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Note that for reasons related with the availability of computing resources, the hindcasts usually have fewer ensemble members per start date than the real-time forecasts, e.g. ECMWF SEAS5 5 has 51 members for the real-time forecasts, and just 25 members for the hindcasts.

Seasonal forecasting systems' versions and updates

blahblahblah (system keyword, etc)

How the seasonal forecasting systems build their ensembles? And how data are produced?

"Burst

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" vs. "lagged" mode

In the last few decades, in the earth system modelling it has been an established technique the use of "ensemble" runs to take into account 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 of the forecasting system is not a single model output but a set of different results which allow to produce a forecast in terms of a probability distribution as opposed to a single deterministic forecast.

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 the uncertainty in the initial conditions, is the use of a "lagged" approach in the start dates.

• "Burst" mode: all the members are initialized (start date) at the same value (e.g. all members initialized on 1st March 2017, this is the case for ECMWF system)
• "Lagged" mode: members are initialized in different start dates (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 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 hindcasts: Some systems are designed so their expected lifetime will be around 4-5 years. Thus, 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 for the whole climate period for that model. In that way, the hindcasts are produced well in advance the real-time forecasts and they constitute a fixed dataset during the lifetime of that seasonal forecasting system
• On-the-fly hindcasts: Some other systems run the necessary set of hindcasts every time they produce a new real-time forecast. They are produce just slightly in advance (a few weeks) 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 resources.

Production schedules in the seasonal forecasting systems contributing to C3S

N members at 1st month

N members at specific dates (different production schedules)

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Table with production per system and per forecast/hindcast type

Seasonal forecasting systems' versions and updates

blahblahblah (system keyword, etc)

Description of the c3s-seasonal dataset in the MARS archive

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