## Bottom stress in ROMS

Frequently Asked Questions about ROMS usage

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ljzhong
Posts: 14
Joined: Tue Nov 25, 2003 3:36 pm
Location: CSIRO

### Bottom stress in ROMS

Hernan,

I am a little confused with the bottom stress calculation in ROMS. There are 3 ways for this calculation using either linear drag coefficient, quadratic bottom drag coefficient, or logarithmic formulation via bottom roughtness. Only one method should be chosen. I find bottom stress is calculated in two places. One is in subroutine set_vbc.F, where bottom stress is proportional to the velocity at the lowest grid near the bottom. And antother is in subroutine step2d.F. Here bottom stress is proportional to the depth integrated velocity, which is true if only barotropic mode is simulated, and obviously it is different from that calculated in set_vbc.F in a 3D simulation. Bottom stress should be the same in both barotropic mode and baroclinic mode in a 3D simulation. Therefore it makes sense to me only in these two situations.

1) Only 2D not 3D simulation is done. In this case, set_vbc.F is not called. bottom stress is only calculated in step2d.F.

2) Choose logarithmic formulation for 3D simulation, then bottom stress is calculated only in set_vbc.F, since step2d.F does not compute bottom stress when linear or quadratic drag coefficient is zero.

If we simulate 3D circulation and choose linear or quadratic bottom drag coefficient instead of bottom roughness, we will run into this situation that bottom stress is different in barotropic and baroclinic mode.

I may misunderstand the code. If I am wrong, please correct me.

Liejun

arango
Site Admin
Posts: 1102
Joined: Wed Feb 26, 2003 4:41 pm
Location: IMCS, Rutgers University
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Hi Liejun,

Yes, there are several ways to add bottom stress in ROMS:

Code: Select all

``````         1) Linear          RDRG > 0,  RDRG2 = 0,   Zob = 0
2) quadratic       RDRG = 0,  RDRG2 > 0,   Zob = 0
3) law of wall     RDRG = 0,  RDRG2 = 0,   Zob > 0
4) bottom boundary layer model
``````
Notice that in schemes 1 to 3, only one value is greater than zero. This is a must in order to have the appropriate code executed.

The logarithmic law of the wall is only apply to 3D configurations. It does not make sense, I think, to apply this to shallow-water applications neither to barotropic components "ubar" and "vbar". It is only applied to bottom baroclinic velocities!!! In 3D applications, the logarithmic bottom stress or other BBL formulations enter in the 2D equations (step2d) in the vertically integrated residual terms "rufrc" and "rvfrc".

Do this makes sense?

Good luck,

Hernan G. Arango
arango@imcs.rutgers.edu

debcox
Posts: 15
Joined: Tue Mar 09, 2004 3:21 pm
Location: University of New South Wales, Australia

### Bottom Velocities

Hi,

In a 3D application with quadratic bottom friction what do the velocities at the lowest level in the model represent?

Are they a shear velocity (u*) or the velocity at the top of an assumed constant stress bottom boundary layer or something different?

cheers,
Deborah

m.hadfield
Posts: 520
Joined: Tue Jul 01, 2003 4:12 am
Location: NIWA
If we simulate 3D circulation and choose linear or quadratic bottom drag coefficient instead of bottom roughness, we will run into this situation that bottom stress is different in barotropic and baroclinic mode.

I may misunderstand the code. If I am wrong, please correct me.
I think it does all work out correctly in the end, owing to the mysterious workings of the 2D/3D coupling. But I can't guarantee that. A while back I ran a test case with a doubly periodic domain initialized with a vertically uniform flow and confirmed that the deceleration of the flow matched the expected values. I could dig that up for you if you want to verify it.
In a 3D application with quadratic bottom friction what do the velocities at the lowest level in the model represent?

Are they a shear velocity (u*) or the velocity at the top of an assumed constant stress bottom boundary layer or something different?
Velocity at top of an assumed boundary layer.

crowley
Posts: 11
Joined: Wed Mar 26, 2003 4:14 pm
Location: DOI/MMS
For a 3D application, the bottom stress is computed in set_vbc.F and is not computed in step2d.F at all.

From step2d.F:

Code: Select all

``````# ifndef SOLVE3D
!
!-----------------------------------------------------------------------
!  Add in bottom stress.
!-----------------------------------------------------------------------
!

``````
It enters the barotropic mode through the magic of 2D/3D coupling (as does surface momentum stress).

This is one of the things that took me quite a while to completely figure out when I was working on the diagnostics.