天山北麓一例黑风暴天气的成因

利用常规观测资料、地面气象自记记录和静止气象卫星云图资料对1998-04-18由南支冷锋爆发性发展生成的干飑线触发的天山北麓黑风暴天气过程进行了分析。结果表明：强沙尘暴和黑风暴过境时风、压、温、湿在短时间内出现突变，跃变幅度黑风暴远大于强沙尘暴，黑风暴嵌于强沙尘暴时变化幅度比强沙尘暴弱。沙尘壁具有等温性，水平厚度4～9km。影响黑风暴的天气因素中，地面大风形成主要源于冷锋的加强和锋后强冷平流，高空急流的加强及其形成的次级环流使高空动量有效下传到地面则是另一个重要原因。混合层是有利于黑风暴在干旱环境形成的大气层结特征，这次过程深厚混合层的形成是深厚干对流和黑风暴产生的主要原因，而它的形成则是长时间地面加热的结果。

A Case Study of a Black Storm at the Northern Foothills of Tianshan Mountains

Using conventional observational data, autographic records and satellite images, the authors studied a black storm caused by a dry squall line associated with a southern cold front at the northern foothills of Tianshan Mountains in Xinjiang on April 18, 1998. The results show that winds, pressure, temperatures and humidity all have abrupt changes when the black storm passes through meteorological stations, changes associated with the black storm are much stronger than that with dust storms. The dust wall is almost isothermal, with horizontal thickness of about 4-9km. Strong surface winds were mainly caused by rapid intensification of the cold front and the cold advection. Meanwhile, the development of a upper level jet and strong downward transport of momentum by the sinking branch of the upper level jet's secondary circulation are also important for the formation of strong surface winds. The mixed layer is the crucial stratification factor affecting the formation of black storm because it can minimize the energy required to develop the deep dry convection and so is favorable for the formation of black storm in drought region. In this case primary reason that the black storm occurres at the northern foothills of Tianshan Mountain is the formation of deep mixed layer, which is induced by the surface heating for a long time. So the deep dry convection can be developed when the cold front passes and the black storm occurres there.