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The orographic
information stems from a data set with a resolution of about 1 km
which contains values of the mean elevation above the mean sea
level, the fraction of land and the fractional cover of different
vegetation types. This detailed data is aggregated ("upscaled") to
the coarser model resolution.
The resulting mean
orography gives quite a realistic description over most of the land
areas, but is insufficient in high mountain areas where the
sub-grid orographic variability becomes important. This is for
example the case when cold air drainage in valleys makes it
difficult for air from outside to penetrate the mountain at its
true geographical height; the cold air effectively acts to "lift"
the orography.
When stable stratified
airflow crosses a mountain ridge gravity waves are excited into the
flow. They play an important role for making the large scale flow
slightly less zonal and increasing the frequency of blocking highs
and cut-off lows. Depending on the local dynamic and thermal
conditions the orography can, rather than make it pass over, block
the low level flow and give rise to local winds.
To represent these
mountain effects the mean orography is supplemented by four
additional sub-grid fields: the standard deviation, anisotropy
(aspect ratio of the obstacle), slope and geographical orientation
of the sub-grid orography. They are added to the mean orography to
provide flow dependent blocking of the air flow and to generate
gravity waves.
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