EAM and SCREAM uses the HOMME dycore package.   HOMME has both Fortran and C++ versions with different options for the prognostic thermodynamic variable (preqx use temperature while theta-l uses potential temperature) and those in turn have some additional capabilities.

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Summary of HOMME capabilities available for different versions.

1. When building Kokkos for theta-l Fortran, the "cpu" target is used by default because running only the SL on the GPU would not give good performance.

New features that did not make the V2 cutoff:

These features will hopefully be turned on for use in V3 development,  after the V2 maintenance branch is created.

1. Physics mass adjustment due to phase change: apply locally to dp3d instead of to surface pressure. (reduces magnitude of artificial pressure work term)

2. Adopt tensorHV for NE30 grids. (it is the default for all other grids)

3. Turn on topographic improvements and switch to rougher topography

Using the THETA dycore in E3SM:     

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dt_dyn:  The CFL condition for dt_dyn with uniform resolutions is well understood and depends on the horizontal resolution and the speed of the Lamb wave (~340m/s).   This assumes the use of the KG5 RK method (tstep_type=5) for hydrostatic, and the related IMEX method (tstep_type=9 or 10) for NH.  

• NE30:   dt_dyn = 300    (probably stable up to 360)

• NE60:    dt_dyn = 150

• NE120:   dt_dyn = 75

• NE240:    dt_dyn = 40

• NE256:    dt_dyn <= 37.536

• NE512:    dt_dyn <= 18.75

• NE1024    dt_dyn <= 9.375

dt_tracer:   The tracer CFL condition is controlled by the maximum advective velocity (~200m/s) which usually occurs near the model top.  For Eulerian advection HOMME uses a RK3 SSP and the CFL also depends on the mesh spacing.  For SL tracers, the limit is governed by the size of the halo exchange.   

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For the THETA dycore, we need to determine the stable dt_remap, dt_dyn, dt_vis and dt_vis_tom.    We then take dt_vis_q=dt_vis and with SL tracers, we will assume that dt_tracer = 6*dt_dyn. 

1. Run 10 days with very small, inefficient timesteps.   Take dt_remap=dt_dyn and use small values for  dt_dyn, dt_vis and dt_vis_tom.  Create a new IC file (see INITHIST in EAM).   Use this new IC file for all future runs below

2. dt_vis_tom:  Use the CFL condition (take S=2) printed in atm.log to determine the largest safe value of dt_vis_tom.   In practice the code appears to be unstable around S=3.3, and it is good not to run right at the stability limit.     

3. dt_dyn:   Keeping all other timesteps below their known stable limits (which takes some care due to how they are all set) make a serious of runs increasing just dt_dyn until the run crashes.  May need to make relatively long runs (several months) to ensure stability.  

4. dt_vis:  Keeping all other timesteps fixed  find the largest stable value.  For THETA, with the recommended viscosity coefficients, dt_vis = dt should be stable.  For some RRM grids, dt_vis might be need to be slightly smaller due to mesh distortion.  When the viscous CFL is violated (dt_vis too large), the run usually crashes within a couple of steps.  

5. The procedure outlined above can find timesteps that are borderline unstable, but don’t blow up do to various dissipation mechanisms.  Hence it is a good idea to run 3 months, and look at the monthly mean OMEGA500 from the 3rd month. This field will be noisy, but there should not be any obvious grid artifacts.  Weak instabilities can be masked by the large transients in flow snapshots, so it best to look at time averages.  

6. dt_remap:  Using 2*dt_dyn is a conservative choice that is well tested up to ne240.   Decreasing dt_remap results in more frequent vertical remaps resulting in increased vertical dissipation.   If dt_remap is too large, the code may crash with negative layer thickness errors.  The code has a "dp3d" limiter that can prevent some of these crashes.  If this limiter is triggered (warnings in e3sm.log file), that can mean either dt_remap is too large of one of the CFL conditions has been violated and the code is unstable.       

During this tuning process, it is useful to compare the smallest ‘dx’ from the atmosphere log file to the smallest ‘dx’ from the global uniform high resolution run.  Use the ‘dx’ based on the singular values of Dinv, not the ‘dx’ based on element area. If the ‘dx’ for newmesh.g is 20% smaller than the value from the global uniform grid, it suggests the advective timesteps might need to be 20% smaller.  The code prints out CFL estimates that are rough approximation that can be used to check if you are in the correct ballpark.

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With RRM grids, the timesteps will be controlled by the highest resolution region.  So with an RRM grid with refinement down to NE120, the timesteps should be close to what we run on a uniform cubed-sphere NE120 grid.   The timesteps may need to be slightly smaller because of the deformed elements in the transition region.   With a hiqh quality RRM mesh ( Max Dinv-based element distortion metric <= 4, see Generate the Grid Mesh (Exodus) File for a new Regionally-Refined Grid) we can usually run with the expected dt_dyn and dt_tracer values, and only the viscosity timesteps need to be slightly reduced.  

Spreadsheet for looking at scaling with resolution of constant and tensor coefficient HV:

https://docs.google.com/spreadsheets/d/1LHTl2_A065pfdWC69OHmXvNL_v1484cXg7ZowhyEbPU/edit?usp=sharing

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WARNING on dtime: in the table below we give "dtime", the physics timestep. This is also a namelist option in EAM, but it will be ignored.  The only way to set dtime is to set ATM_NCPL = ( 24*60*60  / dtime) in env_run.XML

PREQX Default Settings (for reference)

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se_tstep=-1             ( timesteps set via se_nsplit, rsplit,  qsplit)
dt_remap_factor=-1
dt_tracer_factor=-1

Documentation on parameter tuning

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• SE Atmosphere Grid Overview (EAM & CAM)

• Plotting data on SE native grid

• KE Spectra - best practices

• Transition to TempestRemap for Atmosphere grids