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Hi,

my question concerns the definition of the grib keys 'validityDate' and 'step' for monthly statistics of seasonal forecast data. In particular, the values for the grib keys 'validityDate' and 'verifyingMonth' in the monthly statistics to me seem to be inconsistent with each other. Take the following example request to get monthly forecasts of 2-m temperature from the UKMO model initialized on 1 July 2022 (nominal start date):

import cdsapi

c = cdsapi.Client()

c.retrieve(
    'seasonal-monthly-single-levels',
    {
        'format': 'grib',
        'originating_centre': 'ukmo',
        'variable': '2m_temperature',
        'product_type': 'monthly_mean',
        'year': '2022',
        'month': '07',
        'leadtime_month': [
            '1', '2', '3',
            '4', '5', '6',
        ],
        'grid': [1.0,1.0],
        'area': [89.5,0.5,-89.5,359.5]
    },
    't2m_2022_07_ukmo_.grib'
)

Inspecting the grib keys of 't2m_2022_07_ukmo_.grib' for the different ensemble members (as suggested here)

grib_ls -p origin,type,shortName,number,indexingDate,indexingTime,dataDate,dataTime,validityDate,fcmonth,verifyingMonth t2m_2022_07_ukmo_.grib

shows that 'validityDate' and 'verifyingMonth' are different. While 'validityDate' (to my mind) suggests that there is no data for July and the first 60 members are valid for August, 'verifyingMonth' indicates that the first 60 ensemble members are valid for July (which makes more sense to me).

ecCodes seems to handle this information correctly when using grib_to_netcdf (i.e., the first step in the resulting netcdf file is July 2022) but when opening the grib file with cdo or python's xarray (cfgrib backend) the data gets a valid date coordinate that I assume it derives from 'validityDate' (or the actual addition of 'step' to 'dataDate').

Am I correct in assuming that the first forecast step in the downloaded file above is the monthly mean temperature for July? If so, what is the actual defintion of the grib keywords 'validityDate' (and related to this the meaning of 'step') for the monthly statistics? Is it (always) the end date of the monthly aggregation window (which would be the only way I could make sense of it here)? Is this specified somewhere?

Thanks a lot for your help!

ole