Rationale for a subgrid scale orography parameterization that includes turbulent form drag, gravity wave drag and low level flow blocking

Parameterizations of subgrid scale mountains are commonly used in large scale numerical weather prediction and climate models. They try to represent quite separate processes: the enhancement of the turbulent drag by orography, gravity waves and low level flow blocking. Among the gravity waves some schemes eventually separate between the upward propagating waves and the trapped lee waves.  Using a recent theoretical methodology that addresses the interaction of stratified boundary layers with mountains, a theory that handles the transition from neutral to stratified dynamics and trapped waves, we propose a formalism that can include all these effects.  As in most parameterizations it separates the flow between a linear part and a blocked part.  Here  the linear part handles enhanced turbulent drag in the neutral case and gravity waves in the stratified case, trapped lee waves in the transition. In this presentation we evaluate the mountain drag associated to all these processes as well as the fraction of the drag that stays within the boundary layer instead of being radiated in the far field.  We also try to  evaluate the blocked part by combining the sheltering effects that dominate when stratification is small and the blocking effects that dominate when stratification is large.