Clusters, or groups, of similar forecasts are computed within the ensemble of 51 (i.e. the 50 perturbed forecasts plus the control forecast); For five different areas of clustering - domains

- General European Area          (75.00N  20.00W 30.00N 45.00E)

- North West Europe          (70.00N  27.50W 40.00N 10.00E)

- North East Europe                  (72.50N  0.00W      50.00N 45.00E)

- South West Europe                  (57.50N  15.00W 32.50N 17.50E)

- South East Europe                  (57.50N  2.50E      32.50N 42.50E)

Up to 6 clusters are produced, depending on the overall spread of the ensemble. The measure of similarity between individual forecasts is based on the geopotential at 500hPa over the particular domain area for the period from day 5 to day 7, so that the clusters are fully consistent within that range. For each cluster, the mean and standard deviation of its elements are computed every 12 hours from day 3 to day 7 for 500 hPa geopotential, 500 hPa temperature, 850 hPa temperature, and 1000 hPa geopotential (Table 5.3).

Extensions to the GRIB code enable cluster products to contain details of the cluster number and the numbers of the ensemble members from which a cluster was formed (an ensemble number of 0 refers to the control forecast).

The number of clusters generated varies from day to day. Note that only the generated clusters will be disseminated; There will be no "empty" clusters.

* On pressure levels 1000/500 ** On pressure levels 850/500

Table 5.3 Cluster Mean and Cluster Standard Deviation products

FIELD CODE	MARS ABBREV.	FIELD NAME	UNITS	NO. OF BITS/VAL.
129	Z	Geopotential*	m2s-2	16
130	T	Temperature**	K	12

Horizontal resolution

- 1.5 x 1.5 degrees latitude-longitude grid, or multiples thereof

Verifying times

- T+072 to T+168 (inclusive) at 12 hour intervals

Cluster Mean and Cluster Standard Deviation requirements

The following command language parameters are specific for the cluster mean and cluster standard deviation requirements. Other DIS parameters are listed under the general EPS requirements above.

TYPE

CLUSTER MEAN

CLUSTER STANDARD DEVIATION

[CM]

[CS]

DOMAIN

General European Area

North West Europe

North East Europe

South West Europe

South East Europe

[G]

[A]

[B]

[C]

[D]

(75.0 -20.0 30.0 45.0)

(70.0 -27.5 40.0 10.0)

(72.5 0.0 50.0 45.0)

(57.5 -15.0 32.5 17.5)

(57.5 2.5 32.5 42.5)

NUMBER

OFF

LEVTYPE

PRESSURE LEVEL

[PL]

LEVELIST

1000/500

850/500

(PARAM=Z only)

(PARAM=T only)

GRID

a/b (latitude-longitude grid)

a= grid resolution (deg.) along a latitude line (i.e. East/West)

b= grid resolution (deg.) along a longitude line (i.e. North/South)

1.5/1.5 or multiples thereof;

STEP

* (hours)

l gives forecast time step(s);

72/84/96/108/120/132/144/156/168

NB ALL, is not permitted.

PARAM

Field name abbreviation

NB ALL is not permitted.

Available parameters are listed in the Table 5.2.

GRIB section 1 definition for Cluster Mean and Cluster Standard Deviation products

By convention, ECMWF always uses octet 41 to define which set of local definitions follow. The set of local definitions for EPS Cluster Mean and Cluster Standard Deviation fields:

Table 5.4 GRIB section 1 definition for Cluster Mean and Cluster Standard Deviation products

Octet

Ksec1

element

Contents

ECMWF local GRIB use definition identifier.

2 = Cluster means and standard deviations

44-45

46-49

Class:

1 = Operations

Type:

14 = Cluster means

15 = Cluster standard deviations

Stream:

1035 = Ensemble forecasts

Version number or experiment identifier.

(4 ASCII characters, right justified) Version number or experiment identifier.

(4 ASCII characters, right justified)

Number: Cluster number

Total: Total number of clusters.

Clustering method:

1 = maximum linkage

2 = mixed

3 = small linkage

54-55

56-57

Start timestep considered when clustering

(units as defined for forecast timesteps)

End timestep considered when clustering

58-60

61-63

64-66

67-69

Northern latitude (of domain of clustering in millidegrees)

Western longitude (of domain of clustering in millidegrees)

Southern latitude (of domain of clustering in millidegrees)

Eastern longitude (of domain of clustering in millidegrees)

Number of cluster to which the operational forecast

belongs

Number of cluster to which the operational forecast

belongs

Number of cluster to which the control forecast

belongs

73 - 328n1-nN

54 - (53+N)

N, the number of forecasts belonging to the cluster

List of the N ensemble forecast numbers used in the

cluster (one octet per number); if the control forecast

has been used, this will be listed as number 0. Any

unused octets at the end of the list will be set to 0.

18-01-2011