About minimum depth

[susonic]

Dear ROMS users,

I was wondering why we set minimum depth in ROMS ?

[arango]

Look at the governing equations and think about its spatial discretization in numerical models. What would happen if you have a grid cell with zero depth? What are the values of the vertical level thicknesses?

I gave you a good clue... As a homework, you can figure out the rest...

[susonic]

Thnank you for your reply, Dr. Arango. Good clue.

So, we have to set non-zero value as a minimum depth in order not to make the vertical level thickness zero.

-JH

[arango]

Yes, that's part of the answer. But you still have to check the governing equations. If the vertical thickness (Hz) is zero, what would happen to the vertical viscosity and diffusion terms? Then if you search for oHz in the code, you will notice that the internal array oHz is in several routines of the numerical kernel. Notice that oHz=1/Hz.

This is the kind of analysis that users with curiosity need to carry out to discover what are the assumptions that we need to make when coding the continuum governing equations in numerical models. This kind of analysis is needed to determine was is the appropriate minimum depth (hmin) for a particular application, for instance. This is going to control the vertical time step limitation (CFL) as a function of your specified number of vertical levels. In terrain-following coordinates, we need to have all the vertical levels to exist at the minimum depth.

So now you know why we cannot have a water column with a total thickness of zero

I added an entry to the FAQ web page of WikiROMS and hope that this issue is very clear and never discussed again in the forum.

[susonic]

Thank you for your detailed comment and adding an entry of WikiROMS.
That is kind of you to do so.
And I'm sure that users can get this fundamental information via the WikiROMS.

Regards,

-JH

[Lei_Shi]

Considering a wet-dry ROMS, We can have a grid covering a domain without worrying about minimum depth. Just let the water flow around following fluid mechanics. In that sense there is no minimum depth.

Of course, practically wet-dry scheme usually still has a minimum depth for a grid point to be considered a wet point.

[lizhicheng]

if defind WER_DRY, the Hmin how to be set?