Zenith/Nadir pointing mm-wave radars: Linear or circular polarization?

We consider zenith/nadir pointing atmospheric radars and explore the effects of different dual-polarization architectures on the retrieved variables: reflectivity, depolarization ratio, cross-polar coherence, and degree of polarization. Under the assumption of azimuthal symmetry, when the linear depolarization ratio (LDR) and circular depolarization ratio (CDR) modes are compared, it is found that for most atmospheric scatterers reflectivity is comparable, whereas the depolarization ratio dynamic range is maximized at CDR mode by at least 3 dB. In the presence of anisotropic (aligned) scatterers, that is, when azimuthal symmetry is broken, polarimetric variables at CDR mode do have the desirable property of rotational invariance and, further, the dynamic range of CDR can be significantly larger than the dynamic range of LDR. The physical meaning of the cross-polar coherence is revisited in terms of scattering symmetries, that is, departure from reflection symmetry for the LDR mode and departure from rotation symmetry for the CDR mode. The Simultaneous Transmission and Simultaneous Reception mode (STSR mode or hybrid mode or Z _DR mode) is also theoretically analyzed for the case of zenith/nadir pointing radars and, under the assumption of azimuthal symmetry, relations are given to compare measurements obtained at hybrid mode with measurements obtained from orthogonal (LDR and CDR) modes.