The development of rainfall retrievals from radar at Darwin

The U.S. Department of Energy Atmospheric Radiation Measurement program Tropical Western Pacific site hosted a C-band polarization (CPOL) radar in Darwin, Australia. It provides 2 decades of tropical rainfall characteristics useful for validating global circulation models. Rainfall retrievals from radar assume characteristics about the droplet size distribution (DSD) that vary significantly. To minimize the uncertainty associated with DSD variability, new radar rainfall techniques use dual polarization and specific attenuation estimates. This study challenges the applicability of several specific attenuation and dual-polarization-based rainfall estimators in tropical settings using a 4-year archive of Darwin disdrometer datasets in conjunction with CPOL observations. This assessment is based on three metrics: statistical uncertainty estimates, principal component analysis (PCA), and comparisons of various retrievals from CPOL data.

The PCA shows that the variability in inline-formulaR can be consistently attributed to reflectivity, but dependence on dual-polarization quantities was wavelength dependent for inline-formula $M2inlinescrollmathml\mathrm{normal\; 1}<R<\mathrm{normal\; 10}\mathrm{normal\; mm}{\mathrm{normal\; h}}^{-\mathrm{normal\; 1}}$ 91pt13ptsvg-formulamathimg06ce2f6691114dac298092348ebe90fd amt-14-53-2021-ie00001.svg91pt13ptamt-14-53-2021-ie00001.png . These rates primarily originate from stratiform clouds and weak convection (median drop diameters less than 1.5 mm). The dual-polarization specific differential phase and differential reflectivity increase in usefulness for rainfall estimators in times with inline-formula $M3inlinescrollmathmlR>\mathrm{normal\; 10}\mathrm{normal\; mm}{\mathrm{normal\; h}}^{-\mathrm{normal\; 1}}$ 72pt13ptsvg-formulamathimg6f30925d1f497e9b490f4b64de6fcd5d amt-14-53-2021-ie00002.svg72pt13ptamt-14-53-2021-ie00002.png . Rainfall estimates during these conditions primarily originate from deep convective clouds with median drop diameters greater than 1.5 mm. An uncertainty analysis and intercomparison with CPOL show that a Colorado State University blended technique for tropical oceans, with modified estimators developed from video disdrometer observations, is most appropriate for use in all cases, such as when inline-formula $M4inlinescrollmathml\mathrm{normal\; 1}<R<\mathrm{normal\; 10}\mathrm{normal\; mm}{\mathrm{normal\; h}}^{-\mathrm{normal\; 1}}$ 91pt13ptsvg-formulamathimgb5da49f317f8d458a551de8be4e71d40 amt-14-53-2021-ie00003.svg91pt13ptamt-14-53-2021-ie00003.png (stratiform rain) and when inline-formula $M5inlinescrollmathmlR>\mathrm{normal\; 10}\mathrm{normal\; mm}{\mathrm{normal\; h}}^{-\mathrm{normal\; 1}}$ 72pt13ptsvg-formulamathimgbe07314aff015718c8929228203d6a3f amt-14-53-2021-ie00004.svg72pt13ptamt-14-53-2021-ie00004.png (deeper convective rain).