Radar Signatures of Tropical Cyclone Tornadoes in Central North Carolina

Abstract

During the tropical cyclone season of 2004, there were four tropical cyclones that spawned tornadoes in central North Carolina: Frances, Gaston, Ivan, and Jeanne. This study examines the environmental characteristics and radar signatures from these events. The tornado warning decision-making process is a difficult one during any severe weather event, but it is even more difficult in a tropical cyclone environment because of the subtlety of features and rapid tornadogenesis that can occur. Previous studies that have examined the characteristics of a tropical cyclone environment found that high low-level moisture content, high shear, and a midlevel intrusion of dry air are favorable for tornadoes. The tropical cyclones that are examined in the current study all exhibited these characteristics. Radar signatures associated with these tornadoes were more subtle and weaker when compared with nontropical cyclone tornadoes, but were still discernable. This study analyzed the radar signatures from tornadic and nontornadic storms in a tropical cyclone environment with the purpose of determining the best indicators of tornadogenesis. Three precursors were found to give good lead time for tornado touchdowns: 1) a near gate-to-gate mesocyclone rotational velocity of 20 kt (10.3 m s?1) or greater, 2) a hook or appendage signature in the reflectivity data, and 3) the presence of a velocity enhancement signature of 30 kt (15.4 m s?1) or greater between 7000 ft (2.1 km) and 14 000 ft (4.2 km) AGL. Using these signatures together in the tornado warning decision-making process can increase lead time and accuracy in the tropical cyclone environment.