Comparison of Gage and Radar Methods of Convective Rain Measurement

Abstract

Gage and radar methods of convective rain measurement are compared in the context of the continuing multiple cloud seeding experiment of the Experimental Meteorology Laboratory. An optimal system, combining the best features of both, is recommended.The nature of the Florida convective rainfall to be measured is documented using measurements from a dense raingage mesonet (about 3 km2 per gage over 570 km2) that was operated for a total of 93 days in 1971 and 1973, and the gaging requirements for detection and measurement of 24 h rainfalls in the mesonet are determined using the full complement of gages as the standard. For the measurement of areal convective rainfall greater than 0.25 mm within a factor of 2 on 90, 70 and 50% of the days, gage densities of 31, 91 and 208 km2 per gage, respectively, are required.Radar performance in estimating convective rainfall over south Florida is determined using two collocated, calibrated 10 cm radars (UM/10-cm of the University of Miami and WSR-57 of the National Hurricane Center). In all cases, the radar estimates of rainfall are compared with the rainfall as determined by raingages (densities 3 to 8 km2 per gage) in cluster arrays. The relative performances of the two radars are compared.In 1973, WSR-57 radar-derived rainfalls were computed by hand as in 1972 and by computer using taped radar observations. On a daily basis, 80% of the radar estimates were within a factor of 2 of the cluster standard. The combined accuracy of the WSR-57 radar in 1972 and 1973 in estimating convective rainfall approximated that which one would obtain with a gage density of 65 km2 per gage over an area the size of the mesonet.The daily representation of rainfall by the radar improves if one adjusts it using gages. In the mean, adjustment produced a statistically significant 15% improvement (<1% level with two-tailed ?t? test) in radar accuracy. The adjusted radar measurements then had an approximate gage density equivalence of 25 km2 per gage.The gaging requirements for the estmation of area mean rainfall for an area the size of the EML target (1.3 ? 104 km2) is inferred using the digitized radar observations. To meet a specification that the area-mean rainfall be measured to within a factor of 2 of the true value 99% of the time requires 143 km2 per gage, compared to a requirement of at least 13 km2 per gage for the mesonet.An optimum method of rain measurement is suggested. For the measurement of the rainfall from individual showers anywhere, the gage-adjusted radar is far superior to gages alone. For measurement in a fixed area the size of the mesonet, gages are superior to the radar. To measure rainfall over the EML target either gages alone, or a radar adjusted by gages, can accomplish the task. About 90 evenly spaced gages in the EML target should provide area rain measurements within a factor of 2 of the true value 99% of the time. The radar estimates adjusted by gages should be as accurate as those provided by the network of 90 gages. The final choice as to the measurement system will probably be determined by other considerations such as budget, personnel and terrain over which the measurements are to be made.