Atmospheric Simulations of Extreme Surface Heating Episodes on Simple Hills

Abstract

A two-dimensional nonhydrostatic atmospheric model was used to simulate the circulation patterns (wind and vorticity) and turbulence energy fields associated with lines of extreme surface heating on simple two-dimensional hills. Heating-line locations and ambient crossflow conditions were varied to qualitatively determine the impact of terrain geometry on the development of buoyancy-induced horizontal roll vortices and their turbulence structures. The model simulations indicate that the type of induced circulation that develops near a line of extreme surface heating on a simple hill is very dependent on the location of the heating line. Heating lines located on the crests of simple hills produced symmetric horizontal roll vortices with large values of turbulent kinetic energy. Symmetric vortices did not develop over heating lines located on the slopes of the hills. The introduction ofa light ambientcrossflowradically changed the circulation and turbulence structures. Simulated vorticity and turbulence energy values over a heating line located on the windward slope were very different than those observed over heating lines located on the crest or leeward slope. A low-level vortex developed just downwind of the windward-slope heating line when a light ambient crossflow was introduced, and this vortex became stronger as the steepness of the hill was increased. Although field dataare not available toconfirm the model results, the simulations suggest that terrain effects play an important role in the development and destruction of vortices near lines of extreme surface heating, especially when a light ambient crossflow is introduced. These effects have implications for fire-fighter safety in actual wildland fire episodes.