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OCMIP-2 Gas exchange: forgotten details



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