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 time-step size.
- The acoustic CFL should be less than 0.5, with 4–6 fast timesteps (substeps) according to WRF best practices.
```
erf.fixed_dt           = 0.06  # slow timestep

# These are equivalent and result in a fixed fast timestep size
#   if dx=10, speed of sound ~ 350 m/s
erf.fixed_fast_dt      = 0.01  # ==> CFL~0.35
#erf.fixed_mri_dt_ratio = 6

# Alternatively, let ERF chose the fast timestep
#erf.cfl                = 0.5
```
- We note that ERF LESs with up to 10 fast timesteps have successfully been run but your mileage may vary.

Single-Column Model¶

Currently, ERF does not have the ability to run a true single-column model (SCM). The grid size in the lateral directions must have a minimum number of cells. This will give comparable results, e.g.:
```
geometry.prob_extent = 25  25  400
amr.n_cell           =  4   4   64
geometry.is_periodic =  1   1    0
```
An SCM was successfully run with third-order advection in the horizontal and vertical.