Enable more ergonomic seasonal grouping and resampling #10198
Description
TLDR
I propose merging #8524 (docs) to provide this new API (after review).
from xarray.groupers import SeasonGrouper, SeasonResampler
ds.groupby(time=SeasonGrouper(["DJFM", "MAMJ", "JJAS", "SOND"])).mean()
ds.resample(time=SeasonResampler(["DJF", "MAM", "JJA", "SON"])).mean()Is your feature request related to a problem?
Current Status
Xarray supports a very simple form of seasonal grouping: groupby("time.season") which has a fixed definition of seasons: DJF, MAM, JJA, SON, and doesn't enforce proper ordering of the output (the seasons get sorted as a string to give : DJF, JJA, MAM, SON :/ )
We support a little more complex resampling using Pandas syntax .resample(time="QS-Jan") for example, but I think this is limited to seasons of 3 months long.
User Requests
A quick scan of issues, discussions, and StackOverflow shows that our users want more control over how seasons are specified.
- Don't include "incomplete" seasons in output.
- allow custom season definitions (e.g. of varying length, overlapping seasons).
Here is a list of user requests:
Details
- Closes Ordered Groupby Keys #757
- Closes Computing 'seasonal means' spanning 4 months (with resample or groupy) #6180
- Closes xarray_ Group by season #6664
- Closes How to change or reorder grouping by 'time.season'? #5134
- Closes Error: Changing months combination in xarray season for India Region #6012
- Closes Calculating ONDJFM mean for each season #6865
- StackOverflow:
- https://stackoverflow.com/questions/61139226/how-to-get-season-as-jjas-instead-of-jja-in-xarray
- https://stackoverflow.com/questions/70658388/how-to-select-djfm-season-instead-of-xarray-groupby
- https://stackoverflow.com/questions/69021082/december-january-seasonal-mean
- https://stackoverflow.com/questions/64976340/keeping-time-series-while-grouping-by-season-in-xarray
- https://stackoverflow.com/questions/68455725/how-to-enable-season-selection-as-jjas-instead-of-jja-in-xarray
- https://stackoverflow.com/questions/60342881/efficient-workaround-to-group-by-multiple-time-coordinates-in-xarray
Describe the solution you'd like
The problem of custom seasons is simply that of converting the seasons to proper integer codes. Our relatively new Grouper objects provide this extension point.
I have implemented this in #9524 (docs). The code isn't pretty and probably doesn't scale well for very long time vectors, but I focused on correctness and tests.
Describe alternatives you've considered
This could live outside Xarray, but is such a common ask from our userbase, that it seems worth of inclusion.