Abstract
The question of the connections between raindrop-size distributions (RDSDS) and radar reflectivity–rainfall rate Z–R relationships is explored from the combined approach of rain-forming physical processes that shape the RDSD, and a formulation of the RDSD into the simplest free parameters of the rain intensity R, rainwater content W, and median volume drop diameter D0. This is accomplished through examination of integral parameters deduced from the RDSD associated with the host of Z–R relations found in the literature. These latter integral parameters are deduced from the coefficient and exponent of empirical Z–R relations using a gamma RDSD. A physically based classification of the RDSDs shows remarkable ordering of the D0–W relations, which provides insight into the fundamental causes of the systematic differences in Z–R relations.
The major processes forming the RDSD are examined with respect to a mature equilibrium RDSD, which is taken as the eventual distribution. Emphasis is placed on cloud microstructure (with the two end members being “continental” and “maritime”) and cloud dynamics (with end members “convective” and “stratiform”). The influence of orography is also considered. The Z–R classification scheme can explain large systematic variations in Z–R relations, where R for a given Z is greater by a factor of more than 3 for rainfall from maritime compared to extremely continental clouds, a factor of 1.5–2 greater R for stratiform compared to maritime convective clouds, and up to a factor of 10 greater R for the same Z in orographic precipitation. The scheme reveals the potential for significant improvements in radar rainfall estimates by application of a dynamic Z–R relation, based on the microphysical, dynamical, and topographical context of the rain clouds.
