导致一例强沙尘暴的若干天气因素的观测和模拟研究

OBSERVATIONAL AND NUMERICAL SIMULATION STUDY OF SOME WEATHER FACTORS LEADING A SEVERE DUST STORM

利用常规观测资料和数值模拟方法对 2 0 0 1年 4月 6～ 8日由蒙古气旋强烈发展触发的内蒙古地区强沙尘暴天气过程进行了分析研究。结果表明 :在这次“锋面气旋型”沙尘暴过程中 ,气旋冷锋是该天气系统引发沙尘暴的主要因素 ,冷锋过境对沙尘暴的触发作用远强于气旋发展。影响沙尘暴的天气因素中 ,地面大风形成主要源于气旋发展和锋后强冷平流 ,高空动量的有效下传则是另一重要原因。高空急流加强及其形成的次级环流使高空动量下传到对流层中层 ,其下方形成深厚混合层使这一动量继续下传到地面 ,是高空动量有效下传的机制。“混合层”可以从本质上反映有利沙尘暴形成的大气层结特征 ,其强度和高度很大程度决定沙尘暴的强弱。这次过程深厚混合层的形成是深厚干对流和强沙尘暴产生的主要原因。而它的形成则是长时间地面加热的结果 ,这也是特强沙尘暴仅发生在内蒙古中部偏北地区而不是下垫面条件更为适宜的内蒙古西部的原因。混合层空气的平流作用对内蒙古中部偏北地区深厚混合层的形成具有相当的贡献 ,其作用程度与地形分布密切相关。地面热通量试验证明地面加热不仅对冷锋过境产生的上升运动强度具有直接影响 ,也影响混合层形成和高空动量下传 ,并因此影响沙尘暴强度。

Observational study and numerical simulation were conducted on a severe dust storm process induced by the intensive development of Mongolia cyclone occurred from 6 to 8 April 2001 in Inner Mongolia. The results showed that the cyclonic cold front in this 'cyclonic dust storm' process was the direct and important portion of cyclonic system inducing the dust storm. Because the cyclonic cold front (associated with strong surface wind, strong frontal raising, etc) is favorable for the formation of severe dust storm and so often accords with it. The strong surface wind, which associated with the dust storm strength closely, was mainly caused by the rapid cyclogenesis and the cold advection. Meanwhile, the great downward transporting momentum was another reason that contributed to the formation of strong surface wind. In west Inner Mongolia, the great momentum at upper troposphere was firstly transported to middle troposphere by the descending branch of the upper level jet's secondary circulation, but under it the downward transportation was not notable because the mixed layer (ML) was shallow. In contrast, in the middle north part of Inner Mongolia, the great momentum was downward transported to middle troposphere because of the development and drop of the upper level jet. In lower troposphere, a deep ML formed there and so transported the great momentum from mid lower troposphere down to the surface. It was the important reason that the 'black storm' accrued there. The ML is the crucial stratification factor affecting the formation of dust storm because it can minimize the energy requiring to develop the deep dry convection and so is favorable for the formation of dust storm. It's depth and strength mostly determined the strength of dust storm. In this case another reason that the black storm occurred in the middle north part of Inner Mongolia was the formation of deep ML, which was induced by the surface heating for a long time. The advection of the ML air also contributed to the formation of deep ML there. So, the deep dry convection can developed when the cold front passed and the severe dust storm occurred there. The sensitivity experiment proved that the surface heating was the crucial factor affecting the dust storm in this case. Firstly, it was the main forcing factor of the cold frontogenesis. Secondly, it induced the deep prefrontal ML developing at middle north part of Inner Mongolia. Thirdly, it influenced the strength of surface wind by affecting the downward momentum transportation in lower troposphere.