Difference between effective roughness and reference roughness.

[Cath]

Dear WAsP team
By using wasp and reading (1989)European_Wind_Atlas,I found two concepts about roughness appear repeatedly, one is "effective roughness" and the other is "reference roughness". would you help  to explain the difference between these two roughness?

Thank you for your help.
Best regards.

[Morten]

The effective roughness is the parameter used in the geostrophic drag law that links surface winds to winds above the boundary layer (EWA chapter 8.1). This effective roughness is a simplification of the surface roughness map, and it is calculated for each turbine or reference mast position and each wind sector. In addition, the wind atlas model includes a sub-model for the effect of surface roughness changes (EWA chapter 8.3).

The wind atlas categorises the terrain by a reference roughness value (EWA Figure 3.1).  WAsP will calculate generalised wind climate statistics for a set of reference roughness values and standard heights, and you can see this table if you right-click a generalised wind climate (GWC) object in WAsP.  As discussed above, the WAsP model depends on climate statistics for effective roughness values for each site and wind sector. It therefore interpolates the GWC using data at the reference roughness values and standard heights. You can avoid the associated interpolation errors if you modify the reference conditions, i.e. right-click the project object and select edit configuration> wind-atlas structure. We recommend inclusion of the turbine hub height and the height of the reference anemometer in the set of standard heights. You might also include the most typical surface roughness of your area.
 

Edited May 18 by Morten

[Cath]

Dear Morten，

Thanks very much for your reply. Now I have a very clear understanding of these two concepts.