A Nonlinear Steady Model for Moist Hydrostatic Mountain Waves

We consider the dynamics of hydrostatic gravity waves generated by the passage of a steady, stably stratified, moist flow over a two-dimensional topography. Coriolis effects are neglected. The cloud region is determined by the dynamics, and within that region the Brunt-Väisälä frequency takes on a value smaller than the outside value. In both the dry and cloudy regions the Brunt-Väisälä frequency is constant with height. We consider the moist layer to be either next to the mountain or at midlevels and to be deep enough so that an entire cloud forms in that layer. The nonlinearity in the flow and lower boundary affects the dynamics of these waves and wave drag. The latter is found to depend upon: 1) the location of the moist layer with respect to the ground, 2) the amount of moisture, 3) the degree of nonlinearity and 4) the departure from symmetry in the bottom topography. For symmetrical profiles substantial drag reductions are obtained when the moisture is adjacent to the topography. In all cases, an increase in the nonlinearity increases the drag.