Retrieval of Vertical Rain Rate Profile by Dual-Frequency Radar Data

Retrieval algorithm of vertical rain rate profile by dual-frequency radar was developed and applied to the NIED ground-based radar data. The algorithm uses a mutual constraint that expresses the consistency of the along path attenuations over a common segment sampled at the two frequencies and the assumption that N0* is constant along the profile (Testud, 2004).

Relationships between two of reflectivity factor (Z), specific attenuation (A) and rain rate (R) normalized by the “normalized” intercept parameter N0* (Testud et al., 2001) at X-, Ka- and W-band were obtained by T-Matrix calculations using DSD data measured with disdrometers at several sites in Japan. Uncertainty of the relationships could be decreased by the normalization although the effect of moment of DSD strongly appeared at W-band.

The rain rate retrieval algorithm by dual-frequency radar data was tested by model calculations. The test profiles of apparent reflectivity factor were made by calculated reflectivity factor and specific attenuation from assumed profiles of rain rate and N0*. Then rain rate profiles were retrieved from the test data of apparent reflectivity factor profiles. The retrieved errors were less than 5 % and larger at higher altitude because of the accumulated error of attenuation estimation but results were much better than by Z-R relationship.

Sensitivity tests for N0* variations by 10, 50 % and one order with height were also made and errors of retrieved rain rate were less than about 10, 15, and 50 %, respectively. If the height of N0* change was known, the retrieval error could be suppressed. In the case without data near the ground surface, it was possible to retrieve rain rate profile with small error by assuming constant rain rate in the range of no data by measured rain rate on the ground.

The proposed algorithm was applied to the data observed by the NIED X- and Ka-band radars in Tsukuba city, Japan in June 2001 and reasonable rain rate profile was derived.