EGU General Assembly 2020
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the Creative Commons Attribution 4.0 License.

Mountain waves produced by a stratified shear flow with a boundary layer: transition from downstream sheltering to upstream blocking

Francois Lott, Bruno Deremble, and Clément Soufflet
Francois Lott et al.
  • PSL: Lab. Météo. Dyn., Geosciences, Ecole Normale Supérieure, Paris, France (

A non-hydrostatic theory for mountain flow with a boundary layer of constant eddy viscosity is presented. The theory predicts that dissipation impacts the dynamics over a an inner layer which depth δ is predicted by viscous critical level theory. In the near neutral case, the surface pressure decreases when the flow crosses the mountain to balance an increase in surface friction along the ground. This produces a form drag which can be predicted quantitatively. With stratification, internal waves start to control the dynamics and produce a wave drag that can also be predicted. For weak stratification, upward propagating mountain waves and reflected waves interact destructively and low drag states occur, whereas for moderate stability they interact constructively and high drag states are reached. In very stable cases the reflected waves do not affect the drag much.

The sign and vertical profiles of the Reynolds stress are profoundly affected by stability. In the neutral case and up to the point where internal waves interact constructively, the Reynolds stress in the flow is positive, with maximum around the top of the inner layer, decelerating the large scale flow in the inner layer and accelerating it above. In the stable case, the opposite occurs, and the large scale flow above the inner layer is decelerated as expected for dissipated mountain waves. These opposed behaviors challenge how mountain form drag and mountain wave drag should be parameterized in large-scale models.

The structure of the flow around the mountain is also strongly affected by stability: it is characterized by non separated sheltering in the neutral case, by upstream blocking in the very stable case, and at intermediate stability by the presence of a strong but isolated wave crest immediately downstream of the ridge.

How to cite: Lott, F., Deremble, B., and Soufflet, C.: Mountain waves produced by a stratified shear flow with a boundary layer: transition from downstream sheltering to upstream blocking, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5343,, 2020

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Presentation version 2 – uploaded on 01 May 2020
Replace v0, it was a test to see if I could upload stuff
  • CC1: Comment on EGU2020-5343, Miguel Teixeira, 04 May 2020


    Fascinating presentation on a very difficult topic.

    Because of the way in which it is formulated, I believe your theoretical drag model provides both gravity wave drag and form drag produced by a boundary layer flow over a mountain. Given its nature as an eminently inviscid process, I think it is easy to accept that the gravity wave drag component will be reliably represented. But, since (I think) you are using constant viscosity and diffusivity to represent friction, the form drag component strongly depends on the values of these quantities, and I believe it would not exist in inviscid flow (you may correct me if I am wrong). Given this, how reliable do you think the form drag component is? How would you estimate the value of the viscosity/diffusivity for (fairly) realistic conditions?

    Thanks in advance.

    • AC1: Reply to CC1, Francois Lott, 04 May 2020

      Dear Miguel,

      thank you for the comment.  Yes the form drag in our case here is purely due to viscous effect, I could think about predicting it with more sophisticated turbulence closure, but did not managed yet. In escence I would like to test the prediction of Belcher and Wood (199?). But yes without viscosity wave drag is only there.  Our model can be adapted to free slip cases for instance showing that. Trapped lee  waves can contribute though in near neutral cases, and when the shears are not constants, but stays quite marginal.  This is ongoing stuff.

      I am sorry not being there during the chat, Covid reorganized my agenda.

      All the best

Presentation version 1 – uploaded on 14 Apr 2020 , no comments