# Ocean Modeling Discussion

ROMS/TOMS

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 Post subject: 'nfast' and 'ndtfast' on Barotropic time stepPosted: Fri Nov 30, 2018 2:53 am

Joined: Mon Sep 24, 2018 7:40 pm
Posts: 9
Location: CSIRO - Hobart Site
Dear all,

nfast: Number of barotropic timesteps needed to compute time-averaged barotropic variables centered at time level n+1
ndtfast: Number of barotropic timesteps between each baroclinic timestep.

I am confused with the time-stepping in ROMS.

For example, I set dt = 150s, ndtfast = 30 in ocean.in, so I got dtfast = 5s, and I also got nfast = 42 computed by set_weights.F. In main3d.F, there are 42 barotropic time-steps for each dt, rather than 30 steps. SO my question is that are there 12 time steps(M*-M) double counted for each dt? For the next baroclinic step n+2, the barotropic step starts from m=M or from m=M* on the figure attached?

Could anyone give me a further explain? Thank you very much!

 Attachments: File comment: The split time stepping used in the model. The split time stepping used in the model.png [ 15.72 KiB | Viewed 586 times ]
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 Post subject: Re: 'nfast' and 'ndtfast' on Barotropic time stepPosted: Fri Nov 30, 2018 11:32 am

Joined: Mon Apr 28, 2003 5:44 pm
Posts: 485
Location: Rutgers University
https://www.myroms.org/wiki/Numerical_S ... g_Overview
and the Shchepetkin papers.

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John Wilkin: DMCS Rutgers University
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 Post subject: Re: 'nfast' and 'ndtfast' on Barotropic time stepPosted: Sun Dec 02, 2018 5:46 am

Joined: Mon Sep 24, 2018 7:40 pm
Posts: 9
Location: CSIRO - Hobart Site
wilkin wrote:
https://www.myroms.org/wiki/Numerical_S ... g_Overview
and the Shchepetkin papers.

Hi John,

I read the explanation before but it was complex for me. Now I think I got the main idea roughly:

There are M* barotropic steps for each baroclinic step, and the fast-time averages among these M* steps are transmitted to the baroclinic mode, as well as used as the initial values for barotropic mode during the next baroclinic step.

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 Post subject: Re: 'nfast' and 'ndtfast' on Barotropic time stepPosted: Sun Dec 02, 2018 4:19 pm

Joined: Wed Feb 26, 2003 4:41 pm
Posts: 1075
Location: IMCS, Rutgers University
Nope, there are actually M* barotropic timesteps The reason for it is that the cosine-square shape filter needs to provide time-averaged values of ubar, vbar, and zeta centered at the n+1 baroclinic timestep, which gives us second-order temporal accuracy for time-averaged barotropic motions. You may check routine set_weights.F to see how the time weights are computed. The above diagram is not clear enough.

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 Post subject: Re: 'nfast' and 'ndtfast' on Barotropic time stepPosted: Sun Dec 02, 2018 10:26 pm

Joined: Mon Sep 24, 2018 7:40 pm
Posts: 9
Location: CSIRO - Hobart Site
arango wrote:
Nope, there are actually M* barotropic timesteps The reason for it is that the cosine-square shape filter needs to provide time-averaged values of ubar, vbar, and zeta centered at the n+1 baroclinic timestep, which gives us second-order temporal accuracy for time-averaged barotropic motions.

Hi Hernan,

Thank you very much!

So the time-averaged values of ubar, vbar, and zeta you mentioned above will be used as initial value for the barotropic modes from m=M to m=M+M* during next baroclinic step n+2, right?

If I am wrong, where does the initial value come from during the next baroclinic step n+2?

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 Post subject: Re: 'nfast' and 'ndtfast' on Barotropic time stepPosted: Mon Dec 03, 2018 2:14 am

Joined: Wed Feb 26, 2003 4:41 pm
Posts: 1075
Location: IMCS, Rutgers University
Again, Nope The governing equations are timestep from n (right-hand-side terms) to n+1 (left-hand-side term: time rate of change). The vertically-integrated equations are integrated with smaller timestep to resolve fast dynamics due to gravity wave phenomena. For 3D total momentum coupling, the ubar, vbar, and zeta need to be time-averaged at n+1. So we need to timestep the vertically integrated equations beyond the time associated with n+1 so the time-averaged quantities are centered exactly at n+1. This is achieved with the cosine-squared filter so we don't need to go all the way to n+2 to achive such average. It is cleaver and efficient Maybe you need read the literature of split-explicit timestepping or find someone at your intitution to explain it to you.

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