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

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

• • Use of area for regional subselection is safe
  • Use of grid for horizontal interpolation is currently NOT recommended:
    • It can lead to errors when trying to download MetOffice seasonal forecast data. You can find more 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 has been introduced in the CDS. [This is a feature expected to be implemented Implementation of system selection is expected imminently]
    • If you have Hindcast data previously downloaded hindcast data using Web API, the may have used a different interpolation method might have changed, so you . You would need to assess whether there are relevant differences or not with from the same data retrieved using CDS API

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Currently the interactive CDS forms doesndon't allow users to select NetCDF as data format for their retrievals. Regardless of thatHowever, the value of of the 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 Users must be aware of the following:

• • Conversion to NetCDF is considered experimental and it is not guaranteed to work in all situations
  • 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]implemented [Implementation of system selection is expected imminently]
  • It is NOT recommended to use when downloading data related to more than one start time in a single request.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

• 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 section 2a here explains the mapping of syntax 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"

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

    
    

  • 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.
      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.

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