Evaluation of the 29-km Eta Model. Part I: Objective Verification at Three Selected Stations

Abstract

This paper describes an objective verification of the National Centers for Environmental Prediction 29-km Eta Model from May 1996 through January 1998. The evaluation was designed to assess the model?s surface and upper-air point forecast accuracy at three selected locations during separate warm (May?August) and cool (October?January) season periods. In order to enhance sample sizes available for statistical calculations, the objective verification includes two consecutive warm and cool season periods. The statistical evaluation identified model biases that result from inadequate parameterization of physical processes. However, since the model biases are relatively small compared to the random error component, most of the total model error results from day-to-day variability in the forecasts and/or observations. To some extent, these nonsystematic errors reflect the variability in point observations that sample spatial and temporal scales of atmospheric phenomena that cannot be resolved by the model. On average, Meso Eta point forecasts provide useful guidance for predicting the evolution of the larger-scale environment. A more substantial challenge facing model users in real time is the discrimination of nonsystematic errors that tend to inflate the total forecast error. It is important that users maintain awareness of ongoing model updates because they modify the basic error characteristics, particularly near the surface. While some of the changes in error were expected, others were not consistent with the intent of the model updates and further emphasize the need for ongoing sensitivity studies and localized statistical verification efforts. Objective verification of point forecasts is a stringent measure of model performance, but when used alone, is not enough to quantify the overall value that model guidance may add to the forecast process. Therefore, results from a subjective verification of the Meso Eta Model over the Florida peninsula are discussed in the companion paper by Manobianco and Nutter.