Improvements in Quantitative Precipitation Forecasts with the Eta Regional Model at the National Centers for Environmental Prediction: The 48-km Upgrade

Abstract

Since 9 June 1993, the eta coordinate regional model has been run twice daily at the National Centers for Environmental Prediction (NCEP, previously the National Meteorological Center) as the NCEP's ?early? operational model. Its performance is regularly monitored in a variety of ways, with particular attention given to precipitation forecasts. Throughout this period, the eta model has demonstrated significantly increased accuracy in forecasting daily precipitation amounts compared to NCEP's Nested Grid Model (NGM). The model has shown a smaller but equally consistent advantage in skill against that of NCEP's global spectral model. Precipitation scores of these three operational models for the 6-month period March-August 1995 are presented. This interval is chosen because the 6-month-long periods September-February and March-August have been used in previous model comparisons and because an upgraded version of the eta model, run at 48-km resolution, was also regularly executed twice daily during the March-August 1995 period. It is thus included and highlighted in the present comparison. The 48-km eta carries cloud water as a prognostic variable and is coupled to a 12-h eta-based intermittent data assimilation system. It replaced the 80-km eta as the NCEP's early operational model on 12 October 1995. Compared to the then-operational 80-km eta, the 48-km eta has demonstrated substantially increased skill at all eight precipitation categories for which verifications are made. The increase in skill was greatest for the most intense precipitation, at the threshold of 2 in. (24 h)?1. A 24?48-h forecast of accumulated precipitation, resulting from Hurricane Allison as it was crossing the extreme southeastern United States, is shown as an example of a successful forecast of intense precipitation by the 48-km model. Reasons for the advantage of the eta model over its predecessor, the NGM, are reviewed. The work in progress is outlined.