The next extended-range configuration for IFS Cycle 48r1
Vitart, Frédéric ; Balmaseda, Magdalena A. ; Ferranti, Laura ; Fuentes, Manuel
Extended-range forecasts in ECMWF's Integrated Forecasting System (IFS), which are predictions beyond two weeks but less than a season, became operational in October 2004 to fill the gap between medium-range and seasonal forecasting. Originally, they were produced by running coupled ocean-atmosphere integrations up to day 32 at a TL159 horizontal resolution (about 110 km) every Wednesday at 00 UTC. At that time, the extended-range forecasting system was run separately from the medium-range and seasonal forecasting systems. Since 2004, the configuration of the ECMWF extended-range forecasting system has changed several times. A particularly important change took place in 2008, when the medium-range and extended-range forecasting systems were merged. Extended-range forecasts were then produced by extending medium-range forecasts up to day 32, at a lower atmospheric horizontal resolution beyond day 15. In 2011, the frequency of extended-range real-time forecasts was increased to twice a week, and in 2015 the forecast length was increased from 32 to 46 days. Since 2004, the model resolution of extended-range forecasts has also increased, from about 100 km to about 36 km and from 40 to 137 vertical levels. The current configuration of real-time extended-range forecasts is the following:<br>? 46-day integrations<br>? Every Monday and Thursday at 00 UTC<br>? 51-member ensemble<br>? Tco639 resolution (about 18 km) up to day 15 and Tco319 (about 36 km) after day 15<br>? 137 vertical levels.<br>Re-forecasts are used for calibration of real-time forecasts and for extended-range forecast skill assessment. In the current configuration, they are produced twice a week, starting the same day and month as the real-time forecasts, but for the past 20 years and with an 11-member ensemble. The availability of the new Atos high-performancecomputing facility in Bologna (Italy) provides an opportunity to enhance the ECMWF extended-range forecasting system further. The proposed changes, which will be introduced with IFS Cycle 48r1 in 2023, are the following:<br>? Daily instead of twice-weekly frequency<br>? 101 instead of 51 ensemble members<br>? Extended-range integrations will be run separately from higher-resolution medium-range forecasts<br>? Tco319 (about 36 km) horizontal resolution from day 0 to 46.<br>This configuration was supported by users, who expressed their feedback in a survey conducted in 2021. The enhancement in the extended-range configuration complements the strategy followed in the medium range, since it focuses on increasing the ensemble size and forecast frequency rather than increasing the model resolution. This article will discuss the main motivations behind these changes.</p>
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