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EAM and SCREAM uses the HOMME dycore package which contains the EAM v1 hydrostatic dycore (PREQX) and the proposed v2 nonhydrostatic dycore (THETA).  These dycores share much of the HOMME infrastructure and have several timesteps:  

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• With the default HV coefficient of 1e15 m^4/s for nu, nu_div, nu_q, nu_p and nu_s at NE30, and assuming this coefficient is scaled as dx^3 or dx^3.2 power under mesh refinement:
  • dt_vis = dt_vis_q = dt_dyn      
• In the PREQX dycore, we usually run with nu_div >> nu, requiring a much smaller dt_vis that must be tuned specially.  This should not be needed with the THETA dycore
• The TOM sponge layer uses a laplacian operator with a fixed (resolution independent) coefficient, nu_top.  This creates a severe CFL condition - if we double the resolution, we must decrease dt_vis_tom by 4x.    For a 72L configuration, hypervis_subcycle_tom (THETA) or hypervis_subcycle (PREQX) must be set so that:  
  • NE30:    dt_vis_tom <= 500     
  • NE60:    dt_vis_tom  <= 125   
  • NE120:   dt_vis_tom <= 31      
  • NE256:   dt_vis_tom <= 6.9      
  • NE512:   dt_vis_tom <= 1.7
  • NE1024:  dt_vis_tom <= 0.43 
• Using the nu_top CFL condition printed by the code, experiments suggest the operator is stable up to S=2.89, but unstable at S=3.38.  
• For the theta model we recommend (dependent on future testing) to take nu_top=2.5e5 up to NE30. For resolutions > NE30, choose nu_top at the CFL limit using S=1.9
• NOTE regarding RRM grids:  The HV coefficient is strongly resolution dependent, decreasing like dx^3 or dx^3.2.  For RRM grids we thus use a tensor hyperviscosity formulation that contains this scaling and applies it locally as the grid resolution changes.  Tensor hyperviscosity is enabled by setting hypervis_scaling>0  (typically 3 or 3.2).   For the tensor hyperviscosity, the coefficients (nu, nu_div, nu_q, nu_p, nu_s) are now specified in 1/s  units that are indepdent of resolution.  For hypervis_scaling=3.2, we currently recommend setting all coefficients to 8e-8.   Currently E3SM only uses tensor hyperviscosity for RRM grids, but we should consider using it for all grids.  

Recommended timestep tuning for new grids

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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 coefficients need to be slightly reduced.  

Recommended settings (THETA)   

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