The Regional Analysis System for the Operational “Early” Eta Model: Original 80-km Configuration and Recent Changes

Abstract The analysis component of the National Centers for Environmental Prediction (NCEP) operational “early” 80-km eta model, as implemented in July 1993, is described. This optimum interpolation (OI) analysis is fully multivariate for wind and geopotential height (univariate for specific humidity) and is performed directly on the eta model's vertical coordinate. Although the eta OI analysis and model performance has been generally favorable when compared to the Limited-Area Fine Mesh Model (LFM) and the Nested Grid Model (NGM), deficiencies in the eta OI analysis fields have been observed, especially near the surface. A series of improvements to the eta OI analysis is described. A refinement to the eta model orography, which created a more realistic depiction of the model terrain, is also discussed along with the impact of these changes on analysis and model performance. These changes were implemented in the early eta system in September 1994. The operational configuration of the new mesoscale (29 km)...