Polarimetric Hail Size Estimation for the NEXRAD Network

Recent improvements made to the NEXRAD network have created the opportunity for improved operational detection of several types of hazardous weather, including hail. The most important of these improvements are the upgrade to dual-polarization technology and new scanning options such as Supplemental Adaptive Intra-volume Low-level Scan (SAILS) and Automated Volume Scan Evaluation and Termination (AVSET) that increase the update frequency of low-level data. Large hail has a very distinct polarimetric signature at S-band, with very high reflectivity, low differential reflectivity, and low correlation coefficient. This polarimetric signature is typically more pronounced for very large hail than for smaller hail. By incorporating this signature into the Polarimetric Identification System (POLARIS), and designing an innovative algorithm to combine polarimetric hail signatures near the ground with traditional reflectivity-based methods to identify hail aloft, Weather Decision Technologies has developed POLARIS Hail Detection (PHD). PHD is a national hail size grid that takes advantage of the more frequent low-level scans by updating every 2.5 minutes. A description of the PHD algorithm and some examples of its performance compared to reflectivity-based methods will be presented.