Abstract
A 10-year study of elevated severe thunderstorms was performed using The National Centers for Environmental Information Storm Events Database. A total of 80 elevated thunderstorm cases were identified, verified, and divided into “Prolific” and “Marginal” classes. These severe cases occurred at least 80 km away from, and on the cold side of, a surface boundary. The downdraft convective available potential energy (DCAPE), downdraft convective inhibition (DCIN), and their ratio are tools to help estimate the potential for a downdraft to penetrate through the depth of a stable surface layer. The hypothesis is that as the DCIN/DCAPE ratio decreases, there exists enhanced possibility of severe surface winds. Using the initial fields from the Rapid Refresh numerical weather prediction model, datasets of DCIN, DCAPE, and their ratio were created. Mann-Whitney U tests on the Prolific versus Marginal case sets were undertaken to determine if the DCAPE and DCIN values come from different populations for the two different case sets. Results show that the Prolific cases have values of DCIN closer to zero, suggesting the downdraft is able to penetrate to the surface causing severe winds. Thus, comparing DCIN and DCAPE is a viable tool in determining if downdrafts will reach the surface from elevated thunderstorms.
Highlights
The generalized definition of elevated convection is “convection that originates from an atmospheric layer above the boundary layer.” [1] An early climatology of such thunderstorm events above a frontal surface by [2] showed that such storms typically occurred poleward of a surface boundary
While surface weather induced by elevated convection is most commonly associated with heavy rainfall [3,4,5,6], some studies have indicated that severe hail, winds, and even tornadoes have been observed with elevated thunderstorms [7,8,9]
This study will further establish a tool for predicting severe criterion winds by measuring the potential for a downdraft to penetrate through the depth of the stable surface layer by comparing downdraft convective available potential energy (DCAPE) and downdraft convective inhibition (DCIN)
Summary
The generalized definition of elevated convection is “convection that originates from an atmospheric layer above the boundary layer.” [1] An early climatology of such thunderstorm events above a frontal surface by [2] showed that such storms typically occurred poleward of a surface boundary (often a warm front). This study will further establish a tool for predicting severe criterion winds by measuring the potential for a downdraft to penetrate through the depth of the stable surface layer by comparing DCAPE and DCIN. The hypothesis is that a progressively decreasing |DCIN|/|DCAPE| ratio (tending to zero) will indicate a strong potential for severe surface winds, while a |DCIN|/|DCAPE| ratio that exceeds 1.0 will virtually eliminate the possibility for significant convective winds at the surface. Such a result is implied in the work of [14].
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