Unexpected coordinates - CAMS global forecast from CDS with restricted area

I used the CDS app web interface to fetch CAMS global forecast data, but the data I got was on a grid which didn't match what I expected.

It seems that the new data I got from CDS was for some reason shifted so that the bottom left corner exactly matches the bottom left corner of the area I requested.

I have another dataset of CAMS global forecast data for about the same area, and the datasets are not aligned. The two datasets I have don't contain the same parameters so I don't know if the parameters' values were interpolated to a new grid or left as-is.


My question:

What is the expected behavior of the CDS app when requesting for a specific area? should I specifically align the area I ask for with the CAMS global grid?

Thank you


Below is some more information to explain the issue.


My CDS form was translated into the following API request:

c.retrieve(
    'cams-global-atmospheric-composition-forecasts',
    {
        'date': '2021-07-08/2021-07-12',
        'type': 'forecast',
        'format': 'netcdf_zip',
        'variable': [
            'organic_matter_aerosol_optical_depth_550nm', 'sulphate_aerosol_optical_depth_550nm',
        ],
        'time': [
            '00:00', '12:00',
        ],
        'leadtime_hour': [
            '0', '1', '10',
            '11', '12', '13',
            '14', '15', '16',
            '17', '18', '19',
            '2', '20', '21',
            '22', '23', '24',
            '25', '26', '27',
            '28', '29', '3',
            '30', '4', '5',
            '6', '7', '8',
            '9',
        ],
        'area': [
            41.97, -121.51, 38.53,
            -115.53,
        ],
    },
    'download.netcdf_zip')


A sample of the resulting coordinates:

     latitude  longitude
0       41.73    -121.51
1       41.33    -121.51
2       40.93    -121.51
3       40.53    -121.51
4       40.13    -121.51
..        ...        ...
130     40.13    -115.91
131     39.73    -115.91
132     39.33    -115.91
133     38.93    -115.91
134     38.53    -115.91


And this is how it looks on a plot (x=lon, y=lat), comparing the data I had before ("existing data") and the new one I got from CDS ("downloaded from CDS"):

Hi Shaked. Yes, the expected behaviour when requesting sub-areas is as you have seen - the data is interpolated to a grid aligned to the south west corner. If you want multiple retrievals to have coincident grid points they must all have the same south west corner. If you don't want the data to be interpolated, the south west corner must match the position of an existing grid point.

Luke.


Hi Luke,

Thank you for answering! I will adjust my queries accordingly.