“09.11.10”石家庄特大暴雪中尺度风场分析

Analysis on Mesoscale Wind Field of "09.11.10" Blizzard in Shijiazhuang

利用常规观测、雷达资料以及四维变分方法反演的风场资料,对2009年11月10—12日石家庄特大暴雪过程的形势场和中尺度风场结构进行了详细分析。从影响系统来看,此次特大暴雪过程分为两个阶段:回流降雪和西来槽降雪。回流降雪是特大暴雪的主要时段,此阶段西部山区降雪明显大于东部平原;而西来槽降雪阶段全区降雪比较均匀。由雷达反射率因子和径向速度可见:10日降雪具有对流性质,而且回波不断地自西部山区向石家庄市区移动,产生"列车效应",造成市区及西部降雪强度较大;11—12日回波强度弱,降雪强度也较弱。分析雷达四维变分反演风场得到如下结论:(1)反演风场能够准确展现东风影响的时间和高度、西来槽影响的始末,对预报员定性外推降雪的强度起到非常好的参考作用。(2)低层水平反演风场上,存在一个比较窄的东风带,回波沿东风带自西向东移动,石家庄市区处于北到东北风的"回流墙"附近,东移回波在此堆积,移速减慢,影响时间较长;同时,市区附近存在风向性和风速性辐合,致使回波在石家庄附近加强或维持。(3)反演风场垂直剖面图上,东风回流降雪阶段,低层东风区上空存在一个"反气旋",强回波位于"反气旋"所包围的范围。

The situation and mesoscale wind field of the blizzard that occurred from 10 to 12 November 2009 in Shijiazhuang are analyzed in detail by using the conventional observation, radar and 4D VAR inversion wind field data. This snowstorm can be divided into two phases according to the influence system： back- flow and west-trough snowfall. The backflow snowfall is the main period when the snowfall in the western mountains is obviously heavier than in the eastern plains. In comparison, the west-trough snowfall is more uniform in the whole region. It can be seen from the radar reflectivity factor and radial velocity on 10 No- vember that the echoes have convective characteristics, continuously moving towards Shijiazhuang from the western mountain, generating the ＂train effect＂ and causing the intensive snow in the urban and western areas. From 11 to 12, the intensity of snowfall is abated along with echoes weakening. On the basis of the 4D VAR inversion wind field, the following results are obtained： （1） The retrieval wind field can accurate- ly demonstrate the time and height of east wind, and the whole influence process of west-trough, which is a good reference for forecasters to extrapolate the intensity of snowfall qualitatively. （2） There is a narrow east wind band in low-level retrieval horizontal wind field, and the echoes move from west to east along the east wind band. The urban area of Shijiazhang is influenced for a long time due to its location near the north-northeast ＂backflow wall＂ where the echoes are slowed downand piled up. Meanwhile, the echoes are strengthened or maintained around Shijiazhuang since there exist wind-direction and wind-speed conver- gences. （3） In the vertical profile of retrieval wind field, there is an ＂anticyclone＂ over the low-level east wind during the backflow snowfall period, and the strong echoes are located in the ＂anticyclone＂ zone.