OFFSHORE RESOURCE ASSESSMENT - Wind Resource Assessment: A Practical Guide to Developing a Wind Project - page 194

14.1 NATURE OF THE OFFSHORE WIND ENVIRONMENT
Offshore wind environments differ from onshore environments in a number of ways.
One difference is that the surface roughness (which determines the drag exerted by
the surface on the lowest layer of the atmosphere) of open water is much smaller than
that of most land surfaces. A typical roughness length assumed in numerical modeling
is 0.001 m, although the value varies with wave height and therefore with wind speed.
In contrast, most land surfaces have a roughness ranging from 0.03 m to over 1.0 m
(Table 13-1). The low roughness of water means that the wind shear offshore tends
to be lower than that observed on land. Average wind shear exponents over water
typically range from 0.07 to 0.15, compared to 0.10 to 0.60 over land (Table 10-3).
Turbulence is generally lower as well.
Another difference is that the daily cycle of surface temperature variation is usually
attenuated offshore because water has a much greater heat capacity than soil and main-
tains a more constant temperature throughout the day. This characteristic produces, in
turn, smaller variations in atmospheric stability and wind shear. Whereas on land, the
mean wind shear can vary greatly between night and day (Fig. 11-3), such patterns
are not usually as evident offshore. In general, the average shear exponent is lower in
tropical waters (0.07-0.10) than that in temperate and cold waters (0.10-0.15). This
is because in the tropics, the water is warm and the atmosphere close to neutrally
stable year-round. In colder climates, seasonal variations in the relative temperature
of air and water modify the stability and shear, producing higher average shears on
the whole.
Because of the lack of terrain, winds and other meteorological conditions tend to
be more spatially uniform offshore, especially farther than around 5 km from the
land. This is fortunate for wind project development, as it means that fewer measure-
ment stations are generally required to characterize the resource accurately within
a project area. Even so, surprisingly complex wind phenomena can occur. Here are
some examples.
• Mountain and Island Blocking. Coastal mountains and islands can act as a barrier
creating a zone of low wind speeds both upwind and downwind. This effect can
extend many kilometers offshore depending on the atmospheric conditions and
the size of the barrier. Figure 14-1 shows an example of blocking by mountains
on the island of Maui, Hawaii, USA.
• Gap Flows. Gaps between and around coastal mountains and islands can con-
centrate the wind and generate high wind speeds offshore. Figure 14-1 shows
such channeling between the islands of Maui, Lanai, and Molokai. Many other
examples exist, some with familiar names such as the Mistral off the coast of
southern France and the Levant through the Strait of Gibraltar.
• Coastal Barrier Jets. When synoptic conditions favor a flow more or less parallel
to the coastline, the terrain can act to concentrate the flow and create a low level
jet with high wind speeds. Figure 14-2 shows a numerical simulation and a
synthetic aperture radar (SAR) image of such a jet off the north shore of the
St. Lawrence River in Canada.