[MITgcm-support] MITgcm-support Digest, Vol 267, Issue 7

Thu Oct 2 13:03:56 EDT 2025

Dear Jean-Michel,

Thank you very much for your detailed explanation—it is extremely helpful.

I understand now the distinction between aim_useFMsurfBC and aim_useMMsurfFc. Your clarification about which surface boundary-condition files are required (or replaced by prognostic variables) makes things much clearer.

Since in my case the SST input is a yearly averaged field with only a single record (constant in time), I will follow your suggestion as follows, so that the model will simply use the prescribed SST without repetition issues:
 &AIM_PARAMS
  aim_useFMsurfBC = .TRUE.,
  aim_useMMsurfFc = .FALSE.,
# aim_surfPotTemp = .TRUE.,
  aim_energPrecip = .TRUE.,
  aim_splitSIOsFx = .TRUE.,
  aim_LandFile   = 'landFrc.St36Ma_CLSD_CS32_revised.bin',
  aim_albFile    = 'Albedo.Ex10.bin',
# aim_vegFile    = '',
#
#  SST.Ex10.ocn_sym.bin: Original annual average SST data (shape: (32,192))
   aim_sstFile   = 'SST.Ex10.ocn_sym.bin',
# aim_lstFile    = '',
# aim_oiceFile   = '',
# aim_snowFile   = '',
# aim_swcFile    = '',
  aim_dragStrato = 2592000.,
  aim_clrSkyDiag = .TRUE.,
  aim_diagFreq   = 3110400000.,
  aim_select_pCO2 = 1,
  aim_fixed_pCO2  = 1000.E-6,
# atmpCO2init    = 800.E-6,
  aim_surfForc_NppCycle = 1,
 /

This really clarifies the setup I need—thank you again for taking the time to explain all these details.

I have one more question: instead of prescribing SST via data.aimphys (using aim_sstFile), would it also be valid to provide SST as a relaxation forcing in the data file, for example with:

 tauThetaClimRelax = 1.,
 thetaClimFile     = 'SST.Ex10.ocn_sym.bin',

Or is it preferable to always prescribe SST through data.aimphys?
(To be honest, I had previously run my simulation this way before your explanation.)

Best regards,
Fisher

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>  Today's Topics:
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>     1. Re: Questions on configuring AIM surface boundary conditions
>        (Jean-Michel Campin)
>  
>  
>  ----------------------------------------------------------------------
>  
>  Message: 1
>  Date: Tue, 23 Sep 2025 16:47:26 -0400
>  From: Jean-Michel Campin <jmc at mit.edu>
>  To: mitgcm-support at mitgcm.org
>  Subject: Re: [MITgcm-support] Questions on configuring AIM surface
>  	boundary conditions
>  Message-ID: <20250923204726.GA37914 at ocean.mit.edu>
>  Content-Type: text/plain; charset=us-ascii
>  
>  Hi Fisher,
>  
>  I will try to clarify few things:
>  > It seems that parameters like aim_useFMsurfBC and aim_useMMsurfFc control
>  > the use of boundary conditions, but I could not find detailed explanations
>  > for these flags. Could you clarify their meanings?
>  
>  The description (in AIM_PARAMS.h + repeated in STDOUT) is rather short:
>  C     aim_useFMsurfBC :: select surface B.C. from Franco Molteni
>  C     aim_useMMsurfFc :: select Monthly Mean surface forcing (e.g., NCEP)
>  These 2 logical parameters allows to select between a simplified monthly
>  mean forcing, that repeats itself over a year ( aim_useMMsurfFc=T, aim_useFMsurfBC=F)
>  and the original Speedy code (from Franco Molteni) surface forcing (aim_useFMsurfBC=T,
>  aim_useMMsurfFc=F, i.e., the default).
>  However, if you are using pkg/land (useLand=T, as in verification/aim.5l_cs/input/),
>  and assuming you don't turn off parts of the land code (keeping all 4 logical
>  parameters: land_calc_{grT,grW,snow,alb} set to True, i.e., the default),
>  many surface fields will be replaced by the land prognostics variables so that the
>  effective number of surface-BC fields to provide is reduced.
>  
>  And similarly, if you are using pkg/thsice (useThSIce=T, as in
>  verification/aim.5l_cs/input.thSI/), few more surface fields will be replaced
>  by pkg/thsice prognostics variables.
>  
>  > Also, should the boundary-condition data be provided as monthly values,
>  > or is another temporal format expected?
>  With aim_useMMsurfFc=T, monthly-mean values is the only option, with 1 file
>  per month (with 3 characters month suffix in the file name, see:
>  verification/aim.5l_LatLon/input settings).
>  
>  With aim_useFMsurfBC=T, all time records are in a single file. it still assumes
>  repeating forcing cycle, but since the repeating period is adjustable, if you provide
>  enough records to exceed the length of the simulation there will not be any repetition.
>  This is controlled by:
>  C     aim_surfForc_TimePeriod :: Length of forcing time period (e.g. 1 month)
>  C     aim_surfForc_NppCycle   :: Number of time period per Cycle (e.g. 12)
>  and the length of the repeating cycle is the product of the 2.
>  
>  To summarize, with aim_useFMsurfBC=T, the surface BC files you may provide are:
>   aim_LandFile : land fraction, loaded once (1 record) and kept fixed
>   aim_albFile  : surface albedo over land, loaded once (1 record) and kept fixed
>   aim_sstFile  : SST, multiple records
>  
>  Not used if useThSIce=T:
>   aim_oiceFile : seaice fraction, multiple records
>  
>  Not used if useLand=T:
>   aim_lstFile  : land surface Temp, multiple records
>   aim_swcFile  : soil water content, multiple records
>   aim_vegFile  : vegetation fraction, loaded once (1 record) and kept fixed
>   aim_snowFile: snow depth over land, multiple records
>  Note: some might still be used as Land default initial condition (at startTime=0,
>       myIter=nIter0=0), something I have not checked.
>  
>  Regarding this:
>  > If I want to prescribe only SST from data while letting the model internally
>  > compute albedo, land-surface temperature, Sea Ice fraction, and other variables,
>  > what is the correct way to set this up?
>  It seems to me that you will need both useLand=T and useThSIce=T (in data.pkg),
>  is this the case ?
>  And you still need to provide land-albedo (that will be modified where there
>  is snow).
>  
>  Finally, regarding the attached "data.aimphys", what is the type of time-variation
>  you are targeting for the SST ?
>  If it's constant, I will just change aim_useFMsurfBC=F to True and set:
>   aim_surfForc_NppCycle=1
>  (not the most efficient way but should do the job).
>  
>  Cheers,
>  Jean-Michel
>  
>  On Thu, Sep 18, 2025 at 01:14:17PM +0000, ht wrote:
>  > Dear Community,
>  >
>  > I am currently trying to set up atmospheric and land-surface simulations using MITgcm, taking MITgcm/verification/aim.5l_cs as a reference but with my own topography and SST distribution.
>  >
>  > I have several questions regarding the configuration files, especially data.aim:
>  >
>  > -
>  >
>  > It seems that parameters like aim_useFMsurfBC and aim_useMMsurfFc control the use of boundary conditions, but I could not find detailed explanations for these flags. Could you clarify their meanings? Also, should the boundary-condition data be provided as monthly values, or is another temporal format expected?
>  >
>  > -
>  >
>  > If I want to prescribe only SST from data while letting the model internally compute albedo, land-surface temperature, Sea Ice fraction, and other variables, what is the correct way to set this up?
>  >
>  > -
>  >
>  > I have attached my current data.aimphys file. Could you please check whether it is correctly configured for this purpose or if modifications are needed?
>  >
>  > Thank you very much for your time and support.
>  >
>  > Best regards,
>  > Fisher
>  
>  
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>  End of MITgcm-support Digest, Vol 267, Issue 7
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