Abstract

ECMWF will introduce a new seasonal forecast system (System 4) in November 2011 (*), following a line of research and development in extended-range predictions which has spanned almost three decades. The new system is expected to provide advances in the quality of forecast products, feed back onto the ECMWF core activities in terms of atmospheric model diagnostics and ocean modelling/analysis tools, allowing ECMWF to maintain a position of excellence in international programmes and activities on long-range forecasting. This paper illustrates the main methodologies and modelling configurations adopted in the new system, and presents a preliminary assessment of the system performance. The paper is broadly divided into three parts. The first part describes the configuration of the coupled model, ocean analysis system, and re-forecast set, illustrating the main areas of progress with respect to the current seasonal system (System 3). The second part of the paper deals with ” core” results on the coupled model biases and forecast scores for SST and atmospheric variables. It is shown that System 4 delivers improved predictive skill for a majority of parameters and regions, with the additional advantage of a better match between ensemble-mean errors and ensemble spread and therefore increased reliability. This occurs in spite of a cold bias in the tropical Pacific sea surface temperature, which originates from too strong trade winds simulated by the ECMWF atmospheric model in the central and western Pacific. The third part discusses some aspects of System 4 configuration and forecast skill which are worth a more detailed analysis. These include the configuration and scientific results from the new ocean re-analysis (ORA-S4), an assessment of predictive skill for large-scale rainfall anomalies and tropical storm properties, and results of experimentation with momentum flux correction aimed at investigating the impact of IFS tropical wind biases on the coupled system. In summary, the preliminary assessment of System 4 presented in this paper indicates that System 4 is able to deliver on all major goals that a state-of-the-art seasonal forecast system is expected to attain. As with any new operational system, however, the progress is not uniform, and this highlights the important role of extended-range simulations in providing feedbacks for future improvements in the physical and dynamical aspects of model formulation. (*) This Memorandum corresponds to the report presented to the ECMWF Scientific Advisory Committee on 3 October 2011. System 4 has actually been implemented operationally on 1 November 2011 and made available to Member States on 8 November 2011.

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Molteni2011New
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