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Hurricane-Spawned Tornadoes

David J. NovlanDepartment of Atmospheric Science, Colorado State University, Fort Collins, Colo. 80521

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William M. GrayDepartment of Atmospheric Science, Colorado State University, Fort Collins, Colo. 80521

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Abstract

Hurricane-spawned tornadoes are most frequent at the time when hurricanes initially cross land and undergo rapid filling. This paper presents data composite information on all available rawinsonde and pibal reports surrounding this type of tornado genesis in the United States and Japan. Information has also been gathered on tropical storms entering land which did not produce tornadoes.

The most important difference between storms which produce tornadoes and those which do not is a very large increase of the vertical shear of the horizontal wind between the surface and 4–5 thousand feet. This averages about 40 knots for the tornado cases, but is much less in the cases which do not produce tornadoes. Differences in vertical stability are observed to be small.

An overland hurricane dissipation model is proposed whereby the boundary layer frictional inflow towards the hurricane center occurs without the usual ocean sensible heat gain and is not, as over the ocean, isothermal. Over land the inward spiraling air parcels cool. This reverses the usual hurricane boundary layer baroclinicity and allows for large observed low-level positive vertical wind shear during the short period of rapid filling. This large magnitude vertical wind shear appears to be required for tornado formation. It should be used as a forecast tool in hurricane tornado prediction.

Abstract

Hurricane-spawned tornadoes are most frequent at the time when hurricanes initially cross land and undergo rapid filling. This paper presents data composite information on all available rawinsonde and pibal reports surrounding this type of tornado genesis in the United States and Japan. Information has also been gathered on tropical storms entering land which did not produce tornadoes.

The most important difference between storms which produce tornadoes and those which do not is a very large increase of the vertical shear of the horizontal wind between the surface and 4–5 thousand feet. This averages about 40 knots for the tornado cases, but is much less in the cases which do not produce tornadoes. Differences in vertical stability are observed to be small.

An overland hurricane dissipation model is proposed whereby the boundary layer frictional inflow towards the hurricane center occurs without the usual ocean sensible heat gain and is not, as over the ocean, isothermal. Over land the inward spiraling air parcels cool. This reverses the usual hurricane boundary layer baroclinicity and allows for large observed low-level positive vertical wind shear during the short period of rapid filling. This large magnitude vertical wind shear appears to be required for tornado formation. It should be used as a forecast tool in hurricane tornado prediction.

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