[MITgcm-devel] seaice_get_dynforcing.F: CDAIR SEAICE_drag and OCEAN_drag

[Fenty, Ian G (3244)]

Sea ice developers,

I would like to make two observations about air-sea ice momemtum fluxes

----------------------------------------------------------------------------------------------------------------
Observation 1: Using the variable name CDAIR - described as the 
local wind stress coefficient - in seaice_get_dynforcing.F is 
unnecessarily confusing since it is not a air-ice drag coefficient.
---------------------------------------------------------------------------------------------

If u,v winds are prescribed, the variable CDAIR is determined by

CDAIR = SEAICE_rhoAIR * SEAICE_drag * sqrt(u_wind^2 + v_wind^2)

Normally, the variable SEAICE_drag is referred to as C_D and called 
the air-ice momentum transfer coefficient.

Following the calculation of CDAIR, the surface momentum flux is calculated as,

taux = CDAIR * |u_wind| 

So clearly, CDAIR is just a temporary variable and not a wind stress coefficient and 
I think use a different name for it.

----------------------------------------------------------------------------------------------------------------
Observation 2: The meaning of CDAIR completely changes when wind stresses are 
prescribed and that, again, is unnecessarily confusing.
----------------------------------------------------------------------------------------------------------------

In ECCO v4 and ASTE, wind stress - not u,v winds - are prescribed as
forcings. In that case, seaice_get_dynforcing defines CDAIR as,

CDAIR(i,j) = SEAICE_drag      /OCEAN_drag

In the above, CDAIR is clearly a ratio of two momentum transfer coefficients.

Following the calculation of CDAIR, the surface momemtum flux is
calculated by scaling fu by CDAIR:

taux = CDAIR * fu
tauy = CDAIR * fv

If one prescribes wind stress, why do we scale the wind stress by the ratio of SEAICE_drag /OCEAN_drag? In doing so, are we not making the implict assumption that the prescribed wind stresses are only equivalent open water stresses? 

Should we not trust that the stresses prescribed by the user are the actual momemtum fluxes to the surface (whatever the surface) and not the equivalent open water momemtum fluxes?  

If the prescribed wind stresses are the actual atmospheric momemtum fluxes, shouldn't taux = fu?

If so, shouldn't we remove the SEAICE_drag/OCEAN_drag ratio? So as to prevent someone from getting a factor 2 too large air-sea ice stress (2 is the default ratio of SEAICE_drag/OCEAN_drag).


-Ian