[MITgcm-support] Evaporation in sea ice regions

[Naughten, Kaitlin A.]

Hi Martin,


Thanks very much, I will use 1000*EXFevap + SIfwSubl in future. No need to see the python script.


All the best,

Kaitlin


Dr Kaitlin Naughten
Ocean-Ice Modeller
British Antarctic Survey

________________________________
From: MITgcm-support <mitgcm-support-bounces at mitgcm.org> on behalf of Martin Losch <Martin.Losch at awi.de>
Sent: 30 July 2019 16:22:22
To: MITgcm Support <mitgcm-support at mitgcm.org>
Subject: Re: [MITgcm-support] Evaporation in sea ice regions

Hi Kaitlin,

with latest code, EXFevap only contains ocean evaporation, and -(SIatmFW - 1000*EXFpreci - 1000*EXFroff) should still be correct. I think that you need 1000*EXFevap + SIfwSubl for the total evaporation, because SIfwSubl also contains the evaporation of water after all of the ice has sublimated away. When I run the verification experiment global_ocean.cs32x15/input.seaice and save all the required fields the difference between these two combination is order 1e-20. (the evaporation is of order 5e-5). Do you want to see my python script?

I don’t think that the sea ice model cares about the evap-forcing and will compute it’s own sublimation regardless of what you specify.

Martin

> On 24. Jul 2019, at 16:58, Naughten, Kaitlin A. <kaight at bas.ac.uk> wrote:
>
> Hi Martin,
>
> Thanks very much for your response. I tried calculating the evaporation-plus-sublimation 3 different ways (with the newest version of the code to make sure that EXFevap was treating sea ice correctly, and with all necessary unit conversions between the variables):
>
>        • EXFevap
>        • EXFevap + SIacSubl
>        • -(SIatmFW - EXFpreci - EXFroff)
>
> All of them look similar, but they have significant differences over the sea ice zone (sometimes by over 100%, and not in a consistent direction). I am not sure which one is correct.
>
> Regardless, comparing to ERA-Interim makes me think that my model has a strong bias in evaporation. I would like to try forcing with evaporation using EXF_READ_EVAP, rather than allowing this bias to continue and mess up the surface freshwater balance. My follow-on question is: should the evaporation forcing field include sublimation? (I think ERA-Interim does, but I am confirming with some atmosphere people.) Does the sea ice model care about the evaporation forcing at all, or does it calculate its own sublimation regardless?
>
> Many thanks for all your help,
> Kaitlin
>
> Dr Kaitlin Naughten
> Ocean-Ice Modeller
> British Antarctic Survey
> From: MITgcm-support <mitgcm-support-bounces at mitgcm.org> on behalf of Martin Losch <Martin.Losch at awi.de>
> Sent: 23 July 2019 13:56:40
> To: MITgcm Support <mitgcm-support at mitgcm.org>
> Subject: Re: [MITgcm-support] Evaporation in sea ice regions
>
> Hi Kaitlin,
>
> I agree with you that this is a little confusing.
>
> Until recently (and you’ll have to check the version of your code), the exf-diagnostics did not know about the presence of sea ice (neither did the exf-bulkformulae, so that you can have quite funny values over sea ice). I think this has changed recently (PR #203 was merged on Jun19, see <https://github.com/MITgcm/MITgcm/pull/203>).
>
> Before that
>
> 'SIatmFW ‘ is the net freshwater flux from atmosphere and land (includes runoff and precip), including the entire sublimation. You could just subtract precip and runoff (which you normally prescribe) and use that as total evapo-sublimation.
>
> 'SIhl    ‘ is the latent heat flux (including evap and sublimation) into the ocean, could also be converted to freshwater flux
>
> SIfwSubl, SIacSubl, SIrsSub are potential, actual and residual fw flux by sublimation.
> SIfwSubl tells you how much latent heat there was available for sublimation (in units of fw).
> SIacSubl is the actual sublimation flux that enters SIhl
> SIrsSubl is what’s left of SIfwSub, after all of the snow and ice have been sublimated away. This is then used to evaporate from the now exposed ocean.
>
> SIatmFW contails SIfwSubl (so sublimation of snow, ice, and if necessary ocean) and
> SIatmFW-EXFprecip-EXFrunoff is the correct diagnostic (although I am not sure).
>
> (I hope that this is correct)
>
> Martin
>
> > On 23. Jul 2019, at 11:51, Naughten, Kaitlin A. <kaight at bas.ac.uk> wrote:
> >
> > Hello,
> >
> > I am trying to calculate evaporation in my domain, which contains both sea ice and open ocean. I can't figure out what the EXFevap diagnostic represents in sea ice covered regions. Is it the evaporation which actually happens (i.e. considering the sea ice) or the evaporation which would have happened if there was no sea ice (based on the SST below the ice)?
> >
> > Furthermore, is sublimation included in the evaporation fields, or is this completely separate? I see some sublimation diagnostics (SIfwSubl, SIacSubl, SIrsSubl) but am not sure if adding them to evaporation would be double-counting.
> >
> > Many thanks,
> > Kaitlin Naughten
> >
> > Dr Kaitlin Naughten
> > Ocean-Ice Modeller
> > British Antarctic Survey
> >
> >
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