Winds in Complex Terrain - Wind Energy Meteorology: Atmospheric Physics for Wind Power Generation - page 77

Fig. 4.3 Local and regional
wind systems induced by
mountains at night-time (top)
and at daytime (below) during
calm weather and mainly
cloud-free skies. Open arrows
denote regional winds
towards or away from the
mountains, bold arrows more
local out-valley (top) and in-
valley (below) winds and thin
arrows on the mountain
flanks indicate purely local
slope winds. The thin arrows
above the mountains indicate
the direction of vertical
motion

disappear within minutes with the appearance (and then disappearance) of thermal
forcing. 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 dissolve
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 pro-
long the existence of temperature inversions in valleys. During the evening,
downslope winds develop. See also the description of katabatic winds in

Sect. 4.1.2 below.
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, but 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 h
after sunset and valley winds (sometimes called up-valley winds or better in-valley
winds) 3-4 h after sun rise. Both winds require clear-sky conditions so that heating
by incoming short-wave radiation and cooling by outgoing long-wave radiation
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 the
total 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 regional-scale wind system between a mountain range and the surrounding
planes in Fig. 2.3 has some similarity with the land-sea wind system depicted in