The ENS is run twice daily giving forecasts to Day15 based on 00UTC and 12UTC data times.
The ENS consists of an ensemble of 51 forecasts (current spatial resolution 18km) . It consists of a control member (CTRL) which is unperturbed and 50 perturbed members that are similar to the CTRL but their initial states and model physics have been perturbed to explore the currently understood range of uncertainty in the observations and the model. The ensemble members provide a range of possible future weather states. When averaged over many forecasts each of the perturbed members has lower skill than the CTRL (or, for that matter, HRES), though any individual forecast might show a higher skill. There is a range of products to present the information from the ensemble in different ways, appropriate for different uses including probabilities, clusters of alternative scenarios, and tropical cyclone and extra-tropical cyclone products. Certain important products are based on, or directly show, anomalies relative to the ENS model climate (M-climate), such as meteograms and the Extreme Forecast Index and Shift of Tails (EFI/SOT ). From these one can infer, for example, whether conditions (temperature, rainfall, etc.) are likely to deviate from the average for the location and time of year, and whether such an anomaly could be extreme. The ENS products and presentation in chart or diagram form are described later in the user guide.
ENS forecasts are run coupled to the Wave model (ECWAM) and the Dynamic Ocean model (NEMO) which includes modelling of sea ice (LIM2). Atmosphere-ocean coupling occurs every hour. This high-frequency coupling generally has a positive impact on the forecast development of synoptic-scale systems, such as tropical cyclones.
The ENS runs are extended at lower resolution on Mondays and Thursdays from Day15 to Day46 based on 00UTC data time (the extended range forecasts).
Definitive information on the ECMWF dissemination schedule and products can be found in the the Catalogue of ECMWF real-time products (ENS: Set iii).
(Note: In older material there may be references to issues that have subsequently been addressed)