Winds in Complex Terrain - Wind Energy Meteorology: Atmospheric Physics for Wind Power Generation - page 73

Chapter 4
Winds in Complex Terrain
More and more onshore wind turbines are built away from flat regions near the
coasts in complex (i.e., hilly or mountainous) terrain. The most favourite sites in
complex terrain are at elevated positions such as hill tops. Therefore, this Chapter
introduces a few of the main flow features which influence wind energy yields in
complex terrain.
Wind over complex terrain is influenced by changes in surface properties (such
as roughness and land use) and the height elevation of the site above sea level
(such as hills, ridges, mountains, and escarpments). We will use the term
'topography' to address the whole variation in surface properties and elevation,
and we will use the term 'orography' to address especially height elevation.
Changes in surface properties without any orographic structures have already been
addressed in Sect. 3.5 on internal boundary layers. Here in this Chapter, topo-
graphic and purely orographic influences on the wind field will be discussed.
In between roughness and orography we might think of having a third class of
topographic features which can be termed flow obstacles, e.g. such as buildings or
larger trees (Petersen et al. 1998b ). See Sects. 3.6 and 3.7 for a basic treatment of
such obstacles.
The complexity of hilly and mountainous terrain does not allow for a
straightforward application of the wind profile laws introduced in Chap. 3 . Usu-
ally, analytical or numerical flow models must be used for the assessment of wind
and turbulence conditions at a given site. Three-dimensional numerical wind field
models can roughly be stratified into three classes. The simplest ones are mass-
consistent flow models which generate a divergence-free flow over orography from
given measurements. They do not involve dynamic equations such as ( 2.1 )-( 2.4 ).
For reliable solutions they need a larger number of observations. The next class are
hydrostatic flow models which solve the dynamic Eqs. 2.2 and 2.3 . Equation ( 2.4 )
is substituted by the hydrostatic Eq. ( 2.1 ). They only work for larger scales of say a
few kilometres or more. For smaller scales, full non-hydrostatic models with the
full Eqs. ( 2.2 )-( 2.4 ) have to be used.