Land boundary for ROMS online floats

[jxiong]

Dear ROMS users,

I wondered if anyone knows how ROMS online floats treat the land boundary? I got the particle trajectory by activating FLOATS (please see attached figure) but couldn't figure out how exactly the particle interacts with the land boundary from the source code. Is it reflective or some other rules?

Thanks in advance for the kind help.
Best,
Jilian

[wilkin]

The 4th order predictor corrector scheme for ROMS online floats interpolates the gridded velocity field to each float's position within a cell and integrates over the model time step DT in such a way that as a float approaches a land boundary the velocity normal to the boundary tends zero (by virtue of the boundary condition) so the float never quite bumps into the coast. It can freely slip along parallel to the coast (free-slip or no slip would not matter for coast interactions, only the speed).

Since the floats are integrated on every time step, it's not possible for a float to cross more than one cell in one DT because that would imply the velocity violates CFL stability. So the local interpolation within a cell, and high order time stepping, is sufficient to prevent float/coast interactions. I can't recall ever seeing floats get stuck.

[jxiong]

Thanks very much, Dr. Wilkin!