Observations and high-resolution simulations of a severe turbulence and downslope windstorm event

Mountain waves can be severe aeronautical hazards occasionally due to intense turbulence and wind shear often thought to be associated with internal wave breaking. The characteristics of mountain waves forced by three-dimensional topography are investigated through a series of high-resolution real data simulations (333 m horizontal grid increment) and Large Eddy Simulation (LES) experiments (isotropic resolution of ~ 100 m) with the non-hydrostatic COAMPS® model. The focus of this poster presentation is on a windstorm that occurred in the mountainous coastal region of Alaska. Observations from the Southern Alaska Regional Jets (SARJET) experiment show that strong downslope flow was characterized by substantial vertical wind shear. High data-rate measurements by the University of Wyoming King Air research aircraft in the region of vertical shear revealed extreme magnitudes of turbulence, most notably turbulent kinetic energy exceeding 50 m2/s-2. The high-resolution simulations capture the large-amplitude mountain waves and indicate that the strong region of vertical wind shear appears to be associated with a rotor-like circulation. The simulations also underscore the importance of the proper representation of topography and the necessity of high-horizontal resolution. Very high-resolution eddy resolving simulations using COAMPS indicate small-scale coherent circulations are generated along the steep mountain slopes and may be a source for the severe turbulence. These LES results suggest that small scale circulations embedded within mountain wave induced shear zones may be a substantial aviation hazard.