Extended Abstract (204K)P6.7
High resolution numerical simulations of thunderstorm outflow boundaries
Bruce D. Lee, University of Northern Colorado, Greeley, CO; and C. A. Finley
Thunderstorm outflow boundaries play a large role in convection initiation while also representing the vehicle for potentially damaging winds and dangerous wind shear impacting the aviation community. Yet, high resolution, three-dimensional numerical modeling studies adequate to resolve the fine-scale (~100-300 m) kinematic structure along these boundaries have not been done. Only recently, have comprehensive plans been considered for observational study of boundaries at the necessary resolution to document the fine-scale instability structures (e.g., IHOP). Understanding of the instabilities indigenous to the outflow boundary leading edge and head section is likely critical for explaining, at least in part, the localized distribution of moist convective forcing along outflow boundaries, the magnitude of both vertical and horizontal shears of importance to aviation, the development of gustnadoes, and possibly even the damage pattern for outflow-associated severe wind events. Of particular interest is lobe and cleft instability (Simpson 1969, 1972). Although a dominant instability type along outflows, lobe and cleft instability has received very little attention in the atmospheric sciences community due likely to the observational and computational restrictions associated with studying phenomenon on this scale.
A three-dimensional, non-hydrostatic, quasi-compressible convective cloud model (MSTFLOW, Lee and Wilhelmson, 1997) is being employed to simulate strong outflow boundaries. An idealized initial experiment framework featuring a neutrally stable environment, no moist microphysics, and a calm ambient environment is being used to produce baseline results that are readily understandable in the absense of the complexity associated with a more extensive parameter space. Preliminary experiments using a 40 m horizontal grid spacing and 40 m vertical grid spacing (near surface) have yielded interesting results. An evolving pattern of lobes and clefts has been resolved that shed light on the intricacies of the flow field at the outflow leading edge. The lobe and cleft structure greatly influences the three-dimensional horizontal and vertical wind field while also creating a pattern of counter-rotating vertical vortices, some of which are shed internally when the clefts collapse. These results as well as those from higher resolution simulations (of 25-30 m grid spacing) will be presented at the conference.
Poster Session 6, Numerical Modeling of Severe Local Storms
Wednesday, 14 August 2002, 3:00 PM-4:30 PM
Previous paper Next paper