Lee wave interferences over double bell-shaped orography

A possibility of lee wave resonance occurring over Owens Valley has emerged from T-REX (and its pilot Sierra Rotors Project) observations and real-data simulations of the strongest lee-wave event of the SRP campaign, SRP IOP 8. In particular the latter display a pronounced sensitivity of the trapped lee-wave response to downstream orography during certain stages of the examined event. Idealized simulations of a stably stratified and strongly sheared flow over double bell-shaped orography were performed with the NRL Coupled Ocean-Atmosphere Mesoscale Prediction System (COAMPS) to examine lee wave interferences over double bell-shaped orography that characterizes the Owens Valley terrain. Idealized wind speed and stability profiles were derived from radio soundings obtained during the most sensitive period of SRP IOP 8.

In this study, the COAMPS simulations were free-slip, two-dimensional, irrotational, and dry. The flow sensitivity to the ridge separation distance, mountain height, mountain asymmetry as well as the upstream wind speed and stability profiles was examined. The results confirm that both the amplitude and horizontal wavelength of trapped lee waves are significantly affected by the introduction of a second mountain, either as an upstream or a downstream secondary obstacle. The ridge separation distance, in concert with the upstream wind shear profile, is shown to exhibit the strongest control of the lee wave wavelength, which exhibits oscillatory character as a function of the ridge separation distance. The same oscillatory behavior is present also in the quasi-state-state value of gravity-wave drag, where the maxima and minima signify constructive and destructive non-linear interference. The separation distances for which these interferences occur are found to be approximately half the horizontal wavelength predicted by linear theory for the upstream profiles including an inversion.