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, post-

processing 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 non-hydrostatic 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 non-hydrostatic 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 Pagehttp://wrf-model.org/index.php