Evaluation of April 1999 rainfall forecasts over South America using the Eta model

On April 1999 an excessive amount of rainfall over northern South America was predicted by the regional Eta model in the 24-h forecasts. This error was detected during the routine monthly forecast evaluation. In this study the error is described and compared with surface observations, images and precipitation estimates from satellites and global model precipitation forecasts. Large scale atmospheric conditions were characterized by anomalous northerly displaced position of the Intertropical Convergence Zone in the proximity of the continent. The excessive rainfall error is reduced with model integration period. This is shown by the equitable threat score and the bias score estimated at different forecast lead times. The error was not influenced by the global model forecasts, as the latter started with smaller amount of rainfall and increased with model integration. Forecast error profiles of model variables such as specific humidity and wind show that the errors increase with forecast lead time despite the improvement in the precipitation forecast errors. The model relaxes the lower troposphere toward a drier atmosphere with weaker trade winds. This fact may suggest an inadequacy of the current global model analyses as initial conditions to the regional model.