用EVAP方法反演冷流暴雪的风场及检验

Wind retrieval and its verification for the ocean-effect snowstorms using the EVAP method

EVAP(Extended VAP)雷达反演方法是在忽略空气垂直运动速度而考虑云雨粒子下落速度影响的条件下,导出从雷达多仰角多普勒速度资料产生不同高度上的水平径向速度计算公式,然后利用VAP方法来反演出等高面上的二维水平风场。利用烟台多普勒天气雷达资料,采用EVAP方法反演了2005年12月山东半岛两个风场背景条件和暴雪落区均有明显差异的冷流暴雪个例水平风场。将雷达实测的径向速度、反射率因子插值到各等高面上,并与反演的同等高面风场叠加显示,以此分析反演的风场是否与原始雷达数据相符。此外,还与地面自动站风场、高空风场等实测资料进行对比,综合检验反演结果的合理性和可靠性,并进一步分析了冷流暴雪的中尺度风场结构。结果表明,反演风场与实测资料相符;EVAP方法对强风和弱风的冷流暴雪个例都可反演出不同高度的中尺度风场结构,反演的西南风与东北风、西北风与东北风之间的切变线和强回波带相对应,揭示了暴雪产生的动力机制,说明EVAP方法对不同风场条件的暴雪中尺度系统都有一定的反演能力。高时空分辨率的EVAP反演风场弥补了天气图上仅能分析出山东半岛西北风的缺憾,且反演过程中一个体扫的计算时间不足1 min,因此EVAP反演的中尺度风场信息适合在冷流降雪的短时临近预报业务中应用。

A formula to calculate the horizontal radial velocity at different heights via the several elevation angles from a Doppler radar was derived with the falling speed of cloud/rain particle covered but the vertical velocity neglected.Then the two dimensional horizontal wind field on the isohypsic surface was retrieved by the VAP technology.This method is called the Extended VAP（EVAP）.Two different snowstorm samples occurred in the Shandong peninsula in December 2005 were retrieved to verify the EVAP method,the radial velocity and reflectivity interpolated and compared with the retrieved wind on the every isohypsic surface.In addition,the retrieved wind fields were compared with the observational data from the automatic meteorological stations and synoptic charts as well.The comparative results show that the retrieved wind fields at different heights accord with the observational data.Mesoscale wind structure of ocean-effect snowstorms is further analyzed in this paper with the result that the shears between the retrieved southwestern and northeastern wind as well as between the retrieved northwestern and northeastern wind are corresponding with the strong reflectivity zones suggesting that the EVAP method is able to correctly retrieve wind of the different weather systems.As a result,the retrieved wind fields of high spatial and temporal resolution remedy the defect that only the wind of larger scale can be analysed conventionally.Calculation of the retrieving wind for one volume scan needs just less than one minute.So the EVAP technology can be applied to the operational nowcasting of ocean-effect snowstorm.