Systems in seasonal forecasts

Imanol_Uriarte_Latorre · 26 February 2025 11:03

I’m downloading the Seasonal forecast daily and subdaily data on single levels (Seasonal forecast daily and subdaily data on single levels) and I want to use the ECMWF forecasts.

Currently the only system that provides data it is system 51, so i downloaded this one.

As I understand from the documentation, these systems are simply different methods of processing the data:

https://confluence.ecmwf.int/display/CKB/Detailed+list+of+parameters
https://confluence.ecmwf.int/display/CKB/Description+of+the+C3S+seasonal+multi-system
https://confluence.ecmwf.int/display/CKB/Description+of+SEAS5-v20171101+C3S+contribution
Announcements - Copernicus Knowledge Base - ECMWF Confluence Wiki (Check 12th October 2022)

But when I download the data, the system is a dimension, and for a single pixel and timestep, I have 51 different values.

Why is so? Are they 51 different simulations? Should I just simply average over that dimension?

I don’t find any information regarding this.

EDIT: For the sake of testing I downloaded data for system 4, and i just realized that it also includes 51 values. Turns out, this is another bug. No matter which system you select in the download page, it downloads all.

Eduardo_Penabad · 26 February 2025 17:08

Hello Imanol!

a quick read about what is “ensemble forecasting” might be helpful to disentangle some of the elements that are puzzling you.

For instance you can read here:

a quick fact sheet here: https://www.ecmwf.int/sites/default/files/medialibrary/2017-03/ecmwf-fact-sheet-ensemble-forecasting.pdf
a bit more detailed explanation about the rational of ensemble forecasting across timescales in the ECMWF systems here: Section 5 Forecast Ensemble (ENS) - Rationale and Construction - Forecast User Guide - ECMWF Confluence Wiki
and a bit of introductory explanation on the specifics of seasonal forecasting and C3S here: Seasonal forecasts and the Copernicus Climate Change Service - Copernicus Knowledge Base - ECMWF Confluence Wiki

But going to your questions/comments, there is absolutetly no bug in the behaviour you described and the data you are looking at is the expected outcome of your data requests.

Regarding “system”. It is the way used to identify a “version” of a forecast system (that is not only the set of models -atmosphere, ocean, sea-ice, etc-, but also the way initial conditions are created or the way uncertainty is sampled with ensemble members). Usually, at a given moment in time, for a given producing centre, only one system is run routinely. You can find in the “real-time” table of the “Summary of available data” details about when a particular system started real-time forecast production.
The keyword “system” as used here is simply a label to identify the different versions and, for instance, as of February 2025, the operational forecasting system from ECMWF is SEAS5 (system=51), from MetOffice is GloSea6 (system=603), from CMCC is SPS3.5 (system=35), from Météo-France is System8 (system=8), from Enviroment and Climate Change Canada are CanESM5.1p1bc (system=4) and GEM5.2-NEMO (system=5), etc
The case of ECMWF system=51 is a special one, as this is not a different forecast system than system=5, it is simply a different labelling of ECMWF SEAS5 (system=5) data used for the current data provision of ECMWF SEAS5 to the C3S activity. You can find more details about this in footnote (5) of the table mentioned above.
As you would have realised by now, after having read the information about ensemble forecasting, and the description of what “system” means, the data you retrieved from the CDS were all the available ensemble members for that specific forecast system and start date. And yes, the bit about start date is relevant here because typically, for a given forecast system, start dates in the hindcast (or reforecast) period have fewer members than for real-time forecasts.

I hope all that sounds helpful.

Regards,
Edu