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Extended-Range Forecast
The ENS runs are extended on Mondays and Thursdays from Day15 to Day46 based on 00UTC data time.The Extended Range extended range (monthly) forecasts bridge provide a range of possible future weather states in the extended range. It bridges the gap between medium-range and seasonal forecasting. However, it is not an extension of the medium-range forecasts but is a completely separate system. It runs daily from
The extended range forecast ensemble:
- is run once daily, base time 00 UTC
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- , producing a 46 day forecast).
- has horizontal resolution of 36
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- Km and vertical resolution of 137 model levels
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- . These values are the same as the IFS version before Cy48r1.
- has land-atmosphere and ocean-atmosphere coupling.
- has 100 ensemble members plus an unperturbed ensemble member (CONTROL).
The ensemble consists of one unperturbed control member (the control) and 100 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 extended range re-forecasts provide a basis for the extended-range model climate (ER-M-Climate.
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climate). The extended range forecast suite is not an extension of the medium-range forecasts but is a completely separate system.
The extended-range forecast provides an overview of the atmospheric evolution for the 32-day period (Day15 to Day46).
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Standard output products focus mainly on the week-to-week changes in the weather. The chart products generally are presented in terms of anomalies relative to the extended-range model climate (ER-M-climate)
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. The anomalies are mainly shown as 7-day means for calendar weeks Monday to Sunday. From these one can infer whether conditions (e.g. temperature, rainfall, etc
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) are likely to deviate from the average for a location and time of year, and whether such an anomaly could be extreme.
Extended Range forecasts are coupled to the Wave model (ECWAM) and the Dynamic Ocean model (NEMO) run with resolution of ~28km.
Since the ENS and Extended Range are run with different resolutions, the corresponding forecast fields are generated using effectively a different model having the The medium range and extended range models have the same structure and physics etc, but which are based on different reduced Gaussian grid in physical space, a different . but different spacial (but not vertical) resolutions and differing land-sea mask, masks and a different orography. Users should be aware of this, especially when generating that the output is effectively from different models. This is especially important if a time-series of products for a single location that crosses the ENS to Extended Range truncation forecast step (e.g. when generating a time-series of a variable or an meteogram spanning Day15), and crosses the boundary between the end of the ensemble 15 day forecast and the start of the extended range. Users need to develop interpolation procedures that take these changes into account.
Extended Range Products
The extended range output is available on the ECMWF web charts and ecCharts. Guidance on the Interpretation of the extended range products is discussed elsewhere in the guide.
Interface between ENS and Extended Range products at Day15
The Extended Range does not start from Day0 but uses the results at Day14 of the ENS forecast as a starting point. However, the resolutions of ENS and Extended Range differ, and the high-resolution spectral representations within the ENS have to be smoothed to the lower resolution of the Extended Range. This is achieved by using a 24-hour overlap period to reduce the impact of this smoothing on the fields that are most sensitive (e.g. convection and large-scale precipitation) so that the changes in forecast parameters between ENS and Extended Range at Day15 is minimised.
The change in resolution creates incorrect values for accumulated fields (such as precipitation) that run beyond Day15 (i.e. spanning forecast time steps 360‐366 hours). To reduce the differences, the accumulated fields are reinitialized at the reduced resolution of the Extended Range for Day15 from interpolated fields obtained from ENS forecast using a conserving interpolation scheme. These fields are stored in Fields Data Base (FDB) and the Meteorological Archival and Retrieval System (MARS) under:
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.
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Similar fields are stored for wave products from WAM.
A list of the overlap fields is available.
Using EFOV fields to obtain a correct accumulation across Day15
For time ranges that do not span Day15, the normal method of calculation should be used. For example:
Accumulation (TPt+72 to TPt+120) = (TPt+120 –TPt+72) for the 72 to 120 hour (start of Day3 to end of Day4) total precipitation. Here the accumulations from ENS are used.Accumulation (TPt+72 to TPt+360) = (TPt+360 –TPt+72) for the 72 to 360 hour (start of Day4 to end of Day15) total precipitation. Here the accumulations from ENS at T+360 may be used.Accumulation (TPt+384 to TPt+432) = (TPt+432 –TPt+384) for the 384 to 432 hour (start of Day16 to end of Day17) total precipitation. Here the accumulations from Extended Range are used.
For time ranges that do span Day15, the calculation should use ENS accumulations etc up to T+360 and then use the ENS accumulation interpolated to the lower resolution of the Extended Range held in EFOV as the base for the rest of the calculation. In other words such calculations have to be split into two parts. For example, consider total precipitation (TP) for 48‐hours, from 336 to 384 hours (start of Day14 to end of Day16) for one ENS member:
Accumulation (TPt+336 to TPt+384) = (TPt+360 – TPt+336) + (TPt+384 –TPVARt+360)
Where: TP is the ENS field. TPVAR is the corresponding EFOV TP field. The subscript indicates the forecast step (hrs). The value TPt+384 is the total accumulation from T+0 to T+384 which will have used the interpolated field (TPVARt+360) at the T+360 change of resolution.
Specialised Products
Specialised products for the Extended Range include information on potential tropical cyclone activity and evolution of the Madden-Julian Oscillation (MJO). The MJO is characterized by an eastward propagation of an area of more organised convection in the tropics, typically initiated over the Indian Ocean and . It is important as it can influence developments elsewhere on the globe:
- monsoon evolution,.
- tropical cyclogenesis near the Americas,.
- El Niño/La Nina development (ENSO),.
- aspects of the synoptic pattern over higher northern and southern latitudes including the North Atlantic Oscillation (NAO).
The MJO has an influence on the forecast skill over the extra-tropics and MJO predictions are skilful well beyond Day20.. Typically the forecast reliability is enhanced when an MJO event is detected in the forecast initial conditions.
Additional Sources of Information
(Note: In older material there may be references to issues that have subsequently been addressed)
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- Read more on the justification for extended range forecasting, the structure of the extended range ensemble, the extended range output.
- Read about the simulation of the Madden-Julian oscillation and its impact over Europe.
- Some information on the principle of ECMWF Variable Resolution Ensemble Prediction System (VAREPS) may be found at:
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