CROCO-NH relaxes the hydrostatic assumption in CROCO

CROCO is the regional

circulation model formerly known as ROMS/AGRIF. It does relax the H assumption by integrating in time the vertical component of the velocity and by computing the pressure via a Poisson equation. The Poisson equation is solved with a multigrid solver. The solver is parallelized, even for large number of cores. It handles sigma coordinates and realistic topographies.

1. The third component of the velocity is prognosed. Because CROCO momentum equation is written in flux form, it is easier to prognose the 'vertical flux' (as opposed to the strict vertical velocity). Changes are in the OCEAN folder
2. The multigrid solver has been developped specifically. It sits in the NHMG folder. It can be compiled and tested as a standalone. See the NHMG/README and use the NHMG/configure.

• The multigrid has many tunable parameters. The user may access them via the NHMG/nhmg_namlist
  • the user is advised to monitor the performances of the multigrid (e.g. output_freq = 10 in the nhmg_namelist).
  • set surface_neumann = .true. in nhmg_namelist this will set a Neuman boundary condition (BC) type for pressure at surface. Note that the proper BC is of Dirichlet type but activating it imposes to shorten the time step, thus making the NH code much more computationally expensive.

• A NH experiment should take roughly 120% more time than a

  Hydrostatic version

• A NH experiment can be run with the exact same time step than the Hydro version
• The multigrid imposes some constraint on the grid size and the number of cores that can be used
  • grid dimensions (nx, ny, nz) should be in the form 2*p or 32**p
  • number of cores in x and y should be in the form 2**p
• The NH physics is activated with the cpp key NHMG (see Run/cppdefs.h)

A workshop on Non-Hydrostatic​ ​ Ocean​ ​ Modeling is organized in Brest (France) in October 2018.