Best Practices

Please note this section is a work in progress and aims to offer guidelines but, in general, your mileage may vary.

Large-Eddy Simulations

  • Advection Scheme

    • A WRF-like configuration is generally robust, but may under-resolve turbulence features.

      erf.dycore_horiz_adv_type  = "Upwind_5th"
erf.dycore_vert_adv_type   = "Upwind_3rd"
erf.dryscal_horiz_adv_type = "Upwind_5th"
erf.dryscal_vert_adv_type  = "Upwind_3rd"

    • Centered difference schemes will generally give some non-physical numerical noise, clearly visible in the free atmosphere, but may also better resolve turbulence features. With Centered_2nd, the simulation may remain numerically stable but without any upwinding or numerical diffusion, these results should be carefully interpreted.

    • For higher-order central differencing alone (i.e., without any added upwinding), at least 5% numerical diffusion should be included to stabilize the solution; this was tested with Centered_6th. Note that this does not necessarily kill the numerical noise and is only for numerical stability. These options are identical to WRF’s diff_6th_opt (default: off) and diff_6th_factor (default: 12%) options.

      erf.use_NumDiff  = true
erf.NumDiffCoeff = 0.05

  • Time Integration

    • Split timestepping offers some computational cost savings but still does not allow you to run with an incompressible/anelastic time-step size in general.

    • The acoustic CFL should conservatively be less than or equal to 0.5, with 4–6 fast timesteps (substeps) according to WRF best practices. If not explicitly specified (through erf.fixed_mri_dt_ratio or erf.fixed_fast_dt, the number of substeps in ERF is chosen based on the same algorithm as WRF. If the user follows the recommendation that dt [s] ~ 6 dx [km], then 4 substeps will be used, giving an effective CFL of approximately 0.5. This meets the stability criteria from Wicker & Skamarock 2002 that, for a 5th-order scheme, the CFL be less than 1.42/sqrt(3) = 0.820.

      erf.fixed_dt           = 0.06  # slow timestep

# These are equivalent and result in a fixed fast timestep size
#   if dx=10, speed of sound ~ 300 m/s
erf.fixed_mri_dt_ratio = 4
#   or
#erf.fixed_fast_dt      = 0.015  # ==> CFL~0.45
#   or, let ERF chose the fast timestep
#erf.substepping_cfl    = 0.5

    • Following the WRF guidelines for dt is conservative. More aggressive time integration–i.e., larger time steps with more substeps–is possible. We note that ERF LESs with 10 or more fast timesteps have successfully been run but your mileage may vary.

Single-Column Model

  • An SCM is set up with a single cell in the lateral directions:

    geometry.prob_extent = 400  400  400
amr.n_cell           =   1    1   64
geometry.is_periodic =   1    1    0

  • An SCM was successfully run with third-order advection in the horizontal and vertical.

2-D Cases

  • A 2-D planar domain can be configured as follows:

    geometry.prob_extent = 10000  100  1000
amr.n_cell           =   100    1    20
geometry.is_periodic =     0    0     0

ylo.type = "SlipWall"
yhi.type = "SlipWall"