Impact of orographic drag on forecast skill
Sandu, Irina ; Zadra, Ayrton ; Wedi, Nils
The drag exerted by topography (orographic drag) plays an important role for many aspects of the large-scale circulation, such as the northern hemisphere winter extratropical circulation, including the position of storm tracks in the North Atlantic. However, the representation of orographic drag processes in models is particularly challenging. First, the resolution of most global climate and numerical weather prediction (NWP) models is not fine enough to represent in the required detail surface features such as hills or mountains and the disturbances they introduce into the airflow. A model can only directly capture the effects of mountains that are several times larger than the model grid spacing (the mean or resolved orography, Box A). The effects of disturbances induced by smaller mountains (subgrid or unresolved orography) have to be parametrized (Box B). Furthermore, it is impossible to directly measure the amount and distribution of drag globally, or even regionally. In the absence of observational constraints, the parametrizations of these effects rely on heavily simplified assumptions mostly based on linear theory and idealized mountains. The extent to which they capture the non-linear effects exerted by complex topography remains unknown. As a result, the representation of orographic drag processes remains one of the major sources of uncertainty in NWP and climate models. Here we show that the representation of the mean orography differs even among models with similar resolution. We also illustrate how much models differ in terms of subgrid surface stress (or friction) and its partition among various processes. Finally, we use ECMWF's Integrated Forecasting System (IFS) to demonstrate that these issues significantly affect forecast skill.
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