HowTo: Run at different horizontal resolutions

What one needs to know to be able to run at a different model resolution

This page contains some information and tips to help you setup a successful run at a different (horizontal) resolution

1. You will need input and forcing files at the sought resolution
2. You will need to adapt the (dynamics) time step
   As explained in the LMDZ Tutorial courses "dynamics: ..." (e.g. here) there is a simple rule of thumb to know to what extent you need to change the "day_step" (number of dynamical steps per day):
   day_step(max(iim,jjm)=N) ~ day_step(max(iim,jjm)=M) * N/M
   Check what the reference values (in the gcm.def.* files in the LMDZ/Deflist directory) are for a given N are to evaluate what is needed for M
   Example: currently, in gcm.def_144x142 i.e. for iim=144,jjm=142 , day_step=672 so to run at twice that resolution you should halve the time steps and thus use day_step=1944 (and adapt iphysiq accordingly, i.e. double it here)
3. You might need to change the dissipation constants
   This is more tricky and less linear than for the time steps; in practice some trial and error will be needed but overall you will need to increase dissipation (i.e. lower le teta* constants)
4. At present, physics need to be run at a timestep of 15 minutes so you need to adjust iphysiq according to the formula
   iphysiq = 900 * day_step / 86400

As guidelines, and starting values, you could use or extrapolate from those given in the table below:

Resolution	day_step	tetagdiv	tetagrot	tetatemp	iphysiq
96 x 95 (CMIP5)	480 ( = 180 s)	5400.	5400.	5400.	5 ( = 15 min)
144 x 142 (CMIP6)	672 ( ~ 128 s)	3600.	21600.	10800.	7 ( = 15 min)
256 x 256 (QUEST)	1344 (~ 64 s)	1200.	7800.	3800.	14 ( = 15 min)

If you have values for other resolutions that work well for you, don't hesitate to contact us so that we can include them in the table