Radar polarimetry at S, C, and X bands: Comparative analysis and operational implications

Radar polarimetry at S band has matured to a point that the US National Weather Service plans to retrofit the WSR-88D network with components for polarimetric measurements. The choice technique is simultaneous transmission and reception of Horizontally and Vertically polarized waves (SHV mode). Other national meteorological services have already procured samples of polarimetric radars (10 or 5 cm wavelength). Further, exploration of inexpensive X-band (3 cm wavelength) radars continues in NOAA to complement existing WSR-88D radars in the regions of poor coverage (as “gap fillers”) or for monitoring rainfall in mountain valleys. Therefore, adaptation of existing S-band polarimetric algorithms for shorter wavelengths is an important practical issue. At shorter wavelengths, effects of attenuation, Mie scattering, cross-coupling between orthogonal polarizations due to simultaneous transmission / reception become more significant compared to S band. Nonuniform beam filling also has much larger impact on the quality of polarimetric measurements at shorter wavelengths, particularly for smaller radars with broader beams. This paper examines and quantifies these problems on simulated realistic fields of polarimetric variables from rain at C and X bands. The simulation is a transformation of fields, measured with the dual-polarization WSR-88D radar, into fields expected to be measured with X and C band systems. Polarimetric algorithms for radar echo classification and DSD retrieval are used in this transformation. It is found that the majority of warm-season convective storms in Oklahoma contain large raindrops (exceeding 4 -5 mm); these produce strong resonance effects at C band which are manifested by anomalously high differential reflectivity Z_DR and very low cross-correlation coefficient ρ_hv. The Mie resonance effects are less pronounced at X band due to smaller Q factor (i.e., larger imaginary part of refractive index of water). It is also shown that the presence of gradients of radar reflectivity Z and differential phase Φ_DP within the radar resolution volume leads to spurious oscillations of Φ_DP and decrease of ρ_hv that can become overwhelming at shorter wavelengths and wider radar beams. It is not a loss of sensitivity due to attenuation but beamwidth effects that might restrict use of polarimetric methods on short-wavelength radars (with relatively wide beams). Effects of cross-coupling due to simultaneous transmission are usually negligible at S band (except for such highly depolarized medium as slanted crystals at cloud tops) but might be quite noticeable at shorter wavelengths. Some of our conclusions are corroborated with examples of real polarimetric data collected at C and X band.