2010年冬季北京初雪预报难点分析

Analysis of Forecast Difficulties for the First Snow of Beijing Area in 2010 Winter

北京地区非典型性降雪是预报中的难题,尤其是偏东风在弱降雪过程中的作用难以把握。本文利用常规观测资料与北京地区特种观测资料,对北京2010年冬季(2010年12月至2011年2月)空报的初雪个例和实际初雪个例进行了诊断分析,得出了一些有意义的结果:偏东风的干湿性质取决于东部上游地区的干湿条件。当上游为干中心时,它是一支干平流。12月12日夜间的降雪空报是由于尽管中低层上升运动显著,但是边界层湿度太小,偏东风实际为一支干平流,对北京地区增湿没有明显贡献。北部干冷空气的快速南下控制北京地区,也是预报出现偏差的重要原因。2月9日初雪过程,是边界层高湿区中,弱的辐合上升运动作用下产生的稳定性降雪。对比发现边界层水汽条件在北京地区冬季降雪中非常重要。当边界层水汽条件差,即使中低层上升运动系统明显,也很难形成有效降水。而在边界层受充沛的暖湿气团控制并配合有弱辐合上升运动,即使中高层并无明显的辐合系统,也可产生明显降水。

Atypical snowfall forecast is a complicated issure in Beijing area. It is especially difficult to deter mine the role played by the east wind in light snow process. Based on the conventional observation data and various intensive sounding data, two cases including a false snowfall prediction and a first snowfall process occurring in winter 2010 are analyzed. The analysis shows that： Wet or dry conditions for the east wind, depend mainly on the humidity of the upstream eastern region. When the upstream eastern region is dry, the east wind is dry, and thus could not provide moisture for Beijing. The cause for the false snowfall prediction is that the east wind is a dry advection which is unfavorable to humidification at night on Decem ber 12, 2010. Low humidity in the boundary layer is the main reason in spite of favorable vertical upward motion. The dry and cold airs in areas north of Beijing moving southwards rapidly and invading are the other major reason for the forecast deviation. Under the favorable condition of abundant water vapor in the boundary layer and weak convergence and ascending motion, a steady rainfall process occurred. The re sults show that moisture conditions in the boundary layer are very important for winter snowfall predic tion. It is difficult to cause effective precipitation even with strong ascending motion in upper air but poor water vapor condition in the boundary layer. In fact, distinct precipitation may occur in case of warm and wet air mass and weak convergence and ascending motion in the boundary layer even without apparent con vergence systems.