Variability of tornadoes induced by U.S. landfalling tropical cyclones
James I. Belanger, Georgia Institute of Technology, Atlanta, GA; and C. D. Hoyos, J. A. Curry, and B. A. Miller
Tornadic activity associated with landfalling tropical cyclones (TCs) from the Gulf of Mexico is examined for the period 1948-2007 in the context of TC size, intensity, and mid-level moisture data at landfall. New datasets are assembled for TC tornadoes and for TC size at landfall as indicated by radius of the outer closed isobar and distance of tornado measurements. Owing to uncertainties in the observed TC tornado record particularly prior to 1990, a multiple regression model for the number of TC tornadoes induced by each Gulf landfalling TC is developed based upon the more reliable tornado data since 1990 using TC size, intensity, and mid-level specific humidity gradient data as predictor variables. The model adequately reproduces the reliable portion (1990-2007) of the tornado record explaining 64% of the variance in the number of observed TC tornadoes for individual TCs and 88% of the variance in the annual number of observed TC tornadoes. This model is then used to reconstruct the TC tornado climatology back to 1948—the start of mid-level moisture measurements in the NCEP/NCAR Reanalysis II dataset. Relative to the previous active period for Gulf TC landfalls of 1948-1964, the current active period since 1995 is associated with an increase in the number of total TC tornadoes and an increase in the median tornadoes per TC landfall according to the reconstructed TC tornado dataset. These changes are linked to an increase in the median size and frequency of large landfalling TCs on the Gulf coast as revealed through radius of outer closed isobar and distance of tornado from TC center measurements. The significance of this increase in TC size and landfall frequency is placed into greater historical context by extending the TC tornado analysis back to 1920 (using a modified regression model that does not include mid-level specific humidity gradient as a predictor) to encompass the entire period of previous elevated Gulf landfall TC activity. While recent TC size increases are not statistically significant when compared to the 1926 to 1944 period, the landfall frequency of large hurricanes and major hurricanes striking the Gulf coast remains higher. The frequency of TC tornadoes particularly during 2004 and 2005 is shown to be at unprecedented levels according to the reconstructed TC tornado climatology.
Session 7B, Tropical cyclones
Tuesday, 13 January 2009, 3:30 PM-5:30 PM, Room 129B
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