CFC gas transfer velocity: wrong coefficient?

[mscheinert]

Context

For the air-to-sea flux of CFCs/SF6 the gas transfer velocity k is computed following Wanninkhof (1992, 2014):

k = a * u² * (Sc/6606)^-0.5

with 'a' being a constant. In Wanninkhof (1992) (W-92) it was suggested to use a=0.39 for long-term averaged winds and a=0.31 for instantaneous winds like those from CORE or JRA. An update of the gas exchange wind speed relation was presented by Wanninkhof (2014), and referring to Sweeney et al. (2007) a smaller value of a=0.251 was proposed. This is the same value that is demanded by James Orr et al. (2017) for simulations participating in the CMIP6 Ocean Model Intercomparison Project (OMIP).

Analysis

In the routine trc_sms_cfc of module trcsms_cfc (NEMOGCM/NEMO/TOP_SRC/CFC/trcsms_cfc.F90) Wanninkhof (2014) is cited for the speed transfer formulae. But instead of a=0.251 the old value a=0.31 from the W-92 publication is used (near line 167):

zak_cfc = ( 0.31 * xconv2 * zv2 / SQRT(zsch) ) * tmask(ji,jj,1)

This would mean that NEMO uses a transfer velocity for CFCs and SF6 that is about 23% larger compared to the one with the recommended factor (e.g. Orr et al. (2017)).

Recommendation

If there is no good reason for this factor being 0.31 (and there might be good reasons, but I'm not aware of any of them since I'm not a tracer guy; just checking the code for a certain configuration used by our group) this line should be:

zak_cfc = ( 0.251 * xconv2 * zv2 / SQRT(zsch) ) * tmask(ji,jj,1)

Otherwise the reference should be changed from W-14 to W-92, shouldn't it?

[lovato]

In 9400:

Error: Failed to load processor CommitTicketReference

No macro or processor named 'CommitTicketReference' found

[lovato]

In 9401:

Error: Failed to load processor CommitTicketReference

No macro or processor named 'CommitTicketReference' found

[lovato]

The code is supposed to use the coefficients described in Wanninkhof (2014) as pointed out by mscheinert.

Thus, subroutine NEMOGCM/NEMO/TOP_SRC/CFC/trcsms_cfc.F90 was corrected to use the gas transfer velocity coefficient (0.251) in both v3.6_stable (r9400) and the trunk (r9401).

To comply with guidelines from Orr et al. (2017), in the model configurations using CFC is now included an atmospheric concentration forcing file based on CDIAC dataset from Bullister (2017) [CFCs_CDIAC.dat],
along with the one based on Input4MIPs forcing already in there [CFCs_CMIP6.dat].

[lovato]

In 9613:

Error: Failed to load processor CommitTicketReference

No macro or processor named 'CommitTicketReference' found