[MITgcm-devel] open ocean albedo formulation

[Menemenlis, Dimitris (3248)]

Jean-Michel, I move thread to devel list.
Ideally, we would want to adopt the same formulation as is used
in atmospheric model, which is computed from sun angle,
i.e., based on time-of-day and time-of year.
Isn't there a caned routine for that floating around in fizhi
   Section 6.5.3.2.11 in
   http://mitgcm.org/public/r2_manual/latest/online_documents/node247.html
that we can borrow?  D.

On Apr 1, 2010, at 10:27 PM, Jean-Michel Campin wrote:

> Hi Dimitris,
> 
> I am aware of this. Daniel Enderton added a "daily average" version
> in AIM (since AIM does not have diurnal cycle). But with more
> "state of the art" AGCM, it's generally included, but only for
> the direct SW (the diffus/scattered part still feel a low albedo
> of ~6%). In practice, it means that the effect is quiet small in
> low lat regions & summer mid lat, and not very significant in mid lat
> winter (since the direct part is not very big because of clouds).
> It certainly could have an effet in high lat, but when it's covered
> with sea-ice, then the effect disappears.
> I personally think that the crude way (linear interp in time) we use
> to interpolate the SW 6-hourly forcing fields introduce a much larger
> error (except may be in high lat).
> I was discussing this point with Oliver the other day (and a simple
> way to reduce this error, which might also affects the biology).
> 
> Cheers,
> Jean-Michel
> 
> On Thu, Apr 01, 2010 at 06:25:40PM -0700, Menemenlis, Dimitris (3248) wrote:
>> On the topic of albedos, please see next email.
>> Atmospheric models include an incidence-angle dependence for albedo,
>> i.e., seasonally and latitudinally-dependent, which we have been happily
>> ignoring in ocean models.  Were any of you aware of that and possibly
>> what impact of ignoring this is on global OGCM solutions?
>> 
>> Not so important in estimation context since estimated shortwave will compensate for this.
>> 
>> D.
>> 
>> On Apr 1, 2010, at 6:15 PM, gael forget wrote:
>> 
>>> oops. I forgot the graphs indeed.
>>> Here they are.
>>> Gael
>>> <albedo_approx_ncep.jpg><albedo_approx_ecmwf_interim.jpg>
>>> On Apr 1, 2010, at 8:57 PM, gael forget wrote:
>>> 
>>>> Patrick,
>>>> 
>>>> it is good that you made me think of the matter of
>>>> albedo choices -- I would have overlooked it.
>>>> 
>>>> I tried to back out approximate albedo values
>>>> that would be consistent with NCEP/ECMWF
>>>> downward vs net short wave flux fields.
>>>> My computation is very rough (e.g. I do not
>>>> have a land mask, so I just selected ocean
>>>> squares) but the results seem typical.
>>>> 
>>>> I attach the two graphs (ncep then ecmwf).
>>>> Except for the weird NCEP open ocean values,
>>>> the baked out values match the default MITgcm values
>>>> rather than the suggested ones. So I am tempted
>>>> to simply revert to the defaults, which are also the more
>>>> realistic values, and let the assimilation go from there.
>>>> 
>>>> What do you think?
>>>> 
>>>> Gael
>>>> 
>>>> On Mar 30, 2010, at 7:40 AM, Patrick Heimbach wrote:
>>>>> 
>>>>> Hi Gael,
>>>>> 
>>>>> just a warning:
>>>>> with current seaice albedo settings I am loosing way too much
>>>>> seaice in the Arctic in Sept. (almost all).
>>>>> I am currently testing a different set of values
>>>>> which have been adapted from ERA-40:
>>>>>     exf_albedo        = 0.15,
>>>>>     SEAICE_dryIceAlb   = 0.9775,
>>>>>     SEAICE_wetIceAlb   = 0.8236,
>>>>>     SEAICE_drySnowAlb  = 0.9169,
>>>>>     SEAICE_wetSnowAlb  = 0.7820,
>>>>> 
>>>>> -p.