Radar simulation studies for dual-frequency radar measurements of snow from space

The Global Precipitation Mission (GPM) core satellite successfully launched on 28 Feburary 2014 from Japan on an H-IIA rocket. The GPM core satellite equips with a dual-frequency precipitation radar (DPR) to measure precipitation. The DPR is a radar operated at frequencies of Ku-band and Ka-band and has a potential to measure more accurate rainfall rate than the space-borne precipitation radar (PR) operated at a single frequency on the TRMM satellite. In addition, the GPM core satellite measures higher latitude region than theTRMM satellite and therefore can measure snow for wide regions. Accurate measurements of snow fall rate with radar, however, are much more difficult than rain because radar received signals are affected by more microphysical parameters, such as DSD, shape, volume fraction of water in snow as well as snow fall rate than rain. Uncertainties of snow fall rate are estimated from single frequency is reported up to be 75%. One of methods for accurate measurements of snow is to use two radars operated at different frequencies such as the DPR. Radar at operated at 35 GHz suffers significant attenuation by rain comparing with for radar at 14 GHz. For snow, attenuation is very small both for radars at 35 GHz and 14 GHz. These different characteristics in attenuation and Ze between Ku and Ka-band radar, can be used to discriminate rain and snow. To examine the potential of the DPR measurements of snow, we made radar simulations studies. Scattering properties of snow was calculated by using, so called, soft oblate model: snow shape was assumed to be oblate and refractive index of snow was calculated assuming M-G theorem for various fraction of water in snow and size of snow. Results of the simulations suggested that differential frequency ratio (DFR:Ze35GHz-14GHz) depends on snow/rain fall rate, fraction of water in snow, size of particles. In particular, significant increases in DFR appear for large size of snow/rain drops. Despite complex features of the DFR, dry snow can be discriminate from rain with combination of the DFR and Ze of Ku-band radar. We have also examined a method to identify snow from measured data of the DPR in Japan.