Temperature and salinity nudging at the boundaries

[zi-hua-liu]

May I ask how to set up the model configuration for nudging on the boundaries? There are 4 open boundaries in my model (regional mom6-sis2-cobaltV2) and only temperature and salinity fields on the 4 boundaries need to be nudged. Below is the corresponding setup for the OBCs, but I am not sure how to add the nudging data files to the boundaries. Any comments are appreciated.

OBC_SEGMENT_001 = "I=N,J=0:N,FLATHER,ORLANSKI,NUDGED,ORLANSKI_TAN,NUDGED_TAN" !
OBC_SEGMENT_001_VELOCITY_NUDGING_TIMESCALES = 1.0, 360.0 ! [days] default = 0.0
! Timescales in days for nudging along a segment, for inflow, then outflow.
! Setting both to zero should behave like SIMPLE obcs for the baroclinic
! velocities.
OBC_SEGMENT_002 = "J=N,I=N:0,FLATHER,ORLANSKI,NUDGED,ORLANSKI_TAN,NUDGED_TAN" !
OBC_SEGMENT_002_VELOCITY_NUDGING_TIMESCALES = 1.0, 360.0 ! [days] default = 0.0
OBC_SEGMENT_003 = "I=0,J=N:0,FLATHER,ORLANSKI,NUDGED,ORLANSKI_TAN,NUDGED_TAN" !
OBC_SEGMENT_003_VELOCITY_NUDGING_TIMESCALES = 1.0, 360.0 ! [days] default = 0.0
OBC_SEGMENT_004 = "J=0,I=0:N,FLATHER,ORLANSKI,NUDGED,ORLANSKI_TAN,NUDGED_TAN" !
OBC_SEGMENT_004_VELOCITY_NUDGING_TIMESCALES = 1.0, 360.0 ! [days] default = 0.0

OBC_TRACER_RESERVOIR_LENGTH_SCALE_OUT = 3.0E+03 ! [m] default = 0.0
! An effective length scale for restoring the tracer concentration at the
! boundaries to externally imposed values when the flow is exiting the domain.
OBC_TRACER_RESERVOIR_LENGTH_SCALE_IN = 3.0E+03 ! [m] default = 0.0
! An effective length scale for restoring the tracer concentration at the
! boundaries to values from the interior when the flow is entering the domain.

Do I just need to enlarge the thickness of the boundaries (currently a single layer), or are there any other parameters that need to be added or modified? Thanks!

[yichengt900]

Hi @zi-hua-liu,

Thanks for bringing this up. There are several ways to do nudging in MOM6, and I think the one @charliestock mentioned is the SPONGE option in MOM6. In short, you will need to add the following runtime parameters to your MOM_override:

#override SPONGE = True
#override SPONGE_UV = False
#override SPONGE_DAMPING_FILE = "damping_full_t_90d.nc"
#override SPONGE_IDAMP_VAR = "Idamp"
#override SPONGE_STATE_FILE = "glorys_sponge_monthly_bnd_1990.nc"
#override SPONGE_PTEMP_VAR = "thetao"
#override SPONGE_SALT_VAR = "so"
#override INTERPOLATE_SPONGE_TIME_SPACE = True
#override SPONGE_DATA_ONGRID = True

where damping_full_t_90d.nc is a file with the sponge damping rates (normally we use strong nudging along the boundary and decrease to 0 in the interior domain), and glorys_sponge_monthly_bnd_1990.nc contains the state to damp toward. You can check these scripts to learn how to generate your own files for nudging. Feel free to let me know if you have any further questions.

[zi-hua-liu]

Thanks for your quick and detailed response! These are super helpful. It works.