Differential Reflectivity Bias Caused by Cross Coupling of H, V Radiation from the Antenna

The preferred embodiment of dual linear polarization technology on the WSR-88D weather radars is the mode whereby horizontal (H) and vertical (V) polarizations are transmitted and received simultaneously (Doviak et al. 2000). This mode is sometimes referred to as “hybrid” (Wang et al. 2006); to shorten notation we label it SHV (Simultaneous Horizontal and Vertical). The USA National Weather Service is slated to begin retrofitting its WSR-88Ds with this mode in about 2010. By far, the overriding reason for choosing the SHV mode is its total transparency to all the current automated algorithms used in the radar network. Other advantages of the SHV mode are: 1) direct measurement of the cross correlation between the copolar signals, 2) 360 unambiguous span for differential phase measurement, 3) decoupling of the differential phase and Doppler velocity measurements, 4) smaller error of estimates, 5) no degradation of the performance of the ground clutter filters, and 6) avoidance of a high power microwave switch and its associated problems. Nonetheless there are also disadvantages. For example, Sachidananda and Zrnic (1985) show, if hydrometeors along a propagation path have a mean canting angle, bias errors in differential reflectivity (ZDR) estimates can be an order of magnitude larger if ZDR estimates are made using the SHV mode rather than alternately transmitting, but simultaneously receiving H, V waves (i.e., the AHV mode). Furthermore, as will be shown, the bias associated with the SHV mode depends on the cross-polar radiation to the first order whereas second order terms are important for the AHV mode. Finally, the SHV mode is not fully polarimetric because it precludes cross-polar measurements.