Dynamics of microburst outflows

A numerical study of microbursts in varied geometries is performed using a dry, non-hydrostatic three-dimensional numerical model with 50 m isotropic grid resolution in a 18km x 8km x 4.5km domain. The geometries studied include colliding microbursts, a gust front/microburst collision, and traveling microbursts in sheared environments. These configurations have either been observed or have been hypothesized by researchers studying storm damage patterns and physical data from field studies. Four traveling microburst simulations are examined with varying source speeds, and kinetic energy and horizontal vorticity budget analyses are performed. Eleven colliding microburst simulations in different spatial and temporal geometries are performed, and three classes of flow are categorized which are functions of the spacing between the microburst downdrafts. A gust front/microburst collision is also examined, patterned after sea breeze/microburst collision encountered by a jetliner which crashed on approach flying through these winds.; Surface outflow velocities for the traveling microburst simulations are found to increase until the source speed approaches the Froude velocity. Many colliding geometries exhibit regions of aircraft hazard due to horizontally divergent and downward winds which extend beyond the expected hazard regions associated with an isolated microburst. The gust front/microburst collision and the faster traveling microbursts exhibit regions of horizontal misocyclonic circulations which are initiated by the upward tilting of horizontal vorticity and the interaction between microburst downdrafts and horizontal roll vortices.; The effect of turbulence closure on a controlled simulation is performed by running four simulations which vary only in turbulence closure, and comparing the results to a higher resolution simulation. Turbulence closure is found to be of second-order importance in the morphology of the major dynamic features of the controlled simulation. The low resolution controlled simulation using the Laplacian/biatomic hybrid closure method best matches the higher resolution simulation in small-scale detail.