[MITgcm-support] Inconsistent freshwater fluxes in/out of ocean+seaice

[Katherine Quinn]

Hi Martin,

I want to diagnose/calculate the total freshwater mass variations of the 
ocean plus seaice (includes ice and snow), not volume variations.
You are correct that SIatmFW - oceFWflx will give the flux into the 
seaice system (ice+snow), however that won't be the same at the mass 
from HEFF since that only includes the ice, not the snow.

More details on my calculations:
oceFWflx, SIatmFW both have units of kg/m^2/s, so a cumulative time 
integral will give the changes in the mass load (kg/m^2) and a spatial 
sum will give the total mass (kg). sIceLoad is mass load of ice+snow and 
has units of kg/m^2.
mass1 = sum_area{rA*maskInC*sIceLoad} +
         cumsum_time{sum_area{rA*maskInC*(-EmPmR)*deltaTclock}}
mass2 = cumsum_time{sum_area{rA*maskInC*SIatmFW*deltaTclock}}
Note: cumulative time integral includes all fluxes up to previous time step.

Actually, I have an idea where the mass inconsistency comes from.  In 
seaice_growth.F, sIceLoad is calculated from HEFF and HSNOW. Then 
*after* that EmPmR (which is -oceFWflx) and SIatmFW are adjusted if 
balanceEmPmR=.true.  I'm going to test turning on/off balanceEmPmR and 
see if my intuition is correct.

Any more ideas welcome.  thanks, Katy

> Date: Wed, 5 Feb 2014 16:13:31 +0100 From: Martin Losch
> <Martin.Losch at awi.de>
>
> Katy,
>
> I am not sure, if I understand what you want to diagnose, but as far
> as I know, oceFWflx is the freshwater flux into the ocean (positive
> downward), while SIatmFW tries to diagnose the freshwater flux out
> of the atmosphere. Since there can be sea ice between atmosphere and
> ocean, this fresh water does not necessarily enter the ocean
> directly. Don?t nail me down on the sign conventions, but I think
> that their SIatmFW - oceFWflx should give you the net freshwater
> flux into (out of?) the ice. That should be the same as the
> variations in ice volume (HEFF), but due to subteties in the
> timestepping and diagnostics output that may not always be the case.
>
> I think there is an example of what to diagnose to do it properly in
> verification/global_ocean.cs32x15/input.seaice/data.diagnostics, but
> I am also attaching a data.diagnostics and a matlab script that I
> once used to check the conservation properties (of seaice).
>
> Martin
>
>
> On Feb 4, 2014, at 9:57 PM, Katherine Quinn <kquinn at aer.com> wrote:
>
>> Hi all,
>>
>> I've noticed an inconsistency in the combined ocean and seaice
>> freshwater mass variations when calculated two different ways
>> (ignoring constant offsets).  The first method is sIceLoad (mass
>> load of seaice) plus the cumulative in time sum of oceFWflx (flux
>> in/out of ocean).  The second method is the cumulative in time sum
>> of SIatmFW (flux in/out of ocean+seaice).
>>
>> I've done these calculations using the diagnostic outputs of the
>> latest MIT ECCO run (version 4, rev 4, iteration 10).  The
>> seasonal cycle amplitudes and phases are off.  FYI, these are
>> spatial averages (i.e. total mass) so lateral advection is not an
>> issue.
>>
>> Any insights appreciated.
>>
>> cheers, Katy
>>
>> -- Katherine J. Quinn Atmospheric and Environmental Research voice:
>> 781-761-2234 131 Hartwell Avenue fax: 781-761-2299 Lexington, MA
>> 02421-3126 e-mail: kquinn at aer.com

-- 
Katherine J. Quinn
Atmospheric and Environmental Research         voice: 781-761-2234
131 Hartwell Avenue                              fax: 781-761-2299
Lexington, MA  02421-3126                     e-mail: kquinn at aer.com