Update Copy of ENS - Ensemble Forecasts

ENS - Ensemble Forecasts

The ensemble forecast suite (ENS) forecasts provide a range of possible future weather states in the medium range.

The ensemble forecast suite runs daily from base times 00 UTC and 12 UTC out to day 15.  It has horizontal resolution of 9 km, 137 model levels in the vertical and has and-atmosphere and ocean-atmosphere coupling.  The ensemble consists of one unperturbed member (the control) and 50 perturbed members that are similar to the control but their initial states and model physics have been perturbed to explore the currently understood range of uncertainty in the observations and the model.  A set of medium range re-forecasts provide a basis for M-Climate.

When averaged over many forecasts each of the perturbed members has lower skill than the control (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).

Additional Sources of Information

(Note: In older material there may be references to issues that have subsequently been addressed)

• Read an amplification of the above regarding medium-range ensembles and associated Charts and Products
• View the ECMWF eLearning module on the Concept of Ensemble Predictions.
• View the ECMWF eLearning module on the Ensemble Forecasting Introduction - Sources of Forecast Uncertainty.