Xarray-zarr example dataset recipe (#108)

* add a working xarray-zarr example

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Co-authored-by: b8raoult <[email protected]>

docs/building/sources/xarray-zarr.rst

@@ -2,5 +2,17 @@
  xarray-zarr
 #############
 
+Here is an example recipe that builds a dataset using one of the many
+regridded versions of ERA5 hosted by Google in Analysis-Ready, Cloud
+Optimized format. See `here
+<https://cloud.google.com/storage/docs/public-datasets/era5>`_ for more
+information.
+
 .. literalinclude:: yaml/xarray-zarr.yaml
    :language: yaml
+
+Note that unlike the ``mars`` examples, there is no need to include a
+``grid`` specification. Also, in order to subselect the vertical levels,
+it is necessary to use the :ref:`join <building-join>` operation to join
+separate lists containing 2D variables and 3D variables. If all vertical
+levels are desired, then it is OK to specify a single source.

docs/building/sources/yaml/xarray-zarr.yaml

@@ -1,3 +1,22 @@
+dates:
+  start: 2021-01-01T00:00:00
+  end: 2021-01-10T12:00:00
+  frequency: 6h
+
 input:
-  xarray-zarr:
-    url: https://...
+  join:
+      - xarray-zarr:
+          url: "gs://gcp-public-data-arco-era5/ar/1959-2022-6h-128x64_equiangular_conservative.zarr"
+          param:
+              - surface_pressure
+              - 2m_temperature
+              - 10m_u_component_of_wind
+              - 10m_v_component_of_wind
+
+      - xarray-zarr:
+          url: "gs://gcp-public-data-arco-era5/ar/1959-2022-6h-128x64_equiangular_conservative.zarr"
+          param:
+              - temperature
+          level:
+              - 1000
+              - 500