2009年冬季华北初雪对流层低层风场及大气层结特征

Wind Field Characteristics in the Lower Troposphere and Atmospheric Stratification of the First Snowfall over North China in Winter 2009

对2009年冬季我国华北一次回流初雪天气进行了数值模拟和诊断分析,重点研究了与回流相关的对流层低层风场的动力热力作用以及大气层结特征。结果表明:初雪是在高空低压槽、低空回流切变线以及地面倒槽锋生的共同影响下产生的,降雪区域随着影响系统向东南方向移动而移动;对降雪起到重要动力作用的偏东风回流主要出现在850hPa及以下层次,其发展一方面与偏西气流形成低空风场辐合线,有利于气流的上升运动,一方面在地形的共同作用下,形成偏南风;偏南风的形成有利于暖空气向降雪区域输送,而暖平流与强冷空气相遇引起了锋生;偏东风回流除了起到重要的动力作用外,热力作用也很明显,其对降雪区域的水汽辐合起到了主要作用;在降雪发生时,对流层低层有明显的降温过程,大气边界层温度降到了0℃以下,850hPa温度降至-5℃左右,该结果表明冬季关注数值模式模拟的对流层低层温度层结特征,将有助于对降水性质做出更准确的预报。

The first snowfall over North China in winter 2009 was investigated through numerical modeling and diagnostic analysis with a focus on the dynamic and thermodynamic effect of the wind field in the lower troposphere and the characteristics of atmospheric stratification. The results show that this snowfall was induced by the combined effect of the trough in the middle troposphere, the shear line in the lower troposphere, and the inverted trough on the ground. With the southeastward movement of the influencing systems, the snow area moved southward. The easterly reflux that had important dynamic effects on the snowfall was mainly located at and below 850 hPa. On the one hand, the easterly reflux and the westerly flux formed a wind convergence line, which favored upward motion. On the other hand, the effect of the topography combined with the easterly reflux triggered a southerly flow that prompted the transport of warm airflow into the snow area with a result of frontogenesis when the warm airflow met the cold airflow. The easterly reflux had thermodynamic effects in addition to the dynamic effects, for it played an important role in the convergence of water vapor over the snow area. There was a pronounced temperature drop in the lower troposphere during the snowfall. The temperature in the boundary layer and at 850 hPa fell below 0 ℃ and to -5 ℃, respectively. This result suggests that to improve the forecasting of the nature of winter precipitation, close attention should be paid to the simulated characteristics of temperature stratification in the lower troposphere.