On the role of X-band radar in extending longer-wavelength radar polarimetric retrievals to lighter rains

Polarimetric approaches for radar rainfall retrievals have been in use for more than 20 years. These approaches were first developed and tested with the longer wavelength radars operating at S- and C-bands because these band frequencies are traditionally used for quantitative precipitation estimations (QPE). Meteorological X-band radars have been traditionally limited in their applicability for QPE due to relatively high attenuation rate of radar signals in rain. In the last five - six years or so, however, several polarimetric X-band radars have been introduced worldwide. Special polarimetric procedures have been developed to correct radar power measurements for effects of attenuation and differential attenuation. These correction procedures greatly mitigate signal attenuation issues and significantly extend the usable range of X-band radar QPE measurements. The total signal loss, however, can occur at longer ranges in heavy rain, thus making X-band radars effective for QPE at generally shorter ranges (~50 km or so) compared to S-band radars (~200 - 250 km or so). In spite of this limitation, X-band polarimetry offers some important advantages over longer wavelength polarimetry at shorter ranges and lighter rainfall rates. The main important advantage is in a factor of about 3 stronger magnitude of differential phase compared to S –band (a factor of about 2 compared to C-band). This fact allows applications of X-band specific differential phase (KDP) based techniques to much lower rainfall rates than at S- and C-bands. KDP-based techniques are known to be less sensitive to details of drop size distributions and independent of the radar absolute calibration thus often providing more accurate QPE than methods based on Z-R relations. This presentation will show some comparative analysis of applications of polarimetric approaches with X- and S-band radar simultaneously observing same rain storms. This analysis is indicating that X-band KDP-based polarimetric techniques are applicable to rainfall rates as low as about 2.5-3 mm/h while the S-band KDP-based estimates are generally unavailable for rainfall rates less than about 8-10 mm/h. X-band differential reflectivity (ZDR) is generally larger than that at S-band for drop sizes greater than about 2 mm thus potentially providing a better differentiation for larger drop sizes