Idealized Case Initialization¶

The WRF model provides two classes of simulations: Real-data and Idealized-data. This chapter focuses on idealized cases.

WRF Idealized Simulation

A WRF model simulation of an “idealized,” simplified, and controlled environment.

Idealized simulations are initiated from an an existing sounding (included with the code), and assume a simplified orography. Below are some ways in which idealized test cases can be used:

Since no external data is required, they are a simple method to ensure the model works correctly on a particular architecture and compiler.
They are useful for dynamical studies, reproducing converged or otherwise known solutions for a broad range of space and time scales.
Idealized cloud modeling
Reproducing known solutions
A starting point/template for other idealized experiments
Testing physics development

1-D, 2-D and 3-D idealized cases are available, and provide a set of default examples for users, with and without topography, and with and without an initial thermal perturbation. For e.g.,

To specify a thermal bubble, use the super cell case.
In the hill2d case, the topography is accounted for properly in setting up the initial 3-D arrays, so that example should be followed for any topography cases.
A symmetry example in the squall line cases tests that indexing modifications are correct.

Keep in mind the following limitations with idealized cases:

They can use any boundary condition except specified.
They are not generally set up to run with sophisticated physics.
- No radiation, surface fluxes, or frictional effects are used (with the exception of the sea breeze, LES, and the global Held-Suarez cases).
- The tropical cyclone case lacks only radiation schemes.
- The sea breeze case includes a full complement of parameterization options.

Available Ideal Test Cases¶

Note

All idealized test cases are available in the WRF/test/em_<case> directory, where <case> represents the specific case directory. See the italicized names beneath each case title below.

2-D Squall Line (x,z)
em_squall2d_x

2D squall line (x,z) using Kessler microphysics and a fixed 300 m²/s viscosity
Periodicity condition used in y so that 3D model produces 2D simulation
v velocity should be zero and there should be no variation in y in the results

2-D Squall Line (y,z)
em_squall2d_y

Same as squall2d_x, except with (x) rotated to (y)
u velocity should be zero and there should be no variation in x in the results

2-D Flow over a Hill
em_hill2d_x

2-D flow over a bell-shaped hill (x,z)
10 km half-width, 2 km grid-length, 100 m high hill, 10 m/s flow, N=0.01/s, 30 km high domain, 80 levels, open radiative boundaries, absorbing upper boundary
Case is in linear hydrostatic regime, so vertical tilted waves with ~6-km vertical wavelength

2-D Gravity Current
em_grav2d_x

Test case is described in Straka et al, INT J NUMER METH FL 17 (1): 1-22 July 15 1993
See the README.grav2d_x file in the test directory

2-D Sea Breeze
em_seabreeze_x

2-km grid size, 20-km top, land/water
Can be run with full physics, radiation, surface, boundary layer, and land options

3-D Quarter-circle Shear Supercell
em_quarter_ss

Left and right moving supercells are produced
See the README.quarter_ss file in the test directory for more information

3-D Baroclinic Waves
em_b_wave

Baroclinically unstable jet u(y,z) on an f-plane
Symmetric north and south, periodic east and west boundaries
100-km grid size, 16-km top, with 4-km damping layer
41x81 points in (x,y), 64 layers

3-D Large Eddy
em_les

100-m grid size, 2-km top
Surface layer physics with fluxes
Doubly periodic

3-D Held-Suarez
em_heldsuarez

Global domain, 625 km in x-direction, 556 km in y-direction, 120-km top
Radiation, polar filter above 45 degrees
Period in x-direction, polar boundary conditions in y-direction

3-D Surface Fire
em_fire

Geoscientific Model Development Discussions
50-m, 4.5-km top
10:1 subgrid ratio, no physics
Open boundaries

3-D Tropical Cyclone
em_tropical_cyclone

Test case described in Jordan, J METEOR 15, 91-97, 1958.
15-km, 25-km top
f-plane (f=0.5e-5, about 20 N), SST=28 C
Full physics with a simple radiative cooling, no cumulus
Doubly periodic

3-D Convective-radiative Equilibrium
em_convrad

1 km grid size, 30 km model top
Tropical condition, small f, weak wind, constant SST
Full physics
Doubly periodic

1-D Single Column Model
em_scm_xy

4-km grid size, 12-km top
Full physics
Doubly periodic

Idealized Case Initialization¶

Idealized cases are typically initialized from an existing sounding (provided with the code), and assume a simplified analytic orography.

Initialization programs for idealized cases are responsible for the following:

Computing a base state/reference profile for geopotential and column pressure
Computing perturbations from the base state for geopotential and column pressure
Initializing meteorological variables u, v, potential temperature, and vapor mixing ratio
Defining a vertical coordinate
Interpolating data to the model’s vertical coordinate
Initializing static fields for the map projection and the physical surface; for many of the idealized cases, these are simplified initializations (such as map factors set to one, and topography elevation set to zero)
Moist potential temperature is computed, if requested that the thermal field use it
Reading namelist settings
Allocating space for the requested domain, with model variables specified at run-time
Generating the initial condition file to be used to run wrf.exe

The initialization program (an executable, ideal.exe) is compiled as part of the WRF code installation. Each idealized case uses its own initialization program, and therefore, ideal.exe for one test case cannot be used for another case. These programs are built using target modules from one of the /WRF/dyn_em/module_initialize_*.F files. Below is a list of available idealized WRF cases, along with the initialization target module that builds each one.

Case	Target Module File
em_b_wave	module_initialize_ideal.F
em_convrad	module_initialize_ideal.F
em_fire	module_initialize_fire.F
em_heldsuarez	module_initialize_heldsuarez.F
em_les	module_initialize_ideal.F
em_quarter_ss	module_initialize_ideal.F
em_tropical_cyclone	module_initialize_tropical_cyclone.F
em_grav2d_x	module_initialize_ideal.F
em_hill2d_x	module_initialize_ideal.F
em_seabreeze2d_x	module_initialize_ideal.F
em_squall2d_x	module_initialize_ideal.F
em_squall2d_y	module_initialize_ideal.F
em_scm_xy	module_initialize_scm_xy.F

Initialization Input¶

Typically the only required input to initialize an ideal case are the namelist.input and the input_sounding files (which are provided with the code).

input_sounding¶

input_sounding files can be any set of reasonable levels that goes up to at least the model top height (ztop in the namelist), and are formatted in the following way:

The first line includes surface pressure (hPa), potential temperature (K), and moisture mixing ratio (g/kg).
Each subsequent line includes five input values: height (meters above sea-level), dry potential temperature (K), vapor mixing ratio (g/kg), x-direction wind component (m/s), and y-direction wind component (m/s).

ideal.exe interpolates data from the input_sounding and extrapolates if enough data is not provided. The base state sounding for idealized cases is the initial sounding, minus moisture, and therefore does not need to be defined separately.

Note

The baroclinic wave case does not use a 1-D input sounding because initial 3-D arrays are read-in from the input_jet file. This means namelist.input cannot be used to change the horizontal or vertical dimensions since they are specified in input_jet.

namelist.input¶

The namelist.input controls the size of the domain, number of vertical levels, model top height, grid size, time step, diffusion and damping properties, boundary conditions, and physics options. A large number of settings already exist in the default namelists found in each case directory.

Compiling Idealized Cases¶

See the Compiling chapter for detailed information on building both the idealized case initialization programs and WRF.

Running Idealized Simulations¶

See Running Idealized Cases for instructions on running an idealized case.

Modifying Ideal Cases¶

One of the advantages of running idealized simulations is the ability to take a basic, controlled test, and modify it according to research interest. Such modifications can include changing the topography, distribution of vertical levels, properties of an initialization thermal bubble, or preparing a case to use additional physics options (e.g., a land-surface model). Apart from namelist-controlled options or soundings, modifications must be done by editing the model code.

Make modifications in /WRF/dyn_em/module_initialize_<case>.F, where <case> is either the case name, or simply ideal. See Idealized Case Initialization for details.
Once inside the file, modify the subroutine init_domain_rk.
If changing the vertical levels, the 1-D array znw must be defined, containing the full levels, starting from 1 at k=1, and ending with 0 at k=kde.
To change the topography, the 2-D array ht must be defined, making sure it is periodic if those boundary conditions are used.
To change the thermal perturbation bubble, search for the string “bubble” to locate the code to modify.

**KKW** Add this section somewhere in the running chapter

Backward-compatibility¶

By default, moist potential temperature is used (use_theta_m=1) by all ideal cases, and the WRF model uses a hybrid vertical coordinate (hybrid_opt=1). To use older wrfinput files (generated with WRF code prior to V4.0) as input to wrf.exe:

the moist theta option must be turned off (use_theta_m=0) in the namelist.input &dynamics record
the hybrid vertical coordinate must be turned off (hybrid_opt=0) in the &dynamics record
force_use_old_data=.true. must be set in the &time_control namelist.input record