The National Severe Storms Laboratory Mesocyclone Detection Algorithm for the WSR-88D

Abstract

The National Severe Storms Laboratory (NSSL) has developed a mesocyclone detection algorithm (NSSL MDA) for the Weather Surveillance Radar-1988 Doppler (WSR-88D) system designed to automatically detect and diagnose the Doppler radar radial velocity patterns associated with storm-scale (1?10-km diameter) vortices in thunderstorms. The NSSL MDA is an enhancement to the current WSR-88D Build 9.0 Mesocyclone Algorithm (88D B9MA). The recent abundance of WSR-88D observations indicates that a variety of storm-scale vortices are associated with severe weather and tornadoes, and not just those vortices meeting previously established criteria for mesocyclones observed during early Doppler radar studies in the 1970s and 1980s in the Great Plains region of the United States. The NSSL MDA?s automated vortex detection techniques differ from the 88D B9MA, such that instead of immediately thresholding one-dimensional shear segments for strengths comparable to predefined mesocyclone parameters, the initial strength thresholds are set much lower, and classification and diagnosis are performed on the properties of the four-dimensional detections. The NSSL MDA also includes multiple range-dependent strength thresholds, a more robust two-dimensional feature identifier, an improved three-dimensional vertical association technique, and the addition of time association and trends of vortex attributes. The goal is to detect a much broader spectrum of storm-scale vortices (so that few vortices are missed), and then diagnose them to determine their significance. The NSSL MDA is shown to perform better than the 88D B9MA at detecting storm-scale vortices and diagnosing significant vortices. Operational implications of the NSSL MDA are also presented. In light of the new WSR-88D observations of storm-scale vortices and their association with severe weather and tornadoes, it is clear that the operational paradigms of automated vortex detection require changes.