Winds in Complex Terrain - Wind Energy Meteorology: Atmospheric Physics for Wind Power Generation - page 74

4.1 Characteristics of Boundary Layers Over Complex
Terrain
Some basic peculiarities of the boundary layer structure over orographically
structured terrain are depicted in Fig. 4.1 . Section 4.1.1 and Fig. 4.3 will introduce
a major feature of winds in mountainous terrain: the thermally driven mountain
and valley winds, and in Sect. 4.1.2 katabatic and drainage winds. Mountain and
valley winds as well as katabatic and drainage winds are generated by the orog-
raphy itself. But there are several other flow features over mountainous terrain,
which come from a mainly mechanical modification of the existing larger-scale
flow by the underlying orographic features. This includes the acceleration of wind
speed in flows passing over hills, mountain tops, ridges and escarpments, the
channelling of winds in valleys, gap flows through narrow passages in mountain
ranges, the general deflection of winds around single hills and larger mountain
ranges. The flow speed-up is described in more detail in Sects. 4.2 and 4.3 below.
Channelling in valleys is a frequent phenomenon that is also visible in wider
valleys such as, e.g., in the Upper Rhine valley in Germany. Channelling takes place
at least to a height of the accompanying mountain ranges to both sides of such valleys.
But often, due to vertical mixing phenomena, channelling extends even above the
height of the side ranges. A major feature of channelling is the great constraint which
modifies the wind direction distribution. Cross-valley winds only appear rarely. In
most of the time we find wind direction along the valley where the selection of one of
the possible two directions either depends on the larger-scale pressure field or on the
local temperature gradient which constrains the direction of mountain and valley
winds passing through a certain location in a valley. The phenomenon of channelling
eases the design of larger wind parks, because only two opposite wind directions have
to be taken into account in the planning phase. Therefore, siting of the turbines in a
wind park in such valleys can be easily optimized. Figure 4.2 gives an example of
channelled flow in an Alpine valley in the case of a mountain and valley wind system.
Gap flows occur in a few special locations in a mountain range. The phe-
nomenon is most frequently found in larger mountain ranges perpendicular to the
main large-scale wind direction. Gap flows can exhibit quite large wind speeds but
are often accompanied by high turbulence as well. As such flows depend deci-
sively on the actual orographic features, no general statements on gap flows can be
made here. Gap flows rather need always a specific investigation by on-site
measurements with meteorological masts or ground-based remote sensing in order
to assess the specific flow features.
An example which combines both the effects of flow channelling in a valley and
of a gap flow are the mistral winds in the Rhone valley in Southern France. The
river Rhone flows through a major gap between the Massif Central to the West and
the French Alps to the East. Mechanisms responsible for the temporal evolution of
the Mistral are related to the evolution of upstream synoptic wind speed and
direction conditions during the event and the upstream Froude number, calculated
in the layer below the upstream inversion height (Caccia et al. 2004 ).