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.
The medium range ensemble including the control member (ENS):
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.
It runs before the medium range ensemble of perturbed members starts (on the schedule of HRES in Cy48 and earlier). Although the ensemble control forecast is available earlier than ensemble forecasts, it should nevertheless be considered as an equally valid member among the other perturbed ensemble members.
The ensemble control forecast provides a wide range of output, some of which is post-processed for ease of interpretation. It does not supply probability information; that requires the products of the perturbed ensemble.
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.
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.
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.
Ensemble forecasts are run coupled to the Wave model (ECWAM), and the Dynamic Ocean model (NEMO) which includes modelling of sea-ice (SI3). 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).
The current medium range ensemble, including the control, has horizontal resolution of ~9km and a vertical resolution in 137 levels. The high horizontal and vertical resolutions have the benefit of feeding energy from smaller to larger scale features (as happens in the real atmosphere). But also risks developing or amplifying spurious small features, or suppressing existing ones that should be retained.
A set of medium range re-forecasts provide a basis for the medium range model climate (M-Climate).
See a comprehensive description of medium range forecast output and medium range ensemble forecast output. Some are additional products that are only available by arrangement.
Note: The term "Ensemble Control" has replaced the term "Ensemble Control (ex-HRES)". The terms "HRES" and "(ex-HRES)" are now obsolete.
(Note: In older material there may be references to issues that have subsequently been addressed)
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