1.1
The role of low-level vertical wind shear in promoting long-lived, severe wind-producing convective systems
Morris L. Weisman, NCAR, Boulder, CO
Rotunno et al. (JAS, 1988) and Weisman et al. (JAS, 1988) put forth a theory, based on the analysis of idealized two- and three-dimensional simulations, that squall line strength and longevity was most sensitive to the strength of the component of low-level(0-3 km AGL) vertical wind shear perpendicular to squall line orientation. An ``optimal'' state was proposed, based on the relative strength of the circulation associated with the storm-generated cold pool and the circulation associated with the ambient shear, whereby the deepest leading edge lifting and most effective convective retriggering occured when these circulations were in near balance. This work was expanded by Weisman (1993) to consider three-dimensional bow-shaped segments within such lines, which are associated with the production of severe surface winds, and found that such structures were also especially preferred in environments with strong, low-level vertical wind shear. More recent studies (e.g., Coniglio and Stensrud, 2001; Evans and Doswell 2001), however, have brought into question the results and interpretations of these previous studies, noting the existence of long-lived, severe wind producing convective systems for weaker low-level shears than suggested by the numerical simulations, and raising the question as to the potential role of deeper-layer shears in promoting system strength and longevity in such situations. In the present paper, I will revisit the role of shear in promoting squall line and bow echo strength and longevity, and the interpretations thereof, through an analysis of an extensive set of 3D simulations that consider a wider range of shears and finer grid resolutions than used in the earlier numerical studies. These results reestablish the primary role of the low-level shear in promoting system strength and longevity, but also clarify that a wider range of environments than strickly ``optimal'' support the development of long-lived, severe convective systems in the simulations, as is also evident from observations.
Session 1, Mesoscale Convective Systems I
Monday, 12 August 2002, 10:30 AM-12:00 PM
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