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Medium Range Ensemble

The medium range ensemble forecast suite 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 15 days.  it consists of 50 perturbed ensemble members and one unperturbed member, known as the ensemble control.  It provides a detailed description of future weather and an assessment of probability.

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• • is run four times daily:
    • Day0 to Day15 based on 00UTC and 12UTC data and results are disseminated T+0 to T+360.
    • Day0 to Day6 based on 06UTC and 18UTC data and results are disseminated T+0 to T+72.
  • has horizontal resolution of 9 Km and vertical resolution of 137 model levels.
  • has land-atmosphere and ocean-atmosphere coupling.
  • has 50 ensemble members plus an unperturbed ensemble control member.
  • runs with data analysed from the Short cut off data analysis (SCDA).
  • the unperturbed Ensemble Control Forecast runs before the medium range ensemble starts.
  • results from the Ensemble Control Forecast runs are used in Early cut off analysis (SCDA) and Long window data analysis (LWDA).
  • dissemination schedule:
    • Ensemble control forecasts, schedules and products: Atmospheric model analysis and 15 day forecast of Ensemble Control Forecast (Sets I-i, I-ii, I-iii as shown for 00UTC and 12UTC base times).
    • Ensemble control forecasts, schedules and products: Atmospheric model analysis and 6 day forecast of Ensemble Control Forecast (Set I-i as shown for 06UTC and 18UTC base times). 
    • Medium range forecasts, schedules and products:  Atmospheric model analysis and 15 day forecast ensemble (Set III as shown for 00UTC and 12UTC base times).
    • Medium range forecasts, schedules and products: Atmospheric model analysis and 6 day forecast ensemble  (Set III as shown for 06UTC and 18UTC base times).  

Ensemble control forecast

The ensemble control model structure is identical to that of the other ensemble members except that initial state and model physics are unperturbed.  The Ensemble Control Forecast might be incorrectly thought of as being the deterministic model yielding deterministic results.   It is not.  The unperturbed, Ensemble Control Forecast should be considered as only one member of the whole ensemble.  

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Any individual ensemble control forecast may not be the most skilful compared to any individual ensemble member forecast.  In isolation it cannot provide an estimate of forecast uncertainty or confidence.

Medium range ensemble forecasts

The ensemble members have identical model structure to the unperturbed control forecast 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 forecasts.   When averaged over many forecasts each of the perturbed members has lower skill than the unperturbed control member.  Nevertheless any individual forecast might show a higher skill.

The ensemble forecasts  provide a wide range of output, some of which is post-processed for ease of interpretation.  It supplies probability information to allow interpretation by the user of risk of severe weather and/or significant hazard.

Medium range forecast products

A range of ensemble products show information in different ways appropriate for different uses.  These include probabilities and 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.

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Note: The term "Ensemble Control" has replaced the term "Ensemble Control (ex-HRES)".   The terms "HRES" and "(ex-HRES)" are now obsolete.

Additional Sources of Information

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

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