ENS - Ensemble Forecasts

The ensemble forecast suite (ENS) provides a range of possible future weather states in the medium range.  This allows investigation of the detail and uncertainty in the forecast during the first 10 days or 15 days.

The medium range ensemble (ENS):

    • is run four times daily:
      • base times 00 UTC and 12 UTC, producing a 10 day forecast (T+0 to T+240) and also a 15 day forecast (T+0 to T+360).
      • base times 06UTC and 18UTC, producing a 6 day forecast (T+0 to T+144). Results from these runs are:
    • has horizontal resolution of 9 Km and vertical resolution of 137 model levels.  These values are the same as those of the High Resolution (HRES) in earlier IFS versions.
    • has land-atmosphere and ocean-atmosphere coupling.
    • has 50 ensemble members plus an unperturbed control member (CTRL).
    • the unperturbed control member (CTRL) corresponds to the High Resolution (HRES).
    • documentation gives details of medium range products and dissemination schedules

The ensemble consists of one unperturbed control member (CTRL) and 50 perturbed members.  These are similar to the control member but differ only that initial states and model physics have been perturbed.  This allows  exploration of the currently understood range of uncertainty in the observations and the model.   A set of medium range re-forecasts provide a basis for the medium range model climate medium range model climate (M-Climate).

The medium range 10 day and 15 day ensemble forecasts run with data analysed from the short cut off data analysis (SCDA).  

When averaged over many forecasts each of the perturbed members has lower skill than the control member.  Nevertheless any individual forecast might show a higher skill.  A range of ensemble products show information in different ways and appropriate for different uses.  These include 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 M-climate.  Particularly important are meteograms and the Extreme Forecast Index and Shift of Tails (EFI/SOT).    The ENS products and chart presentations show where and when weather may deviate from the average for the location and time of year, and if such an anomaly could be extreme.

Ensemble 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 (e.g. tropical cyclones).

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)