Impact of topographic variations on mountain waves and rotors

The sensitivity of rotor characteristics forced by three-dimensional topography are investigated through a series of ultra high-resolution simulations (isotropic resolution of ~50 m) using a large eddy simulation (LES) version of the non-hydrostatic COAMPS model. The focus of this investigation is on the impact of along-barrier terrain variations on the internal structure of rotors and in particular on small-scale intense circulations within rotors that we refer to as “sub-rotors”.

The recent Terrain-Induced Rotor Experiment (T-REX), which took place in the lee of the southern Sierra Nevada mountain range, featured a suite of observational platforms including three research aircraft, surface instruments, lidars, wind profilers, and radiosondes on the upstream and downstream portions of the mountains. Preliminary analysis of observations from the King Air research aircraft during several T-REX rotor events indicates large variability in the downslope flow along the eastern Sierra slopes, with maxima in the horizontal wind shear downstream from several deep passes in the Sierra. These relative depressions in the Sierra crest include the Kearsarge, Sawmill, Baxter, and Taboose passes.

Previous high-resolution numerical simulations conducted using idealized terrain have indicated that horizontal vorticity generation due to the stretching of vorticity is maximized near topographic variations in the cross-wind direction. The regions downstream of along barrier variations were shown to be a preferred location for intense sub-rotors due to vortex stretching. The 2-km horizontal resolution real-time COAMPS forecasts performed during T-REX suggest that the wider passes in the Sierra crest induce modulations in the strength and characteristics of the downslope flow and mountain waves. In this study, LES simulations are conducted using a reference state based on upstream radiosondes from several different rotor events during T-REX. Pairs of simulations using the actual Sierra terrain as well as a uniform Sierra crest are used to isolate the impact of the topographic variations in the Sierra crest on the rotor structure. A vorticity budget is computed to assess the magnitude of the horizontal vorticity generation due to the stretching of vorticity downstream of the passes relative to other processes such as tilting and baroclinic generation.