id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,active_lock_reason,draft,pull_request,body,reactions,performed_via_github_app,state_reason,repo,type
1647805851,I_kwDOAMm_X85iN4Wb,7700,Losing data when reading/converting GRIB2 files to netCDF using `open_dataset`/`to_netcdf` methods,31394655,closed,0,,,2,2023-03-30T15:00:34Z,2023-03-30T17:03:15Z,2023-03-30T17:03:15Z,NONE,,,,"### What is your issue?

Hi all,

I have data on GRIB2 format file and I want to convert it to netCDF format. The original dataset (confirmed by using `pygrib` package) has 12 messages: 6 different isobaric levels each with 2 variables (average and maximum) but when I convert the files using `xarray` I miss 6 out of 12 messages. 

The messages of the original file are `pygrib.open('filename.grib2').read()`:

```
[1:Relative clear air turbulence (RCAT):% (instant):regular_ll:isobaricInhPa:level 15000 Pa:fcst time 6 hrs:from 202001080600,
 2:Relative clear air turbulence (RCAT):% (instant):regular_ll:isobaricInhPa:level 15000 Pa:fcst time 6 hrs:from 202001080600,
 3:Relative clear air turbulence (RCAT):% (instant):regular_ll:isobaricInhPa:level 20000 Pa:fcst time 6 hrs:from 202001080600,
 4:Relative clear air turbulence (RCAT):% (instant):regular_ll:isobaricInhPa:level 20000 Pa:fcst time 6 hrs:from 202001080600,
 5:Relative clear air turbulence (RCAT):% (instant):regular_ll:isobaricInhPa:level 25000 Pa:fcst time 6 hrs:from 202001080600,
 6:Relative clear air turbulence (RCAT):% (instant):regular_ll:isobaricInhPa:level 25000 Pa:fcst time 6 hrs:from 202001080600,
 7:Relative clear air turbulence (RCAT):% (instant):regular_ll:isobaricInhPa:level 30000 Pa:fcst time 6 hrs:from 202001080600,
 8:Relative clear air turbulence (RCAT):% (instant):regular_ll:isobaricInhPa:level 30000 Pa:fcst time 6 hrs:from 202001080600,
 9:Relative clear air turbulence (RCAT):% (instant):regular_ll:isobaricInhPa:level 35000 Pa:fcst time 6 hrs:from 202001080600,
 10:Relative clear air turbulence (RCAT):% (instant):regular_ll:isobaricInhPa:level 35000 Pa:fcst time 6 hrs:from 202001080600,
 11:Relative clear air turbulence (RCAT):% (instant):regular_ll:isobaricInhPa:level 40000 Pa:fcst time 6 hrs:from 202001080600,
 12:Relative clear air turbulence (RCAT):% (instant):regular_ll:isobaricInhPa:level 40000 Pa:fcst time 6 hrs:from 202001080600]
```

 To make the conversion I am running the following commands:

```
import xarray

data = xarray.open_dataset('filename.grib2', engine = 'cfgrib')
data.to_netcdf('netcdf_file.nc')
```

and then to read it from another file I run

```
import netCDF4 as nc
ds = nc.Dataset('netcdf_file.nc', engine = 'netcdf4')
```

In any case both `data` and `ds` objects have less levels (6). Here a screenshot of the `data` object

![issue1](https://user-images.githubusercontent.com/31394655/228876972-cd2021fa-7e2d-41bc-975d-e0ea5981863e.png)

Is `xarray` losing data when reading grib2 file? Is it possible that the problem comes from the fact that the original messages are the same for a given isobaric level? In that case, can I rewrite the messages by adding a flag that specifies that one message is for the average (`ave`) and another is for the maximum (`max`) to the CAT parameter?

Thanks in advance,
Martín Gamboa
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