基于数字高程的波束遮挡订正及在雷达定量估测降水中应用

Application of DEM-Based Beam Occlusion Correction in Radar Quantitative Estimation of Precipitation

地形的起伏使雷达波束受到严重的遮挡,使回波数据质量受到很大干扰。本文使用SRTM任务的DEM数据和谷歌公司的DEM数据分别计算了位于北京南郊的S波段雷达低仰角的波束遮挡率,建立了部分遮挡区域的回波反射率订正关系,并在2018年5月19日北京一次大范围层状云降水过程中,对波束遮挡订正前后的雷达定量估测雨量与地面3个雨量计观测结果进行了定性与定量化对比分析。结果表明:①波束遮挡订正有助于改善反射率因子的空间连续性。波束遮挡订正后的仰角0.5°的反射率与1.5°的反射率之间的差值整体呈现缩小特征,符合层状云降水垂直廓线特点。②09:00—11:00,相比波束遮挡订正前的雷达定量估测雨量(QPE),波束订正后的QPE准确性得到改善,使用分级标准误差与归一化平均偏差评价波束遮挡订正前后QPE与雨量计实测值之间的误差,波束订正后的反射率估测雨量与雨量计实测雨量一致性更好。

Because of radar beam occluded seriously by terrain,the quality of echo data is greatly disturbed.In this paper,the DEM(Digital Elevation Model)data of SRTM(Shuttle Radar Topography Mission)task and Google Company are used to calculate the beam occlusion rate of S-band radar at low elevation angle at Nanjiao of Beijing,and the echo reflectance correction relationship for the partial occlusion area is established.In the process of a large-scale stratiform precipitation in Beijing on 19 May 2018,the qualitative and quantitative comparative analysis of the radar quantitative estimation of rainfall before and after the beam occlusion correction and the observation results from three rain gauges on the ground are performed.The results show:(1)Beam occlusion correction helps improve the spatial continuity of the reflectivity factor.The reflectance difference between 0.5°and 1.5°of elevation angle after the beam occlusion correction is reduced as a whole,which is consistent with the vertical profile of the stratiform cloud precipitation.(2)On the same day,between 09:00 and 11:00,the accuracy of the radar Quantitative Precipitation Estimation(QPE)after beam correction is improved.Using the Grading Standard Error and the Normalized Mean Bias(NMB)to evaluate the error between QPEs before and after the beam occlusion correction and measured values from the rain gauge,the result shows that the reflectivity after beam correction is more consistent with the measured rainfall from the rain gauge.