Are there any conventions for nudging time scales setting?

[leon]

Dear all,
Sometimes, I'm not sure about the nudging time scales in the *.in file of ROMS. Actually, different settling about them will get different resluts. The current around the boundary may not flow out or fully radiated when the nudging time scales are set appropriately. For exmample, when a climatological model (360days per year and foce the model every month) is configured, is it ok if the parameters are set as follows:

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! Nudging/relaxation time scales, inverse scales will be computed
! internally, [1:Ngrids].

       TNUDG == 30.0d0  30.0d0             ! days
       ZNUDG == 30.0d0                     ! days
      M2NUDG == 30.0d0                     ! days
      M3NUDG == 30.0d0                     ! days

! Factor between passive (outflow) and active (inflow) open boundary
! conditions, [1:Ngrids]. If OBCFAC > 1, nudging on inflow is stronger
! than on outflow (recommended).

      OBCFAC == 10.0d0                     ! nondimensional

Or any conventions for the nudging parameters settling?

Thank you very much~

leon~

[kate]

I don't know if it's a convention, but I typically use:

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       TNUDG == 2*360.0d0                    ! days 
       ZNUDG == 360.0d0                      ! days 
      M2NUDG == 360.0d0                      ! days 
      M3NUDG == 360.0d0                      ! days 
      OBCFAC == 120.0d0                      ! nondimensional

which gives outgoing nudging at one year, incoming at 3 days.

[leon]

Hi,Kate. Thank you very much for your reply.

How did you determine the nudging coefficients? By expericences?

And do you use the coefficents for all the simulation time scales? For exmaple, the real time scale and the climatological time scale.

Thanks~

[kate]

It wasn't my experience so much as advice from Patrick Marchesiello.

I don't know what you mean by real time scale and climatological time scale. I use the Tnudg as described for the boundary conditions, which are coming from 5-day SODA files. I use a different value for nudging to sea surface salinity, hacked into set_vbc:

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          stflx(i,j,isalt)=stflx(i,j,isalt)*t(i,j,N(ng),nrhs,isalt)-    &
     &                     Tnudg(isalt,ng)*Hz(i,j,N(ng))*               &
#ifdef NEP5
!  60 days from Tnudg of 360 days
     &                     6.0_r8*                                      &
#endif
     &                     (t(i,j,N(ng),nrhs,isalt)-sss(i,j))

I guess I need to add the BERING to the #ifdef test.