Weather radar polarimetry: path integrated differential phase shift optimum polarization and the elliptical EF-basis

Abstract

Polarimetric pulsed Doppler radar measurements of meteorological targets yield mean backscattering properties of hydrometeors in the considered range cells and provide information on propagation effects along the radar ray of propagation. In this paper, two aspects of choosing an optimum polarimetric measurement basis are examined. First, the polarization dependence of path integrated differential phase shift is studied starting from the 2 x 2 transmission matrix. It is shown that the linear HV-basis maximizes differential propagation phase. For the general case of an elliptical polarization basis, propagation effects can no longer be completely characterized by using the differential propagation phase shift (at frequencies lower that 6 GHz). Second, the choice of a particular measurement basis has a great impact on polarimetric weather radar data quality and, therefore, microphysical information content considering real world polarimetric weather radar limited channel isolation. It follows from the mean backscattering properties of hydrometeors in conjunction with polarimetric weather radar performance that the set of elliptical EFbases (|_E - _F| = _E,F = ±/8) yields particularly suited orthogonal measurement bases for rain radar applications having considerable advantages over the conventionally used linear HV- or circular LR-basis. For ensembles of almost sphere-like hydrometeorological scatterers co- and crosspolar power levels will attain roughly the same amplitude if an elliptical EF-basis is used as measurement basis. Considering limited interchannel isolation, polarimetric weather radar measurements performed in an elliptical EF-basis promise minimum relative errors and improved data quality. However, a trade-off situation does not exist between the elliptical EF-basis and the linear HV-basis. Since the elliptical EF-basis provides optimum measurements with respect to system induced cross-talk distortion, the optimum measurement strategy for polarimetric weather radar applications is to perform the measurement in elliptical EF-basis and then transform the measured time series of 2 x 2 scattering matrices to the linear HV-basis to conveniently estimate the differential propagation phase and established radar observables usually defined in the HV-basis such as, in particular, linear depolarization ratio.