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Mountain and valley winds take a few hours to form. They are a feature of the
whole valley (Vergeiner and Dreiseitl
1987
). Mountain winds (sometimes called
down-valley winds, a better term would be out-valley winds because local slopes
of the valley floor are not decisive (Heimann et al.
2007
)) usually start 3-4 hours
after sunset and valley winds (sometimes called up-valley winds or better in-valley
winds) 3-4 hours after sun rise. Both winds require clear-sky conditions so that
heating by incoming short-wave radiation and cooling by outgoing long-wave radi-
ation can occur. The direction of the winds along a valley axis is dominated by the
fact that heating and cooling of the valley air is more effective in the narrower upper
parts of the valley than in the wider lower parts, because the ratio of air mass to
thermally active surface is larger in the narrower upper parts of a valley (Steinacker
1984
). This differential heating or cooling along the valley axis leads to a pressure
gradient along the valley axis, which in turn drives the winds. Usually, the daytime
in-valley winds are stronger and more turbulent than the nocturnal out-valley winds.
The large-scale wind system between a mountain range and the surrounding
planes has the largest similarity with the land-sea wind system (Fig.
2.9
). This
wind system, which blows towards the mountains during daytime and away from the
mountains at night-time, takes 4-6 hours to develop. It is observable even 100 km
away from the foothills of a mountain range (Lugauer and Winkler
2005
). This wind
system comes into existence because at a given height above sea level, the air over
the mountains is heated more than over the plane. The opposite occurs at night-time.
This differential heating once again leads to a pressure difference at a given height,
and this pressure difference in turn drives a compensating wind.
There must be a compensating wind system for the mountain and valley winds
and for the mountain-plain winds as well. Because this compensating motion takes
place over a larger area, it is too weak to be differentiated from the synoptic-
scale motions. During daytime this compensating motion contributes to downward
motions aloft over the surrounding plains of a mountain range that somewhat lim-
its the vertical growth of clouds at the boundary layer top over these plains. For
such a circulation system in Southern Germany, the term Alpine Pumping has been
proposed (Lugauer and Winkler
2005
).
References
Arnfield AJ (2003) Two decades of urban climate research: a review of turbulence, exchanges of
energy and water, and the urban heat island. Int J Climatol 23:1-26
Arya SP (1995) Atmospheric boundary layer and its parameterization. In: Cermak JE et al. (eds)
Wind Climate in Cities. Kluwer, Dordrecht, 41-66
Atkinson BW (2003) Numerical modelling of urban heat-island intensity. Bound-Lay Meteorol
109:285-310
Banner ML, Peirson WL (1998) Tangential stress beneath wind-driven air-water surfaces. J Fluid
Mech 364:115-145
Batchvarova E, Gryning E-E (2006) Progress in urban dispersion studies. Theor Appl Climatol
84:57-67
Blackadar AK (1962) The vertical distribution of wind and turbulent exchange in a neutral
atmosphere. J Geophys Res 67:3095-3102
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