Orographic Flow Representation in the Alps at Multiple scales (OFRAM)

OFRAM (Orographic Flow Representation in the Alps at Multiple scales) will focus on exploiting these multi-scale observations via a multi-scale approach to numerical modelling using the Met Office’s Unified Model (MetUM) and the new Momentum model. The challenges include quantifying the key processes governing various orographic flows and optimising the representation of these key processes across multiple length-scales. We will use a hierarchy of model simulations: from valley-scale to mountain-range-scale, with grid lengths of 100s of m to 10s of km, and scale-aware parameterization schemes. Operational forecast output will be evaluated, inspiring carefully designed process studies and model sensitivity experiments, with the aim of improving understanding, physical representation and fidelity. Orographic flows are embedded in – and to an extent are driven by – the synoptic-scale circulation; so, the interaction between synoptic- and meso-scales needs to be captured. In addition, the interaction between meso- and local-scales is also critical; as heterogeneities in topography, surface characteristics, and the atmospheric boundary layer (ABL) determine the surface-layer meteorology and meteorological impacts – as well as feeding back upscale to affect the flow development.

One focus that is critical across multiple scale is turbulent mixing. For example, turbulent mixing is instrumental to the warming associated with foehn flows and the interaction of foehn flows with the ambient ABL (e.g., with cold pools); and it is also critical in determining the structure of katabatic flows and windstorms. However, representing mixing in complex terrain is challenging – turbulent fluxes can be significantly different to those over flat terrain. Orographically-forced momentum parameterizations are not well constrained leading to radical differences between models, while orographically-forced scalar fluxes are not properly represented in numerical weather prediction (NWP) models.

The overarching aim of this project is to determine the role of turbulent mixing in orographic flows across multiple scales and how this can be best represented in the MetUM/Momentum modelling framework.