Linear Bottom Drag & Horizontal Viscosity

[kkirk]

Hi,
I'm a novice using ROMS and have two questions. I apologize if these have been answered elsewhere.

1. I'm using a linear bottom drag and setting RDRG in my .in file. Is this bottom drag coefficient only applied to velocity, i.e. Tau,b,x = RDRG * u, or is the depth taken into account, i.e. Tau,b,x = RDRG * u / H, so that the bottom stress is stronger over shallower depths and weaker over deeper depths for a given velocity and drag coefficient? I've looked in Hedstrom, 2018 (Tech Manual) and Haidvogel et al., 2008, which mentions the drag coefficient methods implement formulas, but I can't find where those formulas are explicitly shown.

2. I'm looking at estuarine flow through an inlet and/or down a river that feeds out to the ocean. I'm playing with VISC2 and see that the velocities are steady for higher VISC2 coefficients (e.g. 5) and unsteady for smaller VISC2 (e.g. 0.01). Is there guidance in choosing an appropriate VISC2 coefficient for estuarine flow?

Thanks very much for your time and help.

[jcwarner]

1. Look in ROMS/Nonlinear/set_vbc.F

The bottom stress for a 2D application is
# if defined UV_LDRAG
!
! Set linear bottom stress.
!
DO j=Jstr,Jend
DO i=IstrU,Iend
bustr(i,j)=0.5_r8*(rdrag(i-1,j)+rdrag(i,j))* &
& ubar(i,j,krhs)
...
etc

and for 3D it is
# elif defined UV_LDRAG
!
! Set linear bottom stress.
!
DO j=Jstr,Jend
DO i=IstrU,Iend
bustr(i,j)=0.5_r8*(rdrag(i-1,j)+rdrag(i,j))* &
& u(i,j,1,nrhs)

so the stress does not take into account the /H, but it does take into account what velocity it uses.

2. suggest you try to keep the visc2 as low as possible, as it mixes the momentum and there is no limit to prevent over mixing.
we distribute an estuary test case that can be helpful. a value of visc2=5 seems rather high. how about 0.01, 0.1, something like that. The guidance would be to maintain stability (if needed), and still allow the flow to evolve as it wants to.

[kkirk]

This is great! Thanks so much for your quick and helpful reply!