4.2 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 MPDATA advection scheme is

$$\Delta t < \frac{min(\Delta x, \Delta y)}{4 max(u, v)} .$$ (1)

Maximum values for dt for the two standard CCSM POP grids, assuming $max(u,v) = 0.5 m/s$, are shown in Table 4. The default timestep for CSIM is one hour.

Table 4: Maximum values for ice model timestep dt

Grid	$min(\Delta x, \Delta y)$	$max \Delta t$
gx3v5	28845.9 m	4.0 hr
gx1v3	8558.2 m	1.2 hr

The calculation of the ice velocities is subcycled ndte times per timestep so that the elastic waves are damped before the next timestep. The subcycling timestep is calculated as dte = dt/ndte and must be sufficiently smaller than the damping timescale T, which needs to be sufficiently shorter than dt

$$dte < T < dt$$ (2)

This relationship is discussed in (Hunke(2001)); also see (Hunke and Lipscomb(2002)), section 4.4. The best ratio for [dte : T : dt] is [1 : 40 : 120]. Typical combinations of dt and ndte are (3600., 120), (7200., 240) (10800., 120).

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 (ndyn_dt) 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.

Table 5: Namelist Variables for Writing Output

Varible	Type	Default	Description
diagfreq	Integer	24	Frequency of diagnostics written (min, max, hemispheric sums) to standard output
			24 =$>$ writes once every 24 timesteps
			1 =$>$ diagnostics written each timestep
			0 =$>$ no diagnostics written
histfreq	Character	'm'	Frequency of output written to history file
			'D' or 'd' writes daily data
			'W' or 'w' writes weekly data
			'M' or 'm' writes monthly data
			'Y' or 'y' writes yearly data
			'1' writes every timestep
			'0' no history data is written
hist_avg	Logical	.true.	If true, averaged history information is written out at a frequency determined by histfreq. If false, instantaneous values rather than time-averages are written.
dumpfreq	Character	'y'	Frequency restart data is written to file
			'D' or 'd' writes restart every dumpfreq_n days
			'M' or 'm' writes restart every dumpfreq_n months
			'Y' or 'y' writes restart every dumpfreq_n years
			'0' no restart data is written
dumpfreq_n	Integer	1	Frequency restart data is written to file
print_points	Logical	.false.	print diagnostic data for two grid points