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The ENS consists of an ensemble of 51 forecasts (current spatial resolution 9km) .  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.

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