Reference roughness

[Timm]

Hallo WAsP Team,

I would like to know how the reference roughness is calculated exactly.
I have calculated a roughness rose in WAsP 10.2. Therefore I got the roughness length for each different section in each sector. In the Site effect results I got the reference roughness for each sector. And I can't imagine how this is done. I have read the WAsP Help file but it doesn't give me the answer I was looking for.
How are the sections weighted? And how is the reference roughness calculated?

Thank you for your help.
Best regards.
Timm

[Qiangbing]

The sector-wise reference roughness are calculated by weightedly averaging over distance the
logarithmic of roughness of the sector in question, and then taking the exponential of the
average. The weight of the roughness at distance r is taken to be exp(-r/R), where R is the decay length
for roughness area size, which in our case is taken to be 10,000 meters.

[mango]

Dear Qiangbing,

I calculated a roughness survey by adding a reference site. in one sector, the roughness change data is:

Zone z0 start
1 0.101 0
2 0.244 4406
3 0.105 18648

so , according to your answer, the roughness reference can be calculated as the sum of the integral of:
(ln(z0 * exp( - r/R))* dr, r's range is [r1, r2], R = 10000, and the maximum radius is 20000;

and then calculate the average of the above and the exponential of the mean.
by this way, my results is: 0.07, which is smaller than the results from wasp: 0.156.


I want to know how does the roughness reference influence the speedup by roughness. so kindly could you point out where lies the difference?

thanks in advance.

Caplinav Jer

[Qiangbing]

To do the average you need to integrate ln(z(r))*exp(-r/R)* d(r/R) from zero to infinity, which in your case (3 zones)the result is:
average = log(z1)*(1.-exp(-r1/R)) + log(z2)*(exp(-r1/R)-exp(-r2/R)) + log(z3)*exp(-r2/R)

[mango]

Dear Qiangbing,

thanks very much for your help. Now I get it right.

[mango]

I have another question, what about the significant roughness change criteria?

I have two editions of roughness map, one is the data from *.map data, the other is some data of roughness rose. I want to merge the two source data to one. I set the roughness change criteria to: z1 > 2*z0 or z1 < z0/2, the z0 is the previous roughness length and the z1 is roughness being considered. but the result is not good.

thanks in advance.

Caplinav Jer

[Duncan]

Can you explain the two sources of data which you are trying to combine? Are they both maps?