内蒙古中西部一次强沙尘暴天气过程的诊断分析——物理量场分析

利用NCEP/NCAR再分析资料和自动站观测资料对2009年4月23日-24日出现在我国内蒙古中西部的一次强沙尘暴天气过程,从物理量场方面对此次沙尘暴的影响机理进行了分析。结果表明:这次强沙尘暴的垂直速度、散度、涡度和相对湿度的分布情形与沙尘暴的发生发展有明显的对应关系。此次沙尘暴发生之前,大气低层是下沉运动区,成为弱辐合和辐散并存之势;高层为上升运动区,随着低层的下沉运动转为上升运动,形成低层辐合高层辐散的配置,导致强沙尘暴的发生。从地面至高层转变为下沉运动区时,沙尘暴减弱至消失。涡度场上,沙尘暴发生前,低层是弱的正涡度分布,在沙尘暴发生期间正涡度迅速增强,对应蒙古气旋的发展增强,之后,正涡度减小,此时气旋中心移出内蒙古境内,至24日夜间,气旋东移入海,内蒙古地区的沙尘天气结束。此次沙尘暴发生前和发生期间的低层大气相对湿度在30%以下,近地层的干燥大气,也是内蒙古中西部出现强沙尘暴的条件。由于相对湿度迅速增大至50%以上,沙尘暴开始减弱以至消亡。在沙尘暴发生期间的上空,螺旋度的绝对值达到最大,且螺旋度的垂直分布呈明显的上负下正形势;而在沙尘暴发生前和减弱时,或者螺旋度的垂直分布值较小,或者没有形成螺旋度正负相间的明显分布。

Diagnostic analysis of a server sandstorm in the midwest Inner Mongolia-Quantities of the field

According to the encryption sounding data,the automatic meteorological observing data and the conventional observing data,we analyzed a case of weather events occurred in the midwest Inner Mongolia Autonomous Region on April 23-24,2009.The mechanism of physical field influencing sandstorm,as well as the variations of ground meteorological elements and its internal dynamical and thermal changes in event process were studied.The result showed that:(1) There is very good corresponding relation between vertical velocity,divergence,vorticity,relative humidity and storm.Before the dust storm happened,there were downdraft in the low layers and updraft in the upper,and when the storm happened,the downdraft in the low layers turned to the updraft,which is good for the dust storm.When the updraft in the low layers turned to the downdraft,the dust storm ended;Before the dust storm happened,there were the divergence in the low layers and convergence in the upper,and there were the convergence in the low layers and divergence in the upper while the storm happened,and when the dust storm ended,there was the divergence in the low layers.Before the dust storm happened,the vorticity was positive,and then it was bigger when the dust storm happened,and it was going to weaken while the dust storm died away.The humidity was less than 30% when the dust storm happened;after it ended the humidity was more than 50%.(2) As an important physical parameter in analyzing and predicting strong convective weather,the helicity also was a good indicator in the forecasting of sandstorm.The distribution of helicity over the sandstorm area was negative at higher levels and positive at lower levels,but there was not the phenomenon before or when the sandstorm weakened.The distribution of vertical speed over the sandstorm area was negative at higher levels and positive at lower levels,which was good for the taking place of the sandstorm.There was definite relationship between the evolvement of helicity's negative value at higher levels and the occurrence of sandstorm.