Resonant waves over double bell shaped orography

While there have been numerous studies on the influence of an isolated mountain on the atmosphere, the problem of a double mountain barrier has received only limited attention. Numerous observational data gathered for such an orographic barrier presses for better understanding of the double mountain system and the gravity waves that develop on it. An almost perfect two-dimensional double barrier that has received a lot of observational attention is the Sierra Nevada–Owens Valley–White/Inyo Range system, well known for large-amplitude mountain waves and rotors it generates. Owens Valley has consequently been a place of several field campaigns studying mountain waves and rotors; namely, the Sierra Wave Project (SWP) in the 1950s, Sierra Rotors Project (SRP) in the spring of 2004 and the recent Terrain-induced Rotor Experiment (T-REX) in the spring of 2006.

Observational evidence from these projects indicates that the prevailing gravity wave response in Owens Valley under conditions conducive to trapped lee waves consists of integral number of wavelengths over the valley, suggesting a resonant wave response of the atmosphere to the valley geometry. In this study, we report on the results of idealized high-resolution numerical simulations with double bell-shaped orography, carried out using the NRL's Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS). The sensitivity of the resonant response to the upstream conditions, height of the second mountain, and mountain half-widths was examined. The flow is otherwise set to be two-dimensional, irrotational, and dry.

In the first set of simulations, the height of the first mountain was kept constant while the height of the second mountain was varied to analyze the importance of the second barrier on the resonant wave response. In the second set of simulations, the upstream sounding profile was varied so as to include the typical profiles and their most pronounced features observed during SWP and SRP such as the mountain top inversions. The profiles were generated from a climatology of SWP and SRP soundings that was compiled for this purpose. The third set of simulations, tested the sensitivity of the flow response to the asymmetry of the mountains, thus the half-widths of the first mountain were varied from a symmetric to a highly asymmetric profile, such as is characteristic of the Sierra Nevada mountain range.