Systematic Errors of the ECMWF Operational Model over Tropical Africa

Abstract

Five-day forecasts using the high-resolution T213L31 Operational Global Spectral Model of the European Centre for Medium-Range Weather Forecasts and associated analyses for summer 1995 are compared in this study to assess the model drift. It is shown here that there is extensive model warming of the lower troposphere especially over the Sahel (+7.5°C) and cooling of the middle troposphere in the tropical Southern Hemisphere. The surface heat wave episode diagnosed by the Climate Prediction Center in summer 1995 was severely overestimated (11°C). A likely cause of this major error is the inaccuracy of convection and boundary layer parameterizations in the case of extreme events. It is found that the model overestimates upper-level subtropical westerlies (+3 m s−1), low-level monsoon winds, and the Hadley circulation. The African easterly jet is weaker in the model than observed probably because initial easterly zonal wind errors evolve rapidly leading to underestimation of the jet at medium range. It is noteworthy that among climatological features associated with the tropical African summer season, the location and intensity of tropical easterly jet core over West Africa are correctly simulated and no upward shifting of subtropical jets is present in the 5-day forecasts. It is also found that model climatology of geopotential height in the middle troposphere around 700 hPa is relatively free of large-amplitude bias. Suggestions for possible approaches to remove biases are discussed.