Comparison of Radar-Based Rotation Metrics in Tornadic Storms

The National Weather Service in Huntsville, AL has an interest in comparing several metrics used to determine rotation in tornadic storms. With the introduction of super-resolution data from WSR-88Ds and several methods of merging and measuring rotation, it is helpful to revisit old tools and calibrate them against newer ones. Rotation metrics such as normalized rotation, rotational velocity, and rotational shear were analyzed for this research. This looked at cases based on tornado rating and storm mode and included a comparison between the individual parameters with cumulative frequency statistics, box and whisker data analysis, and regression analysis. GRLevel2 Analyst was used along with archived radar data pulled from the National Centers for Environmental Information (NCEI) database. Within a time period of five years from 2012 to 2016, sixty-two tornado cases from both discrete and quasilinear-convective-system (QLCS) storm modes occurred within the Huntsville County Warning Area (CWA). From this rotation metric data, two individual threshold tables for both storm modes were created. These thresholds provided a means to see if tornado warning verification could be improved based on the three rotation metrics. To check if these thresholds improved tornado warning verification, tornado events from 2017 and 2018 were used as a theoretical test, though the results were mixed. This research reveals some valuable thresholds and statistics for these rotation metrics, and will help forecasters to better understand their use in operation.