Just How Reliable are the C-Band Polarization Parameters in Severe Convective Storms?

Many national weather services are upgrading their radar networks to have a polarization capability using the ‘hybrid' mode, whereby there is simultaneous transmission at horizontal and vertical polarization so that the pulse is ideally polarized at 45deg and reception is in the horizontal and vertical planes. This is much simpler than the traditional ‘alternating' mode using a fast switch to produce successive transmitted pulses that are horizontally (H) and vertically (V) polarized. Excellent data quality has been widely reported using the new hybrid mode, but there are some papers in the literature which indicate that small offsets in differential reflectivity (ZDR) can occur at S-band when the differential phase shift (φdp) is around 90deg as a result of cross talk between the two receive channels due to the finite antenna isolation and/or depolarizing targets.

Analysis of data from the C-band UK operational network polarization radars using hybrid mode shows excellent performance in light to moderate rain, with value of the co-polar correlation (ρHV) exceeding 0.99, the linear depolarization ratio (LDR) below -30dB, ZDR is estimated to better than 0.2dB, and φdp to within a couple of degrees; in the bright band ρHV falls to 0.9 as expected from the mixture of wet oblate melting snowflakes and raindrops. The performance in heavier attenuating rainfall is a cause for some concern. In theory in heavy rain φdp should increase with range as a result of attenuation and be accompanied by increasingly negative values of ZDR due to the differential attenuation; any values of ρHV below 0.9 where Z is very high should indicate Mie scattering hail, and in any lighter rain behind the storm ρHV should recover to 0.99. The observed behavior is rather different. We find that whenever Z exceeds 50dBZ along a ray and φdp is above 45deg then ρHV always falls to 0.9 or lower, and stays at this low level in the lighter rain behind the storm; in this light rain with low ρHV, the values of ZDR are far more negative than theoretically predicted from φdp. We suspect that for a C-band radar operating in hybrid mode, once φdp exceeds 45deg, the effects of cross-talk between the H and V returns may render the polarization parameters unreliable and they should be treated with caution.