Observations and numerical simulations of mountain waves in the presence of directional wind shear

Three research aircraft observed small-amplitude gravity waves over the south-western French Alps on 13 November 1999 during the Mesoscale Alpine Programme (MAP). Radiosonde ascents and Global Positioning System dropsondes deployed by MAP research aircraft indicate that the upstream flow was characterized by topographic blocking below 2 km and low-level directional wind shear associated with a synoptic-scale depression that extended above the Alpine crest. The in situ vertical velocity data from the three research aircraft, and backscatter from lidar onboard the low-level and upper-level aircraft, exhibit gravity-wave signatures in the lower troposphere that are characterized by relatively short horizontal wavelengths on a scale of 5 km or less. The in situ flight-level and backscatter data also suggest a rapid decrease in the wave amplitude with height, due to a directional critical layer that partially absorbs wave energy in the lower to middle troposphere in concert with a decrease in the Scorer parameter with height that traps the wave energy. Numerical simulation results, obtained from the non-hydrostatic Coupled Ocean-Atmosphere Mesoscale Prediction System model using a horizontal resolution of 556 m, indicate the presence of low-level wave breaking above the highest peaks, and confirm the presence of trapped waves and a directional critical level. Results based on linear theory and nonlinear model simulations suggest that the absorption of wave energy associated with the directional critical layer has an important impact on the wave characteristics including the momentum flux.