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

When stably stratified air is forced to rise over a topographic barrier, mountain waves are generated and radiate away from the barrier. Mountain waves may be accompanied by severe downslope winds near the surface, occasionally in excess of 50 m s^-1, that rapidly decelerate in the lee and flow back toward the mountain as part of an intense circulation. These vortices, known as rotors, can be severe aeronautical hazards due to intense wind shear and have been cited as contributing to numerous aircraft accidents, including occurrences involving modern commercial and military aircraft. The characteristics of mountain waves and rotors 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 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 a 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 sub-rotor scale circulations are generated within the shear zone along the leading edge of the mountain wave and may be a source for the severe turbulence. These LES results suggest that sub-rotors embedded within mountain wave induced shear zones may be a substantial hazard for commercial and military aviation operations.