Zenith/nadir-pointing cloud radars: linear or circular polarization?

Description

In this paper we consider scatterers with azimuthal symmetry and explore the effects of transmit polarization (either linear or circular) on the retrieved polarimetric variables: reflectivity, depolarization ratio, cross-polar coherence, and degree of polarization. The results are relevant to the interpretation of signatures from millimeter-wavelength zenith-pointing cloud radars, like the KAZR (Ka-band zenith-pointing radar), MMCR (millimeter-wavelength cloud radar) and WACR (W-band ARM cloud radar) systems in use at the Atmospheric Radiation Measurement (ARM) Climate Research Facility operated by the U.S. Department of Energy, as well as for nadir-pointing precipitation and cloud radars onboard NASA (National Aeronautics and Space Administration) and ESA (European Space Agency) satellite missions.

It is found that, for scatterers with azimuthal symmetry, reflectivity is maximized at linear polarization transmit (therefore preferable for single-polarization systems), whereas the depolarization ratio dynamic range is maximized at circular polarization transmit (probably preferable for dual-polarization systems). The physical meaning of the cross-polar coherence is revisited in terms of scattering symmetries, and use of the degree of polarization to extend the depolarization ratio dynamic range below the cross-polar isolation level of the antenna is illustrated.

Two practical applications emerge from this theoretical analysis. First, at Ka-band (wavelength is 8 mm), the circular depolarization ratio (CDR) of rain displays a large dynamic range due to non-axisymmetric drop oscillations. Since drop oscillations map monotonically to raindrop size, CDR also maps monotonically to raindrop size, and we exploit this dependence to derive a rain-rate estimator at Ka-band. Experimental data from MMCR are compared with Mishchenko T-matrix code to substantiate the result. Second, the polarimetric analysis conducted for scatterers with azimuth symmetry outlines a powerful two-dimensional classification scheme making use of Huynen parameters. Given the stunning variability of ice crystal habits (pristine dendrites, stellars, columns, needles, plates + the processes that transform them: riming and aggregation), it is necessary to develop a model-free classification scheme that identifies some “shape parameter” independently of any assumed model. In our scheme, cloud of spheres and cloud of dipoles can be uniquely identified in the classification plane, thus providing an indication to how “sphere-like” or how “needle-like” the scatterers are.