Origins of aircraft-damaging clear-air turbulence during the 9 December 1992 Colorado downslope windstorm: Numerical simulations and comparison with observations

Results from numerical simulations of the Colorado Front Range downslope wi ndstorm of 9 December 1992 are presented. Although this case was not charac terized by severe surface winds, the event caused extreme clear-air turbule nce (CAT) aloft, as indicated by the severe structural damage experienced b y a DC-8 cargo jet at 9.7 km above mean sea level over the mountains. Derai led measurements from the National Oceanic and Atmospheric Administration/E nvironmental Research Laboratories/Environmental Technology Laboratory Dopp ler lidar and wind profilers operating on that day and from the Defense Met eorological Satellite Program satellite allow for a uniquely rich compariso n between the simulations and observations. Four levels of grid refinement were used in the model. The outer domain use d National Centers for Environmental Prediction data for initial and bounda ry conditions. The finest grid used 200 m in all three dimensions over a 48 km by 48 km section. The range of resolution and domain coverage were suff icient to resolve the abundant variety of dynamics associated with a time-e volving windstorm forced during a frontal passage. This full range of resol ution and model complexity was essential in this case. Many aspects of this windstorm are inherently three-dimensional and are not represented in idea lized models using either 2D or so-called 2D-3D dynamics. Both the timing and location of wave breaking compared well with observatio ns. The model also reproduced cross-stream wavelike perturbations in the je t stream that compared well with the orientation and spacing of cloud bands observed by satellite and lidar. Model results also show that the observed CAT derives from interactions between these wavelike jet stream disturbanc es and mountain-forced internal gravity waves. Due to the nearly east-west orientation of the jet stream. these two interacting wave modes were orthog onal to each other. Thermal gradients associated with the intense jet strea m undulations generated horizontal vortex tubes (HVTs) aligned with the mea n Row. These HVTs remained aloft while they propagated downstream at about the elevation of the aircraft incident, and evidence for such a vortex was seen by the lidar. The model and observations suggest that one of these int ense vortices may have caused the aircraft incident. Reports of strong surface gusts were intermittent along the Front Range dur ing the period of this study. The model showed that interactions between th e gravity waves and how-aligned jet stream undulations result in isolated o ccurrences of strong surface gusts in line with observations. The simulatio ns show that strong shears on the upper and bottom surfaces of the jet stre am combine to provide an episodic "downburst of turbulence." In the present case, the perturbations of the jet stream provide a funnel-shaped shear zo ne aligned with the mean flow that acts as a guide for the downward transpo rt of turbulence resulting from breaking gravity waves. The physical pictur e for the upper levels is similar to the surface gusts described by Clark a nd Farley resulting from vortex tilting. The CAT feeding into this funnel c ame from all surfaces of the jet stream with more than half originating fro m the vertically inclined shear zones on the bottom side of the jet stream. Visually the downburst of turbulence looks similar to a rain shaft plummet ing to the surface and propagating out over the plains leaving relatively q uiescent conditions behind.