Roughness in Complex terrain

[Petros]

Dear WAsP team,

I am currently working on a project in Greece on a very mountainous area.
Additional to the slopes, the windfarm lies very close to a fir forest which ends about 80m from the ridge where a 10m high mast is installed.
As a result we have a Roughness Class 2.5 area really close the the mast position.
Generally the hill ridge is clear so most of the turbines and the mast are found in a roughness class 1 area.
What WAsP returned as a result is a small negative wind profile since wind speed at 80m is calculated slightly lower than the one measured at the mast height. Also the energy density seems to be relatively low to what we expected.
I was trying to find some explanation by myself and what is more convincing to me is that due to the clearing there is a speed up at the lower levels which cannot be applied at 80m due to the small distance between the forest and the mast.
But as i was trying to find a more scientific answer it turned out that the model of roughness that WAsP uses does not contain any information on vertical displacement due to orography.
My question is: Does WAsP calculate the roughness effect on a flat terrain and the ads/subtacts the results from the speed up due to orography? Does the roughness model take into account the complex terrain?
Should i add any displacement height as mentioned in Dellwik's paper on forests and WAsP if i am located in a complex terrain site?

Any input would be welcome
Thanks in advance
Petros

[Ib]

Hi Petros

From what you describe a negative wind shear is quite plausible (as
predicted by WAsP). The 80m clearing from the edge of the forest to the 10m
mast should have very little if any effect at 10m. It is true that the
roughness change model in WAsP gives same results (for a given roughness
configuration) for flat or non-flat terrain (The orographic model does
however take local variations of roughness into account).

Apart from the fact that the orographic model results depend on the
roughness, the two models for roughness and orography are giving independent
correction factors which are added in WAsP. WAsP does not pt consider
displacement. Regarding displacement adding/subtracting: This complication
should be treated the same way in complex and flat terrain. For your
configuration, dependent on the density and height of the forest, you can
expect some effects and it would be a good idea to try to estimate these.

Good luck

Ib

Ib Troen

[Petros]

Hi Troen,
Thank you for your input. Your answer is actually what I expected, but i also have one more question.
When a forest is located in a flat area the trunks of the trees are usually perpendicular to the ground, but when we are on the slopes this changes since the trees grow vertically to the horizon so this decreases the effective height of them. The roughness length of a 15m high forest should not be the same in a flat terrain and a slope with lets say 30 degrees because then we have 13m high forest.
Is this change taken into account? Appart from the roughness length this would influence the IBL (internal boundary layer) too.

[sroot]

could anyone send me something about how to define the roughness.

[Duncan]

If you search for "roughness" in the WAsP help file, you will find several useful pages describing how the roughness inputs can be prepared, and how to judge different landscape roughnesses. If those don't help, post back here with some more detailed question, and we'll try to help.