Storm-Relative Winds and Helicity in the Tornadic Thunderstorm Environment

Brynn W. Kerr NOAA/NWS Storm Prediction Center, Kansas City, Missouri

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Grant L. Darkow Department of Atmospheric Science, University of Missouri, Columbia, Missouri

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Abstract

Environmental flow relative to tornado-producing thunderstorms is examined through the use of the large tornado proximity sounding dataset compiled at the University of Missouri. It is believed that the 184 soundings gleaned from this collection represent the largest, most restrictive database of its kind with observed storm velocities, as determined from microfilm of conventional National Weather Service radar. Using these storm velocities, mean storm-relative wind profiles were derived for the entire data sample and sample subsets based on tornadic intensity, strength of the mean environmental flow, magnitude of CAPE, and direction of storm motion with respect to the mean environmental wind vector. Although it is apparent that a number of tornadoes occur independent of the larger-scale flow, the mean storm-relative wind profiles suggest that there is a preferred storm-relative flow structure for tornadic thunderstorms. Tornadic intensity in association with this structure appears to strengthen as 1) the magnitude of storm-relative helicity grows through an increasingly deep layer of the lower through midtroposphere and 2) mid- and upper-level storm-relative winds strengthen while possessing decreasing directional variability at their respective heights above ground level (4–12 km AGL).

Abstract

Environmental flow relative to tornado-producing thunderstorms is examined through the use of the large tornado proximity sounding dataset compiled at the University of Missouri. It is believed that the 184 soundings gleaned from this collection represent the largest, most restrictive database of its kind with observed storm velocities, as determined from microfilm of conventional National Weather Service radar. Using these storm velocities, mean storm-relative wind profiles were derived for the entire data sample and sample subsets based on tornadic intensity, strength of the mean environmental flow, magnitude of CAPE, and direction of storm motion with respect to the mean environmental wind vector. Although it is apparent that a number of tornadoes occur independent of the larger-scale flow, the mean storm-relative wind profiles suggest that there is a preferred storm-relative flow structure for tornadic thunderstorms. Tornadic intensity in association with this structure appears to strengthen as 1) the magnitude of storm-relative helicity grows through an increasingly deep layer of the lower through midtroposphere and 2) mid- and upper-level storm-relative winds strengthen while possessing decreasing directional variability at their respective heights above ground level (4–12 km AGL).

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