Changes to contribution to C3S seasonal forecast systems from ECMWF

1/November/2022

Changes to operational forecast system from ECMWF

From November 2022 a new version of ECMWF SEAS5 output will substitute the current one. This new version (v5.1) will be labelled with the keyword system=51  

• What are the changes in this new version of ECMWF SEAS5 data?
  • A different interpolation tool has been used to produce the 1-degree grid, both for hindcasts and real-time forecasts.
  • A new 1-degree grid, now consistent with the grid available from the other participating forecast systems. Specifically, it will have grid-points centered at half-degree latitude/longitude, and hence it has 180 values in the latitude coordinate (from -89.5 to 89.5) instead of the current 181 values (from -90 to 90)
  • A couple of additions to complete the list of available variables: Top solar incoming radiation and 1000hPa level for the 5 variables available at pressure levels. Additionally, on top of the values of runoff, for SEAS5.1 both sub-surface and surface runoff will be also available.

• What are the implications for users of ECMWF SEAS5 data? 
  • From November 2022, only system=51 real-time forecast data will be available from the CDS
  •  Users will need to use system=51 hindcast data alongside the new real-time forecasts.
    Data from the current version (system=5 ) should not be used, so those users with previously downloaded hindcast data will need to download the new version.
  • At the time of this announcement, hindcast data for October, November and December has been made available on the CDS
    • The remaining start dates (from January to September) will be processed and published progressively following the calendar order. Details on their availability can be found in the "Hindcasts" table of the Summary of available data

• Why have these changes been introduced?
  • When ECMWF SEAS5 was released in 2017, the C3S 1-degree version was created with the default interpolation tool (emoslib ) available from MARS. With the migration of ECMWF computing infrastructure to its new Data Centre in Bologna, this tool has been discontinued. That means new real-time forecasts produced after the migration can't be produced anymore in a way consistent with the available SEAS5 hindcast in the CDS. After careful evaluation of the impact of the interpolation, it was decided to create a new version of the hindcast data. This has been labelled as SEAS5.1 and encoded with keyword system=51.
  • With this new version of SEAS5 hindcast, a couple of additional improvements were also included:
    • A new 1-degree grid consistent with the one provided for other forecast systems (see details above)
    • A couple of additions to the list of variables (see details above).