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:

Daily/Sub-daily dataMonthly data
DateEnsemble memberDateEnsemble member

19931127
19931127
19931127
19931127
19931122
19931122
19931122
19931122
19931117
19931117
19931117
19931117
19931112
19931112
19931112
19931112
19931107
19931107
19931107
19931107
19931102
19931102
19931102
19931102

0
1
2
3
0
1
2
3
0
1
2
3
0
1
2
3
0
1
2
3
0
1
2
3

19931201
19931201
19931201
19931201
19931201
19931201
19931201
19931201
19931201
19931201
19931201
19931201
19931201
19931201
19931201
19931201
19931201
19931201
19931201
19931201
19931201
19931201
19931201
19931201

0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23

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 storing additionally 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 a new local definition has been devised for lagged start systems (GRIB keyword localDefinitionNumber=12) and its use has been implemented to all the lagged start systems data produced after December 2019.

Sytems affected by this change
  • MetOffice GloSea5 (system=14) used localDefinitionNumber=16 up to December 2019 nominal start date, and started using localDefinitionNumber=12 from January 2020 onwards. (Note this affects not just the forecasts but also the relevant hindcasts)
  • NCEP CFSv2 uses localDefinitionNumber=12 since its introduction into the C3S multi-system (Note that this was achieved by delaying the publication of all monthly data until December 2019)

In the future, systems using the lagged start approach introduced into C3S will also use localDefinitionNumber=12

What has changed?

localDefinitionNumber=16
(before December 2019)

localDefinitionNumber=12
(after December 2019)
Date used in the CDS requestsNominal start date
GRIB keywords: dataDate, dataTime
Nominal start date
GRIB keywords: indexingDate, indexingTime
Real start dateNot availableGRIB keywords: dataDatedataTime


It is important to bear in mind that most common grib tools will read dataDate and dataTime

It is therefore recommended to use tools based on ecCodes (version 2.15.0 or greater), specifically cfgrib (version 0.9.7.4 or greater) which can be used within python's xarray

3. Examples

Example 1: UK MetOffice hindcast data for December 1993 (localDefinitionNumber=16)

CDS API script
import cdsapi

c = cdsapi.Client()

c.retrieve(
'seasonal-monthly-single-levels',
{
'format': 'grib',
'originating_centre': 'ukmo',
'system': '14',
'variable': '2m_temperature',
'product_type': 'monthly_mean',
'year': '1993',
'month': '12',
'leadtime_month': [ '1', '2', ],
},
'c3s_seas_ukmo199312_t2m.grib')
GRIB file contents
> grib_ls -p origin,type,shortName,number,indexingDate,indexingTime,dataDate,dataTime,step,fcmonth,verifyingMonth c3s_seas_ukmo199312_t2m.grib
c3s_seas_ukmo199312_t2m.grib
origin          type            shortName       number          indexingDate    indexingTime    dataDate        dataTime        step            fcmonth         verifyingMonth  
egrr            fcmean          2t              0               not_found       not_found       19931201        0               744             1               199312         
egrr            fcmean          2t              1               not_found       not_found       19931201        0               744             1               199312         
egrr            fcmean          2t              2               not_found       not_found       19931201        0               744             1               199312         
egrr            fcmean          2t              3               not_found       not_found       19931201        0               744             1               199312         
egrr            fcmean          2t              4               not_found       not_found       19931201        0               744             1               199312         
egrr            fcmean          2t              5               not_found       not_found       19931201        0               744             1               199312         
egrr            fcmean          2t              6               not_found       not_found       19931201        0               744             1               199312         
egrr            fcmean          2t              7               not_found       not_found       19931201        0               744             1               199312         
egrr            fcmean          2t              8               not_found       not_found       19931201        0               744             1               199312         
egrr            fcmean          2t              9               not_found       not_found       19931201        0               744             1               199312         
egrr            fcmean          2t              10              not_found       not_found       19931201        0               744             1               199312         
egrr            fcmean          2t              11              not_found       not_found       19931201        0               744             1               199312         
egrr            fcmean          2t              12              not_found       not_found       19931201        0               744             1               199312         
egrr            fcmean          2t              13              not_found       not_found       19931201        0               744             1               199312         
egrr            fcmean          2t              14              not_found       not_found       19931201        0               744             1               199312         
egrr            fcmean          2t              15              not_found       not_found       19931201        0               744             1               199312         
egrr            fcmean          2t              16              not_found       not_found       19931201        0               744             1               199312         
egrr            fcmean          2t              17              not_found       not_found       19931201        0               744             1               199312         
egrr            fcmean          2t              18              not_found       not_found       19931201        0               744             1               199312         
egrr            fcmean          2t              19              not_found       not_found       19931201        0               744             1               199312         
egrr            fcmean          2t              20              not_found       not_found       19931201        0               744             1               199312         
egrr            fcmean          2t              21              not_found       not_found       19931201        0               744             1               199312         
egrr            fcmean          2t              22              not_found       not_found       19931201        0               744             1               199312         
egrr            fcmean          2t              23              not_found       not_found       19931201        0               744             1               199312         
egrr            fcmean          2t              24              not_found       not_found       19931201        0               744             1               199312         
egrr            fcmean          2t              25              not_found       not_found       19931201        0               744             1               199312         
egrr            fcmean          2t              26              not_found       not_found       19931201        0               744             1               199312         
egrr            fcmean          2t              27              not_found       not_found       19931201        0               744             1               199312         
egrr            fcmean          2t              0               not_found       not_found       19931201        0               1488            2               199401         
egrr            fcmean          2t              1               not_found       not_found       19931201        0               1488            2               199401         
egrr            fcmean          2t              2               not_found       not_found       19931201        0               1488            2               199401         
egrr            fcmean          2t              3               not_found       not_found       19931201        0               1488            2               199401         
egrr            fcmean          2t              4               not_found       not_found       19931201        0               1488            2               199401         
egrr            fcmean          2t              5               not_found       not_found       19931201        0               1488            2               199401         
egrr            fcmean          2t              6               not_found       not_found       19931201        0               1488            2               199401         
egrr            fcmean          2t              7               not_found       not_found       19931201        0               1488            2               199401         
egrr            fcmean          2t              8               not_found       not_found       19931201        0               1488            2               199401         
egrr            fcmean          2t              9               not_found       not_found       19931201        0               1488            2               199401         
egrr            fcmean          2t              10              not_found       not_found       19931201        0               1488            2               199401         
egrr            fcmean          2t              11              not_found       not_found       19931201        0               1488            2               199401         
egrr            fcmean          2t              12              not_found       not_found       19931201        0               1488            2               199401         
egrr            fcmean          2t              13              not_found       not_found       19931201        0               1488            2               199401         
egrr            fcmean          2t              14              not_found       not_found       19931201        0               1488            2               199401         
egrr            fcmean          2t              15              not_found       not_found       19931201        0               1488            2               199401         
egrr            fcmean          2t              16              not_found       not_found       19931201        0               1488            2               199401         
egrr            fcmean          2t              17              not_found       not_found       19931201        0               1488            2               199401         
egrr            fcmean          2t              18              not_found       not_found       19931201        0               1488            2               199401         
egrr            fcmean          2t              19              not_found       not_found       19931201        0               1488            2               199401         
egrr            fcmean          2t              20              not_found       not_found       19931201        0               1488            2               199401         
egrr            fcmean          2t              21              not_found       not_found       19931201        0               1488            2               199401         
egrr            fcmean          2t              22              not_found       not_found       19931201        0               1488            2               199401         
egrr            fcmean          2t              23              not_found       not_found       19931201        0               1488            2               199401         
egrr            fcmean          2t              24              not_found       not_found       19931201        0               1488            2               199401         
egrr            fcmean          2t              25              not_found       not_found       19931201        0               1488            2               199401         
egrr            fcmean          2t              26              not_found       not_found       19931201        0               1488            2               199401         
egrr            fcmean          2t              27              not_found       not_found       19931201        0               1488            2               199401         


Example 2: NCEP hindcast data for December 1993 (localDefinitionNumber=12)

CDS API script
import cdsapi

c = cdsapi.Client()

c.retrieve(
    'seasonal-monthly-single-levels',
    {
        'format': 'grib',
        'originating_centre': 'ncep',
        'system': '2',
        'variable': '2m_temperature',
        'product_type': 'monthly_mean',
        'year': '1993',
        'month': '12',
        'leadtime_month': [ '1', '2', ],
    },
    'c3s_seas_ukmo199312_t2m.grib')


GRIB file contents
> grib_ls -p origin,type,shortName,number,indexingDate,indexingTime,dataDate,dataTime,step,fcmonth,verifyingMonth c3s_seas_ncep199312_t2m.grib
c3s_seas_ncep199312_t2m.grib
origin          type            shortName       number          indexingDate    indexingTime    dataDate        dataTime        step            fcmonth         verifyingMonth  
kwbc            fcmean          2t              0               19931201        0               19931127        0               840             1               199312         
kwbc            fcmean          2t              1               19931201        0               19931127        6               840             1               199312         
kwbc            fcmean          2t              2               19931201        0               19931127        12              840             1               199312         
kwbc            fcmean          2t              3               19931201        0               19931127        18              840             1               199312         
kwbc            fcmean          2t              4               19931201        0               19931122        0               960             1               199312         
kwbc            fcmean          2t              5               19931201        0               19931122        6               960             1               199312         
kwbc            fcmean          2t              6               19931201        0               19931122        12              960             1               199312         
kwbc            fcmean          2t              7               19931201        0               19931122        18              960             1               199312         
kwbc            fcmean          2t              8               19931201        0               19931117        0               1080            1               199312         
kwbc            fcmean          2t              9               19931201        0               19931117        6               1080            1               199312         
kwbc            fcmean          2t              10              19931201        0               19931117        12              1080            1               199312         
kwbc            fcmean          2t              11              19931201        0               19931117        18              1080            1               199312         
kwbc            fcmean          2t              12              19931201        0               19931112        0               1200            1               199312         
kwbc            fcmean          2t              13              19931201        0               19931112        6               1200            1               199312         
kwbc            fcmean          2t              14              19931201        0               19931112        12              1200            1               199312         
kwbc            fcmean          2t              15              19931201        0               19931112        18              1200            1               199312         
kwbc            fcmean          2t              16              19931201        0               19931107        0               1320            1               199312         
kwbc            fcmean          2t              17              19931201        0               19931107        6               1320            1               199312         
kwbc            fcmean          2t              18              19931201        0               19931107        12              1320            1               199312         
kwbc            fcmean          2t              19              19931201        0               19931107        18              1320            1               199312         
kwbc            fcmean          2t              20              19931201        0               19931102        0               1440            1               199312         
kwbc            fcmean          2t              21              19931201        0               19931102        6               1440            1               199312         
kwbc            fcmean          2t              22              19931201        0               19931102        12              1440            1               199312         
kwbc            fcmean          2t              23              19931201        0               19931102        18              1440            1               199312         
kwbc            fcmean          2t              0               19931201        0               19931127        0               1584            2               199401         
kwbc            fcmean          2t              1               19931201        0               19931127        6               1584            2               199401         
kwbc            fcmean          2t              2               19931201        0               19931127        12              1584            2               199401         
kwbc            fcmean          2t              3               19931201        0               19931127        18              1584            2               199401         
kwbc            fcmean          2t              4               19931201        0               19931122        0               1704            2               199401         
kwbc            fcmean          2t              5               19931201        0               19931122        6               1704            2               199401         
kwbc            fcmean          2t              6               19931201        0               19931122        12              1704            2               199401         
kwbc            fcmean          2t              7               19931201        0               19931122        18              1704            2               199401         
kwbc            fcmean          2t              8               19931201        0               19931117        0               1824            2               199401         
kwbc            fcmean          2t              9               19931201        0               19931117        6               1824            2               199401         
kwbc            fcmean          2t              10              19931201        0               19931117        12              1824            2               199401         
kwbc            fcmean          2t              11              19931201        0               19931117        18              1824            2               199401         
kwbc            fcmean          2t              12              19931201        0               19931112        0               1944            2               199401         
kwbc            fcmean          2t              13              19931201        0               19931112        6               1944            2               199401         
kwbc            fcmean          2t              14              19931201        0               19931112        12              1944            2               199401         
kwbc            fcmean          2t              15              19931201        0               19931112        18              1944            2               199401         
kwbc            fcmean          2t              16              19931201        0               19931107        0               2064            2               199401         
kwbc            fcmean          2t              17              19931201        0               19931107        6               2064            2               199401         
kwbc            fcmean          2t              18              19931201        0               19931107        12              2064            2               199401         
kwbc            fcmean          2t              19              19931201        0               19931107        18              2064            2               199401         
kwbc            fcmean          2t              20              19931201        0               19931102        0               2184            2               199401         
kwbc            fcmean          2t              21              19931201        0               19931102        6               2184            2               199401         
kwbc            fcmean          2t              22              19931201        0               19931102        12              2184            2               199401         
kwbc            fcmean          2t              23              19931201        0               19931102        18              2184            2               199401         

This document has been produced in the context of the Copernicus Climate Change Service (C3S).

The activities leading to these results have been contracted by the European Centre for Medium-Range Weather Forecasts, operator of C3S on behalf of the European Union (Delegation agreement signed on 11/11/2014). All information in this document is provided "as is" and no guarantee or warranty is given that the information is fit for any particular purpose.

The users thereof use the information at their sole risk and liability. For the avoidance of all doubt, the European Commission and the European Centre for Medium-Range Weather Forecasts have no liability in respect of this document, which is merely representing the author's view.