Dual-Polarization Radar Rainfall Estimation over Tropical Oceans

Dual-polarization radar rainfall estimation relationships have been extensively tested in continental and subtropical coastal rain regimes, with little testing over tropical oceans where the majority of rain on Earth occurs. A 1.5-yr Indo-Pacific warm pool disdrometer dataset was used to quantify the impacts of tropical oceanic drop-size distribution (DSD) variability on dual-polarization radar variables and their resulting utility for rainfall estimation. Variables that were analyzed include differential reflectivity Zdr; specific differential phase Kdp; reflectivity Zh; and specific attenuation Ah. When compared with continental or coastal convection, tropical oceanic Zdr and Kdp values were more often of low magnitude (,0.5 dB, ,0.38 km ) and Zdr was lower for a given Kdp or Zh, consistent with observations of tropical oceanic DSDs being dominated by numerous, small, less-oblate drops. New X-, C-, and S-band R estimators were derived: R(Kdp), R(Ah), R(Kdp, zdr), R(z, zdr), and R(Ah, zdr), which use linear versions of Zdr and Zh, namely zdr and z. Except for R(Kdp), convective/stratiform partitioning was unnecessary for these estimators. All dual-polarization estimators outperformed updated R(z) estimators derived from the same dataset. The best-performing estimator was R(Kdp, zdr), followed by R(Ah, zdr) and R(z, zdr). The R error was further reduced in an updated blended algorithm choosing between R(z), R(z, zdr), R(Kdp), and R(Kdp, zdr) depending on Zdr . 0.25 dB and Kdp . 0.38 km 21 thresholds. Because of these thresholds and the lack of hail, R(Kdp) was never used. At all wavelengths, R(z) was still needed 43% of the time during light rain (R, 5 mm h, Zdr , 0.25 dB), composing 7% of the total rain volume. As wavelength decreased, R(Kdp, zdr) was used more often, R(z, zdr) was used less often, and the blended algorithm became increasingly more accurate than R(z).