Abstract
The problem of representing Subgrid Scale Orography (SSO) in General Circulation Models (GCM) and Numerical Weather Prediction Models (NWPM) is discussed using a very accurate dataset (185mxl85m) of the Alps. The linear approach shows that for horizontal scales varying between few Kms and 100Km mountains excite gravity waves which transport significant angular momentum. Nevertheless, this linear view is not sufficient for real mountains, and upstream blocking, low level wave breaking and flow splitting around the mountains often occur. At smaller scales, the orography strongly affects the Boundary Layer turbulence. These nonlinear effects lead to drag enhancement which are significant compared to the gravity wave drag. Efforts in the literature to represent these different mechanisms (gravity wave drag, increased roughness length, envelope orography) are also presented.
Recent progress in representing these nonlinear effects in a more consistent manner than in the past is also summarized. This includes development and testing againts data of new SSO scheme at ECMWF. An important feature of the scheme proposed is that it deals explicitly wit a “blocked” low level flow which does not go over the mountain under certain dynamical configurations and which is slowed down. The upper part (non-blocked) of the flow goes over the mountains and forces gravity waves. By comparison to the PYREX data, it appears that the new scheme improves the realism of the ECMWF model. It also has a beneficial impact on the forecast performances. In the LMD-GCM, it improves the occurence of synoptic blockings.
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Lott, F., Miller, M. (1997). The representation of sub-grid scale orography in GCMs. In: Hamilton, K. (eds) Gravity Wave Processes. NATO ASI Series, vol 50. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-60654-0_19
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DOI: https://doi.org/10.1007/978-3-642-60654-0_19
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