Physics Timestep

Where and When to Change Time Steps:

When the model crashes due to large, temporary instabilities, one method to overcome the problem is to change the time step. This is typically done in either the atmosphere or ocean components.

• For atmosphere and land component (CAM/CLM): change ATM_NCPL in env_run.xml.
  

• For ocean (POP): change dt_count in POP namelist.

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• For the atmosphere component (CAM), the physics time step is set by ATM_NCPL and NCPL_BASE_PERIODin env_run.xml. NCPL_BASE_PERIOD specifies base period associated with coupling frequency, the default value is “day”. The ATM_NCPL variable specifies the number of coupling intervals per NCPL_BASE_PERIOD between the atmosphere/land and the coupled system. Based on these two variables, the scripts will automatically compute the time step for the atmosphere and land and populate the namelist files accordingly. For example, if NCPL_BASE_PERIOD is set to “day”, and ATM_NCPL is 48, then the timestep is 1800 seconds.

• For the land component (CLM), the physics time step is the same as the CAM time step; this is automatically set with the CAM time step via ATM_NCPL. You cannot set this separately.

• For the ocean component (POP2), the physics time step is changed in the user_nl_pop file and is based on $OCN_NCPL (found in env_run.xml), dt_count , and dt_option. The default dt_option is “steps_per_day”.

• For the sea ice component (CICE), the physics time step is set by the coupling interval variable $ICE_NCPL found in env_run.xml.