Vortex development mechanisms associated with the 8 May 2009 Central United States derecho event

During the early morning hours of 8 May 2009, intense convective development and its upscale growth across southwestern Kansas brought about the development of an intense derecho that, during the course of the day on 8 May 2009, traversed from southeastern Kansas into the Ohio Valley with millions of dollars in damages left in its wake. The companion presentation of Weisman et al. (2010) describes the evolution of this derecho event from formation to decay as exhibited in both reality and a 3 km convection-permitting WRF-ARW mesoscale model simulation of the case. In this work, using output from the aforementioned model simulation, we aim to quantify how the intense, warm-core line-end vortex associated with the derecho formed and occluded within an environment of weakening instability and weakening vertical wind shear.

To this end, the storm-relative circulation budget analysis described by Davis and Galarneau (2009, JAS) is utilized. A tripling of the circulation throughout the depth of the lower and middle troposphere over a 6-8 h period early on 8 May 2009 is observed as the vortex develops. Positive contributions to the net circulation tendency are observed in the lower troposphere in association with vortex stretching, presumably on both the vortex- and convective-scales, as well as with the horizontal transport (or flux) of relative vorticity. This favorable transport contribution includes both the inward flux of enhanced cyclonic relative vorticity, primarily along inflowing trajectories from the southeast, and the outward flux of enhanced anticyclonic relative vorticity, primarily equatorward of the derecho's descending rear inflow jet and along the back end of its convective line. These results will be presented in conjunction with diagnostics of the thermodynamic environment of both the vortex and its environment in an attempt to qualitatively determine all of the factors responsible for the development of the vortex.

Comparisons will be drawn between this case and cases of weaker MCV development in the literature (e.g. Davis and Galarneau 2009, JAS; Fritsch et al. 1994, JAS; Trier et al. 2000a,b, MWR, Yu et al. 1999, MWR; etc.) to attempt to quantify what specific factors led to the intense vortex development in this case (and not in others) and thus why such an evolution as is observed here does not occur more frequently.