Genesis of Tornadoes Associated with Hurricanes

Abstract

Study of the hurricanes of the last 22 years reveals that nearly every tropical cyclone of full hurricane intensity whose center crosses the United States coast between Brownsville, Texas and Long Island, New York has associated tornadoes. Approximately 60% of the tropical cyclones of only storm intensity when crossing the coast also have tornadoes reported. The climatology of hurricane-tornadoes is brought up to date through 1982. Tornadoes form in areas of the hurricane where the tipping term and convergence terms of the vorticity equation are the largest. This usually happens in air that has had a relatively short trajectory over land. That is, they form far enough inland for the lower layers of air to be slowed by friction with the ground but close enough to the ocean for the upper layers of air to still be moving at approximately the speed they had while above the ocean. This means that most of the tornadoes either form near the center of the hurricane (from the outer edge of the eyewall outward) or in an area between north and east-southeast of the hurricane center. The blackbody temperatures of the cloud tops which were analyzed for several hurricane-tornadoes that formed in the years 1974, 1975 and 1979 did not furnish strong precursor signals of tornado formation, but did follow one of two patterns. Either the temperatures were very low or the tornado formed in areas of strong temperature gradients. Tornadoes with tropical cyclones can occur any hour of the day or night, but occur most frequently between 1200 and 1800 LST. Most are relatively weak but can reach at least the F3 level of intensity. Most form in association with the outer rainbands of the hurricane, but in the period 1973?80 more than 20% were with the inner bands or near the outer edge of the eyewall.