On the Variation of the Parametrization of Rainfall Rate Estimation by dual polarization Techniques

Since the begining of estimation of rainfall rate by radar it is known that different relations between reflectivity factor (Z) and rain rate (R) are required for various regions and precipitation types. Even in the same kind of precipitation type Z-R relations can vary from time to time. This is due to variation of the shape of the raindrop-size distribution (RSD).

With the upcoming of polarization diversity techniques it was assumed that rain rate estimation by radar will become more independent on the shape of the raindrop-size distribution. However, this expections were not completely accomplished. Indeed, relations based on the differential reflectivity (ZDR) are less sensitive to RSD variations than Z-R relations. But they depend now on the raindrop shape, which is not completely known. Considering operational aspects, further uncertainties are caused by the fact that ZDR measurements are required with high precision, i.e. better than 0.2 dB. Similar aspects apply when the specific differential propagation phase (KDP) is considered. KDP depends on the raindrop shape and has to be measured with high precision, especially when low rainfall rates will be measured.

In the present study it will be shown how the coefficients of R-Z-ZDR, R-KDP, and R-KDP-ZDR relations depend on the drop shape and the kind of RSD's used for the parametrization. It will be shown that despite the application of polarization techniques there will be still a certain uncertainty in the estimation of rainfall rate by radar.