A. The timestep depends on the horizontal and vertical resolution, the type of grid (full, linear, cubic reduced Gaussian), whether Eulerian or semi-Lagrangian (SL) transport is used and whether the 2 or 3 time-level SL scheme is used. The recommended default is the 2 time-level SL scheme. For this configuration, timesteps that give a good combination of run-time and forecast are: T21 - T159 : 60mins; T255: 45mins; T399: 20mins; T511: 15mins; T1279 10mins.
If the 2 time-level scheme is not used, these timesteps should be halved. If Eulerian, not semi-Lagrangian advection is used, these timesteps should be a quarter. For more details, see the code in the routine sudyn.F90 which has approximate values for the timestep (n.b. the NAMDYN namelist variable TSTEP is in units of secs).
Note the model will calculate the optimum timestep for its particular configuration. This value is written to the NODE_001.01 output file. Look for the line:
PTSTEP_OPT (optimal timestep computed in SUTSTEP) 0.36000000E+04
to see a suggested value in seconds. However, please use these values as a rough guide only. This code has been removed in later versions of IFS.
With longer timesteps, it is possible to get errors in the NODE.001.01 log file regarding trajectories going underground. Whilst some warnings may appear with the recommended timestep values above, the timestep should be reduced with excessive numbers of warnings.