I. Common Characteristics of operational NWP Models from NCEP

Solve "Primitive Equations"

Hydrostatic

Parameterize sub grid scale processes

Subject to initial condition error

Provide physically realistic simulations regardless of model skill II. Specific Modeling System Chracteristics

Numerical method (e.g., Finite Difference, Spectral, or Finite Element)

Numerical accuracy and speed (higher accuracy usually implies less speed)

Vertical Coordinate (sigma, ETA, isentropic, hybrid)

Horizontal and vertical resolution (forecast skill does not necessarily increase with higher resolution)

Terrain representation

Physical parameterizations (PBL, sub-grid scale convection, grid-resolved convection, radiation, diffusion, etc...)

Limited area or global

Initial conditions

III. Operational NCEP Models

A. NGM (Nested Grid Model)

Limited area model running operationally since 1980's

90 km, 16-level

Sigma-coordinate

Finite difference

Kuo cumulus parameterization (low-level moisture convergence)

Grid scale precipitation occurs when RH > 95% with precipitation falling into and evaporating in layers with RH < 95%

Model output used for NGM MOS

B. AVN (Aviation)

Global model running for worldwide aviation and medium range forecast guidance

Sigma coordinate

Spectral

T126 (~80 km), 28 levels

Late data cut-off

Model output used for AVN MOS

C. ETA

Limited area model running operationally for a few years

32-km horizontal resolution, 55 levels(?)

Eta "step mountain" coordinate

Finite difference

Betts-Miller cumulus parameterization

Explicit cloud water prediction

Early data cut-off time for initial conditions

No MOS based statistics

D. RUC (Rapid Update Cycle)

Limited area model running for data assimilation and short range forecasting

40-km horizontal resolution

Hybrid coordinate

Analyses every 1 h, 12-h forecasts every 3 h F. MRF

Global model running for medium range forecast guidance

Similar numerics, physics, and resolution as AVN

Forecast output to 16 days ("dream-prog land")

E. MRF Ensembles

Ensemble forecasts for medium range prediction

Based on MRF/AVN forecast model

17 forecasts (1-T126 MRF at 0Z, 1-T62 MRF at 0Z, 1-T126 at 12Z and 14 forecasts with perturbed initial conditions)

Can be used to examine model sensitivity to intial condition uncertainty

Usefulness of ensembles can be limited by weaknesses of initial condition perturbation methods, number of ensembles, and model imperfections and biases

Ensemble mean is not necessarily the best forecast of the 17

Verification does not necessarily lie within the ensemble members

IV. Thoughts to live and learn by

Model resolution does not necessarily correlate with forecast skill

"Model of the Day" forecast techniques are popular

Large-scale error growth is an important factor limiting the skill of higher resolution models...always examine the quality of a model's large-scale forecast

V. References and on-line links

Mittelstadt, J., 1995: Introduction to Ensemble Forecasting. Western Region Technical Attachment No. 95-29.

Staudenmaier Jr., M., 1996: A Description of the Meso ETA Model. Western Region Technical Attachment No. 96-06.

Staudenmaier Jr., M., 1996: The Explicit Cloud Prediction Scheme in the Meso ETA Model. Western Region Technical Attachment No. 96-29.

Staudenmaier Jr., M., 1996: The Convective Parameterization Scheme in the Meso ETA Model. Western Region Technical Attachment No. 96-23.

Kalnay, E. and Z. Toth, 1996: Ensemble Prediction at NCEP. Preprints, 15th Conference on Weather Analysis and Forecasting. Norfolk, VA., J19-J20.

Kalnay, E., G. DiMego, S. Lord, H.-L. Pan, M. Iredell, M. Ji, D. B. Rao, and R. Reynolds, 1996: Recent Advances in Modeling at the National Centers for Environmental Prediction. Preprints, 15th Conference on Weather Analysis and Forecasting. Norfolk, VA., J3-J8.

Kanamitsu, M., 1989: Description of the NMC global data assimilation and forecast system. Wea. Forecasting, 4, 335-342.

Black, T. L., 1994: The new NMC mesoscale Eta model: Description and forecast examples. Wea. Forecasting, 9, 265-278.

Hoke, J.E., N. A. Phillips, G. J. DiMego, J. J. Tuccillo, and J. G. Sela, 1989: The Regional Analysis and Forecast System of the National Meteorological Center. Wea. Forecasting, 4, 323-334.

Petersen, R. A., G. J. DiMego, J. E. Hoke, K. E. Mitchell, and J. P. Gerrity, 1991: Changes to NMC's Regional Analysis and Forecast System. Wea. Forecasting, 6, 133-141.

Updated May 1, 1997