Lee waves over double bell-shaped orography

Most of the theoretical and idealized numerical studies of two dimensional mountain waves have focused on the problem of atmospheric flow past an isolated ridge. On the other hand, only limited attention has been given to the problem of airflow past a double mountain barrier. An almost perfect two-dimensional double barrier that has lately received considerable observational attention is the Sierra Nevada–White-Inyo system, well known for generation of large-amplitude mountain waves and rotors that were the focus of the recent Terrain-induced Rotor Experiment (T-REX in 2006) as well as its pilot Sierra Rotors Project (SRP in 2004).

Observational evidence from these projects indicates that trapped lee waves play an important role in the formation of rotors. They also suggest that the largest amplitude lee waves have a wavelength close to the ridge separation distance, suggesting a resonant wave response of the atmosphere to the valley geometry.

Here we report on further results of our idealized high-resolution numerical simulations of the double-barrier problem, carried out using the NRL Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS). The flow is taken to be two-dimensional, irrotational and dry. Our earlier results show that the existence of the second mountain exercises a profound influence on the wavelength as well as the amplitude of the trapped lee waves over the valley. In this study, we examine in more detail the sensitivity of the double barrier flow to upstream conditions using several upstream soundings from the T-REX and SRP campaigns. Special attention is given to the mountain top inversion as well as the vertical wind shear profile.