Nesting
Eta Model
Ptop
=
0
Eta Coordinate
And Step Mountains
ground
MSL
=1
Horizontal resolution of 12 km
12-km terrain
Drawbacks of the Eta Coordinate
• The failure to generate downslope wind storms in regions
of complex terrain
• Weak boundary layer winds over elevated terrain when
compared to observations
• The displacement of precipitation maxima too far toward
the bottom of steeply sloping terrain as opposed to the
observed location near the top half of the terrain slope
• The reduction in the number of vertical layers used to
define the model atmosphere above elevated topography
particularly within the boundary layer
WRF Model Family
A Tale of Two Dynamical Cores
Why WRF?
• An attempt to create a national mesoscale
prediction system to be used by both operational
and research communities.
• A new, state-of-the-art model that has good
conservation characteristics (e.g., conservation of
mass) and good numerics (so not too much
numerical diffusion)
• A model that could parallelize well on many
processors and easy to modify.
• Plug-compatible physics to foster improvements
in model physics.
• Designed for grid spacings of 1-10 km
Two WRF Cores
• ARW (Advanced Research WRF)
• developed at NCAR
• Non-hydrostatic Numerical Model (NMM) Core
developed at NCEP
• Both work under the WRF IO Infrastructure
NMM
ARW
The NCAR ARW Core Model:
(See: www.wrf-model.org)
 Terrain following vertical coordinate
 two-way nesting, any ratio
 Conserves mass, entropy and scalars using up to
6th order spatial differencing equ for fluxes. Very
good numerics, less implicit smoothing in
numerics.
 NCAR physics package (converted from MM5 and
Eta), NOAH unified land-surface model, NCEP
physics adapted too
The NCEP Nonhydrostatic Mesoscale
Model: NMM (Janjic et al. 2001)





Hybrid sigmapressure vertical coord.
3:1 nesting ratio
Conserves kinetic energy, enstrophy and
momentum using 2nd order differencing equation
Modified Eta physics, Noah unified land-surface
model, NCAR physics adapted too
Parallelized within WRF infrastructure
Hybrid and Eta Coordinates
Ptop
Ptop
=
0
Pressure domain
=
0
Sigma domain
ground
ground
=1
MSL
=1
WRF Modeling System
WRF Software Infrastructure
Obs Data,
Analyses
Static
Initialization
3DVAR Data
Assimilation
Dynamic Cores
Mass Core
NMM Core
…
Standard Physics Interface
Physics Packages
Post Processors,
Verification
WRF Hierarchical Software Architecture
• Top-level “Driver” layer
– Isolates computer architecture concerns
– Manages execution over multiple nested domains
– Provides top level control over parallelism
• patch-decomposition
Driver Layer
• inter-processor communication
• shared-memory parallelism
initial_config
– Controls Input/Output
• “Mediation” Layer
wrf
alloc_and_configure
Mediation Layer
init_domain
integrate
solve_interface
solve
–
–
–
–
Performs actual model computationsModel Layer
Tile-callable
Scientists insulated from parallelism
General, fully reusable
filter
physics
scalars
recouple
advance w
advance uv
• Low-Level “Model” layer
decouple
– Specific calls to parallel mechanisms
•The National Weather Service dropped Eta in 2006
as the NAM (North American Mesoscale) run and
replaced it with WRF NMM.
•The Air Force uses WRF ARW.
•Most universities use WRF ARW
WRF-NMM
•Same domain as Eta
•Sixty levels like Eta
•Essentially same physics as ETA
•Much better in terrain…doesn’t share
the eta’s problems.
•Clearly inferior synoptic initialization
and synoptic forecast than GFS
NMM WRF
NAM NMM upgrades December 2008, include
• GDAS (GFS analysis) as initial first guess. use of
global analysis (GDAS) for first guess at t-12 hour (the start of
the analysis cycle) improves the evolution of synoptic scale
features in the new NAM-WRF. This is found consistently
throughout the 84-hour forecast.
• Improved physics higher resolution snow
analysis and changes to snow impact on surface
energy budget, increased absorptivity of model
clouds
NMM
• Generally inferior to GFS
Rapid Update Cycle-RUC
RUC
• A major issue is how to assimilate and use the
rapidly increasing array of offtime or continuous
observations (not a 00 and 12 UTC world
anymore!
• Want very good analyses and very good shortterm forecasts (1-3-6 hr)
• The RUC ingests and assimilates data hourly, and
then makes short-term forecasts
• Uses the MAPS mesoscale model…which uses a
hybrid sigma/isentropic vertical coordinate
• Resolution: 13 km and 50 levels
13km RUC
Terrain elevation - 100 m interval
Improvements expected from 13km RUC
- Improved near-surface forecasts
- Improved precipitation forecasts
- Better cloud/icing depiction
- Improved frontal/turbulence forecasts
NCEP computer upgrade
allows RUC13 to run in
same time as current
RUC20
Data Type
~Number
Freq.
-------------------------------------------------Rawinsonde
80
/12h
NOAA profilers
30
/ 1h
VAD winds
110-130
/ 1h
Aircraft (V,temp)
1400-4500 / 1h
Surface/METAR
1500-1700 / 1h
Buoy/ship
100-150
/ 1h
GOES precip water 1500-3000 / 1h
GOES cloud winds
1000-2500 / 1h
GOES cloud-top pres 10 km res / 1h
SSM/I precip water 1000-4000 / 6h
-------------------------------------------------GPS precip water
~300
/ 1h
Mesonet
~5000
/ 1h
METAR-cloud-vis-wx ~1500
/ 1h
--------------------------------------------------
Cloud
analysis
variables
NCEP RUC20
operational
Observations used in RUC
RUC13
(at NCEP
June 2005)
RUC History – NCEP (NMC) implementations
1994 - First operational implementation of RUC
- 60km resolution, 3-h cycle
1998 – 40km resolution, 1-h cycle,
- cloud physics, land-sfc model
2002 – 20km resolution
- addition of GOES cloud data in assimilation
2003 – Change to 3dVAR analysis from previous OI
(April)
2004 – Vertical advection, land use (April)
PBL-depth for surface assimilation (September)
2005 – 13km resolution, new obs, new model physics
(June)
2007 – WRF-based Rapid Refresh w/ GSI to replace RUC
20km RUC
13km RUC
Soil moisture – 22z - 21 Feb 2005
Dark blue = water
More detailed coastline with 13km resolution
WRF RUC
• A new version of RUC has been developed,
but not yet operational that uses the WRF
model instead of the MAPS model.