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Every forecasting system that contributes to C3S will have a different lifetime, so different versions of the systems are expected to be changed, upgraded from by their original institutions. For the real-time forecasts just one version of each one of the contributors will be made available to C3S as at a real-given time forecasts. For instance, in November 2017 ECMWF has changed its operational seasonal forecasting forecast system from system 4 to SEAS5, but both systems will be were kept routinely running in parallel at ECMWF for a while. Despite that factHowever, the only version of ECMWF seasonal forecasts available at C3S from November 2017 onwards will be is from SEAS5.
How
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do seasonal forecasting systems build their ensembles? And how are data are produced?
"Burst" vs. "lagged" mode
In the last few decades, in the earth system modelling it has been an established technique Earth system prediction has established the use of "ensemble" runs to take into account , to quantify the effect of errors due to both the uncertainty in the initial conditions and model deficiencies. This means that the forecasting systems produced a set of "slightly" different runs of the same forecast which form - the members of the ensemble , in a way that the outcome - and thus the output of the forecasting forecast system is not a single model output solution, but a set of different results which allow to produce a forecast in terms of a probability distribution as opposed to a single deterministic forecastsolutions. Since, by design, all ensemble members are equally likely, the forecast offers a distribution of outcomes, rather than a single deterministic answer.
Different techniques are commonly used to build the different members of an ensemble forecast, but one of the most common ways to create a set of slightly different members that mainly maps so that they sample the uncertainty in the initial conditions, is the use of a "lagged" approach in the start dates.:
- "Burst" mode: all the members are initialized (with conditions on the same start date) at the same value , but from slightly different (perturbed) initial states, intended to sample the uncertainty in observations. (e.g. all members initialized on 1st March 2017, ; this is the case for ECMWF's system)
- "Lagged" mode: members are initialized in on different start dates, the differences between which are sufficiently small (e.g. 2 members initialized every day of the month, ; this is the case for Met Office system)
Among all the systems that contribute to the C3S seasonal forecasts, some of them have opted to use a "burst" mode, while some others lag the start dates of the members of their ensembles. For more details, refer to the table below in the "Production schedules" subsection.
Fixed vs. on-the-fly hindcasts
Due to For several reasons, from computer load balance to flexibility in the introduction of changes in the systems, the different seasonal forecast contributors to C3S use different schedules to produce their hindcast sets:
- Fixed fixed hindcasts: . Some systems are designed so their expected lifetime will be around 4-5 years. Thus, once Once the system has been designed and tested, its development gets frozen and exactly that version of the model is used to run all the hindcast members ensemble hindcasts for the whole climate period for that model. In that way, the hindcasts are produced reference period are run. The advantage is that this reference dataset is available well in advance the of real-time forecasts and they constitute a fixed dataset during the lifetime of that seasonal forecasting systembeing 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.
- onOn-the-fly hindcasts: . Some other systems run the necessary set of hindcasts every time they produce 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 . They are produce just is produced, slightly in advance (a few weeks) of the real-time forecast and using exactly the same version of the forecasting system. In this way changes in the system can be introduced more frequently and the computing of the hindcasts can be spanned over a longer period, then balancing the load of the computing resourcesThis also offers the advantage of balancing the requirement for computing resources, but the compromise is the regular change of the model climatology.
Production schedules
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for the seasonal forecasting systems contributing to C3S
In the following table, it is shown The following summarises the information about the ensemble sizes, start dates and production schedule for the seasonal forecasting systems contributing to C3S.
| SYSTEM | FORECASTS | HINDCASTS | |||
|---|---|---|---|---|---|
| ENSEMBLE SIZE and START DATES | PRODUCTION | ENSEMBLE SIZE and START DATES | PRODUCTION | ||
| ECMWF | System 4 | 51 members start on the 1st | real-time | 15 members start on the 1st | fixed dataset |
| SEAS5 | 51 members start on the 1st | real-time | 25 members start on the 1st | fixed dataset | |
| Météo-France | System 5 (a) | 51 members 26 start around the 22th | real-time | 15 members | fixed dataset |
| System 6 | 51 members 1 starts on the 1st | real-time | 25 members 1 starts on the 1st | fixed dataset | |
| MetOfficeMet Office | GloSea5 (b) | 2 members every day (c) | real-time | 7 members on the 1st | on-the-fly produced around 4-6 weeks in advanced |
| CMCC | SPSv3(d) | 50 members start on the 1st | real-time | 40 members start on the 1st | fixed dataset |
| DWD | GCFS1 | 50 members start on the 1st | real-time | 30 members start on the 1st | fixed dataset |
(a) Despite they are being produced in a lagged mode, the data from MeteoFrance system5 is produced Météo-France system 5 is currently encoded and provided at in CDS as if all the members were initialized on the 1st.
(b) The production schedule of the MetOffice Met Office forecasting system doesn't prescribe how to build an ensemble for an a specific nominal start date. The following choices are currently in use for the data archived in C3S:
FORECASTS: the most recent 50 members starting on or before the 1st of the month (NOTE: The original daily/subdaily data from all the daily members, and not just those 50, is processed and made available at C3S). The monthly mean data is currently encoded and provided in CDS as if all the members were initialized on the 1st.
HINDCASTS: 28 members (7 starting on the 1st of the month and 7 on each of the 9th,17th and 25th of the previous month)
(c) Due to the flexibility of MetOffice the Met Office forecasting system, incidences forecast failures on a given data date are not usually recovered by re-running the missed forecast but incrementing the number of members in forecasts, but by running more members on one of the following days.
Example: An incidence happened affecting the 22nd of August/incident affected the 22 August 2017 forecast so no members are available for that date. Instead, there are 4 members available starting on the 23rd of August(d) CMCC seasonal forecast system routinely produces 80 real-time forecast members but just 50 of them are made publicly available as their contribution to C3Swere started on 23 August 2017.