Exploring the Transient Behavior of Z–R Relationships: Implications for Radar Rainfall Estimation

Abstract

The objective of this study is to characterize the signature of dynamical microphysical processes on reflectivity?rainfall (Z?R) relationships used for radar rainfall estimation. For this purpose, a bin model with explicit microphysics was used to perform a sensitivity analysis of the shape parameters of the drop size distribution (DSD) as a function of time and rainfall regime. Simulations show that coalescence is the dominant microphysical process for low to moderate rain intensity regimes (R < 20 mm h?1) and that the rain rate in this regime is strongly dependent on the spectral properties of the DSD (i.e., the shape). The time to equilibrium for light rainfall is at least twice as long as in the case of heavy rainfall (1 h for stratiform vis-à-vis 30 min for thunderstorms). For high-intensity rainfall (R > 20 mm h?1), collision?breakup dynamics dominate the evolution of the raindrop spectra. The time-dependent Z?R relationships produced by the model converge to a universal Z?R relationship for heavy intensity rainfall (A = 1257; b ? 1) centered on the region of Z?R space defined by the ensemble of over 100 empirical Z?R relationships. Given the intrinsically transient nature of the DSD for light rainfall, it is proposed that the vertical raindrop spectra and corresponding rain rates should be modeled explicitly by a microphysical model. A demonstration using a multicolumn simulation of a Tropical Rainfall Measuring Mission (TRMM) precipitation radar (PR) overpass over Darwin for a stratiform event during the Tropical Warm Pool?International Cloud Experiment (TWP-ICE) is presented.