2011年山西省一次连续性降雪过程成因分析

Cause Analysis of a Continuous Snowfall Process in 2011 in Shanxi

利用NCEP/NCAR(1°×1°)逐日6 h再分析资料和常规观测资料,分析了2011年2月25-28日山西的一次连续降雪天气过程。结果发现:此次降雪过程经历了3个阶段,分别是2月25日的倒槽冷锋降雪阶段,26-27日的回流降雪阶段,28日的低空切变降雪阶段。过程涵盖了华北地区大到暴雪的3个类型,分别是倒槽冷锋类、回流类、低空切变线类。25-27日的降雪属东路冷空气影响,28日的降雪则由西北路径冷空气影响所致。暴雪过程有不同的影响系统,26日的暴雪天气是850 hPa东北急流、700 hPa西南急流及暖切变和边界层切变线共同作用的结果,28日的暴雪天气则是500 hPa西北冷平流和西南暖平流导致的地面锋生以及700 hPa暖切变共同作用的结果,条件性对称不稳定是28日降雪维持的机理。对于低空切变类暴雪天气,对流层中层湿核的出现与消失对降雪的开始和结束有较好的指示意义;对于回流类暴雪天气,低层回流对降雪起到了冷垫作用,低空东北急流、西南急流的建立使降雪增幅。26日受东路冷空气影响的回流降雪,从形势特征和高低空配置的完整性分析,预报员相对容易预报暴雪的落区和强度。28日在西北路径冷空气影响下产生的暴雪天气是一个预报难点,对这类天气的预报需关注暖湿空气的活动及其维持机制,假相当位温低谷期的开始对降雪的预报有12～24 h的提前量。

Using the NCEP/NCAR （ 1°×1°） day-to-day 6 h reanalysis data and general observa- tional data, the Shanxi snowfall process was analyzed during 25th -28th Feb 2011. The results are as fol- lowing. This snowfall process contains three stages： first, the surface inverted though and cold front snow stage on 25th Feb, second, the return stream snow stage during 26th-27th Feb, third, low shear snow stage on 26th Feb. The snowfall process contains three types of heavy snow in Huabei area： inverted though cold front snow, the return stream snow, low shear snow. The snowfall during 25th-27th Feb is affected by east cold air, while the snow on 28th Feb is affected by northwest cold air. The heavy snow has a different impact system. The heavy snow on 26th Feb is the result of 850 hPa northeast jet stream, 700 hPa southwest jet stream and warm shear lines at boundary layers; the heavy snow on 28th is surface result of frontogenesis of 500 hPa northwest cold advection and southwest warm advection and result of 700 hPa warm shear. The snow maintenance mechanism on 28th Feb is the conditional symmetric insta- bility. For low shear type of heavy snow, the appearance and disappearance of wet core in the troposphere have good implications of the beginning and ending of the snowfall; for the return stream snow, the low return stream plays a cold pad role, the establishments of low-level northeast jet and southwest jet making increasing for snowfall. The return stream snowfall on 26th Feb affected by east cold air, for the analysis of the situation characteristics and high and low air configuration integrity, forecaster is relatively easy to forecast the area and intensity. The heavy snow on 28th Feb affected by northwest cold air forecasting is difficult, forecasting this kind of weather concerning the activities of the warm and moist air and maintai- ning mechanism. The potential pseudo-equivalent temperature trough advances 12-24 hour for snowfall forecast.