Introduction
The software handling area extraction for gridded datasets will be upgraded to a new version to correct a behaviour that caused an interpolation of the data during the area extraction.
The change only affects gridded datasets archived on regular lat-lon grids such as ERA5 reanalysis, seasonal forecast and some CAMS global datasets.
What is the reason for this change?
Until now, area extraction has triggered an interpolation onto a new grid whose south west corner is that given in the user supplied area. This interpolation has been identified as undesirable behaviour. In order to preserve the original values it has been decided the software should instead only crop the existing data, returning just those grid points that lie inside the area without modification.
What will be the effect on the data?
By changing from interpolation to cropping, the resulting output grid point locations may change, shifting by at most one grid length. The shift will be zero if the area's south west point coincides perfectly with a grid point in the source field. The number of grid points in the output may also change correspondingly.
Updated version vs Previous version behaviour explained
This sections provides a visual representation of the differences between the updated and previous behaviour.
Updated behaviour (from 25/02/26 until next update):
If the requested area has points that are not in the grid of the source field, data is cropped, i.e. points that are within the area selected are returned and the grid/projection information is not modified.
Previous behaviour (until 25/02/26):
If the requested area has points that are not in the grid of the source field, the data was interpolated to respect the southwest corner of the selected area then recreating a grid from that point with the resolution of the source field.
Should you be attempting to request a point location, this will now return an error as there is no data in the area requested as illustrated below.
For ERA5 and ERA5-Land point extractions, please use the post-processed ERA5 hourly time-series data on single levels from 1940 to present and ERA5 Land hourly time-series data from 1950 to present.
Previous behaviour can be reproduced via the CDS API only, by using a specific combination of CDS API keywords. Please see table below.
Summary table for different combinations of the CDS API keywords.
| CDS API keyword | Updated behaviour | Previous behaviour | |
|---|---|---|---|
| AREA | GRID | ||
| NOT PRESENT | NOT PRESENT | Source data | Source data |
NOT PRESENT ANY VALUE | GRID different to source | Interpolation | Interpolation |
N, W, S, E grid-aligned | NOT PRESENT | Crop only | Interpolation and crop |
N, W, S, E not grid-aligned | |||
N, W, S, E grid-aligned | GRID = source | Crop only | Crop only |
N, W, S, E not grid-aligned | GRID = source | Interpolation | Interpolation |
S, W, S, W grid-aligned | NOT PRESENT | Crop only | Crop only |
S, W, S, W not grid-aligned | NOT PRESENT | "Data not found" error | Interpolation |
S, W, S, W grid-aligned not grid-aligned | GRID = source | Interpolation | Interpolation |
Notes
grid-aligned latitude-longitude = User input area SW corner is present in the grid of the source field.
N, W, S, E - bounding box
S, W, S, W - point location
Are my data requests affected?
For API users, view your API request code and follow through the workflow below.
For web users who are downloading data using the Climate Data Store (CDS) web interface, go back to the download form, make your selection and then use the "Show API request code" feature at the bottom of the form. This will display the API keys corresponding to the selections made on the download form. You can then follow through the workflow below.
How to reproduce the previous behaviour
The previous behaviour may be reproduced for datasets archived on regular lat-lon grids only, via the cdsapi by specifying the grid explicitly. This previous behaviour is however not encouraged.
For reanalysis-era5-complete and reanalysis-era5.1-complete, which native grids are non-regular, legacy behaviour cannot be reproduced following software upgrade.
Related articles
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Please read: CDS and ADS migrating to new infrastructure: Common Data Store (CDS) Engine (Copernicus Knowledge Base)
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