有利环流形势下北京降雪空报原因

Causal Analysis of a False Snowfall Prediction under Favorable Circulation Situation

利用常规资料及微波辐射计、风廓线等新型探测资料,分析了2007年2月7日当大尺度环流形势非常有利并且华北东部地区出现大范围降雪的情况下,京津地区未产生降水及导致预报失误的主要原因。大范围环流形势演变分析结果表明,700 hPa以上辐合系统前部的偏南气流将水汽输送到降水区,且回流冷空气形成的冷楔和华北倒槽提供了有利的背景条件。弱冷空气南压导致倒槽填塞没有影响京津地区而且边界层内辐合系统产生的上升气流较弱是造成京津绝大部分地区未出现降水的原因之一。大湿度区层次高、湿层薄是北京城区没有降雪的另一重要原因。造成此次降雪空报的主要原因是:数值模式对边界层相对湿度预报过高,且时效间隔较长、其间的天气形势难以判断;不利于降雪的实况信息显现得过晚。北京东部个别测站出现降水的可能原因是在短时回流条件具备的同时有高空槽过境,但动力抬升条件差。在较强偏南暖湿气流提供水汽的同时,海拔高、水汽易于凝结是北京西北部的几个较高海拔测站出现降雪的原因。

Based on the conventional,microwave radiometer,and wind profiler data,the causes of a snowy weather event occurred on 7 February 2007 in Beijing are analyzed.The analysis of the circulation situation shows that the large-scale circulation situation was favorable to the snowy weather during the eastward shifting of the middle troposphere convergence system,such as the strong warm southerly airflow and near-surface conditions,etc,which are similar to the classical model of snowing in Beijing.The causes of the false precipitation prediction for the most part of Beijing include the filling of the inverted trough resulted from the weak southward cold airflow,the weak ascending current in the convergent system in PBL,and the relatively high and thin wet layer.The main reason for the false snowfall prediction is that the location and intensity of weather systems from NWP have obvious bias and longer lead time,and it is difficult for forecasters to judge the weather situation.The precipitation occurred at some stations in the eastern Beijing is mainly induced by the short-term returning airflow in the condition of abundant vapor while the passing of an upper-level trough with poor dynamic lifting.Under the favorable condition of abundant vapor provided by the warm-wet air flow from south,condensation occurred easily at relatively high altitudes,which is the main reason of snowfall at some high-elevation stations over Beijing.