河北北部两次强降雪过程对比分析

选取河北北部承德市2010年1月3—4日和2015年2月20—21日两次强降雪过程,利用常规观测资料和NECP(1°×1°)逐6 h再分析资料,对环流形势和物理量场进行对比分析。结果表明,两次过程影响系统虽有不同,但500 hPa贝加尔湖附近有冷涡、低层有切变线缓慢东移、地面上贝加尔湖以西存在冷高压,海平面气压场呈"西北高东南低"是其共同特征,也是承德出现强降雪的有利天气形势。物理量场在强降雪期间有以下共同特征:低层水汽通量呈辐合,辐合中心与强降雪有很好对应关系;700 hPa以下为强上升运动,且850 hPa附近有上升中心;850 hPa以上涡度为正值;垂直螺旋度整层为正值或呈"上负下正"结构。

Contrast Analysis of Two Heavy Snowfall Processes in Northern Hebei Province

In order to reveal the development mechanisms of heavy snowfall in Northern Hebei,two heave snow processes on 3 to 4 January 2010 and 20 to 21 February 2015 are selected.A contrast analysis of their meteorological characteristics is made from aspect of synoptic situation and physical mechanism by using NCEP data and conventional observation data.The reasults showed that the cold vortex system at 500 hPa near Lake Baikal,an eastward slowly shear line at mid-low levels,as well as the presence of cold high pressure to the west of Lake Baikal at the surface and sea level pressure shows a "high-northwest low-southeast" pattern were propitious to the heavy snowfall in Chengde City.Relative humidity at 850 hPa Over Chengde is more than 90% and there is moisture flux divergence center or maximum moisture flux center were propitious to the heavy snowfall.From the cross sections of vertical velocity,vorticity,and vertical helicity,it is found that the vertical distribution of down at upper level but up at low level increase snowfall,and the distribution of positive vorticity(vertical helicity)in the whole troposphere is most favorable during snow process.when the vorticity is positive,the vertical helicity shows "up-positive low-negative" structure conducive to upward motion and snowfall.