1. Context.The concept of "nominal" start date in lagged start ensembles

Within the seasonal forecast datasets available at C3S, we have contributions from two families of forecasting systems, depending on what techniques are used to build the members of their ensemble forecast, so that they sample the uncertainty in the initial conditions:

• "Burst" mode: all the members are initialized with conditions on the same start date, 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 on different start dates, the differences between which are sufficiently small (e.g. members initialized every day of the month; this is the case for Met Office system)

Due to a number of reasons, members from ensembles using the "lagged start" approach are bundled together at C3S and labelled with a "nominal" start date (see details here).

The C3S seasonal forecast datasets are based on GRIB files stored at ECMWF MARS archive where that "nominal" start date is used to index the monthly data (monthly statistics and monthly post-processed products like anomalies). Hence, the details about the original initialization date could get lost in those files. Nonetheless, the data at daily and sub-daily time frequencies is always encoded with the real initialization date, as it can be seen in the example below:

2. The encoding in GRIB format using "Local Definitions"

In December 2019, with the release of ecCodes 2.15.0, an extension to the GRIB encoding was introduced to allow additionally storing the original initialization date on those monthly files, while keeping the "nominal" start date as indexing date to be used by the MARS archive. The solution adopted was to create a new GRIB "local definition" for the seasonal forecast monthly data produced by lagged start systems.

Up to that date, all seasonal forecast monthly data files produced at ECMWF, and that includes the data provided by all C3S seasonal forecast contributors, was encoded using GRIB local definitions described in table 16 (GRIB keyword localDefinitionNumber=16 ). With the changes introduced with version 2.15.0 of ecCodes 

3. Examples