WAsP in Tropics (Unstable Atmoshper?)

[IrvingG]

Dear WAsP forum,

I am analyzing a few sites in the tropics on flat terrain farming land, i.e. not complex terrain. It fits with WAsP, but I am not sure if some parameters need to adjustment (especially may be the heat flux offset and RMS values).

Our masts have temperature and wind speed measurements at multiple heights. It suggests slightly unstable condition.

The default RMS and offset heat flux values in WAsP are based on European climates. The tropics are hotter than Europe. Has someone got experiences in this?

In the DTU report 462000 Planning and Development of Wind Farms:Wind resource assessment using the WAsP software, by Niels G. Mortensen, pp.21-22, the heat flux was modified for a site in South Africa (subtropics). The modified heat flux offset of -10 Wm-2 still suggests stable region.

Has WAsP need applied for hotter climate, and especially in unstable atmosphere?

Many thanks and regards,
Irving

[Niels Gylling Mortensen]

Dear Irving,
As you write, I have some experience in hotter climates, where it is sometimes required to tweak the heat flux values to get a better fit of the WAsP modelling to the measured profile. However, you have to be careful... many things have to be checked/done first:
- mast spec's (position, height of instruments, boom orientation, etc.)
- wind measurements (calibration, data quality, missing data, etc.)
- elevation map (size, resolution, detail, quality, etc.)
- roughness map (size, resolution, roughness values, etc.)
- any nearby obstacles or significant mast flow distortion
- adjustment of GWC heights to project spec's


If these basics are ok, it may be in order to tweak the heat flux values. However, it should be done based on a wind profile analysis as described in the report. This is because the WAsP heat fluxes are not exactly the same as the heat fluxes you can measure / estimate in nature (though they are strongly related).


In South Africa, I have used heat flux offset values from -50 to +20 W/m^2, see the Wind Atlas for South Africa (http://orbit.dtu.dk/en/publications/wind-atlas-for-south-africa-wasa-observational-wind-atlas-for-10-met-stations-in-northern-western-and-eastern-cape-provinces(b7c28037-116d-422a-a1d8-450c7050f8c7).html). Note though, that this atlas is still being developed. - You will find that the variation on the unstable side is much smaller than on the stable side.


Remember, that the WAsP analysis is for entire years, so even if you think the climate / atmosphere is 'unstable', in many cases it is not so on average over the year. We are working on improving the models so they can use measured or modelled heat fluxes in the future.


Best regards,
Niels

[Mark Kelly]

Another thing to note is that the 'default' WAsP offset heat flux is for slightly stable on average values (-40 W/m^2, since stable stratification affects the profile more than unstable stratification).
So a value of -10 W/m^2 for Hoffset, for example, gives less shear (less change in speed with height) than the default settings.

[For more details one can also check Kelly & Troen (2016) "Probabilistic stability and 'tall' wind profiles: theory and method for use in wind resource assessment", Wind Energy, 19(2), 227–241.]

With kind regards,

[Fierrinho]

What is the point of adjusting the WAsP parameters if you have previously scaled up your mast height to hub height using the shear measured at mast location?

I can see the point of adjusting the Heat Flux parameters if no information about the shear is available as for example in those cases where only production data is used but if not, not much.

Can anyone clarify this?

Thanks,

[Niels Gylling Mortensen]

Dear Fierrinho,
There seems to be two schools of thought / practices:
1) Shearing up the observed wind climate to hub height at the mast site and then do the flow modelling – which then describes the horizontal change of the wind climate over the terrain. In this case, you assume that the measured shear is representative for the entire wind profile, and the heat flux value will have little influence on the results (as long as the prediction height is the same).
2) Doing the flow modelling directly between the anemometer and the hub height, which then includes both vertical and horizontal extrapolations. For this, one would need an estimate of the heat flux at the site. The choice of heat flux can be validated at the mast site.

Best regards,
Niels