Description of the upgrade

A new set of cluster products for the ENS has been introduced. The new system includes two applications of clustering methodology:

• A daily clustering of the forecast fields from the EPS, similar in principle to the old EPS clustering but using a different algorithm. The daily clustering determines weather scenarios, defined as the ensemble members closest to the centroid of each cluster.

• A set of fixed climatological regimes for each season, computed using 29 years of reanalysis data (ERA-Interim and ERA-40).

This combined approach provides the user with:

• a set of weather scenarios that appropriately represent the ensemble distribution

• an additional objective measure of the differences between scenarios in terms of large scale flow (by using the attribution of each weather scenario to the climatological regimes)

• easy computations of flow-dependent skill measures (under development)

The new products are computed on a new European domain (domain=H). For type cluster means they will replace the old products and a new type cluster representative (type=CR) will be added. Type cluster standard deviations will not be produced.

The new cluster products have been added to the Catalogue of ECMWF Real-Time Products Set III- Atmospheric Model low resolution 15-day forecast (ENS) on 8 Mar 2011.

The old cluster products have been discontinued on 6 Feb 2013.

Old cluster products

The old operational clustering algorithm is based on the RMS differences between the 500 hPa geopotential height ensemble forecasts, averaged from +120h to +168h taking the synoptic continuity into account.

The graphical products for the old clustering algorithm have been discontinued.

Products in GRIB code using local definition 2 are available from MARS and dissemination as follows:

The old cluster products have been discontinued on 6 Feb 2013.

The ENS cluster products replace the old Cluster means and introduce the new type Cluster representatives.

New cluster products

The new clustering method combines a daily clustering of ENS forecasts, similar in principle to the old ENS clustering, with a set of fixed climatological regimes for each season, computed from 29 years of reanalysis data (ERA-Interim and ERA-40). Weather scenarios derived from the daily clustering and defined as the ensemble members closest to the centroid of each cluster are called cluster representative members in MARS language (type=CR).

The new graphical cluster products are available for the time windows 72-96, 120-168, 192-240 and 264-360 hours.

Products in GRIB code using local definition 32 are available from MARS and dissemination as follows:

Note that the new products, based on 500 hPa geopotential, are computed in a new domain H (30°-75°N, 20°W-40°E).

Technical characteristics of the new cluster products

As the new products have been coded using local definition 32, they have to be handled with the following software versions:

• libemos 372 onwards

• grib_api 1.9.0 onwards

Resolution

Unchanged