Simulation of Diurnally Varying Downslope Winds Over Northern Utah

A relatively complete regional model is utilized to simulate downslope winds in two dimensions. The purpose of this study is to investigate the applicability of conclusions gained by prior investigators using simpler models that lacked diurnal solar cycles and have not explicitly included boundary layer physics. The model used in this study retains turbulent kinetic energy and surface energy budgets as well as longwave and shortwave radiative effects, and is applied to a realistic section to topography over Northern Utah. The diurnal cycle, established by the radiation calculations and reinforced with the prognostic turbulence closure terms, is found to be a first order effect and therefore essential for the evolution of windstorm events. These events are characterized by a nocturnal maximum in the forecast wind fields and development of a strong low level temperature inversion. Winds are weakest in the early afternoon at the time of strongest vertical turbulent mixing and most unstable stratification. This study confirms previous findings on the importance of the vertical wind and stability profiles to the amplification of downslope winds. It also investigates the dynamics of the momentum equation, and finds that, over most of the region of strong winds, the Bernoulli and dissipation terms oppose the pressure gradient and terrain following terms, and the Coriolis term is negligible. Preliminary results for the three-dimensional case are presented and appear to validate the results of the two-dimensional case for present goals.