Upstream propagating modes in moist and dry flow over topography

Orographic precipitation patterns can be significantly affected by terrain-induced wave motion. Numerical simulations of idealized flow over topography, with moist, nearly neutral flow in the troposphere capped by a stable stratosphere, have shown upstream propagating gravity wave modes associated with desaturation air upwind of the mountain. Previous studies attributed the existence of these modes to nonlinear effects associated with changes in upstream stability due to localized desaturated regions.

Here we show these upstream propagating waves are also present in both linear and nonlinear dry simulations. Hence the upstream desaturation appears to be a result, rather than a cause, of the upstream propagating modes. The parameter space associated with these waves and implication of these modes on precipitation patterns upwind of terrain will be discussed.