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Time step and coupler frequency control
Each model can set its internal time step(s) through its Fortran namelist (or MPAS stream). For example "dtime" in the atmosphere might be set to 1800 seconds (30 minutes).
How often each component sends/receives data to/from the coupler is first determined by the NCPL_BASE_PERIOD. This is almost always 1 day and is set in env_run.xml.
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In a fully coupled case, the NCPL_ATM =48. With a base period of 1 day, the atmosphere communicates with the coupler very 30 simulated minutes. When dtime The longest time step for the atmosphere (dtime) is also set to 30 minutes (typical for ne30 cases), the the atmosphere communicates with the coupler every atmosphere time step.Making sure stable combinations of timesteps and coupler frequency is available out-of-the-box is up to the model developer. For example, the dtime for ne120 cases is set to 900 seconds (15 minutes) in https://github.com/E3SM-Project/E3SM/blob/master/components/cam/bld/namelist_files/namelist_defaults_cam.xml and the . (The "dtime" variable in the atmosphere namelist is ignored). The atmosphere may (and often does) subcycle parts of its solve within that 30 minutes.
As resolution in the atmosphere increases, the coupler frequency increases. For ne120, the coupler frequency is set to 96 times per day (15 minutes). The atmosphere's time step is also then 15 minutes.
The driver contains code for time-averaging the data to be sent to a component if the coupler frequencies don't match. For example if NCPL_OCN is set to 3 hours while the NCPL_ATM is 1 hour, the coupler will accumulate the atmosphere ocean fluxes calculated every time the atmosphere talks to the coupler and send the ocean the average. Not all component pairs can have different frequencies. The driver assumes the atmosphere, land and sea-ice all have the same coupler frequency.
In the current implementation, the coupling period must be identical for the atmosphere, sea ice, and land components. The ocean coupling period can be the same or greater. The runoff coupling period should be between or the same as the land and ocean coupling period. All coupling periods must be multiple integers of the smallest coupling period and will evenly divide the NCPL_BASE_PERIOD.
Flux calculation
The atmosphere-ocean fluxes are unique in being calculated in the coupler. This was done because for a long time the ocean only talked to the coupler once a day but the atmosphere needed fluxes that changed with the changing lower-atmosphere-level properties. Also the fluxes are calculated on the high-resolution (ocean) grid. So in the coupler SST was held fixed and new heat and momentum fluxes would be calculated as new atmosphere properties were sent to the coupler (and interpolated to the ocean grid). The fluxes are interpolated back to the atmosphere grid and sent to the atmosphere. Meanwhile, the fluxes (on the ocean grid) are summed and then averaged before sending to the ocean.
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