A Modified Dual-Wavelength Technique for Ku- and Ka-Band Radar Rain Retrieval

To overcome a deficiency in the standard Ku- and Ka-band dual-wavelength radar technique, a modified version of the method is introduced. The deficiency arises from ambiguities in the estimate of the mass-weighted diameter D-m of the raindrop size distribution (DSD) derived from the differential frequency ratio (DFR), defined as the difference between the radar reflectivity factors (dB) at Ku and Ka band Z(Ku) - Z(Ka). In particular, for DFR values less than zero, there are two possible solutions of D-m, leading to ambiguities in the retrieved DSD parameters. It is shown that the double solutions to D-m are effectively eliminated if the DFR is modified from Z(Ku) - Z(Ka) to Z(Ku) - Z(Ka) (dB), where is a constant with a value less than 0.8. An optimal radar algorithm that uses the modified DFR for the retrieval of rain and D-m profiles is described. The validity and accuracy of the algorithm are tested by applying it to radar profiles that are generated from measured DSD data. Comparisons of the rain rates and D-m estimated from the modified DFR algorithm to the same hydrometeor quantities computed directly from the DSD spectra (or the truth) indicate that the modified DFR-based profiling retrievals perform fairly well and are superior in accuracy and robustness to retrievals using the standard DFR.