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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- NE30: dt_dyn = 300 (probably stable up to 360)
- NE60: dt_dyn = 150
- NE120: dt_dyn = 75
- NE240: dt_dyn = 40
- NE256: dt_dyn= 4037.5
- NE512: dt_dyn = 2018.75
- NE1024 dt_dyn = 109.375
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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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.
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_tracer=1800 | nu=1e15 se_tstep=300 dt_tracer_factor = 6 | 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 dt_tracer_factor = 6 | 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 yearsruns several years. Occasional problems near coastlines - considering ( ) different topography, reducing dtime, increasing HV |
12km (NE256) | dtime=600 (ATM_NCPL=144) dt_tracer=300 | nu=1.6e12 dt_tracer_factor=8 | nu_tom=4e4 is running at the code's estimate of the CFL limit with S=1.9 | F-EAMv1-AQP1:
FC5AV1C-H01B: NH 128L run for several months dt=37.5/75/300/600. |
6km (NE512) | dtime=300 (ATM_NCPL=288) dt_tracer=150 | nu=2e11 dt_tracer_factor=8 | CFL estimates suggest: dt_vis_tom*nu_top <= 1.7*2.5e5, nu_top=2.5e5 needs hypervis_subcycle_tom=13 | F-EAMv1-AQP1:
FC5AV1C-H01B: NH 128L run for 1 day with dt=18.75/37.5/150/300, then NaNs in microphysics (not yet debugged) |
3km (NE1024) | dtime=150 (ATM_NCPL=576) dt_tracer=75 | nu=2.5e10 dt_tracer_factor=8 | CFL estimates suggest: dt_vis_tom*nu_top <= 0.43*2.5e5 with nu_top=2.5e5, | F-EAMv1-AQP1:
FC5AV1C-H01B: SCREAM physics: run for several days with dt=9.375/18.75/75/75. Frequent (daily) problems near coastlines - considering ( ) different topography, reducing dtime, increasing HV |
PREQX Default Settings (for reference)
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