paolap / climtas

Climate Extremes Timeseries Analysis

climtas

Functions for working with large (> 10 GB) datasets using Xarray and Dask, especially for working in the time domain

Topics

Apply a function grouping by day of year, without massive numbers of dask chunks

Climtas' blocked resample and groupby operations use array reshaping, rather than Xarray's default slicing methods. This results in a much simpler and efficient Dask graph, at the cost of some restrictions to the data (the data must be regularly spaced and start/end on a resampling boundary)

Example notebook: ERA-5 90th percentile climatology

>>> import numpy; import pandas; import xarray
>>> time = pandas.date_range("20010101", "20030101", closed="left")
>>> data = numpy.random.rand(len(time))
>>> da = xarray.DataArray(data, coords=[("time", time)])
>>> da = da.chunk({"time": 365})

>>> from climtas import blocked_groupby
>>> blocked_groupby(da, time='dayofyear').mean()
<xarray.DataArray 'stack-...' (dayofyear: 366)>
dask.array<mean_agg-aggregate, shape=(366,), dtype=float64, chunksize=(365,), chunktype=numpy.ndarray>
Coordinates:
  * dayofyear  (dayofyear) int64 1 2 3 4 5 6 7 8 ... 360 361 362 363 364 365 366

Find and apply a function to events

Climtas includes a number of parallelised building blocks for heatwave detection

>>> from climtas.event import find_events, map_events
>>> temp = xarray.DataArray([28,31,34,32,30,35,39], dims=['time'])
>>> events = find_events(temp > 30)
>>> sums = map_events(temp, events, lambda x: {'sum': x.sum().item()})
>>> events.join(sums)
   time  event_duration  sum
0     1               3   97
1     5               2   74

Memory-saving write to NetCDF

Climtas' throttled saver reduces memory usage, by limiting the number of Dask output chunks that get processed at one time

Examples

See the examples in the notebooks directory for mores ideas on how to use these functions to analyse large datasets