A polarimetric radar, also called dual-polarization radar, transmits radio wave pulses that have both horizontal and vertical orientations. The horizontal pulses essentially give a measure of the horizontal dimension of cloud and precipitation (snow, ice pellets, hail, and rain) particles while the vertical pulses essentially give a measure of the vertical dimension. Since the power returned to the radar is a complicated function of each particles size, shape, and ice density, this additional information results in improved estimates of rain and snow rates, better detection of large hail location in summer storms, and improved identification of rain/snow transition regions in winter storms.
Most weather radars, including NEXRAD, transmit and receive radio waves with a single, horizontal polarization. That is, the direction of the electric field wave crest is aligned along the horizontal axis. Polarimetric radars, on the other hand, transmit and receive both horizontal and vertical polarizations. Although there are many different ways to mix the horizontal and vertical pulses together into a transmission scheme, the most common method is to alternate between horizontal and vertical polarizations with each successive pulse. That is, first horizontal, then vertical, then horizontal, then vertical, etc. And, of course, after each transmitted pulse there is a short listening period during which the radar receives and interprets reflected signals from the cloud.
All weather radars, including NEXRAD, measure horizontal reflectivity. That is, they measure the reflected power returned from the radar's horizontal pulses. Polarimetric radars, on the other hand, measure the reflected power returned from both horizontal and vertical pulses. By comparing these reflected power returns in different ways (ratios, correlations, etc.), we are able to obtain information on the size, shape, and ice density of cloud and precipitation particles.