Considerations for Polarimetric Upgrades to Operational WSR-88D Radars

This paper reports on the steps taken by the National Severe Storms Laboratory (NSSL) to 1) develop open system hardware to facilitate upgrades to the WSR-88D (NEXRAD) radar and 2) improve identification of the type of precipitation and its quantitative measure. An engineering evaluation is made to determine if the WSR-88D antenna assembly with minimum modification could be used in a polarimetric mode. The polarimetric characteristics and radiation patterns of a research WSR-88D are briefly discussed. Considerations for the choice of polarimetric basis and design options are described. A polarimetric scheme employing simultaneous transmission of horizontally (H) and vertically (V) polarized waves is suggested for the WSR-88D, which eliminates an expensive, high-power switch. A theoretical evaluation is performed to determine the effects that feed alignment, drop canting, and backscatter depolarization have on the measurements of polarimetric parameters made with simultaneous transmission and reception of H and V signals. Experiments with the Colorado State University–Universities of Chicago and Illinois radar are performed to compare polarimetric variables obtained with alternate and simultaneous transmissions of H, V waves. Both simultaneous reception in two receivers and alternate reception in one receiver have been used.