Regional subselection and horizontal grid interpolation
Currently the interactive CDS forms doesn't allow users to perform a regional subselection or an interpolation of the selected data. Regardless of that, both action 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 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 data. You can find more information about what causes this error in Known issue D3 here.
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] - If you have previously downloaded hindcast data using Web API, the interpolation method might have changed, so you would need to assess whether there are relevant differences or not with the same data retrieved using CDS API
- It can lead to errors when trying to download MetOffice data. You can find more information about what causes this error in Known issue D3 here.
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:
- 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]
- 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 the data they want to retrieve in one single request can make a huge difference in the performance of the retrieval.
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 in one single request all the data they are interested in which is contained in the same MARS "leaf"
- As general recommendations, users are encouraged to devise their requests as follows:
- Do NOT ask for different start months at the same time
It is much more efficient to request one single month (e.g. November) for all the hindcast years (1993 to 2016) than requestin all 12 months from a single year - Bundle in one single request all variables you are interested, all levels (for pressure level fields), all product types and all leadtimes (hours or months, depending on the CDS product)
- Note that there are some limitations in the size of a request. If you are using directly the CDS forms 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.
- Do NOT ask for different start months at the same time
