[MITgcm-support] Forcing a surface fresh water flux with EmPmRFile

Martin Losch Martin.Losch at awi.de
Tue Jan 12 15:39:59 EST 2021

Hi Marion,

you need to use the nonlinear free surface with this, otherwise you always use a virtual salt flux that can lead to negative salinities, if you are not careful. With the nonlinear free surface, fresh water is added as expected and salinity cannot go below zero (well, not because of the freshwater input anyway).

define NONLIN_FRSURF in CPP_OPTIONS.h and turn it on with nonLinFreeSurf=4 in data. See documentation (section 2) and some of the verification experiments for more details.


> On 12. Jan 2021, at 21:28, Marion Alberty <marion.s.alberty at gmail.com> wrote:
> Hello all,
> I am trying to run a channel model configuration where I have a cold, freshwater input over half the domain that is constant in time. To do this I have turned on EmPmR with 
> EmPmRFile       = 'M_dT5e+00C.bin’,
> The file is a 2-D array of flux values in m/s where values<=0 to indicate the addition of freshwater to the system.
> I also want my surface fresh water flux to have a temperature of 0 deg C so I include this line in my data file:
> temp_EvPrRn=0.,
> Over the first 5 days, salinity in the surface layer looks ok, with values of 0 psu, however salinity in the top grid cells become increasingly negative after this point. At the same time, the temperature of the surface layer in the region of forcing drops below 0 deg C after a day or so becoming unrealistically cold (-15 deg C) by day 5. 
> If any one can offer some guidance for where I’ve gone wrong, I would greatly appreciate it. The full set up for the simulation can be found here:
> https://github.com/marionalberty/MITgcm/tree/front_relax_SODA2/verification/front_surfFlux_highres
> Thanks in advance for any assistance,
> Marion Alberty (she/her), Ph.D.
> Postdoctoral Research Associate
> Princeton University
> NOAA Geophysical Fluid Dynamics Laboratory
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> MITgcm-support at mitgcm.org
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