Offshore Winds - Wind Energy Meteorology: Atmospheric Physics for Wind Power Generation - page 93

5.1 Characteristics of Marine Boundary Layers
First of all, the sea surface is much smoother than the land surface. This leads to
higher wind speeds at a given height above the surface, to smaller turbulence
intensities and to shallower surface layer depths. Thus, offshore wind turbines
usually experience less wind shear over the rotor area. But sea surface roughness is
wind speed-dependent due to the formation of waves. Diurnal cycles of temper-
ature and atmospheric stability are nearly absent due to the large heat storage
capacity of water. The infinite moisture source at the sea surface tends to bias
static stability of the MABL towards unstable stratifications. Figure 5.1 gives the
principle features of the vertical structure of the MABL. Adjacent to the sea
surface we find the wave sublayer within which the direct influence of single
waves through pressure forces is dominant. This sublayer is roughly five wave
amplitudes deep. Above the wave sublayer we find the constant flux or Prandtl
layer which is often much shallower than the respective layer over land (see Fig.
3.1 ). This depth can be in the order of just 10 m for stable stratification and in low
to moderate winds. The upper 90 % of the MABL are covered by the Ekman layer
within which the wind slightly turns and reaches the geostrophic wind at its top.
Like the constant flux layer the entire MABL is usually much shallower than the
ABL over land.
5.1.1 Sea Surface Roughness and Drag Coefficient
The typical roughness length of the sea surface for moderate wind speeds is in the
order of a tenth of a millimetre to a millimetre (see Fig. 5.2 left). In contrast to
land surfaces the roughness of the sea surface is not constant but varies over
several decades depending strongly on the wind speed, because of the evolving
wave size, height and shape. Consequently, the surface roughness length, z 0
increases with increasing wind speed. Waves are generated mainly by frictional
forces exerted by the wind on the ocean surface, thereby transporting momentum
from the atmosphere downwards into the water column (Bye and Wolff 2008 ).
This transport is downward as long as the waves are still young and wind-driven,
i.e. if the wind speed is faster than the phase speed of the waves. For old waves or
swell, no clear relation with the wind speed can be expected (Oost et al. 2002 ;
Sjöblom and Smedman 2003 ). Furthermore, this downward transport depends also
on the thermal state of the MABL, because this state influences the ability of the
atmosphere to replenish the momentum loss at its lower boundary with momentum
from higher atmospheric layers. For unstable stratification (air colder than the sea),
this downward transport is larger and the waves are expected to be higher than for
stable stratification. This presumption has initially been proven by the analysis of
North Atlantic weather ship data by Roll ( 1952 ).