[MITgcm-support] Step change in OBC

[Olivier Marchal]

Hello MITgcm-ers,

I am trying to set up a step change in the OBC (at the southern 
boundary) for a flow problem with no surface forcing. A sponge layer is 
prescribed at the southern boundary in order to absorb an outflow at 
that boundary. An inflow is prescribed along the same boundary but at 
another location than the outflow. The desired step change is the 
magnitude of the inflow (from 0 to 50 Sv).

Here is the current set-up:

1) code/packages.conf includes gfd, cal, exf, and obcs

2) /input/data.pkg:

&PACKAGES
useCAL=.TRUE.,
useEXF=.TRUE.,
useOBCS=.TRUE.,
&

3) input/data.cal:

&CAL_NML
The Calendar='model',
startDate_1=0000101,
startDate_2=000000,
&

4) input/data.exf:

&EXF_NML_01
&
&EXF_NML_02
&
&EXF_NML_03
&
&EXF_NML_OBCS
obcsSstartdate1=0000101,
obcsSstartdate2=000000,
obcsSperiod=432000., (i.e., step change desired after 5 days of integration)
&

5) input/data.obcs:

&OBCS_PARM01
...
useOBCSprescribe=.TRUE.,
useOBCSsponge=.TRUE.,
OBCSsponge_S=.TRUE.,
...
OBCSuFile = 'uSouth.bin',
OBCSvFile = 'vSouth.bin',
OBCStFile = 'tSouth.bin',
OBCSsFile = 'sSouth.bin'
&

&OBCS_PARM03
spongeThickness=10,
Urelaxobcsinner=43200.,
Urelaxobcsbound=8640.,
Vrelaxobcsinner=43200.,
Vrelaxobcsbound=8640.,
&

The .bin files in input/data.obcs contain fields for two time slices 
produced from input/gendata.m.

Here is the issue: the run blows up after 1 iteration ("SOLUTION IS 
HEADING OUT OF BOUNDS"). Use of a ridiculously small time step (deltaT 
well below CFL criterion) yields a similar result. If no step change is 
prescribed (inflow fixed at 50 Sv from the start of the run), then the 
run goes fine.

Could anyone help me understand this result?

Thanks for any pointers,

Olivier.




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