Refactor grib_to_netcdf time_counter calculation

Update `time_counter` calculation to use forecast step ranges. This mitigates
the fact that `cfgrib` no longer provides correct forecast time step values from
ECCC GRIB2 files. This is a rework of commit 406e7e5 that tried to resolve the
issue using the `valid_time` GRIB variable.

This change makes our timestamp processing compatible with non-hourly step
handling that was introduced in `cfgrib=0.9.12.0`.

This change removes the need for version pinning of `cfgrib=0.9.11.0`
that was introduced in PR#276. By doing that, issue #336 is resolved.

Author: douglatornell

nowcast/workers/grib_to_netcdf.py

@@ -266,7 +266,14 @@ def _calc_nemo_ds(
             fcst_date, fcst_hr, fcst_step_range, msc_var, full_grid, config
         )
         nemo_datasets[nemo_var] = _calc_nemo_var_ds(
-            msc_var, grib_var, nemo_var, grib_files, full_grid, georef_ds, config
+            msc_var,
+            grib_var,
+            nemo_var,
+            grib_files,
+            fcst_step_range,
+            full_grid,
+            georef_ds,
+            config,
         )
     nemo_ds = xarray.combine_by_coords(
         nemo_datasets.values(), combine_attrs="drop_conflicts"
@@ -328,7 +335,7 @@ def _trim_grib(ds, y_slice, x_slice):
         # Select region of interest
         ds = ds.sel(y=y_slice, x=x_slice)
     # Drop coordinates that we don't need
-    keep_coords = ("valid_time", "latitude", "longitude")
+    keep_coords = ("time", "step", "valid_time", "latitude", "longitude")
     ds = ds.reset_coords(
         [coord for coord in ds.coords if coord not in keep_coords],
         drop=True,
@@ -337,13 +344,21 @@ def _trim_grib(ds, y_slice, x_slice):
 
 
 def _calc_nemo_var_ds(
-    msc_var, grib_var, nemo_var, grib_files, full_grid, georef_ds, config
+    msc_var,
+    grib_var,
+    nemo_var,
+    grib_files,
+    fcst_step_range,
+    full_grid,
+    georef_ds,
+    config,
 ):
     """
     :param str msc_var:
     :param str grib_var:
     :param str nemo_var:
     :param list grib_files:
+    :param tuple fcst_step_range:
     :param boolean full_grid:
     :param :py:class:`xarray.Dataset` or None georef_ds:
     :param dict config:
@@ -369,14 +384,18 @@ def _calc_nemo_var_ds(
         grib_files,
         preprocess=_partial_trim_grib,
         combine="nested",
-        concat_dim="valid_time",
+        concat_dim="step",
         engine="cfgrib",
         backend_kwargs={"indexpath": ""},
     )
+    start, stop = fcst_step_range
+    time_counter = grib_ds.time.values + numpy.array(
+        [numpy.timedelta64(fcst_step, "h") for fcst_step in range(start, stop + 1)]
+    )
     nemo_da = xarray.DataArray(
         data=grib_ds[grib_var].data,
         coords={
-            "time_counter": grib_ds.valid_time.values,
+            "time_counter": time_counter,
             "y": grib_ds.y,
             "x": grib_ds.x,
         },
@@ -399,7 +418,7 @@ def _calc_nemo_var_ds(
             nemo_var: nemo_da,
         },
         coords={
-            "time_counter": grib_ds.valid_time.values,
+            "time_counter": time_counter,
             "y": grib_ds.y,
             "x": grib_ds.x,
             "nav_lon": nav_lon,
@@ -412,8 +431,8 @@ def _calc_nemo_var_ds(
         # Drop unneeded variables that come from full continental domain GRIB files.
         # Drop time separately from lons/lats because drop_vars() fails is any of the vars
         # in the list don't exist.
-        nemo_ds = nemo_ds.drop_vars(["valid_time"])
-        nemo_ds = nemo_ds.drop_vars(["longitude", "latitude"])
+        nemo_ds = nemo_ds.drop_vars(["time", "valid_time"])
+        nemo_ds = nemo_ds.drop_vars(["latitude", "longitude"])
     except ValueError:
         # We don't care if some or all of them don't exist in the dataset
         pass