Stratified flow over a mountain with a gap:: Linear theory and numerical simulations

Linear theory and numerical simulations are used to imestigate gap flows. Both gaps embedded in a long mountain ridge and gaps located between two isolated mountain peaks are considered. The study mainly focuses on level gaps so as to isolate three-dimensional effects from two-dimensional gravity-wave propagation. Special attention is given to qualitative difference between the linear and the nonlinear flow regimes, the linear regime being defined by the absence of gravity-wave breaking. In the linear regime. wind and pressure Perturbations along a level gap axis are found to be qualitatively similar to those over a mountain ridge or peak, but of lower amplitude and longer horizontal wavelength. They are caused by vertically propagating gravity waves radiating from the mountain ridges (peaks) towards the gap axis and by low-level confluence within the gap. In the nonlinear regime, however, these two processes are of minor importance due to upstream blocking and low-level wave e breaking, Instead, the pressure difference across the mountain ridge primarily drives the gap flow, which tends to be decoupled from the flow over the adjacent ridge. In contrast to the linear regime, gap winds are as strong as or even strongger than the winds over the mountain ridge. Surface friction is found to reduce the near-surface wind speed more strongly for mountain ridges than for gaps.