Chapter 5: WRF Model

The WRF-NMM is a fully compressible, non-hydrostatic mesoscale model with a hydrostatic option (Janjic et al. 2001, Janjic 2003a,b). The model uses a terrain following hybrid sigma-pressure vertical coordinate. The grid staggering is the Arakawa E-grid. The same time step is used for all terms. The dynamics conserve a number of first and second order quantities including energy and enstrophy (Janjic 1984).

The WRF-NMM code contains an initialization program (real_nmm.exe; see Chapter 4) and a numerical integration program (wrf.exe). The WRF-NMM model Version 2.2 supports a variety of capabilities. These include:

· Real-data simulations

· Non-hydrostatic and hydrostatic (runtime option)

· Applications ranging from meters to thousands of kilometers

WRF-NMM Dynamics in a Nutshell:

Time stepping:

Horizontally propagating fast-waves: Forward-backward scheme

Vertically propagating sound waves: Implicit scheme

Horizontal: Adams-Bashforth scheme

Vertical: Crank-Nicholson scheme

TKE, water species: Explicit, iterative, flux-corrected (called every two time steps).

Advection (space) for T, U, V:

Horizontal: Energy and enstrophy conserving, quadratic conservative, second order

Vertical: Quadratic conservative, second order

TKE, Water species: Upstream, flux-corrected, positive definite, conservative

Diffusion:

Diffusion in the WRF-NMM is categorized as lateral diffusion and vertical diffusion. The vertical diffusion in the PBL and in the free atmosphere is handled by the surface layer scheme and by the boundary layer parameterization scheme (Janjic 1996a, 1996b, 2002a, 2002b). The lateral diffusion is formulated following the Smagorinsky non-linear approach (Janjic 1990). The control parameter for the lateral diffusion is the square of Smagorinsky constant.

Divergence damping:

The horizontal component of divergence is damped (Sadourny 1975). In addition, if applied, the technique for coupling the elementary subgrids of the E grid (Janjic 1979) damps the divergent part of flow.

Physics Options

All available WRF System physics package options are listed below. Some of these options have not yet been tested for WRF-NMM. Indication of the options that have been tested, as well as the level of the testing, is included in the discussion below.

It is recommended that the same physics be used in all grids (coarsest and nests). The only exception is that the cumulus parameterization may be activated on coarser grids and turned off on finer grids.

Microphysics (mp_physics)

0. No microphysics

1. Kessler scheme: A warm-rain (i.e. no ice) scheme used commonly in idealized cloud modeling studies (Kessler 1969, Wicker and Wilhemson 1995).

2. Lin et al. scheme: A sophisticated scheme that has ice, snow and graupel processes, suitable for real-data high-resolution simulations (Lin et al. 1983, Rutledge and Hobbs 1984, Tao et al. 1989, Chen and Sun 2002).

3. WRF Single-Moment (WSM) 3-class simple ice scheme: A simple efficient scheme with ice and snow processes suitable for mesoscale grid sizes (Hong et al. 1998, Hong et al. 2004).

4. WRF Single-Moment (WSM) 5-class scheme. A slightly more sophisticated version of option 3 that allows for mixed-phase processes and super-cooled water (Hong et al. 1998, Hong et al. 2004). (This scheme has been preliminarily tested for WRF-NMM.)

5. Ferrier scheme: A scheme that includes prognostic mixed-phase processes (Ferrier et al. 2002). This scheme was recently changed so that ice saturation is assumed at temperatures colder than -30ºC rather than -10ºC as in the original implementation. (This scheme is well tested for WRF-NMM, used operationally at NCEP.)

6. WSM 6-class graupel scheme: A new scheme with ice, snow and graupel processes suitable for high-resolution simulations (Lin et al. 1983, Dudhia 1989, Hong et al. 1998). (This scheme has been preliminarily tested for WRF-NMM.)

8. Thompson et al. scheme: A scheme with six classes of moisture species plus number concentration for ice as prognostic variables (Thompson et al. 2004). (This scheme has been preliminarily tested for WRF-NMM.)

98. NCEP 3-class simple ice scheme (to be removed): An older version of WSM-3.

99. NCEP 5-class scheme (to be removed): An older version of WSM-5.

Longwave Radiation (ra_lw_physics)

1. RRTM scheme: Rapid Radiative Transfer Model. An accurate scheme using look-up tables for efficiency. Accounts for multiple bands, trace gases, and microphysics species (Mlawer et al. 1997). (This scheme has been preliminarily tested for WRF-NMM.)

99. GFDL scheme: Geophysical Fluid Dynamics Laboratory (GFDL) longwave. An older version multi-band, transmission table look-up scheme with carbon dioxide, ozone and water vapor absorptions (Fels and Schwarzkopf 1975, Schwarzkopf and Fels 1985, Schwarzkopf and Fels 1991). Cloud microphysics effects are included. (This scheme is well tested for WRF-NMM, used operationally at NCEP.) Note: If it is desired to run GFDL with a microphysics scheme other than Ferrier, a modification to module_ra_gfdleta.F is needed to comment out (!) #define FERRIER_GFDL.

Shortwave Radiation (ra_sw_physics)

1. Dudhia scheme: Simple downward integration allowing for efficient cloud and clear-sky absorption and scattering (Dudhia 1989). (This scheme has been preliminarily tested for WRF-NMM.)

2. Goddard Shortwave scheme: Two-stream multi-band scheme with ozone from climatology and cloud effects (Chou and Suarez 1994).

99. GFDL scheme: Geophysical Fluid Dynamics Laboratory (GFDL) shortwave. A two spectral bands, k-distribution scheme with ozone and water vapor as the main absorbing gases (Lacis and Hansen 1974). Cloud microphysics effects are included. (This scheme is well-tested for WRF-NMM, used operationally at NCEP.) Note: If it is desired to run GFDL with a microphysics scheme other than Ferrier, a modification to module_ra_gfdleta.F is needed to comment out (!) #define FERRIER_GFDL.

Surface Layer (sf_sfclay_physics)

1. Monin-Obukhov Similarity scheme: Based on Monin-Obukhov with Carslon-Boland viscous sub-layer and standard similarity functions from look-up tables (Skamarock et al. 2005).

2. Janjic Similarity scheme: Based on similarity theory with viscous sublayers both over solid surfaces and water points (Janjic, 1996b, Chen et al. 1997). (This scheme is well tested for WRF-NMM, used operationally at NCEP.)

3. NCEP Global Forecasting System (GFS) scheme: The Monin-Obukhov similarity profile relationship is applied to obtain the surface stress and latent heat fluxes using a formulation based on Miyakoda and Sirutis (1986) modified for very stable and unstable situations. Land surface evaporation has three components (direct evaporation from the soil and canopy, and transpiration from vegetation) following the formulation of Pan and Mahrt (1987). (This scheme has been preliminarily tested for WRF-NMM.)

Land Surface (sf_surface_physics)

1. Thermal Diffusion scheme: Soil temperature only scheme, using five layers (Skamarock et al. 2005).

2. Noah Land-Surface Model: Unified NCEP/NCAR/AFWA scheme with soil temperature and moisture in four layers, fractional snow cover and frozen soil physics (Chen and Dudhia, 2001). (This scheme has been preliminarily tested for WRF-NMM.)

3. RUC Land-Surface Model: Rapid Update Cycle operational scheme with soil temperature and moisture in six layers, multi-layer snow and frozen soil physics (Smirnova et al. 1997, 2000). (This scheme has been preliminarily tested for WRF-NMM.)

99. NMM Land Surface Scheme: The NMM LSM package is based in the pre-May 2005 Noah Land Surface Model (LSM) in the operational NAM/Eta with soil temperature and moisture in 4 layers, fractional snow cover and frozen soil physics (Ek et al. 2003) and is quite similar to the unified Noah LSM (option 2 above). (This scheme is well tested for WRF-NMM, used operationally at NCEP.)

Planetary Boundary Layer (bl_pbl_physics)

1. Yonsei University scheme (YSU): Next generation MRF-PBL. Non-local-K scheme with an explicit entrainment layer and parabolic K profile in unstable mixed layer (Skamarock et al. 2005).

2. Mellor-Yamada-Janjic Scheme: One-dimensional prognostic turbulent kinetic energy scheme with local vertical mixing (Janjic 1990, 1996a, 2002). (This scheme is well-tested for WRF-NMM, used operationally at NCEP.)

3. NCEP Global Forecast System scheme: First-order vertical diffusion scheme of Troen and Mahrt (1986) further described in Hong and Pan (1996). The PBL height is determined using an iterative bulk-Richardson approach working from the ground upward whereupon the profile of the diffusivity coefficient is specified as a cubic function of the PBL height. Coefficient values are obtained by matching the surface-layer fluxes. A counter-gradient flux parameterization is included. (This scheme has been preliminarily tested for WRF-NMM.)

99. MRF scheme: An older version of YSU with implicit treatment of entrainment layer as part of non-local-K mixed layer (Hong and Pan 1996).

Note: Two-meter temperatures are only available when running with MYJ scheme (2).

Cumulus Parameterization (cu_physics)

0. No cumulus parameterization. (Tested for WRF-NMM)

1. Kain-Fritsch scheme: Deep and shallow sub-grid scheme using a mass flux approach with downdrafts and CAPE removal time scale (Kain 2004, Kain and Fritsch 1990, 1993). (This scheme has been preliminarily tested for WRF-NMM.)

2. Betts-Miller-Janjic scheme: Adjustment scheme for deep and shallow convection relaxing towards variable temperature and humidity profiles determined from thermodynamic considerations (Janjic 1994, 2000). (This scheme is well tested for WRF-NMM, used operationally at NCEP.)

3. Grell-Devenyi ensemble scheme: Multi-closure, multi-parameter, ensemble method with typically 144 sub-grid members (Grell and Devenyi 2002). (This scheme has been preliminarily tested for WRF-NMM.)

4. Simplified Arakawa-Schubert scheme: Penetrative convection is simulated following Pan and Wu (1995), which is based on Arakawa and Schubert (1974) as simplified by Grell (1993) and with a saturated downdraft. (This scheme is well tested for WRF-NMM.)

Below is a summary of physics options that are well-tested for WRF-NMM and are used operationally at NCEP:

&physics	Identifying Number	Physics options
mp_physics (max_dom)	5	Microphysics-Ferrier
ra_lw_physics	99	Long-wave radiation - GFDL (Fels-Schwarzkopf)
ra_sw_physics	99	Short-wave radiation - GFDL (Lacis-Hansen)
sf_sfclay_physics	2	Surface-layer: Janjic scheme
sf_surface_physics	99	Land-surface – NMM LSM
bl_pbl_physics	2	Boundary-layer - Mellor-Yamada-Janjic TKE
cu_physics	2	Cumulus - Betts-Miller-Janjic scheme
num_soil_layers	4	Number of soil layers in land surface model

Description of Namelist Variables

The settings in the “namelist.input” file are used to configure WRF-NMM. This file should be edited to specify: dates, time step, number and size of domains, output options, and physics options. When modifying the “namelist.input” file, be sure to take into account the following points:

“time_step”: The general rule for determining the time step of the coarsest grid follows from the CFL criterion. If d is the grid distance between two neighboring points (in diagonal direction on the WRF-NMM's E-grid), dt is the time step, and c is the phase speed of the fastest process, the CFL criterion requires that:

(c*dt)/[d/sqrt(2.)] ≤1

This gives: dt ≤ d/[sqrt(2.)*c]

A very simple approach is to use 2.25 x (grid spacing in km) or about 330 x (angular grid spacing) to obtain an integer number of time steps per hour.

For example: If the grid spacing of the coarsest grid is 12km, then this gives dt=27 s

The following are pre-tested time-steps for WRF-NMM:

Approximate Grid Spacing (km)	DELTA_X (in degrees)	DELTA_Y (in degrees)	Time Step (seconds)
4	0.026726057	0.026315789	9-10s
8	0.053452115	0.052631578	18s
10	0.066666666	0.065789474	24s
12	0.087603306	0.075046904	25-30s
22	0.154069767	0.140845070	60s
32	0.222222222	0.205128205	90s

“e_we and e_sn”: Given WRF-NMM’s E-grid staggering, the end index in the east-west direction (e_we) and the south-north direction (e_sn) for the coarsest grid need to be set with care.

When using the WRF Preprocessing System (WPS), the coarsest grid dimensions should be set as:

e_we (namelist.input) = e_ew (namelist.wps),

e_sn (namelist.input) = e_sn (namelist.wps).

For example: If the parent grid e_we and e_sn are set up as follows in the namelist.wps

e_we = 124,

e_sn = 202,

then e_we and e_sn in namelist.input should be:

e_we = 124,

e_sn = 202.

When using the WRF-NMM Standard Initialization (SI), the coarsest grid dimensions should be set as:

e_we = XDIM+1,

e_sn = YDIM+1.

For example: If the parent grid XDIM and YDIM are set up as follows in your wrfsi.nl

XDIM = 123,

YDIM = 201,

then e_we and e_sn in namelist.input should be:

e_we = 124,

e_sn = 202.

For nested grids, there are no special rules to follow when choosing e_we and e_sn.

“dx and dy”: For WRF-NMM, dx and dy are the horizontal grid spacing in degrees, rather than meters (unit used for WRF-ARW). Note that dx should be slightly larger than dy due to the convergence of meridians approaching the poles on the rotated grid. The grid spacing in namelist.input should have the same values as in namelist.wps (if using WPS) or wrfsi.nl (is using SI).

When using WPS,

dx (namelist.input) = dx (namelist.wps),

dy (namelist.input) = dy (namelist.,wps).

When using the WRF-NMM SI,

dx = MOAD_DELTA_X,

dy = MOAD_DELTA_Y .

When running a simulation with multiple (N) nests, the namelist should have N values of dx, dy, e_we, e_sn separated by commas.

For more information about the horizontal grid spacing for WRF-NMM, please see Chapter 3, WRF Preprocessing System (WPS)

“nio_tasks_per_group”: The number of I/O tasks (nio_tasks_per_group) should evenly divide into the number of compute tasks in the J-direction on the grid (that is the value of nproc_y). For example, if there are 6 compute tasks in the J-direction, then “nio_tasks_per_group” could legitimately be set to 1, 2, 3, or 6. The user needs to use a number large enough that the quilting for a given output time is finished before the next output time is reached. If one had 6 compute tasks in the J-direction (and the number in the I-direction was similar), then one would probably choose either 1 or 2 quilt tasks.

The following table provides an overview of the parameters specified in namelist.input. Note that “namelist.input” is common for both WRF cores (WRF-ARW and WRF-NMM). Most of the parameters are valid for both cores. However, some parameters are only valid for one of the cores. Core specific parameters are noted in the table. In addition, some physics options have not been tested for WRF-NMM. Those options that have been tested are highlighted by indicating whether they have been “fully” or “preliminarily” tested for WRF-NMM.

Variable Names	Value (Example)	Description
*&time_control*		Time control
run_days	2	Run time in days
run_hours	0	Run time in hours Note: If run time is more than 1 day, one may use both run_days and run_hours or just run_hours. e.g. if the total run length is 36 hrs, you may set run_days = 1, and run_hours = 12, or run_days = 0, and run_hours 36.
run_minutes	00	Run time in minutes
run_seconds	00	Run time in seconds
start_year (max_dom)	2005	Four digit year of starting time
start_month (max_dom)	04	Two digit month of starting time
start_day (max_dom)	27	Two digit day of starting time
start_hour (max_dom)	00	Two digit hour of starting time
start_minute (max_dom)	00	Two digit minute of starting time
start_second (max_dom)	00	Two digit second of starting time
end_year (max_dom)	2005	Four digit year of ending time
end_month (max_dom)	04	Two digit month of ending time
end_day (max_dom)	29	Two digit day of ending time
end_hour (max_dom)	00	Two digit hour of ending time
end_minute (max_dom)	00	Two digit minute of ending time
end_second (max_dom)	00	Two digit second of ending time Note: All end times also control when the nest domain integrations end. Note: All start and end times are used by *real_nmm.exe. One may use either run_days/run_hours etc. or end_year/month/day/hour etc. to control the length of model integration, but run_days/run_hours takes precedence over the end times. The program real_nmm.exe* uses start and end times only.
interval_seconds	10800	Time interval between incoming real data, which will be the interval between the lateral boundary condition files. This parameter is only used by “real_nmm.exe”.
history_interval (max_dom)	60	History output file interval in minutes
frames_per_outfile (max_dom)	1	Output times per history output file, used to split output files into smaller pieces
tstart (max_dom)	0	This flag is only for the WRF-NMM core. Forecast hour at the start of the NMM integration. Set to >0 if restarting a run.
restart	.false.	Logical indicating whether run is a restart run
restart_interval	60	Restart output file interval in minutes
io_form_history	2	Format of history file wrfout 2 = netCDF
io_form_restart	2	Format of restart file wrfrst 2 = netCDF
io_form_input	2	Format of input file wrfinput_d01 2 = netCDF
io_form_boundary	2	Format of boundary file wrfbdy_d01 1. Binary format 2. netCDF format 4. PHD5 format 5. GRIB-1 format
auxinput1_inname	“wrf_real_input_nm.d01.<date>”		Name of input file from SI
auxinput1_inname	“met_nmm_.d01.<date>”		Name of input file from WPS
debug_level	0	Control for amount of debug printouts 0 - for standard runs, no debugging. 1 - netcdf error messages about missing fields. 50,100,200,300 values give increasing prints. Large values trace the job's progress through physics and time steps.

*&Domains*		Domain definition
time_step	18	Time step for integration of coarsest grid in integer seconds
time_step_fract_num	0	Numerator for fractional coarse grid time step
time_step_fract_den	1	Denominator for fractional coarse grid time step. Example, if you want to use 60.3 sec as your time step, set time_step = 60, time_step_fract_num = 3, and time_step_fract_den = 10
max_dom	1	Number of domains (1 for a single grid, >1 for nests)
s_we (max_dom)	1	Start index in x (west-east) direction (leave as is)
e_we (max_dom)	124	End index in x (west-east) direction (staggered dimension)
s_sn (max_dom)	1	Start index in y (south-north) direction (leave as is)
e_sn (max_dom)	62	End index in y (south-north) direction (staggered dimension) For WRF-NMM this value must be even.
s_vert (max_dom)	1	Start index in z (vertical) direction (leave as is)
e_vert (max_dom)	61	End index in z (vertical) direction (staggered dimension). Note: This parameter refers to full levels including surface and top.
dx (max_dom)	.0534521	Grid length in x direction, in degrees for WRF-NMM.
dy (max_dom)	.0526316	Grid length in y direction, in degrees for WRF-NMM.
grid_id (max_dom)	1	Domain identifier.
p_top_requested	5000	P top used in the model (Pa) (WPS related)
ptsgm	42000.	Pressure level (Pa) in which the WRF-NMM hybrid coordinate transitions from sigma to pressure (WPS related).
eta_levels	1.00, 0.99, …0.00	Model eta levels. (WPS related).
num_metgrid_levels	40	Number of vertical levels in the incoming data: type nudump –h to find out. (WPS related)
parent_id (max_dom)	1	ID of the parent domain. Use 0 for the coarsest grid.
i_parent_start (max_dom)	1	Defines the LLC of the nest as this I-index of the parent domain. Use 1 for the coarsest grid.
j_parent_start (max_dom)	1	Defines the LLC of the nest in this J-index of the parent domain. Use 1 for the coarsest grid.
parent_grid_ratio (max_dom)	3	Parent-to-nest domain grid size ratio. For WRF-NMM this ratio must be 3.
parent_time_step_ratio (max_dom)	3	Parent-to-nest time step ratio. For WRF-NMM this ratio must be 3.
tile_sz_x	0	Number of points in tile x direction.
tile_sz_y	0	Number of points in tile y direction.
numtiles	1	Number of tiles per patch (alternative to above two items).
nproc_x	-1	Number of processors in x–direction for decomposition.
nproc_y	-1	Number of processors in y-direction for decomposition: If -1: code will do automatic decomposition. If >1 for both: will be used for decomposition.

*&physics*		Physics options
chem_opt	0	Chemistry option - not yet available
mp_physics (max_dom)	5	Microphysics options: 0. no microphysics 1. Kessler scheme 2. Lin et al. scheme 3. WSM 3-class simple ice scheme 4. WSM 5-class scheme (Preliminarily tested for WRF-NMM.) 5. Ferrier (Well-tested for WRF-NMM, used operationally at NCEP) 6. WSM 6-class graupel scheme (Preliminarily tested for WRF-NMM.) 8. Thompson et al. scheme (Preliminarily tested for WRF-NMM.) 98. NCEP 3-class simple ice scheme (to be removed) 99. NCEP 5-class scheme (to be removed)
ra_lw_physics (max_dom)	99	Long-wave radiation options: 0. No longwave radiation 1. RRTM scheme (Preliminarily tested for WRF-NMM.) 99. GFDL (Fels-Schwarzkopf) (Well-tested for WRF-NMM, used operationally at NCEP.)
ra_sw_physics (max_dom)	99	Short-wave radiation options: 0. No shortwave radiation 1. Dudhia scheme (Preliminarily tested for WRF-NMM.) 2. Goddard short wave scheme 99. GFDL shortwave radiation scheme (Lacis-Hansen) (Well-tested for WRF-NMM, used operationally at NCEP.)
nrads (max_dom)	100	This flag is only for the WRF-NMM core. Number of fundamental time steps between calls to shortwave radiation scheme. NCEP's operational setting: “nrads” is on the order of “3600/dt”. For more detailed results, use: *nrads=1800/dt*
nradl (max_dom)	100	This flag is only for the WRF-NMM core. Number of fundamental time steps between calls to longwave radiation scheme. Note that *nradl* must be set equal to *nrads*.
tprec (max_dom)	3	This flag is only for the WRF-NMM core. Number of hours of precipitation accumulation in WRF output.
theat (max_dom)	6	This flag is only for the WRF-NMM core. Number of hours of accumulation of gridscale and convective heating rates in WRF output.
tclod (max_dom)	6	This flag is only for the WRF-NMM core. Number of hours of accumulation of cloud amounts in WRF output.
trdsw (max_dom)	6	This flag is only for the WRF-NMM core. Number of hours of accumulation of shortwave fluxes in WRF output.
trdlw (max_dom)	6	This flag is only for the WRF-NMM core. Number of hours of accumulation of longwave fluxes in WRF output.
tsrfc (max_dom)	6	This flag is only for the WRF-NMM core. Number of hours of accumulation of evaporation/sfc fluxes in WRF output.
pcpflg (max_dom)	.false.	This flag is only for the WRF-NMM core. Logical switch that turns on/off the precipitation assimilation used operationally at NCEP.
co2tf	1	This flag is only for the WRF-NMM core. Controls CO2 input used by the GFDL radiation scheme. 0: Read CO2 functions data from pre- generated file 1: Generate CO2 functions data internally
sf_sfclay_physics (max_dom)	2	Surface-layer options: 0. No surface-layer scheme 1. Monin-Obukhov scheme 2. Janjic scheme (Well-tested for WRF-NMM, used operationally at NCEP) 3. NCEP Global Forecast System scheme (Preliminarily tested for WRF-NMM.)
sf_surface_physics (max_dom)	99	Land-surface options: 0. No surface temperature prediction 1. Thermal diffusion scheme 2. Noah Land-Surface Model (Preliminarily tested for WRF-NMM.) 3. RUC Land-Surface Model (Preliminarily tested for WRF-NMM.) 99. NMM Land Surface Model (Well-tested for WRF-NMM, used operationally at NCEP.)
bl_pbl_physics (max_dom)	2	Boundary-layer options: 0. No boundary-layer 1. YSU scheme 2. Mellor-Yamada-Janjic TKE scheme (Well-tested for WRF-NMM, used operationally at NCEP) 3. NCEP Global Forecast System scheme (Preliminarily tested for WRF-NMM.) 99. MRF scheme (to be removed)
nphs (max_dom)	10	This flag is only for WRF-NMM core.