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* Friendlier io title. * Fix lists. * Fix *args, **kwargs "inline emphasis..." * misc * Reference xarray_extras for csv writing. Closes pydata#2289 * Add metpy accessor. Closes pydata#461 * fix transpose docstring. Closes pydata#2576 * Revert "Fix lists." This reverts commit 39983a5. * Revert "Fix *args, **kwargs" This reverts commit 1b9da35. * Add MetPy to related projects. * Add Weather and Climate specific page. * Add hvplot. * Note open_dataset, mfdataset open files as read-only (closes pydata#2345). * Update metpy 1 Co-Authored-By: dcherian <dcherian@users.noreply.github.com> * Update doc/weather-climate.rst Co-Authored-By: dcherian <dcherian@users.noreply.github.com>
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.. _weather-climate: | ||
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Weather and climate data | ||
======================== | ||
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.. ipython:: python | ||
:suppress: | ||
import xarray as xr | ||
``xarray`` can leverage metadata that follows the `Climate and Forecast (CF) conventions`_ if present. Examples include automatic labelling of plots with descriptive names and units if proper metadata is present (see :ref:`plotting`) and support for non-standard calendars used in climate science through the ``cftime`` module (see :ref:`CFTimeIndex`). There are also a number of geosciences-focused projects that build on xarray (see :ref:`related-projects`). | ||
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.. _Climate and Forecast (CF) conventions: http://cfconventions.org | ||
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.. _metpy_accessor: | ||
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CF-compliant coordinate variables | ||
--------------------------------- | ||
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`MetPy`_ adds a ``metpy`` accessor that allows accessing coordinates with appropriate CF metadata using generic names ``x``, ``y``, ``vertical`` and ``time``. There is also a `cartopy_crs` attribute that provides projection information, parsed from the appropriate CF metadata, as a `Cartopy`_ projection object. See `their documentation`_ for more information. | ||
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.. _`MetPy`: https://unidata.github.io/MetPy/dev/index.html | ||
.. _`their documentation`: https://unidata.github.io/MetPy/dev/tutorials/xarray_tutorial.html#coordinates | ||
.. _`Cartopy`: https://scitools.org.uk/cartopy/docs/latest/crs/projections.html | ||
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.. _CFTimeIndex: | ||
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Non-standard calendars and dates outside the Timestamp-valid range | ||
------------------------------------------------------------------ | ||
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Through the standalone ``cftime`` library and a custom subclass of | ||
:py:class:`pandas.Index`, xarray supports a subset of the indexing | ||
functionality enabled through the standard :py:class:`pandas.DatetimeIndex` for | ||
dates from non-standard calendars commonly used in climate science or dates | ||
using a standard calendar, but outside the `Timestamp-valid range`_ | ||
(approximately between years 1678 and 2262). | ||
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.. note:: | ||
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As of xarray version 0.11, by default, :py:class:`cftime.datetime` objects | ||
will be used to represent times (either in indexes, as a | ||
:py:class:`~xarray.CFTimeIndex`, or in data arrays with dtype object) if | ||
any of the following are true: | ||
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- The dates are from a non-standard calendar | ||
- Any dates are outside the Timestamp-valid range. | ||
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Otherwise pandas-compatible dates from a standard calendar will be | ||
represented with the ``np.datetime64[ns]`` data type, enabling the use of a | ||
:py:class:`pandas.DatetimeIndex` or arrays with dtype ``np.datetime64[ns]`` | ||
and their full set of associated features. | ||
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For example, you can create a DataArray indexed by a time | ||
coordinate with dates from a no-leap calendar and a | ||
:py:class:`~xarray.CFTimeIndex` will automatically be used: | ||
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.. ipython:: python | ||
from itertools import product | ||
from cftime import DatetimeNoLeap | ||
dates = [DatetimeNoLeap(year, month, 1) for year, month in | ||
product(range(1, 3), range(1, 13))] | ||
da = xr.DataArray(np.arange(24), coords=[dates], dims=['time'], name='foo') | ||
xarray also includes a :py:func:`~xarray.cftime_range` function, which enables | ||
creating a :py:class:`~xarray.CFTimeIndex` with regularly-spaced dates. For | ||
instance, we can create the same dates and DataArray we created above using: | ||
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.. ipython:: python | ||
dates = xr.cftime_range(start='0001', periods=24, freq='MS', calendar='noleap') | ||
da = xr.DataArray(np.arange(24), coords=[dates], dims=['time'], name='foo') | ||
For data indexed by a :py:class:`~xarray.CFTimeIndex` xarray currently supports: | ||
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- `Partial datetime string indexing`_ using strictly `ISO 8601-format`_ partial | ||
datetime strings: | ||
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.. ipython:: python | ||
da.sel(time='0001') | ||
da.sel(time=slice('0001-05', '0002-02')) | ||
- Access of basic datetime components via the ``dt`` accessor (in this case | ||
just "year", "month", "day", "hour", "minute", "second", "microsecond", | ||
"season", "dayofyear", and "dayofweek"): | ||
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.. ipython:: python | ||
da.time.dt.year | ||
da.time.dt.month | ||
da.time.dt.season | ||
da.time.dt.dayofyear | ||
da.time.dt.dayofweek | ||
- Group-by operations based on datetime accessor attributes (e.g. by month of | ||
the year): | ||
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.. ipython:: python | ||
da.groupby('time.month').sum() | ||
- Interpolation using :py:class:`cftime.datetime` objects: | ||
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.. ipython:: python | ||
da.interp(time=[DatetimeNoLeap(1, 1, 15), DatetimeNoLeap(1, 2, 15)]) | ||
- Interpolation using datetime strings: | ||
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.. ipython:: python | ||
da.interp(time=['0001-01-15', '0001-02-15']) | ||
- Differentiation: | ||
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.. ipython:: python | ||
da.differentiate('time') | ||
- Serialization: | ||
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.. ipython:: python | ||
da.to_netcdf('example-no-leap.nc') | ||
xr.open_dataset('example-no-leap.nc') | ||
- And resampling along the time dimension for data indexed by a :py:class:`~xarray.CFTimeIndex`: | ||
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.. ipython:: python | ||
da.resample(time='81T', closed='right', label='right', base=3).mean() | ||
.. note:: | ||
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For some use-cases it may still be useful to convert from | ||
a :py:class:`~xarray.CFTimeIndex` to a :py:class:`pandas.DatetimeIndex`, | ||
despite the difference in calendar types. The recommended way of doing this | ||
is to use the built-in :py:meth:`~xarray.CFTimeIndex.to_datetimeindex` | ||
method: | ||
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.. ipython:: python | ||
:okwarning: | ||
modern_times = xr.cftime_range('2000', periods=24, freq='MS', calendar='noleap') | ||
da = xr.DataArray(range(24), [('time', modern_times)]) | ||
da | ||
datetimeindex = da.indexes['time'].to_datetimeindex() | ||
da['time'] = datetimeindex | ||
However in this case one should use caution to only perform operations which | ||
do not depend on differences between dates (e.g. differentiation, | ||
interpolation, or upsampling with resample), as these could introduce subtle | ||
and silent errors due to the difference in calendar types between the dates | ||
encoded in your data and the dates stored in memory. | ||
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.. _Timestamp-valid range: https://pandas.pydata.org/pandas-docs/stable/timeseries.html#timestamp-limitations | ||
.. _ISO 8601-format: https://en.wikipedia.org/wiki/ISO_8601 | ||
.. _partial datetime string indexing: https://pandas.pydata.org/pandas-docs/stable/timeseries.html#partial-string-indexing |
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