Differential reflectivity bias caused by cross coupling of H, V radiation in the antenna

One of the most critical elements of a polarimetric Doppler radar is the antenna. Once an antenna is selected and installed it is a costly and time consuming process to modify or replace it. Thus, it is important to initially establish requirements on the polarimetric performance of the antenna to insure that the required accuracy of polarimetric parameters is attained. Not only is it important to have low sidelobes and a narrow copolar beamwidth, but the cross-polar radiation characteristics of the antenna must not reduce the effectiveness of the Polarimetric Doppler radar for making significant improvements in rainfall measurements and classification of hydrometeors. An ideal polarimetric antenna is one that only receives signals in its H port if horizontal (H) waves are incident on it, and receives only signals in its V port if vertically polarized waves are incident. But in practice both ports will receive signals if either H or V polarized waves are incident. The amount of this coupling causes errors in polarimetric measurement, in particular the measurement of differential reflectivity ZDR, the parameter most critical to accurate rainfall measurements. To that end, a brief review of the effects of the ZDR bias on classification of hydrometeors and on quantitative rainfall measurements are given. Suggestions for tolerable values of this bias are made. This establishes a baseline goal for the improvement of quantitative precipitation estimation.

Then examined is the ZDR bias due to coupling of the vertically and horizontally polarized fields through the copolar and cross-polar radiation patterns. Of utmost interest is the bias produced by radar simultaneously transmitting horizontally and vertically polarized fields, as this configuration has been chosen for the USA national network of radars (WSR-88D). The bias strongly depends on the type of cross-polar radiation pattern. Two patterns, documented in the literature, are considered. One has a single lobe centered on the pattern's copolar main lobe. Two causes of this coaxial cross-polar radiation are exposed: 1) antenna tilt, and 2) non orthogonal H and V ports on the feed horn. The coaxial pattern is shown to produce significant bias. The other cross-polar pattern consists of four peaks, axially symmetric with respect to the main beam lobes. This pattern is attributed to the properties of the parabolic reflector. Nevertheless, it produces quite low ZDR bias. It is this latter pattern that should be representative of the ones expected on the WSR-88D when the network is upgraded to have polarimetric capability.