Evaluation of Precipitation Measurements with the WSR-57 Weather Radar

Abstract

Quantitative data collected with the WSR-57 radar at Atlantic City from five rainstorms and two snow-storms are compared with precipitation data from 60 recording rain gages within 100 mi of the radar. Hourly rainfall amounts of from 0.01–0.02 inches are detected by the radar in at least 95 per cent of the cases at all radar ranges out to 70 mi. Hourly amounts of from 0.04–0.05 inches are detected in at least 95 per cent of the cases at all ranges out to 100 mi. The relationship between radar echo intensity and rainfall rate varies from storm to storm. Although the radar appears to have excellent potential for determination of area-average rainfall, reflectivity measurements provide only coarse estimates of point rainfall intensity. The radar estimates of hourly rainfall averages, over a 750 sq mi area within 60 mi of the radar, are within the confidence limits of the average of 10 gage measurements, when a best-fitting radar-rainfall relationship is used for each storm. Use of one grand average relationship for all storms provides estimates of the average areal rainfall whose accuracy corresponds to those of a single rain gage located near the area center. An analysis of errors made in transferring PPI photographs to digitized arrays and in measuring the echo intensity in steps of 6 db indicates that a reduction in the size of these errors would not substantially improve the accuracy of the radar measurements. An important unresolved problem concerns the development of techniques for quick determination, under field conditions, of the most accurate reflectivity-rainfall relationship for a particular storm. A chart based on the average relationship developed in this study is presented for converting echo intensities measured with a WSR-57 to rainfall intensities.