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• Regional subselection and horizontal

  Horizontal grid interpolation

Currently the interactive CDS forms doesndon't allow users to perform a regional subselection or an interpolation of the selected data. Regardless of However, that , both action can be performed using the CDS API keywords area and keyword grid with the same syntax used by ECMWF MARS and Web API, as explained here..

import cdsapi

c = cdsapi.Client()

c.retrieve(
    'seasonal-monthly-single-levels',
    {
        'format':'grib',
        'originating_centre':'cmcc',
        'system':'3',
        'variable':'2m_temperature',
        'product_type':'monthly_mean',
        'year':'2007',
        'month':'11',
        'leadtime_month':['1','2','3','4','5','6'],
		'area' : '75/-25/10/60', # 'North/West/South/East' (i.e. 'top-left corner lat/top-left corner lon/bottom-right corner lat/bottom-right corner lon'
		'grid' : '2.5/2.5' # Interpolate to a regular 2.5x2.5 degree horizontal grid
    },
    'download.grib')

Users Despite that feature being available via CDS API, users must be aware of the following:

...

• • Use of grid for horizontal interpolation is currently NOT recommended:
    • It can lead to errors when trying to download MetOffice data . You can find more produced before January 2018 (system=12). More information about what causes this error can be found in Known issue D3 hereD2.These errors will be heavily reduced once the option to select system (model version) had be implemented in the CDS. [This is a feature expected to be implemented imminently]
    • It can lead to failing data retrievals if more than one model version (system) is requested. Note that if system is not explicitly specified it defaults to all available systems.
    • Access to C3S seasonal data was also available through ECMWF Web API until 5 March 2019. Data previously downloaded using that Web API access may have used a different interpolation method; If you have previously downloaded hindcast data using Web API, the interpolation method might have changed, so you would need to assess whether there such differences are relevant differences or not with the same data retrieved using CDS API

• Conversion to NetCDF

Currently the interactive CDS forms doesn't allow users to select NetCDF as data format for their retrievals. Regardless of that, the value of format keyword in CDS API scripts can be set to "netcdf" to obtain a netCDF version of the archived GRIB data.

Despite that feature being available via CDS API, users must be aware of the following:

...

• • • for your choice of data. This issue is relevant in the context of the required consistency between interpolation of real-time forecasts and the associated hindcasts.

• Conversion to NetCDF

Users must be aware that conversion to NetCDF is considered experimental and it is not guaranteed to work in all situations.

In this context,

• • It is NOT recommended to download hindcast data in netCDF, while the option to select system (model version) in the CDS is not implemented [This is a feature expected to be implemented imminently]
  • Despite the experimental status of this conversion to NetCDF, there are some situations where it can be safe to use it. Specifically,
    
    • When you don't retrieve at the same time different start dates
  • include in a single request more than one model version (system). Note that for hindcasts, data from more than one system may exist in the archive.
  • It is NOT recommended to include in a single request data related to more than one start time.

• Efficiency tips

The C3S seasonal forecast dataset is currently based on GRIB files archived at ECMWF's MARS archive. This implies, in some situations, accessing data not available online (on disk) but archived in the tape library.

The way the user combines selects the data they want items to retrieve include in one a single request can make a huge difference in the performance of the retrieval. To improve performance:

• • Users are encouraged to explore the C3S catalogue using the former Web API catalogue tool here: https://apps.ecmwf.int/data-catalogues/c3s-seasonal/?class=c3
    The guidance given in this page can help you to understand the mapping between that Web API catalogue and the CDS forms.
    All the data shown at the same time in a window like the picture below, constitutes what it is called a "leaf" in the MARS "tree". This is usually guaranteed to be in a single archived file, and hence in one single tape.
    Users are encouraged to bundle

    • combine in one single request all the data they are interested in which is contained in the same MARS "leaf"

    • avoid combining data from two different "leaves".

      Expand
      title See here how a MARS leaf looks like in the ECMWF catalogue
      Image Modified

      
      

  • As general recommendations, users Users are encouraged to devise their requests as follows:
    • Do NOT ask for different start months at in the same timerequest when retrieving data from the "daily" datasets.
      It is much more efficient to request one single month (e.g. November) for all the hindcast years (1993 to 2016) than requestin requesting all 12 months from a single year
    • Bundle Combine in one single request all variables you are interestedof interest, all levels (for pressure level fields), all product types and all leadtimes (hours or months, depending on the CDS product)
      Note . However, note that there are some limitations in the size of a request. If you are using directly the CDS forms directly, you will be informed when you request is too big before submitting it. If you are using CDS API you can use the forms to discover whether the request you are trying to submit is affordable or not.

• Guidelines to decode monthly data

More information here.