Two- and Three-Dimensional Simulations of the 9 January 1989 Severe Boulder Windstorm: Comparison with Observations

Abstract

Simulations of the 9 January 1989 Colorado Front Range windstorm using both realistic three-dimensional (3D) orography and a representative two-dimensional (2D) east?west cross-sectional orography are presented. Both Coriolis forcing and surface friction (drag law formulation) were included for all experiments. The model results were compared with analyses of Doppler lidar scan data available from the surface to 4 km MSL provided by the Environmental Technology Laboratory of the National Oceanic and Atmospheric Administration (NOAA). The fully three-dimensional simulations with realistic orography used time-dependent inflow boundary conditions. These experiments were designed, in part, to assess the ability of mesoscale models to predict the onset and general characteristics of downslope windstorms. The present experiments highlight the sensitivity of wind storm onset and positioning of surface gusts to both model resolution and surface physics, which is in agreement with previous findings. These realistic orography experiments show that the major east?west canyons in the vicinity of Boulder produce a north?south broken structure to the strong updraft jump patterns. However, as the model resolution is increased from 3.33 to 1.11 km, the modulating effects of the canyons, with the exception of the Big Thompson, actually decreased. This tendency is attributed to an increasingly dominant role of the nonlinear internal fluid dynamics as the model resolution increases. Comparisons of model simulations with the lidar observations showed good agreement on the spatial and temporal scales of lee eddies. A north?south scale of ?10 km occurred in both the realistic orography model results and observations. A relatively strong Coriolis effect was shown to result from the super- and subgeostrophic flows caused by the nonlinear gravity wave dynamics. A northerly wind component of as much as 12 m s?1 at low levels over the foothills and plains is shown to be a direct result of Coriolis forcing. The turning of the wind with height as a result of this effect is supported by the observations. The transition from two to three dimensions showed some dramatic changes to the structure of the windstorm gusts in the idealized 2D orography simulations. The 3D simulations showed a smooth distribution of energy centered about a scale of ?3 km. These gust structures were close to isotropic in the horizontal as they propagated out onto the plains. Again this type of structure was supported by the observations. Three sources of surface gustiness are discussed in the paper. Surface gusts produced by vortex tilting and advected out of the wave-breaking region, as described in previous studies, occur in the present simulations. This mechanism is evidenced by the accompanying strong vertical vorticity. Propagating gust structures, similar in appearance to those obtained by others, are also obtained in both the 2D and 3D experiments using the idealized 2D orography. Rather than resulting from local Kelvin?Helmholtz instabilities, the propagating gusts in the present experiments appear to arise from high-amplitude lee waves that propagate as a result of the transient character of the wave-breaking region modulating the shape of the effective waveguide.