甘肃民勤“4.24”沙尘暴过程的数值模拟分析

Numerical Simulation Analysis of a Dust Storm on 24 April 2010 in Minqin of Gansu

利用2010年4月24—25日民勤基准站地面逐时温度、相对湿度、风速风向等观测资料和NCEP/NCAR FNL逐6 h再分析资料(1°×1°),基于WRF模式对2010年4月24日发生在甘肃民勤及周边地区的一次沙尘暴过程进行数值模拟试验,在检验模式模拟能力的基础上,从空间和时间上分析此次沙尘暴发生发展机制。结果表明:在沙尘暴发生前,民勤上游地区300 h Pa高空急流加强,急流前的高空辐散抽吸作用是此次沙尘暴起沙的主要动力机制;阿尔泰山附近的冷气团沿祁连山脉北侧进入民勤,并形成冷锋系统;在民勤及附近地区,对流层中下层受冷平流作用,相对湿度持续增加,而近地面存在一热低压系统,地面风速较小,温度较高。在冷锋开始过境时,沙尘暴爆发,高空急流携带的高空动量下传使得地面风速急剧增加,是沙尘暴发生的直接因素。在沙尘暴过程中,民勤地区相对湿度在垂直方向上呈上下湿、中间干的"漏斗状";近地面锋前暖空气和700 h Pa冷平流的相互作用,使锋面逆温出现时间较晚。

Based on the hourly temperature,relative humidity,pressure,wind direction and wind speed in Minqin station of Gans Province and 6- hourly NCEP / NCAR FNL reanalysis data with 1° × 1° spatial resolution from 24 to 25 April 2010,the sandstorm weather process occurring in Minqin and surrounding areas from 24 to 25 April 2010 was simulated by Weather Research and Forecast( WRF)model. On the basis of testing the performance of WRF model in simulating the hourly change of meteorological elements before and after the sandstorm process,the basic characteristics and its occurrence and development mechanism were discussed in time and space.The results showed that the hourly temperature,pressure,relative humidity and wind speed simulated by WRF were close to the observation in Minqin before and after the sandstorm process,and their change trends were more consistent. Before the sandstom,the jet stream on 300 h Pa over Minqin and surrounding area strengthened,and the upper divergence pumping action in the right side of the front area of upper level jet stream was main dynamical mechanism of the dust emission. The cold air mass near the Altai Mountains invaded Minqin region along the north of Qilian mountains,and formed cold front system. The relative humidity continuously rose due to the effect of cold advection at the middle and low troposphere,while the wind speed was smaller and temperature was higher due to the near ground thermal low pressure system over Minqin and surronding areas. The sandstorm erupted when the cold front began to transit in Minqin,and the surface wind speed increased dramatically due to the momentum down from the upper level jet stream,which was direct factor of the sandstorm occurrence. In the process of sandstorm,the virtical structure of relative humidity over Minqin and surrounding areas showed the funnel shape with wet in upper and lower level and dry in middle level. The near ground warm air in front of the cold front and cold advection on 700 h Pa interacted in Minqin,which leaded to the late occurrence of frontal temperature inversion.