Systematic Model Forecast Errors of Surface Cyclones in NMC's Nested-Grid Model, December 1988 through November 1990

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.