2-DIMENSIONAL AND 3-DIMENSIONAL SIMULATIONS OF THE 9 JANUARY 1989 SEVERE BOULDER WINDSTORM - COMPARISON WITH OBSERVATIONS

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) we re 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 Nation al Oceanic and Atmospheric Administration (NOAA). The fully three-dime nsional simulations with realistic orography used time-dependent inflo w boundary conditions. These experiments were designed, in part, to as sess the ability of mesoscale models to predict the onset and general characteristics of downslope windstorms. The present experiments highl ight the sensitivity of windstorm onset and positioning of surface gus ts to both model resolution and sur-face physics, which is in agreemen t with previous findings. These realistic orography experiments show t hat the major east-west canyons in the vicinity of Boulder produce a n orth-south broken structure to the strong updraft jump patterns. Howev er, as the model resolution is increased from 3.33 to 1.11 km, the mod ulating effects of the canyons, with the exception of the Big Thompson , actually decreased. This tendency is attributed to an increasingly d ominant role of the nonlinear internal fluid dynamics as the model res olution increases. Comparisons of model simulations with the lidar obs ervations showed good agreement on the spatial and temporal scales of lee eddies. A north- south scale of approximately 10 km occurred in bo th the realistic orography model results and observations. A relativel y strong Coriolis effect was shown to result from the super- and subge ostrophic flows caused by the nonlinear gravity wave dynamics. A north erly wind component of as much as 12 m s-1 at low levels over the foot hills and plains is shown to be a direct result of Coriolis forcing. T he turning of the wind with height as a result of this effect is suppo rted by the observations. The transition from two to three dimensions showed some dramatic changes to the structure of the windstorm gusts i n the idealized 2D orography simulations. The 3D simulations showed a smooth distribution of energy centered about a scale of approximately 3 km. These gust structures were close to isotropic in the horizontal as they propagated out onto the plains. Again this type of structure w as supported by the observations. Three sources of surface gustiness a re discussed in the paper. Surface gusts produced by vortex tilting an d advected out of the wave-breaking region, as described in previous s tudies, occur in the present simulations. This mechanism is evidenced by the accompanying strong vertical vorticity. Propagating gust struct ures, similar in appearance to those obtained by others, are also obta ined in both the 2D and 3D experiments using the idealized 2D orograph y. Rather than resulting from local Kelvin-Helmholtz instabilities, th e propagating gusts in the present experiments appear to arise from hi gh-amplitude lee waves that propagate as a result of the transient cha racter of the wave-breaking region modulating the shape of the effecti ve waveguide.