Calendar and dates

The GCM needs to know the current date since the forcings, declination of the sun, surface conditions, ozone and aerosol fields, depend on the date.

You can run the GCM with a real calendar or with a fictive calendar where years have a constant number of days, either 360 or 365. With a fictive calendar, everything happens as if the Earth moved a little bit faster along its orbit around the sun.

Choose the calendar as soon as you prepare surface conditions with the program ce0l. In the file run.def, use the parameter calend (character variable). For years with constant length, 360 or 365 days:

calend=earth_360d

or:

calend=earth_365d

For the real calendar (with leap-years), run ce0l for each year you want boundary conditions for:

calend=gregorian
anneeref=<the right year>

where anneeref is an integer variable in run.def.

When you create an initial state with program ce0l, you also choose the date of that state, using the variable anneeref and another integer variable in run.def, dayref, the day number in the year. Note that the date of the created initial state is not read from the date of the data in files ECDYN.nc or ECPHY.nc (see Initial state and surface conditions).

When the GCM runs, it normally reads the intial date of the run from the file start.nc, and writes the final date in the file restart.nc. So you do not change calend, nor anneeref, nor dayref when you continue a simulation, starting from the end of a previous simulation. anneeref and dayref serve as a reference point in time for a time-step counter in start.nc.

Restarting from an arbitrary date

You can set raz_date = 1 in run.def. Then the GCM uses dayref and anneeref in run.def for the initial date. This way, you can start the GCM at any date from any restart.nc and restartphy.nc. But watch out for the initial shock if there is a large seasonal difference between the original date of the restart files and the reset date. The shock will come from the discrepancy between the state of the atmosphere and the forcing conditions.

Do not forget to set raz_date back to 0 if you want to continue a simulation.