The Polarimetric Characteristics of Chaff Using the WSR-88D Network

Chaff is a radar countermeasure typically used by the military in training exercises around the United States. Due to its resonant cut lengths on the order of centimeters, chaff appears prominently on the S-band WSR-88D radars. Chaff returns tend to look similar to weather echoes in the reflectivity factor and radial velocity fields, and can masquerade as clutter, stratiform precipitation, or deep convection to the radar operator or radar algorithms. Knowledge of chaff characteristics, specifically in the polarimetric estimates, is useful to radar users and automated algorithms to discriminate between chaff and weather echoes. Upgrades to the WSR-88D network have provided dual-polarimetric capabilities across the Unites States, leading to the collection of a large database of chaff cases. This database of over 2 million data points is analyzed in order to determine the characteristics of chaff in the reflectivity factor and polarimetric estimates on large scales. Particular focus is given to the dynamics of differential reflectivity (ZDR) in chaff and its dependence on height and fallout time, with a specific emphasis on the common observation of negative ZDR in chaff. A discussion regarding the observed characteristics is presented, and hypotheses for the observed ZDR dynamics are put forth.