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Dual-Polarization Radar Rainfall Estimation over Tropical Oceans

2017; American Meteorological Society; Volume: 57; Issue: 3 Linguagem: Inglês

10.1175/jamc-d-17-0160.1

1558-8432

Elizabeth Thompson, Steven A. Rutledge, Brenda Dolan, Merhala Thurai, V. Chandrasekar,

Soil Moisture and Remote Sensing

Abstract 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 Z dr ; specific differential phase K dp ; reflectivity Z h ; and specific attenuation A h . When compared with continental or coastal convection, tropical oceanic Z dr and K dp values were more often of low magnitude (<0.5 dB, <0.3° km −1 ) and Z dr was lower for a given K dp or Z h , 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 ( K dp ), R ( A h ), R ( K dp , ζ dr ), R ( z , ζ dr ), and R ( A h , ζ dr ), which use linear versions of Z dr and Z h , namely ζ dr and z . Except for R ( K dp ), 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 ( K dp , ζ dr ), followed by R ( A h , ζ dr ) and R ( z , ζ dr ). The R error was further reduced in an updated blended algorithm choosing between R ( z ), R ( z , ζ dr ), R ( K dp ), and R ( K dp , ζ dr ) depending on Z dr > 0.25 dB and K dp > 0.3° km −1 thresholds. Because of these thresholds and the lack of hail, R ( K dp ) was never used. At all wavelengths, R ( z ) was still needed 43% of the time during light rain ( R < 5 mm h −1 , Z dr < 0.25 dB), composing 7% of the total rain volume. As wavelength decreased, R ( K dp , ζ dr ) was used more often, R ( z , ζ dr ) was used less often, and the blended algorithm became increasingly more accurate than R ( z ).