Fully coherent simultaneous polarimetric mesurements in weather radars

Temporal decorrelation of weather echoes, due to random motion of raindrops (or hydrometeors in general) is a relevant issue in weather radar applications, since it has a direct influence on the accuracy with which radar observables can be measured. In particular, when polarization information of interest has to be extracted from fully polarimetric weather radar data, temporal decorrelation becomes of primary importance since the alternate H-V polarisation transmission scheme that must be adopted to provide a complete measurement of the scattering matrix halves (with respect to a non polarimetric system) the sampling time of each polarization channel and introduces decorrelation between the two columns of the matrix. The latter effect is of interest in radar meteorology for its influence on the statistics of the product of the two copolar elements and on the measurement accuracy of the differential phase shift. Some authors have highlighted the problems introduced by temporal decorrelation on the measurement of the scattering matrix of rainfall echoes and of the parameters of interest in polarimetric radar meteorology, showing that the effects of decorrelation can be relevant also in quite common rainfall events. Correction methods based on Doppler phase interpolation have also been proposed to mitigate such effects and they are effective to some extent; however, they fail in preserving acceptable accuracy for the measurement of the scattering matrix elements when the decorrelation time of echoes is still quite high with respect to the radar pulse repetition interval. Based on such elements, we propose in this paper a method for a fully polarimetric coherent and simultaneous measurement of weather echoes that overcomes the problems introduced by the alternate polarisation transmission scheme and provides simultaneous sampling of the scattering matrix elements at a rate equal to the system puIse repetition frequency. Such method, that was proposed by us for different radar appIication fields, is discussed here with reference to the potential advantages it can provide for weather radar applications: its working principle is analysed in the framework of radar meteorology accounting for both polarimetric measurement issues and propagation related problems.