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

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.