How to remove xarray dimension after adding another without deleting the data variables-CodePudding

I have data from ECMWF which when read into xarray looks like this

Dimensions:     (time: 424, step: 12, latitude: 3, longitude: 2)
Coordinates:
    number      int64 0
  * time        (time) datetime64[ns] 1990-03-01T06:00:00 ... 1993-04-22T18:0...
  * step        (step) timedelta64[ns] 01:00:00 02:00:00 ... 11:00:00 12:00:00
    surface     float64 0.0
  * latitude    (latitude) float64 41.0 40.75 40.5
  * longitude   (longitude) float64 -96.92 -96.67
    valid_time  (time, step) datetime64[ns] 1990-03-01T07:00:00 ... 1993-04-2...
Data variables:
    i10fg       (time, step, latitude, longitude) float32 4.876 4.637 ... 3.959
Attributes:
    GRIB_edition:            1
    GRIB_centre:             ecmf
    GRIB_centreDescription:  European Centre for Medium-Range Weather Forecasts
    GRIB_subCentre:          0
    Conventions:             CF-1.7
    institution:             European Centre for Medium-Range Weather Forecasts
    history:                 2022-04-23T22:18 GRIB to CDM CF via cfgrib-0.9.9...

I've stacked the time and step dimensions to make a new one called forecast. I then added another dimension called valid and set that equal to the coordinate valid_time. Both valid and fcst are the same length, but I now want to drop the fcst dimension. However, when I do this, the data variable also gets deleted. Does anyone know how to fix this? Here is my sample code. There might be a better way to do what I'm doing, but I'm still pretty new to xarray!

ds = ds.stack(fcst=("time", "step")).transpose("fcst", "latitude", "longitude")
ds.expand_dims(valid=ds['valid_time']).drop_dims('fcst')

which leaves me with

<xarray.Dataset>
Dimensions:    (valid: 5088, latitude: 3, longitude: 2)
Coordinates:
  * valid      (valid) datetime64[ns] 1990-03-01T07:00:00 ... 1993-04-23T06:0...
    number     int64 0
    surface    float64 0.0
  * latitude   (latitude) float64 41.0 40.75 40.5
  * longitude  (longitude) float64 -96.92 -96.67
Data variables:
    *empty*

I've tried setting valid as the index then dropping fcst but it still deletes the data variables.

any help would be appreciated!

CodePudding user response：

All variables in an xarray Dataset must be indexed by named dimensions. You can use ds.reset_index` drop any labeled coordinates associated with a dimension, but this isn't what you want. You can't simply do away with a dimension without losing the variables which are indexed with this dimension.

From the xarray docs on data structures:

dimension names are always present in the xarray data model: if you do not provide them, defaults of the form dim_N will be created.

Instead, you can swap a non-indexing coordinate for an indexing coordinate using swap_dims. This will swicth valid_time and fcst in your array, such that valid_time becomes the dimension which indexes fcst and any variables previously indexed by fcst (i10fg in your case).

So the answer is, after the stack but without expand_dims or drop:

ds.swap_dims({'fcst': 'valid_time'}).drop('fcst')