Evaluation of the National Centers for Environmental Prediction Eta model for Subtropical South America

The performance of the National Centers for Environmental Prediction (NCEP) Eta model adapted to South America was examined using information from radiosondes, rain-gauges and satellites. The model's domain covers all South America and part of the adjacent oceans at a horizontal resolution of 80 km and 38 vertical levels. Its vertical coordinate, called "Eta", is a generalization of the sigma coordinate, but its surfaces are quasi-horizontal. Orography can then be defined as "steps" that are particularly well suited to reproduce sharp slopes like those of the Andes mountains. Initial fields and boundary conditions, updated every six hours, are taken from NCEP's global reanalyses.

The model forecasts reproduce all observed precipitation centers away from the Tropics, although in some cases the forecasts may be deficient with respect to magnitude. Of particular relevance for this study is the finding that spatial correlations between the model forecast and observed precipitation over Cuenca del Plata (the basin in southeastern South America) are almost as high as those obtained for the Mississippi River basin using forecasts of NCEP's operational Eta model. Other local and global models often produce spurious precipitation maxima near the Andes, but that was not apparent in our integrations.

The Eta model shares a deficiency common to many other models in its inability to simulate relatively dry conditions, in terms of precipitation, over northeast Brazil and the wet conditions over central/northwestern Brazil; additionally, the Atlantic ITCZ is missing. Given that the forecast of tropical moisture contents and moisture transports are similar to observational climatologies, it is suggested that the problem may be due to parameterizations. In particular, the convective scheme parameterization and the representation of surface processes seem to be the main reason for the model's limited performance in the tropics. It is also possible that the heat source due to the excessive precipitation over northeast Brazil could have associated subsidence on the neighboring areas of central Brazil and the Atlantic ITCZ.