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Keywords:

  • monthly forecasting;
  • ocean-atmosphere coupling;
  • heat waves;
  • MJO

Abstract

A combined medium-range and monthly-forecasting forecasting system is now operational at the European Centre for Medium-Range Weather Forecasts. Previously, these two systems were run separately. The new combined system provides skillful predictions of small-scale, severe-weather events in the early forecast range, accurate large-scale forecast guidance up to day 15 twice a day, and large-scale guidance up to day 32 once a week. In addition, the daily medium-range forecasts starting at 0000 utc are now coupled to an ocean general-circulation model after day 10 and persisted sea-surface temperature (SST) anomalies instead of persisted SSTs are applied when the atmospheric model is run in uncoupled mode. Average results indicate that the monthly forecasting scores are slightly higher in the Extratropics with this new combined system. In particular, the new system seems to produce better monthly forecasts for extreme events such as the 2003 heatwave over Europe or the 2007 wet summer over England. However, the monthly forecasting scores in the Tropics are slightly lower than with the previous system, most likely because of the lack of ocean-atmosphere coupling during the first ten days of the forecasts. This demonstrates that ocean-atmosphere coupling may be needed in medium-range forecasting and future plans include coupling the atmosphere to the ocean model from day 0. Average results based on 30 cases indicate that the medium-range forecasts benefit from the introduction of the ocean-atmosphere coupling after day 10, most especially the prediction of low-level temperature in the Tropics. Copyright © 2008 Royal Meteorological Society