WIND RESOURCE

Roughness/Roughness rose


Roughness
         High above ground level, at a height of about 1 km, the wind is hardly influenced by the surface of the earth at all. However, in the lower layers of the atmosphere wind speeds are affected by the friction against the surface of the earth. In the wind industry one distinguishes between the roughness of the terrain, the influence from obstacles and the influence from the terrain contours, which is also called the orography of the area.
         In general, the more pronounced the roughness of the earth's surface, the more the wind will be slowed down. Forests and large cities obviously slow the wind down considerably, while concrete runways in airports will only slow the wind down a little. Water surfaces are even smoother than concrete runways and will have even less influence on the wind, while long grass and shrubs and bushes will slow the wind down considerably.



Roughness class and roughness lengths
        In the wind industry, people usually refer to roughness classes or roughness lengths, when they evaluate wind conditions in a landscape. A high roughness class of 3 to 4 refers to landscapes with many trees and buildings, while a sea surface is in roughness class 0. Concrete runways in airports are in roughness class 0.5. The term roughness length is really the distance above ground level where the wind speed theoretically should be zero.
        In the next table the roughness classes and roughness lengths are given for different types of terrains, as given in the European Wind Atlas.


Roughness Class Roughness Length (m) Energy Index (%) Landscape type
0
0.0002
100
Water surface
0.5
0.0024
73
Completely open terrain with a smooth surface (e.g. concrete runways in airports, mowed grass)
1
0.03
52
Open agricultural area without fences and hedgerows and very scattered buildings. Only softly rounded hills.
1.5
0.055
45
Agricultural land with some houses and 8 m tall sheltering hedgerows with a distance of approx. 1250 m
2
0.1
39
Agricultural land with some houses and 8 m tall sheltering hedgerows with a distance of approx. 500 m
2.5
0.2
31
Agricultural land with many houses, shrubs and plants, or 8 m tall sheltering hedgerows with a distance of approx. 250 m
3
0.4
24
Villages, small towns, agricultural land with many or tall sheltering hedgerows, forests and very rough and uneven terrain
3.5
0.8
18
Larger cities with tall buildings
4
1.6
13
Very large cities with tall buildings and skycrapers


The roughness rose
        If the wind speed is measured exactly at hub height over a long period at the exact spot where a wind turbine will be standing, then very exact predictions of energy production can be made. Usually, however, wind measurements made somewhere else in the area have to be recalculated. In practice, that can be done with great accuracy, except in cases with very complex terrain, i.e. very hilly or uneven terrain.
The roughness rose         Just like a wind rose maps the amount of wind energy coming from different directions, a roughness rose describes the roughness of the terrain in different directions from a prospective wind turbine site. Normally, the compass is divided into 12 sectors of 30 degrees each, like in the picture, but other divisions are possible. In any case, they should match the wind rose, of course. For each sector an estimate of the roughness of the terrain is made. After that, the Wind Speed Calculator shown later can be used to estimate for each sector how the average wind speed is changed by the different roughness of the terrain.



Averaging Roughness in each sector
        In most cases, however, the roughness will not fall neatly into any of the roughness classes, so a bit of averaging needs to be done. Special attention should be given to the roughness in the prevailing wind directions. In those directions a map should be looked to measure how far away we have unchanged roughness.


Accounting for roughness changes within each sector
Island in front of wind turbines         Imagine that there is a sea or lake surface in the western sector (i.e. roughness class 0) some 400 m from a turbine site and 2 km away there is a forested island, as shown in the picture.
        If west is an important wind direction, then the change in roughness class from 1 to 0 to 3 should be taken into account. This requires more advanced models and software than shown on this web site. It is also useful to be able to use the software to manage all the wind and turbine data.

                                             West sector for roughness rose
     


Accounting for wind obstacles
        It is extremely important to account for local wind obstacles in the prevailing wind direction near the turbine (closer than 700 m or so), for making accurate predictions about energy output. This subject will be further explained later.





Video frames
- Typical values of roughness length: values and notes

Handouts
- Wind power meteorology, part II: Siting and models