[MITgcm-devel] ocean_emissivity consistency

[Martin Losch]

Hi Jean-Michel,

obviously I also stumbled over this (again). The default of this parameter is 0.97*stefanBoltzmann.

Also there is a "0.97" in various places of the seaice-code, that I intend to replace by the appropriate run time parameter (I think I'll need a new one: effectiveEmissivity = 1.).

I just talked to a meteorologist (Gert König-Langlo), and he said this:

- lwdown (as computed from some atmospheric model) should not be scaled with anything, unless you parameterize it by some near surface air temperature, then you can multiply is with some (unknown) "effective emissivity", order 0.7. 

- if the outgoing lw-radition is parameterized as emissivity*stefanBoltzmann*Tsrf^4, then you need to include a term (1-emissitivity)*lwdown as the reflected "Gegenstrahlung" (incoming longwave), so that the net lwFlux = ocean_emissivity*sigma*SST^4 - lw_down + (1-ocean_emissivity)*lwdown=your expression. But it's sufficient to have emissivity=1. (in line with LY04)

I guess that with the current implementation in pkg/exf/exf_radiation.F, we can only have ocean_emissivity=1., for pkg/seaice/seaice_solve4temp and seaice_budget_ocean, it's probably all correct, as long as the ocean/ice_emissivity is =0.97. I'll try to fix the seaice-part, so that other emissivities are possible (and will set wrong parameters to reproduce old results in the verification experiments). What about pkg/exf? It should be easy to add a term (1-e)*lwdown, right?

Martin

PS. Gert also mentioned that the short wave radiation is likely wrong because we never consider multiple reflection/scattering, but I am not so sure about that (because I do not know what atmospheric models do here).


On Dec 19, 2011, at 3:02 PM, Jean-Michel Campin wrote:

> Hi Martin,
> 
> I don't much how thing goes regading long-wave in pkg/seaice,
> but in pkg/exf, when ocean_emissivity < 1, we should also 
> apply the ocean_emissivity factor to the downward long-wave
> flux when computing the absorbed lw flux:
> LW_net(+=up) = ocean_emissivity*( sigma*SST^4 - lw_down )
> 
> In Large & Yeager 2004, eq.12, they recommand to use 
> ocean_emissivity=1 for this reason. And over sea-ice,
> their equation 23 is clear, with ice_emissivity on both
> side. Do we have the same issue in pkg/seaice ?
> 
> Cheers,
> Jean-Michel
> 
> On Mon, Dec 19, 2011 at 06:49:01AM -0500, Martin Losch wrote:
>> Update of /u/gcmpack/MITgcm/pkg/seaice
>> In directory forge:/tmp/cvs-serv17544/pkg/seaice
>> 
>> Modified Files:
>> 	seaice_readparms.F 
>> Log Message:
>> make ocean_emissivity*stefanBoltzmann the default for
>> SEAICE_emissivity, if useEXF; requires setting a wrong parameter value
>> in global_ocean.cs32x15/input.seaice/data.seaice for backward
>> compatiblity
>> 
>> 
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