Radar Observations of Tornado-Warned Convection Associated with Tropical Cyclones over Florida

Abstract

tatistical evaluations of tornado warnings issued during recent tropical cyclone events yielded an above-average false alarm ratio. This study analyzed tornado-warned convection associated with Tropical Storms Debby (2012) and Andrea (2013) using superresolution and dual-polarization data from Weather Surveillance Radar-1988 Doppler radars located throughout the Florida peninsula to identify precursor characteristics and signatures that would distinguish tornadic events prior to tornadogenesis. A series of radar-based interrogation guidance at varying ranges from radar is presented to help facilitate the reduction of the tornado-warning false alarm ratio without compromising the probability of detection. For convection within 74.1 km from the nearest radar, low-level velocity characteristics include a rotational velocity ≥ 10.3 m s?1 (20 kt), shear across the rotation ≥ 0.010 s?1, and a contracting rotation diameter. The convection should also exhibit supercell reflectivity signatures and at least a mesocyclone velocity enhancement signature or horizontal separation of greater ZDR and KDP values. Guidance at a range from 74.1 to 129.6 km is similar except for not requiring the presence of a supercell reflectivity signature and the change of the rotational velocity guidance to ≥7.7 m s?1 (15 kt) at the 0.5°-elevation angle. Convection at a range beyond 129.6 km only requires a rotational velocity ≥ 7.7 m s?1 (15 kt) at the 0.5°-elevation angle. Evaluation of the radar interrogation guidance for tornadic events and tornado-warned convection for six tropical cyclones reduced the number of false alarm events by 28.9% and reduced the false alarm ratio from 0.740 to 0.669.