Description of the upgrade

The operational release of AIFS ENS v2 will introduce scientific and architectural changes to the AIFS ensemble model.

New and revised output variables include:

•  The introduction of a new wave component, including 10 wave variables, marking ECMWF's first operational data-driven wave forecasts.
•  The addition of 2 new snow variables to the existing land component.
• The addition of 2 variables already present in AIFS Single (vsw and cp), to harmonise the two models.
• An improved representation of the stratosphere, with the addition of pressure level fields at 10hPa.

Architectural changes include:

• Replacing the afCRPS loss function used in AIFS ENS v1 with a multi-scale loss. See https://arxiv.org/pdf/2506.10868.
• Imposing the same variable bounding as is used by the AIFS Single model, improving physical consistency.
• Revised graph features, utilising more edges in the decoder.
• An improved training regime, using 1 years of additional training data compared to AIFS ENS v1.

This upgrade also includes a technical fix:

• In AIFS ENS v1.0, some parameters have incorrect encoding of the ecCodes key productDefinitionTemplateNumber, which is used to describe the type of data.
  • Parameters 100u, 100v, hcc, lcc, mcc, rowe, sf, ssrd, strd, tcc and tp are encoded with productDefinitionTemplateNumber=0 or 8, which correspond to single forecasts.
  • The correct encoding for ensemble forecasts is productDefinitionTemplateNumber=1 or 11, which will be applied to all AIFS ENS output products with this upgrade.
  • See https://codes.ecmwf.int/grib/format/grib2/ctables/4/0/ for a description of these template numbers.