pdoubrawa

Hello, does anyone know what WAsP uses as exact values for - Coriolis parameter in the various formulas in the wind atlas application/analysis ? (e.g. 1e-4 s-1) - Absolute surface temperature for the stability calculations ? (e.g. 288 K) Also, is the air density kept at 1.225 kg/m3 for all of the calculations? I appreciate any insight into these little however relevant mysteries. Thank you Paula

While trying to recreate some numbers produced by WAsP (when going from tab to lib), I found the final Weibull parameters quite sensitive to the value I used for the Coriolis parameter in the geostrophic drag law. Does WAsP use a constant value for f (say 1.2e-4 s-1) or is it calculated based on the latitude of the site? Thank you! Paula

Hello, Morten Thanks for clarifying that. Still about the first point, which of the following two is correct: 1) WAsP obtains histograms for each roughness class and then calculates Weibull parameters per sector. Then for the different standard heights the shape parameter is kept constant and the scale parameter is calculated for the different heights according to the log law. 2) WAsP obtains a histogram for each combination of roughness class and standard height (redistributing frequencies across sectors and across wind speed bins) and only then does it fit the Weibull distribution. I am using constant roughness, no terrain, and no heat flux in WAsP so I am just trying to "recreate" the most simple possible case scenario. My results should be close to WAsP's but there is still a significant offset in the scale parameter. Thanks again Paula

Hi, Morten My numbers for A and k are still off when compared to WAsP's. Can you please answer a couple more questions? At which point in the process of going from tab to lib does WAsP calculate the weibull parameters for the first time? From that point on, does it ever go back to probability distributions again, or are all calculations performed on A and k values? It seems that the first thing WAsP does is calculate the first and third moments per sector given the frequencies in the tab file. Then it redistributes those according to the veer angle going up to the geostrophic wind, and then again according to the backing angle coming down to the surface for a given roughness class (is this correct?). However, moments are statistical measures of the sector, while the veer angle is a function of G and u* and therefore also a function of u. There is no one veer angle value per sector. Does WAsP use an average veer angle or the veer angle for a specific speed to do this redistribution? Finally, when you say that the third moment and the probability of winds greater than the mean are conserved, are they conserved for a given roughness class no matter what height we are considering, or are they conserved for any roughness class and height? If the latter is the case, then the third moment calculated straight out of the tab file and redistributed across sectors according to the veer angle, will be the only third moment ever used in the entire calculation of the wind atlas? Thank you so much for helping me move along with my research! Paula

Thank you, Morten! I am starting to understand it a bit better. I am sorry to be asking so many questions, I am just trying to understand what assumptions are being made behind the calculations. Since the third moment and probability of winds higher than the mean are conserved, the only thing that changes with height is the first moment (mean) of the distribution. In order to calculate this new mean for the desired height, does WAsP use the new wind speed bin limits obtained through the logarithmic profile, along with the original sector-wise probabilities from the tab file? Paula

Thank you very much, Morten. I realized that my Question 1 above was not very clear. What I was actually asking, was about the redistribution of frequencies within one sector, but across the wind speed bins. Once the log law is used to calculate the wind speed for a certain height and roughness, what before was 1m/s may now be 1.2m/s for example. Does that mean that 20% of the frequency that was previously in bin 1 should now be moved to bin 2, and so on? Thank you again! Paula

Hello, Morten Thank you for your answer. Finally, I am still a bit confused about how WAsP calculates the Weibull parameters from the frequency distribution. I have two questions, can you please take a look at them? Question 1: After redistributing the frequencies across sectors based on the veer angle, WAsP uses the logarithmic wind profile to calculate wind speed values for each of the standard heights. Then, does it redistribute the frequencies in the same manner that it was done for wind direction according to the veer angles? Question 2: From what I understand, WAsP uses the method of moments. The A,k calculation is based on the mean of the wind speed, the mean of the cubed wind speed, and the probability of winds higher than the mean. How is this obtained from the frequency distribution for a certain height and roughness? Again, thank you for your help! Paula

I have yet another question about this... Equations (8.5) in page 567 of the European Wind Atlas provide the geostrophic wind in a rotated coordinate system aligned with the surface wind. In order to rotate the geostrophic wind coordinates back to a cartesian system, do you use the original mast time series of wind direction to determine the angle of rotation (thus assuming the direction at anemometer height to be the same as the direction at the surface) ? Thank you very much! Paula

Thank you very much for your answer, it was very helpful.

Hello, I am having a hard time understanding why the sectorwise frequency distributions are different for different roughness classes in the lib file. I have thoroughly read the documentation and the European Wind Atlas trying to understand what those lines mean and I have not succeeded. I have extensively tried to recreate the frequency distributions in the lib file but I have not succeeded because I do not understand what they represent. My questions are: - The sectorwise frequency distributions per roughness class are representative of which height? (is it the original anemometer height of the data, or the geostrophic level?) - How does wasp calculate these wind direction changes from one roughness to another? The only reference for wind direction changes that I found in the European Wind Atlas was on page 567 and it refers to the angle between the surface and geostrophic winds. Thank you very much for your help! Paula