Suggestions for unifying the parametrizations of turbulent orographic form drag, mountain wave drag, and flow blocking

Abstract Parametrizations of subgrid‐scale mountains are commonly used in numerical weather prediction and climate models. They try to represent quite separate processes; namely, the enhancement of the turbulent drag by orography, gravity‐wave drag, and the effects of low‐level flow blocking. Among the gravity wave schemes, some of them distinguish between upward‐propagating waves and trapped lee waves. This article makes use of a recent theoretical methodology to propose a formalism that includes all these effects. This theory handles enhanced turbulent drag in the neutral case, gravity waves in the stratified case, and trapped lee waves in the transition. Mountain drag associated with all these processes is estimated analytically, as well as the fraction of the drag that stays within the boundary layer instead of being radiated in the far field. Although the theory used is adapted to gentle hills with small slope, we also try to evaluate the blocked layer depth by combining the sheltering effects that dominate when stratification is small and the blocking effects when stratification is strong.