how WAsP handles many roughness changes in short distances

[Gil]

Dear Team,

based on old knowledge and after discussion with a colleague, i am a bit confused about how WAsP handles many roughness changes in short distances.

from a colleague, she mentioned if 10 different roughness changes were observed by WAsP in short distances (let's say in 3km), WAsP did not consider roughness changes any further in distances (in this case from 3km up to 20km - typical map size and boundary). is this true?

Based on recent improvement and implementation of QGIS instead of Map editor, the user can digitize more detailed roughness. if the above is applied, then i am wondering the impact.

could you explain me and advise me how WAsP handles many roughness changes in short distances?

regards,

Gyeongil

[Morten]

Your colleague is right. WAsP simplifies the detailed surface roughness map to a local polar map around each site of interest. We sometimes call this simplified map a roughness rose, and it has common roughness values within each sector and fewer roughness changes in the radial direction. The original roughness-simplification method is described in chapter 5 of European Wind Atlas - DTU Findit around page 81. The method may have been updated later, but the principle is the same.

The maximum radius of the roughness rose is something like 30 km. We recommend that the map prepared by QGIS has a margin around any site of interest equal to 100-150 times the hub height or anemometer height.   

Edited March 4 by Morten

[Gil]

Dear Morten,

Thanks for your confirmation and answers.

if "10 different roughness changes were observed by WAsP in short distances (let's say in 3km), WAsP did not consider roughness changes any further in distances" is true, 

then (Question 1) what is the impact on AEP if further distance roughness changes are not considered (e.g. from 3km up to 30km)? when I was in WAsP course in DTU, i have learned that most critical information of roughness chances is border (100 ~ 150 x Hub heights) as this affects in transition boundary layer. in this case it seems WAsP did not correctly consider.

(Question 2) to my understanding roughness rose is not anymore valid since WAsP 11.x and later version. Does WAsP 12.x use now again roughness rose?

Could you clarify two additional questions?

Regards,

Gyeongil

[Gil]

to make more clear for question 1, which values do WAsP take from 3km to 30km if values from roughness lengths cannot be considered? , e.g. zero roughness value or last roughness value observed at 3km or something else?

[Morten]

The roughness rose will cover the same area as the user-supplied map out to a radial distance of 30 km. It replaces the detailed roughness patterns of the real map. The European Wind Atlas explains how the roughness values in the sector segments are calculated.   

As you learnt in the WAsP course, the provided map must include a margin around any site of interest equal to 100-150 times the hub or anemometer height. For each sector, the flow model assumes a wind profile based on the far-upwind reference surface roughness, and this basic profile is corrected by internal boundary layers. You can see this in the site view>site effects view.   

The roughness rose is still used inside the model, but the developers choosed to remove it from the WAsP 12 user interface. I sometimes miss it for pedagogical reasons.