Offshore Winds - Wind Energy Meteorology: Atmospheric Physics for Wind Power Generation - page 127

considerably colder than the surrounding land surface and the wind is directed
from the land to the sea (Smedman et al. 1995 ). Brooks and Rogers ( 2000 ) have
observed low-level jets over the Persian Gulf as well.
The jets can form at distances of several tens of kilometres from the coastline
when warm air is advected over the colder water surface. Most likely, such marine
low-level jets are spatial analogues to the nocturnal low-level jets over land, which
occur due to temporal changes in surface layer stratification. When the air flow
passes the coastline and reaches the colder water surface, an internal boundary
layer forms and the same decoupling between the surface layer and the rest of the
boundary layer above takes place which happens in the evening when the ground
cools down due to radiative energy losses.
5.7 Summary for Marine Boundary Layers
The marine atmospheric boundary layer (MABL) shows remarkable differences
compared to the atmospheric boundary layer (ABL) over flat homogeneous terrain.
Wind speeds are higher and turbulence intensities in the MABL are lower than in
the same height over the surface in the onshore ABL (given the same synoptic
forcing through a large-scale horizontal pressure gradient). Vertical wind shear
over the rotor area of a modern large 5 MW wind turbine is in most cases con-
siderably lower for offshore conditions than over land. In addition, all these MABL
characteristics are not constant but vary with wind speed because the roughness of
the sea surface increases with wind speed. A further complication may arise from
the wave age, i.e. the ratio between the wind speed and the phase speed of the
waves. Therefore, the actual wind conditions always depend on atmospheric
conditions and on the properties of the wave field.
Wind shear due to thermal winds (see Sect. 2.4 for an estimation of the size of
thermal wind) has to be taken into account over the seas because the wind shear
due to surface friction is rather low. Usually colder air masses coincide with low-
pressure areas and warmer air masses with high-pressure areas in temperate lati-
tudes. Therefore, thermal winds usually contribute to an increase of wind speed
with height.
In coastal areas up to about 50-100 km away from the coast, internal boundary
layers (see Sect. 3.5 ) and low-level jets at the top of these layers ( Sect. 3.5.1 ) may
occur when the wind blows from the land. The top height of these internal
boundary layers in the coastal MABL is often within the rotor area.
The low level of turbulence in the MABL is advantageous for single wind
turbines, because it leads to reduced loads on the structure of the wind turbine. But
the low turbulence level may turn into a major disadvantage for the planning and
operation of larger offshore wind parks, because the supply of kinetic energy
towards large wind parks by vertical turbulent fluxes is considerably lower in the
ABL. This will be explained in more detail in the following chapter.