Polarimetric Radar at Attenuated Wavelength as a Hydrological Sensor

Abstract

A new method for the estimation of the rain rate using a polarimetric radar at attenuated wavelengths is proposed. At attenuated wavelengths, the differential reflectivity for horizontal and vertical polarization, ZDR, is the addition of a term depending on the drop shape, ZDRs, and a term depending on the differential attenuation for the two polarizations. ADP. The differential attenuation can be interpreted as an estimation of the integral of the rain rate along the radar beam. The principle of the method is the following. along the radar beam a first estimation of the rain rate R, in each range gate, is calculated from the single-wavelength radar reflectivity factor Z using the usual iterative attenuation correction scheme and a combination of a Z?R and an A?R relation, where A is the single polarization attenuation coefficient. This result permits one to obtain an estimation of ZDRs. The difference between the measured value of ZDR and ZDRs, is a measure of ADP. Then the integral of R along the radar beam, deduced from the single polarization reflectivities, is constrained to be equal to the integral of R deduced from ADP. This constraint is used to adjust iteratively one of the two parameters of the Z?R relation (let a be this parameter), the other being maintained constant at a mean value. The, adjusted α enables the calculation of R in each gate. The radial observed with the radar can he partitioned, and the algorithm applied independently to the individual segments with one value of αA computed for each segment. The method is independent from the radar calibration and from the attenuation by undetected clouds since it is based on a differential measurement. This algorithm is also usable as a qualitative hail detector, as well as a detector of anomalous propagation. Simulations of these various functions are presented.