence near the ground is one of important reasons in the prolonged fog process.During the period of formation and development of fog,convergence ascent motion below 900 hPa and divergence sinking motion above 900 hPa are favor of forming inversion layer in the layer where ascent and sinking motion encounter.And the inversion layer is conductive to the accumulation of lower-level water vapor.The dissipation is leaded by the convergence ascent motion at upper level,divergence sinking motion below 900 hPa and shortwave radiation.Medium or high cloud affects the formation and strengthening of inversion near the ground and postpones the formation and dissipation of fog.Warm advection is favor of forming inversion layer.