Parametrization of subgrid-scale orography effects in HIRLAM
Laura Rontu, Kai Sattler, and Robert Sigg
Abstract
Three approaches to the parametrization of the effects related to
orography in a limited-area atmospheric model are discussed: the
mountain waves, effects of blocking of the flow by mountain ridges
under stable atmospheric conditions and orographic turbulence. A new
mesoscale orography parametrization was tested and found to have a
small but physically reasonable effect on the simulated flow. The
results of numerical experiments show that the orography-related
parametrizations are strongly coupled and compensate each other. The
parametrized turbulent processes were found to dominate the model's
momentum budget. Together the schemes retard significantly the low
level atmospheric flow over mountains. The effects of parametrized
wave breaking to the upper atmospheric flow were found insignificant
in the cases studied. The parametrized mesoscale orography effects
decrease when the horizontal resolution increases towards
10 km. (Non-hydrostatic) models with a resolution finer than that
would need another approach for parametrization and resolving the
orography-related effects, with a clear separation of the scales of
orography contributing to the different processes.
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