Tropical forecasting at ECMWF: The influence of physical parametrization on the mean structure of forecasts and analyses

The period 1984–1985 has seen a dramatic improvement in the quality of the ECMWF operational forecasts for tropical regions. This improvement is discussed in terms of the model changes during this period. Revisions include the introduction of a parametrization of shallow cumulus convection, modifications to the parametrization of deep cumulus convection, a new cloud scheme, and an increase in horizontal resolution with a change in spectral truncation from T63 to T106. Impact of the various model changes is assessed through a set of forecast experiments and through parallel 10-day forecasts over a 20-day period. The results show that the improvements in tropical forecasts are mainly through the reduction of systematic errors in response to a more realistic tropical diabatic forcing. The impact of these changes on the analyses is also assessed. This is shown to be substantial, particularly for the thermal state.