问题
I have files which are made of 10 ensembles and 35 time files. One of these files looks like:
>>> xr.open_dataset('ens1/CCSM4_ens1_07ic_19820701-19820731_NPac_Jul.nc')
<xarray.Dataset>
Dimensions: (ensemble: 1, latitude: 66, longitude: 191, time: 31)
Coordinates:
* ensemble (ensemble) int32 1
* latitude (latitude) float32 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 ...
* longitude (longitude) float32 100.0 101.0 102.0 103.0 104.0 105.0 106.0 ...
* time (time) datetime64[ns] 1982-07-01 1982-07-02 1982-07-03 ...
Data variables:
u10m (time, latitude, longitude) float64 -1.471 -0.05933 -1.923 ...
Attributes:
CDI: Climate Data Interface version 1.6.5 (http://c...
history: Wed Nov 22 21:54:08 2017: ncks -O -d longitude...
Conventions: CF-1.4
CDO: Climate Data Operators version 1.6.5 (http://c...
nco_openmp_thread_number: 1
NCO: 4.3.7
When I use open_mfdataset the files are concatenated along the time dimension and the ensemble dimension is dropped (possible because it has a size of 1)?
>>> xr.open_mfdataset('ens*/*NPac*.nc')
<xarray.Dataset>
Dimensions: (latitude: 66, longitude: 191, time: 10850)
Coordinates:
* latitude (latitude) float32 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 ...
* longitude (longitude) float32 100.0 101.0 102.0 103.0 104.0 105.0 106.0 ...
* time (time) datetime64[ns] 1982-07-01 1982-07-02 1982-07-03 ...
Data variables:
u10m (time, latitude, longitude) float64 -1.471 -0.05933 -1.923 ...
I'm not sure if it possible to concat along the ensemble dimension as well?
I did a simple test using merge as given here Error on using xarray open_mfdataset function but it fails:
>>> ds = xr.open_mfdataset('ens1/*NPac*')
<xarray.Dataset>
Dimensions: (ensemble: 1, latitude: 66, longitude: 191, time: 1085)
Coordinates:
* ensemble (ensemble) int32 1
* latitude (latitude) float32 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 ...
* longitude (longitude) float32 100.0 101.0 102.0 103.0 104.0 105.0 106.0 ...
* time (time) datetime64[ns] 1982-07-01 1982-07-02 1982-07-03 ...
Data variables:
u10m (time, latitude, longitude) float64 -1.471 -0.05933 -1.923 ...
>>> ds2 = xr.open_mfdataset('ens2/*NPac*')
<xarray.Dataset>
Dimensions: (ensemble: 1, latitude: 66, longitude: 191, time: 1085)
Coordinates:
* ensemble (ensemble) int32 2
* latitude (latitude) float32 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 ...
* longitude (longitude) float32 100.0 101.0 102.0 103.0 104.0 105.0 106.0 ...
* time (time) datetime64[ns] 1982-07-01 1982-07-02 1982-07-03 ...
Data variables:
u10m (time, latitude, longitude) float64 3.992 2.099 -0.3162 ...
>>> ds3 = xr.merge([ds, ds2])
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/nethome/rxb826/local/bin/miniconda3/lib/python3.6/site-packages/xarray/core/merge.py", line 513, in merge
variables, coord_names, dims = merge_core(dict_like_objects, compat, join)
File "/nethome/rxb826/local/bin/miniconda3/lib/python3.6/site-packages/xarray/core/merge.py", line 432, in merge_core
variables = merge_variables(expanded, priority_vars, compat=compat)
File "/nethome/rxb826/local/bin/miniconda3/lib/python3.6/site-packages/xarray/core/merge.py", line 166, in merge_variables
merged[name] = unique_variable(name, variables, compat)
File "/nethome/rxb826/local/bin/miniconda3/lib/python3.6/site-packages/xarray/core/merge.py", line 85, in unique_variable
% (name, out, var))
xarray.core.merge.MergeError: conflicting values for variable 'u10m' on objects to be combined:
first value: <xarray.Variable (time: 1085, latitude: 66, longitude: 191)>
dask.array<shape=(1085, 66, 191), dtype=float64, chunksize=(31, 66, 191)>
Attributes:
long_name: 10m U component of wind
units: m s**-1
second value: <xarray.Variable (time: 1085, latitude: 66, longitude: 191)>
dask.array<shape=(1085, 66, 191), dtype=float64, chunksize=(31, 66, 191)>
Attributes:
long_name: 10m U component of wind
units: m s**-1
I'm using v0.10.0 (thanks for the recent update!)
回答1:
xarray.open_mfdataset does not support 2d merges. What you will need to do is use concat along the second dimension:
import os
import xarray as xr
ens_list = []
for num in range(1, 11):
ens = 'ens%d' % num
ens_list.append(xr.open_mfdataset(os.path.join(ens, '*NPac*')))
ds = xr.concat(ens_list, dim='ensemble')
This is a common problem that xarray users run into. It is quite difficult, however, to write a generalized ND concat routine.
回答2:
I wrote the following function as a workaround for my own use case: https://gist.github.com/jnhansen/fa474a536201561653f60ea33045f4e2
It works with arbitrary dimensions, but currently requires that the same variables exist in each file/dataset.
In my case I have a number of tiles (split along e.g. lat, lon, and time):
ds = auto_merge('data/part*.nc')
This will execute immediately as it returns only a view of the data (just like xarray.open_mfdataset would do).
回答3:
xarray does now support N-D concatenation. As your data has 1-D dimension coordinates, you can simply do
ds = xr.open_mfdataset('ens*/*NPac*.nc', combine='by_coords')
and it should combine them in order automatically! It should even work for the ensemble dimension, as you gave that a coordinate too.
Also see this answer to a very similar question.
来源：https://stackoverflow.com/questions/47545138/python-xarray-open-mfdataset-concat-along-two-dimensions