Wind Regimes - Wind Energy Meteorology: Atmospheric Physics for Wind Power Generation - page 17

south-north wind component with height in this situation. This leads to a backing
of the wind with height. In the opposite case of warm air advection the wind veers
with height.
2.5 Boundary Layer Winds
The wind speed in the atmospheric boundary layer must decrease to zero towards
the surface due to the surface friction (no-slip condition). The atmospheric
boundary layer can principally be divided into three layers in the vertical. The
lowest layer which is only a few millimetres deep is laminar and of no relevance
for wind energy applications. Then follows the surface layer (also called constant-
flux layer or Prandtl layer), which may be up to about 100 m deep, where the
forces due to the turbulent viscosity of the air dominate, and within which the wind
speed increases strongly with height. The third and upper layer, which usually
covers 90 % of the boundary layer, is the Ekman layer. Here, the rotational
Coriolis force is important and causes a turning of the wind direction with height.
The depth of the boundary layer usually varies between about 100 m at night with
low winds and about 2-3 km at daytime with strong solar irradiance.
Scale analysis of the momentum Eqs. ( 2.2 - 2.4 ) for the boundary layer show the
dominance of terms III, V, and VII. Sometimes, for low winds in small-scale
motions and near the equator, the pressure force (term III) is the only force and a
so-called Euler wind develops, which blows from higher pressure towards lower
pressure. Such nearly frictionless flows rarely appear in reality. Usually an equi-
librium between the pressure force and the frictional forces (terms III and VII) is
observed in the Prandtl layer, and an equilibrium between the pressure force, the
Coriolis force and the frictional forces (terms III, V, and VII) is observed in the
Ekman layer. The Prandtl layer wind is sometimes called antitriptic wind. No
equation for the antitriptic winds analog to ( 2.5 , 2.6 )or( 2.8 , 2.9 ) is available, since
neither term III nor term VII contains explicitly the wind speed.
The Prandtl layer is characterised by vertical wind gradients. The discussion of
Prandtl layer wind laws which describe these vertical wind speed gradients is
postponed to Chap. 3 . The vertical gradients are much smaller in the Ekman layer,
so that it is meaningful to look at two special cases of ( 2.2 ) and ( 2.3 ) in the
following section.
In a stationary Ekman layer the terms III, V, and VII balance each other,
because term I vanishes. This layer is named from the Swedish physicist and
oceanographer W. Ekman (1874-1954), who for the first time derived mathe-
matically the influence of the Earth's rotation on marine and atmospheric flows.
A prominent wind feature in the Ekman layer is the turning of wind direction with
height.