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Buoyancy and Shear Characteristics of Hurricane-Tornado Environments

Eugene W. McCaul Jr.National Center for Atmospheric Research, Boulder, Colorado

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Abstract

Detailed composite profiles of temperature, moisture, and wind are constructed for tornado environments in tropical cyclones that affected the United States during the period 1948–86. Winds are composited in components radial and tangential to the tropical cyclone center at observation time. Guided by observed patterns of tornado occurrence, composites are constructed for a variety of different stratifications of the data, including proximity to tornadoes, position relative to the cyclone center, time of day, time after cyclone landfall, cyclone translation speed, and location of landfall. The composites are also compared to composite soundings from Great Plains tornado environments. A variety of sounding parameters am examined to see which are most closely related to the patterns of tornado distribution.

Lower-tropospheric vertical shears are generally stronger in the tropical cyclone tornado environments than on the Great Plains. Vertical shear and helicity parameters, along with 700-hPa wind speed, show the best correlations with the reported intensity of the tropical cyclone tornado outbreaks. Buoyancy for the tropical cyclone tornado cases is much smaller than that seen with Great Plains tornado events and shows a weak negative correlation with tornado outbreak severity.

Composite thermal and wind profiles computed in each quadrant relative to cyclone motion reveal distinct signatures, with relatively small thermal instability near the cyclone centers and enhanced shear and helicity in the tornado-prone right front quadrant. The patterns of wind profile differences from quadrant to quadrant resemble those that would result from the interaction of the cyclone with a steering current containing unidirectional shear that is roughly parallel to the direction of cyclone motion.

The number and intensity of tropical-cyclone-spawned tornadoes am generally found to increase with increasing cyclone intensity and size. Cyclone translation speed is not well correlated with tornado productivity. Although most tornadoes occur on the day of cyclone landfall, some cyclones are able to maintain wind shear profiles that are favorable for tornadic storm development through several days after landfall. Atlantic coast cyclones produce fewer tornadoes than their counterparts from the Gulf of Mexico, in part apparently as a result of a deficiency in postlandfall exposure of the right-front quadrant to land.

Abstract

Detailed composite profiles of temperature, moisture, and wind are constructed for tornado environments in tropical cyclones that affected the United States during the period 1948–86. Winds are composited in components radial and tangential to the tropical cyclone center at observation time. Guided by observed patterns of tornado occurrence, composites are constructed for a variety of different stratifications of the data, including proximity to tornadoes, position relative to the cyclone center, time of day, time after cyclone landfall, cyclone translation speed, and location of landfall. The composites are also compared to composite soundings from Great Plains tornado environments. A variety of sounding parameters am examined to see which are most closely related to the patterns of tornado distribution.

Lower-tropospheric vertical shears are generally stronger in the tropical cyclone tornado environments than on the Great Plains. Vertical shear and helicity parameters, along with 700-hPa wind speed, show the best correlations with the reported intensity of the tropical cyclone tornado outbreaks. Buoyancy for the tropical cyclone tornado cases is much smaller than that seen with Great Plains tornado events and shows a weak negative correlation with tornado outbreak severity.

Composite thermal and wind profiles computed in each quadrant relative to cyclone motion reveal distinct signatures, with relatively small thermal instability near the cyclone centers and enhanced shear and helicity in the tornado-prone right front quadrant. The patterns of wind profile differences from quadrant to quadrant resemble those that would result from the interaction of the cyclone with a steering current containing unidirectional shear that is roughly parallel to the direction of cyclone motion.

The number and intensity of tropical-cyclone-spawned tornadoes am generally found to increase with increasing cyclone intensity and size. Cyclone translation speed is not well correlated with tornado productivity. Although most tornadoes occur on the day of cyclone landfall, some cyclones are able to maintain wind shear profiles that are favorable for tornadic storm development through several days after landfall. Atlantic coast cyclones produce fewer tornadoes than their counterparts from the Gulf of Mexico, in part apparently as a result of a deficiency in postlandfall exposure of the right-front quadrant to land.

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