Reflectivity dependence of reflectivity gradients observed by radar profilers

Current operational estimates of rain accumulation using scanning radars depend on the application of empirical Z-R relations. The use of the Z-R relationship requires that the scanning radar be absolutely calibrated in an optimal fashion. It has long been recognized that reflectivity gradients that relate the scanning radar observed reflectivities at altitude with reflectivities at the surface are an important source of error in the quantitative precipitation estimates (QPE) using scanning radars. In this paper we summarize the reflectivity gradients seen in several TRMM Field campaigns by radar profilers anchored by collocated surface disdrometers. The observations reveal that the reflectivity gradients have a systematic dependence upon reflectivity in the sense that at high reflectivities the gradient is largest. This effect is largely responsible for the reflectivity dependent bias that has been observed in comparisons between profiler and disdrometer reflectivities. This connection is illustrated from data collected during the TRMM ground validation field campaigns and more recently at Wallops Island.