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Seasonal forecasts and the Copernicus Climate Change Service (C3S)
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The C3S seasonal forecast products are based on data from several state-of-the-art seasonal prediction systems. Multi-system combinations, as well as predictions from the individual participating systems, are available. The centres currently providing forecasts to C3S are: ECMWF, The Met Office and Météo-France; in the coming months data produced by , Deutscher Wetterdienst (DWD) and , Centro Euro-Mediterraneo sui Cambiamenti Climatici (CMCC) will be included in the C3S multi-system), National Centers for Environmental Prediction (NCEP), Japan Meteorological Agency (JMA) and Environment and Climate Change Canada (ECCC), and you can find the details about when each system was introduced in the information contained in the Summary of available data.
Each model simulates the Earth system processes that influence weather patterns in slightly different ways, makes slightly different approximations, leading to different kinds of model error. These errors typically increase with the increase of integration time, so that the accumulated model errors become significant in comparison to the signal that the model is meant to predict. Some such errors are shared by the different models but others are not, so combining the output from a number of models enables a more realistic representation of the uncertainties due to model error. In most cases, such combined forecasts are, on average, more skilful than forecasts from the best of the individual models.
Currently, the C3S seasonal service offers graphical forecast products, available on the C3S web site, and public access to the forecast data, via the C3S Climate Data Store (CDS).
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- fixed hindcasts. Some systems are designed so their expected lifetime will be around 4-5 years. Once the system has been designed and tested, ensemble hindcasts for the whole reference period are run. The advantage is that this reference dataset is available well in advance of real-time forecasts being issued, and its properties (biases, skill) can be quantified once for repeated use. As this is a very expensive exercise, it cannot be repeated too often and thus the system remains fixed for a long period of time.
- on-the-fly hindcasts. Some systems prioritise more frequent upgrades, which means that the hindcast sets have to be run more frequently. To achieve this in practice, the full hindcast set is run every time a new real-time forecast is produced, slightly in advance (a few weeks) of the real-time forecast and using exactly the same version of the forecasting system. This also offers the advantage of balancing the requirement for computing resources, but the compromise is the regular change of the model climatology.
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The following summarises the information about the ensemble sizes, start dates and production schedule for the seasonal forecasting systems contributing to C3S.
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SEAS5
(CDS system: 5)
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System 5
(CDS system: 5)
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51 members (a)
26 start on the first Wednesday after the 19th
25 start on the first Wednesday after the 12th
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15 members start on the first Wednesday after the 19th (a)
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51 members
1 starts on the 1st
25 start on the 25th
25 start on the 20th
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25 members
1 starts on the 1st
12 start on the 25th
12 start on the 20th
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System 7
(CDS system: 7)
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51 members
1 starts on the 1st
25 start on the last Thursday of the previous month
25 start on the penultimate Thursday of the previous month
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25 members
1 starts on the 1st
12 start on the last Thursday of the previous month
12 start on the penultimate Thursday of the previous month
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7 members on the 1st
7 members on the 9th
7 members on the 17th
7 members on the 25th
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on-the-fly
produced around 4-6 weeks in advance
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SPSv3
(CDS system: 3)
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CFSv2(b)
(CDS system: 2)
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4 members start each day
1 member per start hour: 0, 6, 12, 18
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4 members start every 5 days (e)
1 member per start hour: 0, 6, 12, 18
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This document has been produced in the context of the Copernicus Climate Change Service (C3S). The activities leading to these results have been contracted by the European Centre for Medium-Range Weather Forecasts, operator of C3S on behalf of the European Union (Delegation agreement signed on 11/11/2014). All information in this document is provided "as is" and no guarantee or warranty is given that the information is fit for any particular purpose. The users thereof use the information at their sole risk and liability. For the avoidance of all doubt, the European Commission and the European Centre for Medium-Range Weather Forecasts have no liability in respect of this document, which is merely representing the author's view. |
(a) Despite being produced in a lagged mode, the data from Météo-France forecasting systems is currently encoded and provided in CDS as if all the members were initialized on the 1st.
(b) The production schedules of forecasting system with lagged start dates don't prescribe how to build an ensemble for a specific nominal start date. The choices currently in use for the C3S products for those forecasting systems can be found within the information about the concept of nominal start date.
(c) Due to the flexibility of the Met Office forecasting system, forecast failures on a given date are not usually recovered by re-running the missed forecasts at a later date, but by running more members with initial conditions of the day of recovery.
Example: An incident affected the 22 August 2017 forecast so no members are available for that date. Instead, 4 members were initialised on 23 August 2017.
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