Blending height in WAsP

[bmvanwijk]

Hi,

I was wondering what the blending height is that WAsP uses to match the logaritmic profiles of one site with another.

Trying to match profiles from WAsP's output myself (no orography) gave me a height of 50km, which I thought was rather large. In texts, I often find smaller blending heights (for example 60 m. by the Dutch meteorological office for some calculations).

Kind Regards,
Bas

[Ib]

Hello Bas

WAsP does not use a "blending height". The term "blending height" was first (I think) introduced by Wieringa (1986) - at that time at the KNMI. He used the concept for matching of log profiles in the analysis of gustiness over inhomogeneous (flat) terrain. He, and later others, uses the concept to arrive at an estimate of an "effective" roughness over some area made up of patches of various roughnesses. Wieringa indeed found that his data was best represented if the blending height was set to a fixed value of 60m. Others have introduced blending heights depending on horizontal scale of the patches. For non neutral atmospheric stabilities some workers have found that the blending height concept may break down (requiring large unphysical values)...

WAsP uses a different methodology for analysis of inhomogeneous terrain - involving several steps:

1) Analysis of input roughness contour lines (in map file) to give averaged roughnesses per direction sector as function of distance

2) Fitting of the sectorwise roughness variations to piecewise constant values, setting (up to ten) steps at distances where most significant changes occur

3) Using the theory of "internal boundary layers" (originating at each roughness step) to calculate the ratio of windspeed at the considered point (x,y,z) to the concurrent far upstream windspeed (beyond all roughness change steps)at the same z.

4) Using these ratios, together with the "far upstream" roughnesses ("mesoroughness") estimated under 2 and 3 to relate to the "geostrophic wind" via the "geostrophic darg law".
(Which is slightly more complicated than a vertical extrapolation along a log profile)

5) Relating from geostrophic to "standard" homogeneous conditions using the geostrophic draglaw
(assuming same geostrophic wind from 4) in reverse.

6) Introducing stability effects (not accounted for in the above steps) via a statistical model derived from surface layer theory and the geostrophic draglaw of how wind profiles deviate when surface fluxes are not zero
(neutral).

...

Details can be found in the European Wind Atlas (Troen and Petersen 1989)

...

Thus depending on how you estimate a blending height you may get varying results and possibly seemingly "unphysical" - as WAsP does not use the concept explicitly.

Best wishes

Ib

[bmvanwijk]

Dear Ib,

Thanks for your extensive reply. The blending height method is indeed quite limited. I realize that since detailed spatial information of roughness around a site is available, a more precise approach can be used in WAsP.

Thanks,
Bastiaan