The role of near-surface wind shear on low-level mesocyclone generation and tornadoes: Renascentia

Despite tremendous progress toward understanding the processes that generate low-level rotation in tornadic thunderstorms over the past 30 years, the precise dynamical mechanism that transforms the available environmental vertical wind shear into a vertical column of rotating air beneath the storm's updraft remains elusive. Observations are crucial toward the development of hypotheses about this process. However the broad range of spatial/temporal scales and the state variables needed to unravel the flow dynamics are impossible to observe completely. Therefore numerical simulation is the primary tool for understanding the internal dynamics within convective storms. While numerical simulations have improved via grid resolution and more accurate physics during the past decades, the wide range of environmental parameters observed to generate rotating supercells remain a challenge modelers designing parameter studies seeking to develop a general theory relating low-level rotation to the local storm environment (McCaul and Cohen 2002). Additional complications arise from the need to represent processes which are unresolvable on the chosen grid, e.g., the representation of the grid averaged effects of condensation, fusion, evaporation, melting, surface fluxes of humidity and heat, and momentum. The sensitivity of the numerical solutions to the treatment of these unresolved processes, particularly at modest horizontal and vertical grid resolutions, is a particular challenge when attempting to generalize results (Adlerman et al. 2001). For example Adlerman et al. clearly demonstrates the sensitivity of the low-level mesocyclone rotation to the choice of surface roughess length (drag coefficient). The effects are significant and alter the strength of rotation both positively and negatively depending on other parameters. It is unclear whether this sensitivity is real or merely the result from the poor representation of the wall turbulence near the ground. Similarily, the relative importance of solenoidally generated horizontal vorticity from evaporation of precipitation versus the inflow of ambient environmental horizontal vorticity contained in the storm's inflow is also difficult to ascertain due to microphysical parameterization errors.

Wicker (1996; 1998) shows that even in environments having significant 0-3 km storm-relative helicity (e.g., SRH > 300 m2 s-2), the development of near-ground rotation within the storm's mesocyclone still depends on the orientiation of the inflow's horizontal vorticity vector relative to the horizontal velocity gradient in the storm's low-level updraft. This sensitivity is modulated by the storm's low-level buoyancy gradient – often the soleniodal generation of horizontal vorticity in this region is sufficient to overcome any sub-optimal environmental wind shear (Rotuno and Klemp 1985). Guided by the resolution recommendations outlined in Bryan et al. (2003) the parameter space discussed in Wicker (1996; 1998) will be reinvestigated using a high-resolution numerical mesh with an improved microphysical parameterization. The work will investigate in detail how the near surface shear in the environment modulates mesocyclone intensity and lifecycle. In this study the effects from storm-generated solenoids are initially minimized using thermodynamic environments constructed from the methods outlined in McCaul and Weisman (2001) and McCaul and Cohen (2002) that reduce the evaporation potential near the surface. After the initial dynamical analysis using trajectory and circulation analyses, the sensitivity of the results to the thermodynamic environment, microphysics parameters, and other model parameters is shown using a methodology similar to Adlerman and Droegemeier (2002).

References available upon request.