How to set up outgoing nudging in nud netCDF

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d.kobashi
Posts: 55
Joined: Tue Sep 28, 2010 11:59 pm
Location: Texas A&M University

How to set up outgoing nudging in nud netCDF

#1 Post by d.kobashi » Thu Nov 07, 2019 4:13 pm

Hi,

I would like to set up spatially-varying nudging using netCDF (ocean_nud.nc) and would like to ask you how I can set up outgoing nudging (passive. outflow) when I activate RudNud in ocean.in.

I checked d_nudgcoef.m and in d_nudgcoef.m,

Code: Select all

% In the North Atlantic DAMEE_4 application the nudging is done in the
% southern and northern domain edges over a 8-point linearly tapered
% nudging scales of 5 to 60 days.

 inner = 1/60;                        % 60 days at interior limit
 outer = 1/5;                         %  5 days at boundary
 width = 8;                           %  8 points
So my understanding is that "outer" is incoming nudging (active) along open boundaries and linearly tapered to interior grids (interior parameter) in 8 grids.

My question is how I can set up outgoing nudging (passive). I created a nudging netcdf file, but I thought that's only for incoming nudging.

If I would like to set up 1 day incoming nudging at the open boundary and tapered in 6 grids to zero (in inverse time scale. i.e. no nudging beyond 6 grids) and outgoing nudging at 60 days, where can I specify outgoing nudging?

It used to be TNUDG, M3NUDG, OBCFAC to set up nudging. e.g. if I want to set up 180 days outgoing nudging (passive) and 1 day incoming nudging (active), I can choose TNUDG=M3NUDG=180 and OBCFAC=180.

Of course, I am trying to set up spatially-varying nudging so I wouldn't use them. I used to modify ana_nudgcoef.h to set up nudging analytically, but now I am trying to do that by creating ocean_nud.nc as recommended by Hernan (https://www.myroms.org/projects/src/ticket/627)

Thanks in advance.

-DJ

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kate
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Location: IMS/UAF, USA

Re: How to set up outgoing nudging in nud netCDF

#2 Post by kate » Thu Nov 07, 2019 6:14 pm

The answer to this question is in the source code, say in t3dbc_im.F. It looks like it is getting the outgoing value from the nudging field you have specified and the incoming value is obcfac times that. Remember that it has been scaled to be an inverse timescale before being used, so an obcfac of 120 makes it a much shorter timescale for incoming nudging.

d.kobashi
Posts: 55
Joined: Tue Sep 28, 2010 11:59 pm
Location: Texas A&M University

Re: How to set up outgoing nudging in nud netCDF

#3 Post by d.kobashi » Thu Nov 07, 2019 6:29 pm

Thanks, Kate. Yes I actually checked the source code already and my reply did not go through for unknown reason. Indeed, in u3dbc_im.F, there are lines,

Code: Select all

                IF (LBC(iwest,isUvel,ng)%nudging) THEN
                  IF (LnudgeM3CLM(ng)) THEN
                    obc_out=0.5_r8*                                     &
     &                      (CLIMA(ng)%M3nudgcof(Istr-1,j,k)+           &
     &                       CLIMA(ng)%M3nudgcof(Istr  ,j,k))
                    obc_in =obcfac(ng)*obc_out
So, as you said, M3_NUDGCOEF and other variables are for outgoing nudging. . And I need to use OBCFAC to set up incoming nudging. I simply assumed that nudging is incoming because many users care about incoming more than outgoing for nesting. But when I tried to use d_nudcoef.m, I got 1 (1/day) along the open boundaries. So I assume that's for incoming nudging, but I guess the following note in the matlab script must have been for outgoing nudging.... my bad.

Code: Select all

% In the North Atlantic DAMEE_4 application the nudging is done in the
% southern and northern domain edges over a 8-point linearly tapered
% nudging scales of 5 to 60 days.

 inner = 1/60;                        % 60 days at interior limit
 outer = 1/5;                         %  5 days at boundary
 width = 8;                           %  8 points
I recently checked CROCO and it seems to be much simpler to set up nudging compared to ROMS although ROMS is more customizable (i.e. We can change nudging spatially. it can depend on depth as well). I sometimes like a simpler and straight-forward way...

Anyway, thanks again for your help, Kate.

Cheers,

-DJ

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