The concept of “normalized” distribution to describe raindrop spectra: A tool for hydrometeor remote sensing

Abstract

The concept of normalization of DSD developed in this paper is founded upon two reference variables, the liquid water content LWC and the mean volume diameter Dm. We show mathematically that it is appropriate to normalize by N0∗ ∝ LWC/Dm4 with respect to particle concentration and by Dm, with respect to drop diameter. The major poi of our approach is that it is totally free of any assumption about the shape of the DSD. This new normalization has been applied to the ensemble of the airborne microphysical data of TOGA-COARE (21 flights of the NCAR Electra). The classification of the TOGA-COARE raindrop spectra into 4 categories (one stratiform, and three convective [0–10mm/h, 10–30mm/h, 30–100mm/h]) allowed us to identify the following features: (i) There is a distinct behavior of N0∗ between stratiform and convective rains; a typical “stratiform” value is 3×106M−4, while it is 2×107M−4 in convective.(ii) Between the convective rain categories, there a clear trend for Dm, to increase as the rainfall rate. (iii) The “average” normalized shape of the DSD is remarkably stable between the four rain categories. This normalized shape departs from the exponential, but also from all the analytical shape considered up to now (as Gamma, log-normal, modified Gamma).