92nd American Meteorological Society Annual Meeting (January 22-26, 2012)

Thursday, 26 January 2012: 4:00 PM
Observations and Large-Eddy Simulations of Wave-Induced Boundary-Layer Separation
Room 339 (New Orleans Convention Center )
Vanda Grubisic, NCAR, Boulder, CO; and S. Serafin and L. Strauss

Wave-induced boundary-layer separation in flow over orography has received significant attention in recent years, especially in relation to the formation of atmospheric rotors. Traditionally depicted as horizontal eddies in the lee of mountain ranges, rotors are characterized by intense turbulence and pose a known threat to aviation. This study focuses on the first observationally documented case of wave-induced boundary-layer separation, which occurred on Jan 26 2006 in the lee of the Medicine Bow Mountains in SE Wyoming. Observations from the University of Wyoming King Air (UWKA) aircraft, in particular, the remote sensing measurements with the dual-Doppler Wyoming Cloud Radar (WCR), indicate strong wave activity, downslope winds in excess of 30 m s-1 within 200 m above the ground, and near-surface flow reversal in the lee of the mountain range. The fine resolution of WCR data (on the order of 40x40 m2 for two-dimensional velocity fields) reveals fine-scale coherent vortical structures which are embedded within the rotor zone and whose intensity contributes to the severity of turbulence therein.

A series of semi-idealized three-dimensional large-eddy simulations of the Medicine Bow case was carried out using the CM1 model. Simulations represent the flow of an air mass with invariant profiles of wind speed and potential temperature over an isolated mountain ridge: the atmospheric soundings match the available observations and the ridge has the same size and shape as the Medicine Bow range. Model runs consider a simplified two-dimensional geometry where the complex topographic obstacle is represented as a smooth linear mountain ridge, but they are fully three-dimensional allowing for realistic turbulence dynamics. The simulated flow field is strikingly similar to the observed, with the simulations reproducing strong downslope flow detaching from the ground, with a patch of considerably lower wind intensities and embedded reverse flow further downstream. The near-surface rotor circulation is associated with an undular bore near the mountain top level that is triggered by breaking of a hydrostatic mountain wave aloft, at an altitude between 2000 and 5000 m. Once separated from the ground, the thin sheet of positive horizontal vorticity breaks down into several small vortices within the rotor region. Several phenomena of interest can be discerned in the simulated flow, including non-steadiness in the position of boundary-layer separation and shooting downslope flow, the latter associated with Kelvin-Helmholtz instability developing upstream of the obstacle along a stable shear layer above the mountain top height. A set of sensitivity experiments with increasing surface friction have been carried out to assess the impact of surface friction on the onset of BLS, through the deceleration of the surface flow and reduction of the wave amplitudes downstream of the obstacle.

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