Source for Heat Flux Values

[Matt]

Hi,

I'm working on a project in the Caribbean and I'm pretty sure I need to change the default values for heat flux in WAsP. However, I'm not sure what to change them to. I've looked up the NCAR dataset here: http://www.esrl.noaa.gov/psd/data/gridded/data.ncep.reanalysis.html, but it isn't clear to me which particular dataset to look at. I think I need the surface heat flux, but should I use sensible or latent or both? Also, should I somehow average these values over a yearlong timespan?

Any insight would be appreciated.

-Matt Nipper

[Duncan]

Hello, sorry you haven't received a proper answer to this yet. I wonder if this paper offers any help?

http://www.wasp.dk/Support/DownloadFiles/GiebelGryning-ShearAndStabInHighMetMastsAndWAsP_Delft2004.pdf

[Gregor Giebel]

Hi Matt,

sorry, I had promised Duncan to reply quickly, but forgot.

The short story is that the results of the paper show that you have to change the parameters quite strongly to get any appreciable effect, and in the end the values will be quite different from the measured ones. You could just treat them as tuning parameters, and try to optimise your self-prediction, but the variation needed is probably small.

Please also be aware that the paper mentioned by Duncan was calculated with older code, which had some calculation deficits for one side of the parameter space. We didn't do the calculations again with the new code.

Hope this helps,
Gregor

[lavisura]

I have three queries.
1. Many here have discussed about changing the stability parameters corresponding to stable conditions. Is it possible or advisable to do it for unstable conditions for say fine tuning?
2. Assuming I am trying to optimise the self-prediction, please suggest if this is right. First I check the default self-prediction value. Then I try to vary the parameter to a certain value and recalculate and check which is closer to the actual. IF this method does give me a certain value of the parameter to be more closer, theoretically is it fine that I use this correction for my further calculations instead of default values?
3. http://help.emd.dk/knowledgebase/content/WindPRO_WAsP_Parameters.pdf talks about the effects of changing some parameters. According to 'best practices' and what is mentioned in 4.2.1 of the above paper, is changing the standard heights necessary (generally) or (especially) if the stability parameters have been altered?
Hope I have expressed my doubts clearly. Do please correct me wherever I am wrong.
Thank you

[boss8]

glad to join.















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[Mark Kelly]

Hello lavisura and all---


1. Currently, splitting the data is not generally recommended within the current WAsP (v10.1) framework: WAsP is designed to put stable- and unstable-condition observations together to give a representative long-term estimate. (However, one can in general divide into stable and unstable subsets, which is in part what the forthcoming improved WAsP-11 alternate stability model does; see Kelly and Gryning 2010, Boundary-Layer Meteorology.)
Within the current framework (v10.1) one would need to make separate "stable" and "unstable" wind-climate (.tab/.owc) files, and then tune the offset and RMS heat flux parameters for each set, compared to the observed profiles for stable and unstable conditions--but again I stress that doing so basically falls outside the design envelope of currently available (up to v10.1) WAsP.


2. If you are trying to improve self-prediction, you hopefully have multiple measurement heights; note that the stability parameter should not alter anything for the same height. You can first vary H_offset and secondly H_rms to optimize the self-prediction, but beware of extrapolating much beyond Delta z above the target height (where Delta z is the difference in height agl between the 2 sites), and beware/caution for the case of extrapolating beyond z_m if both measurements fall below z_m (where z_m~=65-75m over land, see European Wind Atlas Ch.8). If you did use this, it would only apply to this particular site/wind-climate, but again would work much better with 2 met masts--more uncertain with just one.


3. Including measured and hub heights in the standard heights can reduce numerical (log-)interpolation uncertainty, but this is rather small compared to the (unrelated) effect of modifying stability parameters.
Again as Gregor commented earlier, the WAsP parameters in the current framework do not correspond directly to measured heat fluxes, so they do not necessarily reflect reality--contrary to the EMD/WindPro description (though the new [yet unreleased] alternate/v.11 treatment uses measured fluxes). Otherwise the EMD recommendation 4.3.2 is good: adjusting the heat-flux parameters to fit one measured vertical profile can sometimes make flow modelling results worse than using WAsP standard parameter settings (especially if only one mast, 2 heights, see #2 above); H_off and H_rms might be changed only for very cold/warm climates.