Can polarisation help NEXRAD to provide better rainfall estimates?

Anthony Illingworth

Abstract

It will be difficult to use the polarisation parameters on Nexrad for rainrates below 10 mm/hr because the signals are small and noisy. However, we have found that by looking at the distribution of the Z and the noisy differential reflectivity (ZDR) signals over a 5 km square area we are able to derive the raindrop concentration, Nw, quite accurately and hence compute the value of "a" which could be used in the empirical Z = aR ^1.5 relationship. We find remarkable consistency with the Nw derived from a coincident profiler thus confirming the precision and validity of the Z/ZDR technique. Traditionally rainfall rates from radar are evaluated using gauges or a disdrometer, but the disdrometer yields rather different values of Nw, thus demonstrating that the smaller sampling volume of ground based gauges or disdrometers limits their use for evaluating rainfall algorithms.