Idealized Model-Based Examination of Mesovortex Generation

Mesovortices associated with quasilinear convective systems can produce localized areas of strong straight-line winds and enhanced wind damage. Two mechanisms have been proposed in previous studies to explain mesovortex genesis: 1) baroclinic vorticity generation within the cold pool, which is tilted upward or downward near the gust front, and 2) a barotropic mechanism in which ambient low-level vertical wind shear is tilted to generate vortices along the gust front. Prior work examining mesovortices in 41 events suggested that the barotropic mechanism is associated with strong low-level shear parallel to the gust front over the lowest 500 m.

Results will be presented from idealized numerical simulations performed with CM1 to evaluate the dynamics of mesovortex formation and the role of the baroclinic and barotropic mechanisms. Experiments were conducted using a parameter space involving low-level vertical wind shear and cold pool temperature deficit. We hypothesize that a moderately strong cold pool is most favorable for the baroclinic generation of horizontal vorticity while remaining susceptible to tilting to produce vertical vorticity, similar to the hypothesized generation of low-level rotation in supercells. For the barotropic mechanism, based on our observational results and prior work of Wheatley and Trapp (2008), we hypothesize that ambient low-level shear, particularly parallel to the orientation of the gust front, is of greatest importance to the genesis of mesovortices through upward tilting of horizontal vorticity and the release of shearing instability.

Wheatley, D. M., and R. J. Trapp, 2008: The Effect of Mesoscale Heterogeneity on the Genesis and Structure of Mesovortices within Quasi-Linear Convective Systems. Mon. Wea. Rev., 136, 4220-4241.