On the structural characteristics and dynamics of sub-rotors

The dynamics of rotors forced by three-dimensional topography are investigated through a series of high-resolution idealized simulations with the non-hydrostatic COAMPS model. The focus of this investigation is on the internal structure of rotors and in particular on the dynamics of small-scale intense circulations within rotors that we refer to as “sub-rotors”. Simulations are conducted using an upstream reference state representative of the conditions under which rotors form in the real atmosphere; in particular a vertical profile approximating the conditions upstream of the Colorado Front Range on 1200 UTC 3 March 1991. The topography is specified as a 1000-m high elongated ridge with a half-width of 15 km on the upstream portion and 5 km on the downstream side. In several experiments, a 500-m circular peak with a half-width of 7.5 km is used to investigate the sensitivity of the rotor dynamics to topographic variations in the cross-flow direction. As many as five nested grids are used with a characteristic minimum isotropic resolution of 50 m in order to resolve the internal rotor structure and sub-rotors.

The simulation results indicate a thin sheet of high-vorticity fluid develops adjacent to the ground along the lee slope and then ascends abruptly as it is advected into the updraft at the leading edge of the first trapped lee wave. This vortex sheet is primarily forced by mechanical shear associated with frictional processes at the surface. Sub-rotor circulations develop along the leading edge of the “parent” rotor due to an instability of the vortex sheet, likely related to a type of Kelvin-Helmholtz instability. These sub-rotors are advected downstream or back toward the mountain within the parent rotor and occasionally intensify as exhibited by a several fold enhancement of horizontal vorticity. A preliminary vorticity budget indicates that horizontal vorticity generation due to the stretching of vorticity is 2-3 times larger than tilting and 5-10 times larger than baroclinic generation. The horizontal vorticity generation is enhanced near the edges of the wake emanating from the circular peak due to vortex stretching of the parent rotor and also further maximized due to stretching associated with three-dimensional turbulent eddies. The results suggest that preferred regions of intense sub-rotors may exist near topographic features that enhance vortex stretching.