Systematic Surface Anticyclone Errors in Nested Grid Model Run at NMC: December 1988–August 1989

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.