Effects of Raindrop-Size Distribution Variation within the Radar Scattering Volume on Radar Observables

Abstract

Dual linear polarization weather radars measure as primary observables the mean power P?H, and P?V, corresponding to returns at horizontal and vertical polarizations, respectively. Differential reflectivity ZDR is defined as the ratio between these two measurements. Under the assumption of an exponential drop-size distribution, characterized by the two parameters N0 and D0, it has been shown that ZDR may be used to estimate the median volume diameter D0, following which the parameter N0 and, therefore, other drop-size distribution-dependent quantities, may be determined from the horizontal reflectivity ZH. In this paper the effects of reflectivity gradients, due to the variation of the drop-size distribution within the radar scattering volume, on the radar observables (ZH, ZDR) and derived rainfall rates are examined for radar observations with a stationary antenna. The bias of the estimates, their standard errors, and the optimum receiver response are computed for power law and logarithmic receivers. Finally, for the special case of the square law receiver, the contrasting effects due to either similar or opposing signs of the gradients of the parameters N0 and D0 are evaluated.