Initialization Pathways¶

This section describes different ways for defining or reading in the initial data for an ERF simulation. When you run the ERF executable you must specify an inputs file which includes the specification of which initialization pathway ERF will take.

Custom Initialization¶

When not reading the initial data as described in the section below, the initialization in ERF has two steps: creation of the background state and creation of optionally non-zero initial perturbations from the background state.

If erf.init_type = Uniform the user must provide values in the inputs file, prob.rho_0 and prob.T_0, to specify the background density and temperature which will be assumed to be constant in space throughout the domain. Base state pressure is computed from the EOS.

If erf.init_type = ConstantDensity the user must provide prob.rho_0 to specify the background density which will be assumed to be constant in space throughout the domain. If gravity is set to be non-zero then the density will be vertically integrated to generate the background pressure as described in Construction of hydrostatically stratified base state. Once density and pressure are known, the base state potential temperature will be computed from the EOS.

If erf.init_type = Isentropic, the background state is computed by iterating to find base state pressure and density which satisfy both the dry EOS and hydrostatic equilibrium (HSE).

If erf.init_type = MoistBaseState, the background state is computed by iterating to find base state pressure and density which satisfy both the moist EOS and hydrostatic equilibrium (HSE).

If erf.init_type = InputSounding, then the thermodynamic profiles in the provided erf.input_sounding_file are used to set initial conditions and the base state depending on erf.sounding_type. For an Ideal sounding (default), a stratified, hydrostatically balanced base state is reconstructed from the 1-D input sounding data as described in Construction of hydrostatically stratified base state. The initial fields match the base state. This configuration corresponds to WRF’s ideal.exe initialization.

If the sounding is Isentropic or DryIsentropic, a set of thermodynamically consistent, isentropic (constant \(\theta\)) conditions are determined. An initial pressure profile is calculated by integrating \(p_{surf}\) and \(\theta_{surf}\) from the surface; the resulting base-state pressure profile is related to \(\rho\theta_d\) through the equation of state. This reference profile may or may not take into account the input water vapor mixing ratio profile, depending the choice of isentropic or dry isentropic, respectively. Using the input profiles of potential temperature and water vapor mixing ratio, we determine the initial dry air density. These three quantities are used to initialize the solution fields. Note that the base state corresponds to the integrated \(p(z)\) and \(\theta(z)=\theta_{surf}\).

If the sounding is ConstantDensity, then the initial density field is uniformly set to 1.0; the potential temperature (and water vapor mixing ratio) field(s) are set to the sounding values.

Note

You can optionally replace only the velocity fields (u, v, w) by reading them from an existing checkpoint. This is useful when restarting with updated thermodynamics or a new base state while keeping a prior wind field.

Add a line such as the following to your inputs file:

erf.init_vels_from_checkpoint = chk00010

The value should be the checkpoint directory name (relative to the run directory or an absolute path). When set, ERF reads the velocity fields from that checkpoint and uses the usual initialization pathway for all other fields.

In any of these cases, the user can specify any perturbations from the base state by editing the routines that live in the Source/Prob directory and are called in Exec/ERF_Prob.cpp

Initialization From Real Data¶

There are three options for ingesting the full 3D initial data from a NetCDF file. In these cases, no additional initial conditions must be supplied by the user but the file Exec/ERF_Prob.cpp must still be present for the build.

erf.init_type = WRFInput
- In this case both the base state and the full state are read in from a NetCDF file generated by the WPS preprocessing system. This case is designed for realistic atmospheric flow problems and allows for terrain-fitted coordinates, map factors and a non-zero base state. In addition to the initial conditions, time-varying boundary conditions can be read from a NetCDF file specified using erf.nc_bdy_file and time-varying sea surface temperatures can be read from a NetCDF file specified using erf.nc_low_file. We also note that wrfinput files that have later start times than the base level can be read and the new level will be initialized once the start time of the finer level has been reached.
erf.init_type = Metgrid
- In this case the problem is initialized with data contained in the first NetCDF file provided via erf.nc_init_file_0. Lateral boundary conditions are derived from the sequence of NetCDF files provided via erf.nc_init_file_0. The sequence of erf.nc_init_file_0 should be output from the WRF Preprocessing System (WPS) listed chronologically starting with the earliest timestamp. A minimum of two files are required to derive lateral boundary conditions.
erf.init_type = NCFile
- In this case the base state defaults to zero and the full state is read in from a much simplified NetCDF file. Right now, only the density, horizontal and vertical velocity components, potential temperature, and water vapor mixing ratio can be read in. This case is designed for idealized problems and does not allow for terrain-fitted coordinates or map factors. The variables in the NC file should have dimensions of (Time, bottom_top, south_north, west_east). Variable names include RHO, U, V, W, T, and QV. Optional HSE variables include RHO_HSE, T_HSE, and P_HSE; the base state will be calculated if it is not specified.

Workflows¶

For a summary of initialization strategies for real-data simulations, see the table below.

7 Simulation Workflows¶
	Large-Scale Data (reanalysis, HRRR, …)	Intermediate Processing	Weather Simulation
WRF	Manual download	WPS + `real.exe`	`wrf.exe`
WRF –> ERF	Manual download	WPS + `real.exe`	`erf_exec` (init from wrfinput)
	Manual download	`ndown.exe`	`erf_exec` (init from wrfinput)
WPS –> ERF	Manual download	WPS	`erf_exec` (init from metgrid)
E3SM –> ERF	`run_e3sm`	Under development	`erf_exec`
ERF standalone	Python tools	Python tools (under development)	`erf_exec`