Transcript Changing to WRF

NOAA/NWS Change to WRF
13 June 2006
What’s Happening?
• WRF replaces the eta as the NAM
– NAM is the North American Mesoscale
“timeslot” or “Model Run” at NOAA/NWS’s
National Center for Environmental Prediction
(NCEP).
– NMM is not a MODEL !
– A version of WRF is replacing eta as the
model run at NCEP in the NMM timeslot.
What is WRF ?
• WRF is Weather Research and
Forecasting Model.
• WRF is a “infrastructure” system to
conduct Numerical Weather Prediction.
– This includes analysis, prediction, postprocessing and verification
• WRF is a “community” system that allows
easier transition of research into
operations
WRF is not a “model”
• WRF is configurable in hundreds of
different ways.
• It is not a single model.
• Two (2) “Cores”
– NMM
– ARW
• Variety of “Physics” to handle meso and
micro scale processes related to
Convection and the Boundary Layer.
Initial Conditions
• New Analysis System
• Getting the starting point correct is critical
to getting the forecast correct.
• Minor and “non-observable” initial errors
lead to erroneous forecast
• Grid Point Statistical Interpolation (GSI) is
used for the NCEP NMM-WRF.
GSI – What’s New?
• Improved first guess and balance with
observations.
• Improved handling of moisture by allowing
temperature and satellite radiances to
affect the first guess.
• At least initially, precipitation effects (from
observations and radar) are not being
used (they were for the eta).
GSI – What’s New?
• Changes in GSI include:
– 4 km Snow Analysis (was 24 km)
– ~5 NM Sea Surface Temperature Analysis
(was 30 NM)
– Addition on GPS Precipitable Water Sensors.
New Snow Cover Analysis
What’s Different About WRF?
• The Vertical Coordinate System
– Eta had a “Stepped” vertical coordinate
system
– WRF uses a hybrid sigma – or terrain
following.
• Higher Resolution at High Terrain
• More Resolution in the Stratosphere
The Vertical Coordinate Systems
Terrain
• Because of the sigma coordinate system,
terrain is more realistic in WRF
Terrain in the Northeast
http://www.meted.ucar.edu/nwp/pcu2/wrftopo/topocompare.htm
The WRF-NAM is non-hydrostatic!
• What Does this mean?
• Vertical Motions are handled differently –
they’re explicitly predicted in the nonhydrostatic version.
• Stronger and More Details in the Vertical
Motion Fields.
• Important where there’s strong forcing
– Mountain Waves
– MCS or explosive cyclogenesis.
What to expect from the nonhydrostatic WRF?
• Look for more intense and detailed omega
in the NAM-WRF
• Different and more intense mountain
waves.
• Waves throughout the troposphere
• Changes to wind fields.
What’s Not Changed
• Horizontal Resolution – 12 km
• Domain is the same
• Physics
– Convective Parameterization
– Planetary Boundary Layer
Model Output Statistics (MOS)
• None
• eta based MOS will continue until the end
of the year from a special low resolution
(32 km) version of eta.
What else to expect?
• Improved details in high terrain compared
to eta.
– Primarily West Coast, but do expect
differences in even small terrain
• Noisier fields.
– Small feature will not be “damped” out like
they were in the eta.
Problems?
• Heavier Precipitation
• Explosive Cyclogenesis
Heavier Precipitation
• While WRF has same convective scheme
as eta, but non-hydrostatic effect can
result in significant additional amounts in
the QPF Field.
Extra Convective Precipitation in
NCEP’s NMM-WRF
Explosive Cyclogenesis
• Tropical Systems will tend to over
intensify.
– Look to NHC and GFS
• Extra Tropical Systems become overly
compact.
Example of Intensified Tropical
System
Explosive Cyclogenesis
dprog/dt
• Consistency and trends in an NWP model
have long been used as a subjective
forecasting technique by forecasters.
• This may provide little of no skill.
Thomas M. Hamill. 2003: Evaluating Forecasters' Rules of Thumb: A Study
of d(prog)/dt. Weather and Forecasting: Vol. 18, No. 5, pp. 933–937.
Conclusions
• The NCEP NMM-WRF implemented on
Jun 13th is a very different Model than eta.
• It is not a magic solution.
• Improvements?
– Yes!
– Higher Terrain
– Mountain Wave
– Details
Conclusions
• What to watch out for?
– Excessive Rainfall
– Higher vertical velocities in strong systems
– Spurious Tropical Cyclones
– Over intensified Cyclones.
References
• Operational Models Matrix: Characteristics of
Operational NWP Models:
http://www.meted.ucar.edu/nwp/pcu2/index.htm
• More Detailed Presentation at COMET:
http://meted.ucar.edu/nwp/NAMWRF_short/
• WRF Home Page
http://wrf-model.org/index.php