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ratio of the effected air volume to the thermally active surface area in mountain-
ous terrain. As this topic concentrates on boundary layer aspects, gravity wave and
Foehn generation in thermally stably stratified flows over mountains will not be
addressed here. The reader is rather referred to overview papers on these large-scale
effects of mountain ranges, e.g. the classical one by Smith (
1978
).
Mountains lead to three types of thermally driven secondary circulation systems,
which modify the vertical structure of the mountainous ABL: slope winds, mountain
and valley winds, and - like land-sea wind systems - a diurnally changing system
of winds between mountain ranges and the surrounding plains (see Fig.
2.11
, which
is an extension of the classical sketch from Defant (
1949
) that depicted only the first
two of these three secondary circulation systems). These three phenomena occur on
three different spatial scales although they all three have the same temporal scale of
one day. Slope winds (thin arrows in Fig.
2.11
) develop on a slope spatial scale of a
few metres up to about 1 km. Mountain and valley winds (
full arrows
in Fig.
2.11
)
emerge on a spatial scale of a few hundred metres up to a few hundred kilometres in
long valleys. Mountain-plain winds have the largest scale of a few tens of kilometres
to more than one hundred kilometres (open arrows in Fig.
2.11
).
Slope winds come into existence due to the heating by insolation or radiative
cooling of a sloping surface in mountainous terrain. These winds emerge and disap-
pear within minutes after the thermal forcing started or stopped. They form part of a
secondary circulation in a valley cross section. Upslope winds during daytime may
lead to compensating sinking motion over the centre of the valley (Vergeiner
1982
).
This is often the reason why clouds resolve over the valley centre, but form over hill
crests. This sinking motion contributes to a stabilization of the thermal stratification
in the valley atmosphere and can prolong the existence of temperature inversions in
valleys. At night-time downslope winds develop.
Fig. 2.11
Diurnal variation
of local and regional wind
systems in and around
mountain ranges for
night-time (
upper frame
)and
daytime (
lower frame
).
Thin
arrows
: upslope and
downslope winds,
black
arrows
: upvalley and
downvalley winds,
white
arrows
: regional winds
between the mountain range
and the surroundings plains
(mountain pumping). Upslope
winds during daytime support
the formation of clouds over
summits and crests
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