Survey of Relationships Between Rainfall Rate and Radar Reflectivity in the Measurement of Precipitation

Abstract

Numerous investigations have been made in the last two decades from both a direct measurement of the radar reflectivity and the rainfall amount, as well as indirect measurements of the raindrop size spectra. Calculations of the reflectivity factor and rainfall rate from these spectra can be made and the relationships determined. Both methods are discussed and a summary of the relationships presented. These relationships show differences in excess of 500% in rainfall rate at the same reflectivity. These large differences are primarily associated with differences in geographic locality. In addition, there are smaller differences on the order of 150% that can be attributed to different types of rain or different synoptic conditions. Some data are available which are indicative of the differences in the relationship on a given day, depending upon the location within the storm which is sampled. This is briefly described and in only one case out of 18 is there a significant difference. Estimates of the effects of evaporation, accretion and coalescence on the relationship are made and show some of the reasons for the differences in the relationships noted at different geographical locations. The accuracy of the relationships is investigated with attention directed to the evaluation of total storm amounts. It is shown, in general, that the relationships introduce less uncertainty than the uncertainty in obtaining a radar measurement of the reflectivity.