Can we get the mass balance from the model output U,V,W?

[cathyyangfeng]

I recently confused about mass conservation problem in ROMS.

I use the model output U,V,W on each grid, and calculated Pu/Px + Pv/Py + Pw/Pz problems on designed model grid. I could not get a completely zero on each grid. Can we get an idealized mass conservation from this way in calculation?

[linzhenhua]

I recently confused about mass conservation problem in ROMS.

I use the model output U,V,W on each grid, and calculated Pu/Px + Pv/Py + Pw/Pz problems on designed model grid. I could not get a completely zero on each grid. Can we get an idealized mass conservation from this way in calculation?

I think the total volume should be conserved because the vertical velocity is calculated according to volume conservation rule(Pu/Px + Pv/Py + Pw/Pz=0). The non-zero value is possibly because you use different discretization scheme than that used in ROMS.

[cathyyangfeng]

Yes, I agree with what you said. I did not use the central finite difference as the model did.

I believe the model is volume conserved. But if we select a small region and use Int(velocity x dArea). why usually we could not get a totally zero?

Int(velocity x dArea) = Int(u*dydz) + Int(v*dxdz) + Int(w*dydz) at each boundary place.

If we interested in the mass flux goes in from different boundaries, what's going to be the right way for calculation?

[linzhenhua]

The volume for a specific region is not conserved because there is a free surface.