Invariance of the double-moment normalized raindrop size distribution through 3D spatial displacement in stratiform rain

Abstract

ouble-moment normalization of the raindrop size distribution (DSD) summarizes the DSD in a compact way, using two of its statistical moments and a ?generic? double-moment normalized DSD function. We present results of an investigation into the invariance of the double-moment normalized DSD through horizontal and vertical displacement in space, using data from disdrometers, vertically pointing K-band micro rain radars (MRRs) and an X-band polarimetric weather radar. The invariance of the double-moment normalized DSD is tested over a vertical range of up to 1.9 km and a horizontal range of up to 100 km. The results suggest that for practical use, with well chosen input moments, the double-moment normalized DSD can be assumed invariant in space in stratiform rain. The choice of moments used to characterize the DSD affects the amount of DSD variability captured by the normalization. We show that in stratiform rain, it is possible to capture more than 85% of DSD variability in DSD moments zero to seven using the technique. Most DSD variability in stratiform rain can thus be explained through the variability of two of its statistical moments. The results suggest similar behaviour exists in transition and convective rain, but the limited data samples available do not allow for robust conclusions for these rain types. The results have implications for practical uses of double-moment DSD normalization, including the study of DSD variability and microphysics, DSD-retrieval algorithms, and DSD models used in rainfall retrieval.