Neutral and stratified turbulent boundary layer flow over low mountains ; Ecoulements de couche limite neutres et stratifiés sur une montagne

International audience ; A uniform approximation of flow over gentle hills with a turbulent closure based on mixing length theory is derived. It shows that the transition from form drag associated with mountain induced changes in boundary layer friction to mountain gravity wave drag can be captured by theory. We confirm that the first is associated with downstream sheltering with acceleration at the hill top, the second with upstream blocking and strong downslope winds. We also show that the downslope winds penetrate well into the inner layer. The theory shows that the altitude at which the incident flow needs to be taken to calculate the drag is related to the inner layer depth at which dissipations equilibrate disturbance advection. We also demonstrate that the parameter that captures the transition, in our case is a Richardson number, compares the altitude of the turning levels of the gravity waves to the mountain length. Our solutions are used to describe the wave field aloft and the distribution of the Reynolds stress in the vertical. Some discussion on how to combine neutral and stratified effects in the parameterization of subgrid scale orography in large scale models is given.