Changing the timestep

dt is the timestep in seconds for the ice model thermodynamics. The thermodynamics component is stable but not necessarily accurate for any value of the timestep. The value chosen for dt depends on the stability of the transport and the grid resolution. A conservative estimate of dt for the transport using the upwind advection scheme is:

Δt < (1) 4 max(u, v)

Maximum values for dt for the two standard CESM POP grids, assuming max(u, v) = 0.5m/s , are shown in Table 2. The default timestep for CICE is 30 minutes, which must be equal to the coupling interval set in the CESM configuration files.

Table 2: Maximum values for ice model timestep dt

Grid	min(Δx, Δy)	maxΔt
gx3v5	28845.9 m	4.0 hr
gx1v3	8558.2 m	1.2 hr

Occasionally, ice velocities are calculated that are larger than what is assumed when the model timestep is chosen. This causes a CFL violation in the transport scheme. A namelist option was added (xndt_dyn) to subcycle the dynamics to get through these instabilities that arise during long integrations. The default value for this variable is one, and is typically increased to two when the ice model reaches an instability. The value in the namelist should be returned to one by the user when the model integrates past that point.