Abstract
Thompson and Edwards (2000) first noted a prominent hodograph kink separating primarily speed shear from primarily directional shear in the environments of some supercells producing significant tornadoes. Responding to this observation, we compared similar thermodynamic and shear environments between the Moore, Oklahoma tornado of 3 May 1999 and non-tornadic supercell thunderstorms occurring in north Texas on 23 April 2003. The results suggest that certain characteristics of the kink could discriminate between tornadic and non-tornadic supercells. This combination of features consisted of a strong (> 10 m s-1) nearly straight-line hodograph below 500 m above ground level (AGL) and storm-relative inflow orthogonal to the base of this hodograph segment at 10 m, yielding almost purely streamwise storm-relative inflow.
 We evaluated this hypothesis by analyzing 67 severe convective events, 65 of which were supercells, in Oklahoma from 1997-2004, and dividing the events into non-tornadic, weak-tornadic (F0-F1), and significant-tornadic (F2-F5) storm classes. The results show improved discrimination between storm classes for 10-500 m storm-relative helicity and bulk shear magnitude when compared to 10-1000 m calculations of the same. Also, histograms of the critical angle (defined by the storm-relative inflow vector at 10 m and 10-500 m shear vector) revealed that the tornadic storms, and in particular the significant tornadic storms, tended to be characterized by angles near 90°, whereas the non-tornadic storms were not. Although the results are based on a relatively small sample, they suggest that a careful consideration of the evolution of the low-level hodograph in both time and space in relation to the storm motion can potentially be a valuable aid in forecasting supercell tornadoes.
Highlights
Significant (F2-F5) tornadoes are low-probability, localized events, they often have a devastating impact on human infrastructure and welfare
In order to better distinguish between storm types, Thompson et al (2003) combined two shear parameters (0-1 km storm-relative helicity (SRH), 0-6 km bulk shear) with three thermodynamic parameters to produce a composite parameter called the Significant Tornado Parameter (STP), which was updated later to use “effective” layers (Thompson et al 2004)
Examining the distribution of SRH is important because this parameter is related to the strong low-level straight-line hodograph oriented normal to the 10-m inflow vector identified in the hypothesis
Summary
Significant (F2-F5) tornadoes are low-probability, localized events, they often have a devastating impact on human infrastructure and welfare. In order to better distinguish between storm types, Thompson et al (2003) combined two shear parameters (0-1 km SRH, 0-6 km bulk shear) with three thermodynamic parameters (mean layer CAPE, mean layer CIN, and mean layer LCL, where a “mean layer” parcel is an average of the lowest 100 mb AGL) to produce a composite parameter called the Significant Tornado Parameter (STP), which was updated later to use “effective” layers (Thompson et al 2004) Both versions of the STP were found to be successful in providing enhanced discrimination between storm classes, with the updated STP providing a nominal improvement in false alarm ratio. In an overview of the 3 May 1999 Oklahoma tornado outbreak, Thompson and Edwards (2000) noted that hodographs associated with four significant tornado events (including 3 May) were characterized by a low-level hodograph kink between 1 and 1.5 km.1 Hodographs indicate this kink occurred at the interface between a lower layer dominated by speed shear and a higher one dominated by directional shear. If this is roughly a straight line, the positioning
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