Toy model simulations of surface vorticity generation in downdrafts

Full-physics simulations of supercells suggest that surface vertical vorticity (a pre-condition for tornadogenesis) is rather unsteady and tied to parcels emanating from downdrafts adjacent to the main updraft. The configuration of the resulting surface vorticity maxima, including their orientation, intensity, and storm-relative motion, results from some combination of baroclinic and barotropic processes in the downdraft. Operationally, it is also known that parameters based upon the low-level wind profile are somewhat skillful in identifying significant tornado days; the physical explanation for this skill is not yet totally clear. The difficulty with hypothesis tests using full supercell simulations is that, when the environment is varied, many storm-scale features change. We are therefore undertaking idealized sensitivity tests in which simple downdrafts are introduced with various configurations into environments with different magnitudes and orientations of low-level shear. Such work bears some resemblance to previous idealized studies (Walko 1993, Markowski et al. 2010), but here we undertake a broad matrix of sensitivity tests that specifically focus on the resulting structures of simulated surface vorticity maxima.

Our initial experiments support the following tentative conclusions. Realistic surface vortices are produced in this toy model, and the vortices are fundamentally downdraft/outflow phenomena. Barotropic and baroclinic production of vorticity co-exist in these simulations; surface vorticity often is small beneath the zone of barotropic production, whereas surface vorticity associated with baroclinic production moves away rapidly in many conditions. However, it appears that the wind profile (particularly the vertical wind shear) may provide special cases where flow through the downdraft creates a steady surface vorticity maximum that is anchored immediately adjacent to the downdraft. Such a condition would seem to be more favorable for tornado genesis and maintenance.