Winds in Complex Terrain - Wind Energy Meteorology: Atmospheric Physics for Wind Power Generation - page 78

Fig. 5.34 below. This wind system, which blows towards the mountains during
daytime and away from the mountains at night-time, takes 4-6 h to develop. It is
sometimes observable even 100 km away from the foothills of a larger mountain
range (see Lugauer and Winkler 2005 for an example from the European Alps).
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 planes. 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 usually 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 limits 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 ).
4.1.2 Katabatic Winds
Drainage and katabatic flows are purely thermally generated orographic flow
features in a mountain boundary layer which have similarity with the above
introduced slope winds. They are based on the fact that colder air is heavier than
warmer air. Longwave radiative energy losses to space lead to cooling of land,
snow and ice surfaces and a compensating downward sensible heat flux, which
cools the atmospheric surface layer as well and forms a temperature inversion. In
the presence of slopes this induces a horizontal temperature gradient producing a
downslope horizontal pressure gradient force (Anderson et al 2005 ; Renfrew and
Anderson 2006 ) which usually drives shallow drainage flows. These drainage
flows are often too shallow in order to be used for wind energy generation.
Drastic examples of deeper drainage flows are the katabatic flows of Antarctica
and Greenland. The domed topography and radiative cooling of the snow surface
make katabatic flows ubiquitous over these regions (Renfrew and Anderson 2006 ).
Katabatic winds can be very gusty.
4.2 Wind Profiles Over a Hill
Winds over complex terrain show large spatial and temporal variations. Never-
theless there exist a few analytical approaches that help to analyse at least first-
order features of attached flow over complex terrain. Non-linearity such as flow
separation cannot adequately be described with analytical models but must be
addressed with non-linear numerical flow models, see e.g., Zenman and Jensen