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Richard H. Grumm and Anthony L. Siebers

Abstract

A quantitative assessment has been made of the surface anticyclone forecast errors found in the operational nested grid model (NGM) run at the National Meteorological Center (NMC). Preliminary results covering a period from 1 December 1988 to 31 August 1989 reveal that the NGM predicts the central pressure of surface anticyclones to be too low over much of central and eastern North America during the winter and spring, especially along the track of transient anticyclones. The NGM tends to predict surface anticyclone pressure to be too high over the eastern Pacific and portions of the western Atlantic during winter, spring and summer. Pressure errors grow by forecast length and season. The 48-h forecast errors are larger in magnitude and better defined than the 24-h forecasts. The winter and spring pressure errors are better organized and have larger magnitudes than in summer.

Thickness (1000–500 mb) errors over the anticyclone center indicate an overall warm bias, especially over the North American continent and the adjacent western Atlantic Ocean, where anticyclones tend to be transient. Areas of negative thickness errors (cold bias) are found over the oceans and the elevated terrain of western North America. In general, the model places surface anticyclones too far south and east of the verifying position in the colder months.

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Richard H. Grumm and Anthony L. Siebers

Abstract

Results from a study examining the performance of the nested grid model (NGM) and the aviation run of the global spectral model (AVN) in predicting surface cyclones during January 1990 revealed that the AVN slightly outperformed the NGM in forecasting cyclone central pressures and placement. Although both models performed better for deepening systems than filling systems, the AVN outperformed the NGM in predicting the characteristics of filling cyclones.

Overall, the NGM tended to overdeepen surface cyclones. A large part of the pressure error was due to the model's inability to properly fill cyclones and a tendency to forecast systems to deepen when they were observed to be filling.

The AVN tended to underdeepen surface cyclones with the deepening rate errors near 2 mb at 12 h and less than 1 mb by 48 h. The overall pressure errors for deepening cyclones appeared to be linked to a spin-up problem in the AVN and may have also been associated with the AVN cold bias in 1000- to 500-mb thickness forecasts.

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Richard H. Grumm and Anthony L. Siebers

Abstract

Preliminary results from a study examining the performance of the nested grid model (NGM) in predicting cyclones covering a period from 13 November 1988 to 31 Match 1989 reveal that the NGM tends to overdevelop surface cyclones over continental regimes and underdevelop surface cyclones in oceanic regimes. The results also indicate an overall cold bias in the model forecasts of thickness and 850 mb temperatures over the cyclone center. Displacement error data indicate the NGM tends to move cyclones too slowly in the southern half of the forecast domain.

A semiautomated method has been developed at the National Meteorological Center (NMC) to track and verify sea level pressure features in the NGM. The method allows the user to interactively track a system, store its coordinates, and then retrieve information about the system from selected model forecast and analysis grids. This information can then be used to determine systematic forecast errors, compare past forecasts with the most recent forecast, and produce climatological tracks of forecast and observed systems.

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Richard H. Grumm, Robert J. Oravec, and Anthony L. Siebers

Abstract

Systematic errors in the nested-grid model (NGM) forecasts of surface cyclones are examined over a two-year period from 1 December 1988 through 30 November 1990. The parameters examined include the location, central pressure, 850-mb temperature, and the 1000-500-mb thickness over the center of the surface cyclone. The mean cyclone position error was typically 150 km at 12 h and 225 km at 24 h, and grew to about 350 km by 48 h. The overall mean cyclone pressure error was −0.57 and −0.68 mb at 24 and 48 h, respectively.

The results show that the skill of the NGM forecasts of surface cyclones displayed both seasonal and annual variability. The seasonal variability is represented by overall smaller errors in the summer and larger errors in the winter.

The NGM tended to overdeepen surface cyclones in all but the summer months. A large part of the pressure error was due to the model's inability to fill cyclones properly and a tendency to forecast systems to deepen when they were observed to fill. About 15% of the time in the winter months, the NGM forecast cyclones to deepen when they were observed to fill.

The NGM had difficulty detecting the initial development of surface cyclones, especially near the elevated terrain of western North America and along the track of transient cyclones. In these same regions, the NGM tended to forecast cyclones that were not observed. There was a preponderance of both nonobserved and nonforecast cyclones over the elevated terrain of North America, indicating that the NGM has difficulty with orographic effects.

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