The effects of boundary layer on lee waves over double bell-shaped orography

The Sierra Nevada–White-Inyo system is an almost perfect two-dimensional double barrier well known for generation of large-amplitude trapped lee waves and rotors, which were the focus of the recent Terrain-induced Rotor Experiment (T-REX in 2006) and its pilot Sierra Rotors Project (SRP in 2004).

Observational evidence from T-REX as well as attendant numerical studies indicate that trapped lee waves and the process of wave-induced boundary-layer separation play an important role in the formation of rotors. From observations it appears that the largest amplitude lee waves are those with wavelengths close to the ridge separation distance, suggesting a resonant wave response of the atmosphere to the surrounding terrain. Our idealized free-slip simulations, the results of which are presented in a companion paper, confirm that the presence of a second mountain exercises a profound influence on the wavelength as well as the amplitude of trapped lee waves over the double mountain barrier. The ridge separation distance was shown to be a key element that controls the trapped lee wave wavelength. Moreoever, the oscillatory character of the quasi-state-state value of gravity-wave drag as a function of the ridge separation distance reveals the presence of constructive and destructive non-linear interferences and identifies separation distances for which these interferences occur.

Here we report on further idealized high-resolution numerical simulations of the double-barrier problem using a no-slip lower boundary condition. The simulations, carried out using the NRL Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS), are otherwise two-dimensional, irrotational and dry. In this study, we examine in more detail the effect of the frictional boundary layer on the previously established sensitivities of the double barrier flow to the valley geometry and upstream atmospheric structure. Our results indicate that surface friction significantly affects the lee wave wavelengths, reduces lee wave amplitudes and further enhances trapping. The interference pattern is also modified with constructive and destructive interferences appearing for different ridge separation distances than in the free slip simulations. The wave-induced boundary-layer separation is also observed to occur in these experiments together with regions of reversed flow downwind of the separated vortex sheet.