Abstract
As part of a program to evaluate the utility of polarimetric radar for estimating rainfall, the National Center for Atmospheric Research's S-band, dual-polarization radar was deployed in east central Florida during the summer of 1998. The field experiment (PRECIP98) produced a unique dataset of high-resolution polarimetric radar measurements, rain gauge observations, and raindrop disdrometer observations.

Comparisons between radar measurements and radar parameters derived from disdrometer observations (assuming a gamma drop-size distribution and equilibrium axis ratios) revealed that radar estimates of differential reflectivity and specific differential phase were significantly less than those determined from disdrometer observations. Comparisons improved slightly when the calculations allowed for small canting angles and improved markedly for an empirical axis-ratio relation representing more spherical drops.

Rainfalls estimated with previously-published reflectivity-differential reflectivity (Z_HZ_DR) and specific differential phase (K_DP) rain rate relations, developed from simulations with equilibrium axis ratios and widely accepted ranges in DSD parameters, were characterized by overestimates of 50% and underestimates of 30%, respectively. Florida relations, tuned with disdrometer observations, yielded a bias of 8% for the Z_HZ_DR pair; but the bias for K_DP was unchanged. Fine tuning of the Florida relations for drop canting andmore spherical drop shapes readily accounted for the residual bias with Z_HZ_DR estimators. However, a significant underestimate of rainfall (> 14%) remained in the K_DP estimates. The residual K_DP bias is attributed to signal loss in storms dominated by small drops and the growth of errors at weak signal strengths.

Overall, Z_HZ_DR rainfall estimators had higher correlations with rain gaugeobservations (0.92), smaller ranges in bias factors from storm to storm (1.73), and smaller root-mean-square errors for unbiased estimators (6.3 mm) than those based on radar reflectivity or specific differential phase.

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