北京2016年“11·20'初雪预报偏差分析

摘要

通过分析2016年11月19-21日的EC再分析数据、风廓线雷达数据等多种资料,发现北京2016年11·20初雪天气预报偏差的原因包括以下几个方面:①回流“冷垫”缺失、850 hPa切变线偏南、整层可降水量不足等是造成这次雨雪天气过程总降水量偏少的主要原因.②北京平原地区水汽通量辐合相对较弱、雨转雪的时间较预报晚3h左右、-18～-9℃层饱和程度不足、雨转雪后影响系统也已过境北京等是造成北京平原地区,尤其是北京城区降雪量不足的原因.③“回流”冷垫未形成、低层暖平流强盛是造成0℃层高度下降缓慢的主要原因.本文并通过相似天气个例对比发现:此类北京地区高空水汽饱和程度不足的雨转雪天气,雨转雪的时间点预报尤其需关注0℃层高度变化,当0℃层高度下降至100 m以下时,才可能出现雨转雨夹雪或雪的天气.%This paper analyses the forecast deviation of the first snow in 2016 in Beijing using EC reanalysis data and wind profiling radar from 19 to 21 November 2016.The conclusions are as follows.(1) The deficiency of "cold wedge",the location of shear line at 850 hPa by south and the lack of whole layer precipitable water caused the total snowfall amount less than that of the forecast.(2) There are four reasons for the less snowfall amount in urban areas of Beijing.That is,the convergence of water vapor flux was weak,the time for rain to turn into snow was 3 hours later than that of the forecast,the saturation in the layer from-18 to-9℃ was insufficient,and the precipitation weather system had passed through Beijing after rain turned into snow.(3) The slow falling of the 0℃ layer height was due to the failing formation of "cold wedge",and the strong warm advection in low levels.At the same time,the similar weather examples are compared in this paper.When it refers to the phenomenon of rain into snow and the insufficient saturation at high levels,the 0℃ layer height should still be considered in forecasting the time of rain into snow.When the 0℃ layer height drops to lower than 100 m,rain can turn into sleet and snow.