天气雷达回波运动场估测及在降水临近预报中的应用

ESTIMATION OF WEATHER RADAR ECHO MOTION FIELD AND ITS APPLICATION TO PRECIPITATION NOWCASTING

在常用的基于天气雷达反射率因子图像的相关方法跟踪回波运动的TREC(Tracking Radar Echo by Correla-tion)技术基础上,文中发展了一种基于差分图像(时间梯度图像)的相关方法追踪雷达回波运动(Difference Image-based Tracking Radar Echo by Correlations)技术,简称DITREC,并与TREC技术进行比较。个例分析表明,DITREC矢量场消除了TREC矢量场中由于回波型的迅速变化导致的一些无序矢量,使得DITREC矢量场的时间连续性和空间连续性好于TREC矢量场。在TREC矢量场中不存在无序矢量的地方,DITREC矢量场与TREC矢量场基本一致。分别用DITREC矢量场和700 hPa风场作为回波运动估测场外推合肥CINRAD/SA雷达反射率因子复合扫描图,得到1 h外推降水场。以地面雨量计为标准,对外推降水场进行评估,结果表明,DITREC外推小时降水优于700 hPa外推小时降水,但其精度还与所采用的Z-R关系有关。

An extension of TREC （Tracking Radar Echo by Correlations ） technique, named Difference Image-based Tracking Radar Echo by Correlations （DITREC） is presented and evaluated, in which individual features in two difference images from three temporally successive radar reflectivity images are tracked, by finding the maxima in the cross-correlation function between the two successive difference images. Examples are presented where the TREC and DITREC algorithm were applied to two cases： one is the squall line case on 8 July 2003 from Hefei CINRAD/SA radar,and the other the tropical revolving storm case on 19 July 2005 from Whenzhou CINRAD/SA radar. For the two cases, Doppler radar data were available and the radial components （with respect to the radar） computed by the TREC and DITREC could be compared. It is found that the DITREC eliminated those chaotic vectors shown in the TREC which are caused by the rapid changes of reflectivity within the radar echo patterns, and improved the temporal and spatial continuity of the velocity field of echo motion. In general, the DITREC radial componenrs were consistent with the TREC ones, and the TREC and DITREC appeared to work in the lower few kilometers of the tropical revolving storm where the vertical wind shear was relatively weak. On the other hand, the TREC and DITREC vectors determined in squall line situations tended to reflect the mean advection of the rain cell, which could be quite different from the Doppler radar observations. Advecting the hybrid scan reflectivity measured by HeFei Doppler radar with the field of the DITREC vectors and the 700 hPa wind fields, yields the extrapolative hourly precipitations, respectively. The evaluation of the extrapolated hourly precipitations based on the gauge rain data shows that the hourly precipitation extrapolated using the DITREC vectors is closer to the gauge rain data than one by the 700 hPa wind fields, however its precision is also related to the Z-R relation used. The growth and decay of radar echoes are not considered when advecting the hybrid scan reflectivity. The future work may be to investigate how to reduce the errors due to nonlinear temporal changes in the intensity and velocity of rainfall patterns.