Three-Dimensional Numerical Model Simulations of Airflow Over Mountainous Terrain: A Comparison with Observations

Abstract

Numerical simulations of airflow over two different choices of mountainous terrain and the comparisons of results with aircraft observations are presented. Two wintertime casts for flow over Elk Mountain, Wyoming where surface heating is assumed to be zero and one case for airflow over Mt. Withington, New Mexico where surface heating is strong are considered. In the Elk Mountain simulations the flow becomes approximately steady state since the upstream conditions are assumed to be constant and the surface heating is assumed to be zero. The response is significantly different in the two cases. In one case (dynamic Elk) strong lee waves formed with a horizontal separation of ?10 km whereas in the second case (microphysical Elk) mainly weak untrapped waves formed with a vertical wavelength of ?2.5 km. Because of the presence of the lee waves in the first case it is shown that the ridges south of Elk Mountain affect the flow near Elk Mountain. In the second case where there were no strong lee waves, the ridges to the south had very little effect on the flow near Elk Mountain so Elk acted as an isolated peak. The comparison between the simulation and the observations of the Elk Mountain experiments was good. In particular, the model's prediction of the location and intensity of trapped lee waves in the dynamic Elk case was good. In the Mt. Withington simulations, the presunrise response was very weak though there were some weak lee waves. After sunrise, strong longitudinal rolls developed in the lower 1 km. These rolls were parallel to the mean wind direction in the lowest first kilometer and had an initial cross roll separation of 4?5 km for a mixed layer depth of 1.5 km. Later in the morning, after additional surface heating, the longitudinal rolls tended to increase their cross roll separation distance and to break up into a more cellular pattern although still retaining a well-defined roll structure. The ratio of cross roll separation to mixed layer depth was within the typically observed ratio of ?2?3. The overall comparison between the observations and the simulated flow fields in the Mt. Withington case was reasonable although detailed comparisons between individual features met with mixed success. The low-level observations appeared to represent cellular patterns as opposed to the simulated roll patterns although the horizontal scales perpendicular to the simulated rolls compared favorably. This difference in convective regime between the model and observations may be due in part to the very crude surface layer treatment of the model used to treat the unstable boundary layer as well as due to difficulties in choosing representative low-level winds. In the upper levels the comparison was successful in that the observations corroborate the presence of the trapped lee waves simulated by the model.