Numerical Investigations of the Evolution of Mountain-Induced Turbulence Along the Colorado Rockies on 9 December 1992

On December 9, 1992, widespread turbulence associated with orographically forced gravity waves occurred along the Rocky Mountains from Wyoming and Utah south through New Mexico and extended from near the surface into the lower stratosphere. This case was studied by Clark et al., who used numerical simulations and observations to analyze the event in the vicinity of the aircraft incident. The present study uses the stratospheric Non-Hydrostatic Mesoscale Analysis Simulation System (NHMASS) using grid lengths from 18 km to 222 m to simulate this case. Two major wave modes were found in the vertical velocity and divergence fields along the lee of the Colorado Rockies: a standing hydraulic jump and a downstream propagating wave. Cross-sections at grid spacing as coarse as 6 km showed steep isentropes and vertical velocities in the hydraulic jump in excess of 1.5 ms-1, as well as a downstream tilt for the group velocity vector. Model data and theory are used to estimate the group and phase velocities, as well as the angle relative to horizontal of the wave number vector. The downstream propagating wave was considerably weaker than the standing jump, but still contained vertical velocities exceeding 0.6 ms-1. The propagating wave was generated at approximately 1200 UTC and moved eastward at approximately 6.5 ms-1, eventually reaching western Kansas by 2400 UTC. The standing wave mode reached its maximum intensity in Colorado at approximately 1200 UTC. Energy from the gravity wave associated with the hydraulic jump was shown to penetrate the lower stratosphere. Potential vorticity (PV) banners were found to occur on the lee side of the mountains behind the propagating wave front, extending almost completely across the state of Colorado by the end of the period. The maximum wave activity generally occurred in the left exit region of a jet streak entering the area from the northwest. In this study, we hypothesize that the interactions among jet streaks, PV banners and hydraulic jump may generate severe turbulence in the upper troposphere and stratosphere. Numerical sensitivity simulations are performed to verify this hypothesis.