Offset Heatflux (2009-08-31)

[Old Forum Archivist]

Can you explain relevance of the offset heat flux and how the form factor, Frms, is derived? Is it possible to introduce a correction to the adopted Frms value using measured data?

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Richie Cotton wrote:

> Can you explain relevance of the offset heat flux and how

> the form factor, Frms, is derived? Is it possible to

> introduce a correction to the adopted Frms value using

> measured data?



The 'offset' heat flux serves to characterize the climatalogical-mean effect of (mean) stability on the profile. In essence, because the height-dependence (influence) of stable stratification is stronger than that due to unstable conditions, the climatological-mean profiles tend to 'lean' toward the stable side (higher wind speeds). The (default) offset heat flux accounts for this.



The form factor "Frms" serves to (partially) account for the Monin-Obukhov stability function's bias towards stable conditions when modelling the effect of heat flux *fluctuations* upon the mean profile; thus it is less than 1 (i.e. diminished effective positive heat flux). The default value is quasi-empirical.



I would advise strong caution towards immediately trying to modify Frms; changing this and the offset+rms fluxes are the last thing you should try.

Basically, first you should recheck you have followed _all_ of the WAsP best practices, see for example your WAsP course notes; there are a number of adjustments which often occur there

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