The role of low-level shear, mid-level shear, and buoyancy in the intensity of modelled low-level mesocyclones (Formerly Paper P6.15)

A paper by Wicker (1996) showed that the development of low-level mesocyclones within modeled supercells is strongly modulated by the orientation and magnitude of the barotropic vorticity present in the storm's low-level inflow (by barotropic we mean the horizontal vorticity present in the large-scale environment's mean vertical shear). The interaction between the barotropic vorticity within the environment and the baroclinically generated horizontal vorticity produced by the storm-generated cold-pool solenoids is thought to be the principle mechanism controlling the development of low-level mesocyclones. The results showed that the development of low-level rotation can either be enhanced or suppressed simply by changing the magnitude and orientation of the barotropic vorticity in a very shallow layer near the surface. This result strongly suggests that prediction of low-level mesocyclone formation and strength (and potentially their tornado threat) may require detailed knowledge of the boundary layer flow in the near-storm environment.

Using simple unidirectional hodographs, research has continued along these lines to expand the parameters space and vary the mid-level shear. Dozens of simulations have been performed to further investigate the parameter space. Changing the magnitude of the mid-level shear alters the magnitude and location of the surface cold pool relative to the storm's low-level updraft. The location and magnitude of the cold-pool solenoids relative to the storm's inflow and updraft strongly affects the resulting vorticity field which is then tilted and stretched into the storm. The complex interaction between the low-level and mid-level shear creates a wider region within the two-dimensional parameter space which is favorable for the development of strong low-level mesocyclones then might be expected from the low-level shear results. These results will hopefully expand the findings by Brooks et al. (1994).

A final set of experiments add the effects of buoyancy as a third axis to the parameter space. Including the Convective Available Potential Energy (CAPE) should demonstrate the sensitivity of the shear parameters to downdraft and cold pool intensity. The results from this work will hopefully expand our understanding of the environmental parameters which are most favorable for low-level mesocylone development and potentially could lead to improved severe storm and tornado forecasts.

References

Brooks, H. E., C. A. Doswell III, and R. B. Wilhelmson, 1994: The role of midtropospheric winds in the evolution and maintenance of low-level mesocyclones. Mon. Wea. Rev., 122, 126-136.

Wicker, L. J., 1996: The role of near surface wind shear on low-level mesocyclone generation and tornadoes. Preprints, 18th Conf. on Severe Local Storms, San Francisco, CA, American Meteorological Society, 115-119.