Charnock parameter is approximately saturated under high wind speeds.Will Charnock parameter be decreased under gale-force winds? What is the role of the wave age and sea spray on this possible decrease?
Dear George,
I cannot give you any answer to this interresting question. Fairall et al. (1996; 2003) effectively considers that the Charnock parameter can be considered as constant for winds>18m/s. However the in-situ data on which his bulk algorithm is based sampled wind conditions up to 18-20 m/s (see his paper Fairall, C.W., E.F. Bradley, J.E. Hare, A. Grachev, and J.B. Edson, 2003: Bulk parameterization of air-sea fluxes of TOGA COARE: updates and verification for the COARE algorithm. J. Clim., 16, 571-591.).
This is the case for most of the bulk algorithms found in the litterature. Wind conditions sampled for estimating turbulent fluxes are generally less than ~ 20 m/s. So you must consider that any relationships between z0 and u* or Charnock versus u are uncertain.
If I can give you some recommandation, consider studies from the Russian school of fluxes, i.e: Soloviev , Benilov, Makin... In their papers perhaps you can find some considerations on the relationship Charnock versus gale force winds.
Regards
G.Caniaux
Hi Georges:
At high winds (above 25ms-1) the sea surface roughness saturates hence the Charnock coefficient becomes constant. This does not mean that for higher winds the momentum flux becomes constant because it increases as the wind speed squared. A good reference on surface roughness saturation is the paper by Donelan et al. 2004 published in Geophys. Res. Lett. Measurements in hurricanes have shown a decrease of drag coefficient at very strong winds this topic is discussed in papers by Emanuel, Soloviev and others.
Regards
Héctor
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