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*Subject*: OCMIP-2 Gas exchange: forgotten details*From*: orr@cea.fr (James Orr)*Date*: Wed, 13 May 1998 15:03:07 +0200 (CEST)

Dear Ray, Please find below two details that seem to be forgotten in your proposals for OCMIP-2 gas exchange. (1) Wind speed variance: The gas exchange formulation for OCMIP-2 should take into account the variance of the wind speed as we did for OCMIP-1. The basic equation should be <<K>>=a*(<V>+<<U>2>) The monthly climatology of K, <<K>>, is deduced from the monthly climatology of wind speed variance V (denoted as <V> for the average of the monthly V values), and from the monthly climatology of <U>*<U> (which is the average of the square of the monthly U values). The equation above is derived in the OCMIP-1 "README.satdat" file, as are the details of how one gets from SSMI to monthly and climatological values. (2) Coefficient: The coefficient of 0.39 in your equation (3) is incorrect. Wanninkhof (1992) gave that coefficient for use with "long-term averaged wind speed". Wanninkhof gave a coefficient of 0.31 for use with spot measurements (satellite winds); the latter coefficient takes into account wind speed variance via a Rayleigh distribution function. The coefficient to be used for OCMIP-2 should be slightly different. The principal difference is that information on observed wind speed variance is included with the satellite (SSMI) wind speeds (Boutin and Etcheto). Details on how we determined the coefficient for CO2 during the 1st phase of OCMIP can be found in the for OCMIP-1 "README" file. The value of the coefficient "a" will be different than that we derived for the OCMIP-1 CO2 runs, because for non-CO2 tracers, there is no chemical enhancement term. For later comparison between tracers during OCMIP-2, I suggest that we forget about chemical enhancement for CO2 (and C-14) as well. Jean-Claude Dutay (IPSL/LSCE) has computed the coefficient "a" adjusted to obtain an average value of the CO2 gas transfer coefficient Kg (=K*S) of 0.061 mol/(m^2 yr uatm) as obtained by Broecker et al. (1986). For OCMIP-1, we did the same but for coefficient "d" in Wanninkhof's eq. (5), which is like "a" but for the case with chemical enhancement. In any case, Jean-Claude has determined that to be consistent with SSMI wind speed and wind speed variance, a=0.337. Cheers, Jim - ------------------------------------------------------------------------------- James Orr ; Lab. des Sciences du Climat et de l'Environnement mailto: orr@cea.fr ; Unite Mixte de Recherche CEA-CNRS Tel: (33) (0)1 69 08 77 23; Address: LSCE, CEA Saclay Fax: (33) (0)1 69 08 77 16; Bat. 709 - Orme Home: (33) (0)1 30 41 31 72; F-91191 Gif-sur-Yvette Cedex - FRANCE

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