X-Band Polarimetric Radar Measurements of Rainfall

A combined polarimetric estimator for rainfall rate (R) retrievals from polarimetric radar measurements at X band is proposed. This estimator uses the horizontal polarization radar reflectivityZₑ, differential reflectivityZ DR, and specific differential phase shiftK DP, and it intrinsically accounts for changes in how drop oblateness increases with size. Because this estimator uses power measurements (i.e.,ZₑandZ DR), a procedure for correcting these measurements for effects of partial attenuation and differential attenuation using the differential phase measurement is suggested. An altitude correction for estimates of rainfall rates is also suggested. The proposed combined polarimetric estimator that usesK DP,Z DR, andZₑ, an estimator that usesK DPalone for equilibrium drop shapes, and differentZₑ–Rrelations were applied to the 15 rain events observed with the NOAA X-band transportable polarimetric radar during the eight-week field campaign at the NASA Wallops Island facility in Virginia. The observed rains ranged from very light stratiform events to very heavy convective ones with cells producing rainfall rates in excess of 100 mm h−1. The three different ground validation sites were equipped with high-resolution (0.01 in.) tipping-bucket rain gauges. One of these sites also was equipped with disdrometers. In terms of the relative standard deviation, the combined polarimetric estimator provided the best overall agreement with gauge data (22%), closely followed by a case-tunedZₑ–Rrelation (23%) that was determined for each observational case from drop size distributions (DSD) measured in situ by a disdrometer and was available only a posteriori. The use of theK DP-only estimator and a meanZₑ–Rrelation resulted in 30% and 32% relative standard deviations, correspondingly. The combined polarimetric estimator, theK DP-only estimator, and the casetunedZₑ–Rrelation estimator provided about a 6%–9% negative bias in comparison with the gauge data; the meanZₑ–Rrelation estimator provided a larger negative bias (18%).