The Temporal Evolution of Mountain Waves, Lee Vortices and Orographic Blocking

Most previous numerical studies of mountain induced disturbances in stably stratified flow have relied on rather artificial flow initializations. In this paper, we explore the evolution and dissipation of three-dimensional disturbances induced by an isolated ridge in a simple, dynamically consistent, large-scale flow. At the initial time, a stagnant region of the large-scale wind perpendicular to the ridge (U) accelerates to 20 m/s and then decelerates back to zero over an interval of 50 hours. The large-scale static stability (N) is constant throughout the domain. The nonlinearity parameter e=Nh/U ranges from infinity to order unity and back to infinity over the roughly two-day period, or equivalently, over a mean cross-mountain advective time scale of 100.

The transition between highly nonlinear flow (high e) and the quasi-linear wave regime is discussed and compared with observations of tidal flow in Knight Inlet. The Doppler shifting of the mountain wave train and a tendency to develop breaking waves during the deceleration of the large-scale flow are also examined.

Wake vortices are produced in the high-e flow regimes at both the beginning and the end phases of the roughly two-day period. The structure and distribution of these lee vortices during each phase is, however, rather different due to the differences in both the sense of the mean-flow acceleration and to the presence of large-scale diffluence in the accelerating flow (or confluence in the decelerating flow.)