Evaluation of a New Polarimetrically BasedZ–RRelation

Abstract

A new polarimetrically based (or, pol-based) Z–R relation of the form Z = aR^1.5 is described and evaluated where the multiplicative coefficient a is continuously adjusted as the drop size distribution evolves in space and time. The methodology is based on previous studies involving estimation of the normalized gamma drop size distribution parameters (DSD) using radar measurements of Z_h, Z_dr, and K_dp. In moderate-to-intense rainfall, the retrieval of the DSD parameters are formulated to account for the effects of drop oscillations using the “effective” β concept where the axis ratio (r) versus D relation is assumed to be linear and of the form r = 1 − βD in the underlying raindrop shape model. Rayleigh scattering with analytic approximations are used to show that the β estimator in Gorgucci et al. (2000) based on Z_h, Z_dr, and K_dp is of the correct form. The changes in the effective β in a storm cell is studied as the cell evolves from the growth phase to the mature phase (with microburst and rain rates of around 100–120 mm h⁻¹). The systematic shift in β with increasing rain rates in this cell is shown to be consistent with the collisional probability model results of Beard and Johnson (1984). For evaluation of the pol-based Z–R relation, six storm events from the Tropical Rainfall Measuring Mission– Large-Scale Biosphere–Atmosphere (TRMM–LBA) experiment and Texas and Florida Underflights Experiment- B (TEFLUN-B) are analyzed using radar data from the NCAR–S-band Polarimetric (SPOL) radar and a network of gauges specially deployed for these two campaigns. For storm total accumulation, the new pol-based Z–R algorithm gives a normalized bias of 6% (radar overestimate) and normalized standard error of 20%. The corresponding values for a conventional Z–R relation (after stratiform/convective separation) are −18% and 24%. The pol-based Z–R method continuously “tracks” the drop size distribution and so no classification of rain types is necessary.