内蒙古西部一次沙尘暴环流背景和动力机制分析

Circulation Background and Dynamic Mechanisms of a Sandstorm Case in Western Inner Mongolia

利用相关资料对2009年4月23日内蒙古西部地区发生的沙尘暴天气的环流形势及动力机制进行分析。分析表明:强冷空气活动是本次沙尘暴天气发生的主要原因;造成本次沙尘暴的高空系统是斜压性很强的蒙古横槽及槽后的偏北急流,地面系统是强烈发展的蒙古气旋;从动力机制来看,沙尘暴发生时,由于高层辐合、低层辐散而形成的强烈下沉运动,使得高空偏北急流带的动量下传到地面,中低层波动发展形成的次级环流使动量下传效率更高。高空蒙古横槽南压、转竖是冷空气爆发南下的关键,高空偏北急流带以及与急流出口区相对应的地面蒙古气旋的强烈发展是形成沙尘暴的必要条件,地面冷锋后气压梯度的持续加强会加剧冷空气爆发的速度,强烈的高层辐合、低层辐散使得沙尘暴形成的可能性更大。

The weather circulation background and dynamic mechanisms for a sandstorm case occurred in the western Inner Mongolia on 23 April 2009 are analyzed by using observation data.The results show that the main cause for this sandstorm case is strong cold air activities.The upper-level system causing the sandstorm is the deep Mongolian transverse trough and northerly airflow on the east;the surface system is the strongly developing Mongolian cyclone.From the view point of dynamics,when the sandstorm happened,the momentum of northerly airflow aloft was transmitted downward to surface with prevailing descending motion because of convergence at upper levels and divergence at lower levels.The secondary circulation generated by disturbance development at middle and low levels made momentum transmit downward effectively.It is worthy to note that the key points for the storm are the southward movement of the upper Mongolian transverse trough and its rotation to upright position,which caused clod air to break and move southward;the upper northerly jet and the obvious development of the surface Mongolian cyclone around the exit of the jet were the necessary conditions for sandstorm generation;the continuous reinforcement of pressure gradient after the surface cold front speeding up intensified the burst of clod air;and the strong convergence aloft and divergence below increased the possibility for sandstorm generation.