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Speed Adjustments for Local Conditions
It must always be kept in mind that the wind conditions at a particular location may differ substantially from the predicted values because of variations in elevation, exposure, surface roughness, or other factors. The following guidelines should be followed when interpreting the maps.
Obstacles:
The Wind Map assumes that all locations to be considered for wind energy are free of obstacles that could disrupt or impede the wind flow at the height of the wind turbine. "Obstacle" does not apply to trees if they are common to the landscape, since their effects are already accounted for in the predicted speed (however note the discussion of displacement height below). However, a nearby shelterbelt of trees or a large building in an otherwise open landscape would pose an obstacle for a wind turbine. As a rule of thumb, the effect of such obstacles extends to a height of about twice the obstacle height and to a distance downwind of 10-20 times the obstacle height.
Variations in Elevations:
Generally speaking, points that lie above the average elevation within a 400x400 m square will be somewhat windier than points that lie below it. A rule of thumb is that every 100 m increase in elevation above the average will result in an increase in the mean wind speed of 0.50 m/s. This rule of thumb is most applicable to small, isolated hills or ridges in otherwise flat terrain. It should be used with caution in mountains where it is difficult to determine what the "average" elevation is, and where wind flows in any event can be very complex.
Variations in Roughness:
The roughness of the land surface – which is determined mainly by the height and type of vegetation and buildings – has an important impact on the mean wind speed at heights of interest for wind turbines. The MesoMap system assumes a certain roughness for each type of land cover in its land cover data base. Although the land cover and roughness may vary within a 400x400 m grid cell, the MesoMap system uses the grid cell's predominant land cover to determine the overall roughness. Consequently, local wind speed variations will result where the local surface roughness differs substantially from the grid cell's predominant roughness (example: a pasture surrounded by forest). Lower roughness sites will in general experience somewhat stronger winds, while higher roughness sites will have lower winds. The magnitude of these speed variations – which will depend on the nature and heights of the roughness elements and on the distances between roughness changes – are not easily predicted.
Displacement Height:
An additional factor to consider is that the selected heights chosen on the wind maps may not always be the height above ground. Where the vegetation is dense, the "effective ground level" is not the base of the vegetation but the middle of the vegetation canopy because the wind flow is displaced upward. The level of zero wind, called the displacement height, is typically about two-thirds the height of the top of the vegetation. In dense forests the height above ground at which the predicted wind speed actually occurs may be as much as 7-15 m (23-50 ft) higher than indicated on the maps. For example, in an area covered by forest with an average canopy height of approximately 18 m (60 ft), the Wind Map's wind speed prediction at the 65 m (213 ft) level would actually apply to a height of 77 m (253 ft) above ground [65 + 2/3(18)].
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