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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- dt_remap = dtime/nsplit
- dt_tracer = dt_remap/rsplit
- dt_dyn = dt_tracer/qsplit
- dt_vis = dt_dyn/hypervis_subcycle
- dt_vis_q = dt_tracer/hypervis_subcycle_q
- dt_vis_tom = dt_dyn / max(hypervis_subcycle, hypervis_subcycle_tom)
PR in progress: After it is merged, the theta dycore also New namelist variables: EAM now allows the user to directly set the main dycore timesteps with new namelist options "tstep, dt_remap_factor, dt_tracer_factor" replacing "nsplit, rsplit and qsplit":
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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= 40
- NE512: dt_dyn = 20
- NE1024 dt_dyn = 10
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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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Resolution | Timesteps | Namelist settings | Notes | Tested? |
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1 degree (NE30) | dtime=1800 (ATM_NCPL=72) dt_remap=600 | nu=1e15 se_tstep=300 | With dt_remap=1800, we see occasional (every 2-3 years) dp3d limiter activation, meaning that the model levels are approaching zero. This appears to be due to strong divergence above tropical cyclones created by one of the parameterizations. | HS+topo(72L): H and NH F-EAMv1-AQP1: H and NH FAV1C-L: H and NH |
1/4 degree (NE120) | dtime=900 (ATM_NCPL=144) dt_remap=150 | nu=1.5e13 se_tstep=75 | CFL estimates suggest: dt_vis_tom*nu_top <= 31*2.5e5 nu_top=2.5e5 would need | HS+topo(72L): H and NH (with dt_remap=75 and theta limiter to handle unphysical boundary layer) F-EAMv1-AQP1: H and NH, both 72 and 128 levels (1+ years) FC5AV1C-H01B: NH 72L runs several years |
12km (NE256) | dtime=480? dt_remap=80 | nu=1.6e12 | nu_tom=4e4 is running at the code's estimate of the CFL limit with S=1.9 | F-EAMv1-AQP1: NH-128L:
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6km (NE512) | dt_remap=40 dt_tracer=120 dt_dyn=dt_vis=dt_vis_q=20 dt_vis_tom=20 | nu=2e11 | CFL estimates suggest: dt_vis_tom*nu_top <= 1.7*2.5e5, nu_top=2.5e5 needs hypervis_subcycle_tom=13 | F-EAMv1-AQP1: |
3km (NE1024) | dt_remap=20 | nu=2.5e10 | CFL estimates suggest: dt_vis_tom*nu_top <= 0.43*2.5e5 with nu_top=2.5e5, |
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