We're looking at ERA5-LAND 2m-temperature, but can't explain differences between GRIB and NetCDF files downloaded at https://cds.climate.copernicus.eu/cdsapp#!/dataset/reanalysis-era5-land?tab=form with another NetCDF file acquired from the CDS API (via Python client at https://cds.climate.copernicus.eu/api/v2).
Here are 3 time-series in °C from the 3 separate files for 1990-01-01 UTC times at location X=-122.0728 Y=39.75376:
date t2m_api t2m_nc t2m_grib
1: X1990.01.01.00.00.00 13.221094 13.237744 13.237451
2: X1990.01.01.01.00.00 11.665186 11.681625 11.681787
3: X1990.01.01.02.00.00 10.072168 10.095596 10.095850
4: X1990.01.01.03.00.00 9.000879 8.965128 8.964990
5: X1990.01.01.04.00.00 9.196192 9.182172 9.182031
6: X1990.01.01.05.00.00 8.310938 8.317065 8.317285
7: X1990.01.01.06.00.00 7.488183 7.531720 7.531885
8: X1990.01.01.07.00.00 5.521875 5.556852 5.556543
9: X1990.01.01.08.00.00 5.076074 5.118164 5.118311
10: X1990.01.01.09.00.00 5.546777 5.577560 5.577295Below are plots of the 3 files at 1990-01-01 00:00 UTC. They have slightly different spatial extents (and projection for the GRIB), which maybe explain the problem (?):
t2m_api
> extent(r)
class : Extent
xmin : -124.466
xmax : -114.165
ymin : 32.581
ymax : 41.981 t2m_nc
> extent(r)
class : Extent
xmin : -124.46
xmax : -114.16
ymin : 32.48
ymax : 41.98 t2m_grib
> extent(r)
class : Extent
xmin : -124.46
xmax : -114.16
ymin : 32.48
ymax : 41.98
API method (in °C):
GUI NetCDF format (in K):
GUI GRIB format (in K):
We use R raster::extract() to overlay the point location to the rasters (but we have the same results in QGIS).
Differences between the 2 NetCDF files (API vs. GUI):
Differences between NetCDF and GRIB files (GUI):
Is there any reason why we see small differences between the separate files? Max difference is in the order of 0.1°C (between API and GUI) over the month of January 1990, but this is a little concerning for reproducibility's sake.
4 Comments
Kevin Marsh
Hi Melanie,
The grib are the 'original' data, and the netCDF files are produced from these using conversion software. This conversion can introduce small differences. However the same software is used for the conversion whether you use the CDS web or CDS API to request the data, so for exactly the same request, you should get exactly the same file for both methods.
However, if the requests are different (e.g. slightly different area selected, so the data are interpolated) then you may see small differences.
Also, if you use other software (such as R) to retrieve/extract/display the data, this may be the cause of the differences you observe.
Hope that helps,
Kevin
Melanie Bacou
Hi Kevin,
Thanks for confirming that differences can be expected across non-matching requests.
Is it generally "safer" and more robust to use the GRIB format for now, to avoid interpolation approximations when converting to NetCDF? I'm not certain GRIB is available via the API at the moment (seems to default to NetCDF)?
Thx, --Mel.
Kevin Marsh
Hi Melanie,
If you can use the 'original' GRIB files, these will indeed avoid any (minor) issues due to the conversion to netCDF.
You can download GRIB using the CDS API (as shown on the form):
import cdsapi c = cdsapi.Client() c.retrieve( 'reanalysis-era5-land', { 'format': 'grib', 'variable': '2m_temperature', 'year': '2020', 'month': '01', 'day': '01', 'time': '00:00', }, 'download.grib')Hope that helps,
Kevin
Melanie Bacou
Thanks, that's very clear!