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ENS - Ensemble Forecasts

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 9km) .  It consists of a control member (CTRL) which is unperturbed and ensemble forecast suite (ENS) 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 CTRL 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.

The ensemble members provide a range of possible future weather states in the medium range.  When averaged over many forecasts each of the perturbed members has lower skill than the CTRL 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.

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