ECMWF ``ERA5``
==============

In this file, the details of the dataset is explained.

Location of Dataset Files
-------------------------

The global ``ERA5`` dataset is located under the following directory
accessible from Compute Canada (CC) Graham Cluster:

.. code:: console

   # DRAC Graham HPC location
   /project/rpp-kshook/CompHydCore/climateForcingData/ERA5/ERA5_for_SUMMA/2_merged_data # rpp-kshook allocation

   # DRAC Fir HPC location
   /project/rrg-alpie/data/meteorological-data/era5/ # rrg-alpie allocation

   # UCalgary ARC HPC location
   /work/comphyd_lab/data/meteorological-data/era5 # comphyd_lab location

   # Perdue University Anvil HPC location
   /anvil/datasets/meteorological/ERA5_small # Anvil community storage

and the structure of the dataset hourly files is as following:

.. code:: console

   /path/to/dataset/dir/
   ├── ERA5_merged_195001.nc
   ├── ERA5_merged_%y%m.nc
   ├── .
   ├── .
   ├── .
   └── ERA5_merged_202012.nc

Coordinate Variables and Time-stamps
------------------------------------

Coordinate Variables
~~~~~~~~~~~~~~~~~~~~

The coordinate variables of the ``ERA5`` simulations are ``longitude``
and ``latitude`` representing the longitude and latitude points,
respectively. ### Time-stamps The time-stamps are included in the
original files.

Dataset Variables
-----------------

The NetCDF files of the dataset contain 7 variables needed to force
hydrological models. You may see a list of variables by using the
``ncdump -h`` command on one of the files:

.. code:: console

   foo@bar:~$ module load cdo/2.0.4
   foo@bar:~$ module load nco/5.0.6
   foo@bar:~$ ncdump -h  /project/rpp-kshook/CompHydCore/climateForcingData/ERA5/ERA5_for_SUMMA/2_merged_data/ERA5_merged_195001.nc

Spatial Extent
--------------

The spatial extent of the ``ERA5`` is on latitutes from ``+90`` to
``-90`` and longitudes from ``-180`` to ``179.75``. The resolution is
0.25 degrees.

Temporal Extent
---------------

The time-steps are hourly covering from January 1950 to December 2020.

Short Description on ``ERA5`` Variables
---------------------------------------

In most hydrological modelling applications, usually 7 variables are
needed detailed as following: 1) specific humidity at 2 meters, 2)
surface pressure, 3) air temperature at 2 meters, 4) wind speed at 10
meters, 5) precipitation, 6) downward short wave radiation, and 7)
downward long wave radiation. These variables are available through the
current ``ERA5`` dataset and their details are described in the table
below:

.. list-table:: ERA5 Variables
   :header-rows: 1

   * - Variable Name
     - ERA5 Variable
     - Original Shortname
     - Parameter ID
     - Unit
     - IPCC abbreviation
     - Comments
   * - surface pressure
     - ``airpres``
     - sp
     - 134
     - Pa
     - ps
     -
   * - specific humidity @2m
     - ``spechum``
     - q
     - 133
     - kg/kg
     - huss
     -
   * - air temperature
     - ``airtemp``
     - t
     - 130
     - k
     - tas
     -
   * - wind speed @10m
     - ``windspd``
     - u,v
     - 131,132
     - m/s
     - wspd
     - WIND=SQRT(u^2+v^2)
   * - precipitation
     - ``pptrate``
     - mtpr
     - 235055
     - mm/hr
     -
     - mean total precipitation rate
   * - short wave radiation
     - ``SWRadAtm``
     - msdwswrf
     - 235035
     - W m-2
     - rsds``
     -
   * - long wave radiation
     - ``LWRadAtm``
     - msdwlwrd
     - 235036
     - W m-2
     - rlds``
     -


For a complete catalog of the dataset, see
`here <https://confluence.ecmwf.int/display/CKB/ERA5%3A+data+documentation>`__.

Downloading Original ERA5 Data
------------------------------

The scripts to download the original ERA5 data are contained in the
```ERA5_downloads.zip`` <./ERA5_downloads.zip>`__ file located in the
current directory of the repository. The scripts are written by
`Quoqiang Tang <quoqiang.tang@usask.ca>`__ and `Wouter
Knoben <wouter.knoben@usask.ca>`__ as part of the `Community Workflows
to Advance Reproducibility in Hydrological Modelling
(CWARHM) <https://github.com/CH-Earth/CWARHM>`__. “ERA5 data preparation
includes interactions between an atmospheric model and a land surface
model. … [Original] ERA5 data is available at 137 different pressure
levels (i.e. some height above the surface), as well as at the surface.
The lowest atmospheric level is L137, at geopotential and geometric
altitude 10m and data here relies only on the atmospheric model. Any
variables at a height lower than L137 (i.e., at the surface) are the
result of interpolation between atmospheric model and land model. We
want to use only the outcomes from the ECMWF atmospheric model […].
Therefore, we obtain (1) air temperature, (2) wind speed and (3)
specific humidity at the lowest pressure level (L137), (4)
Precipitation, (5) downward shortwave radiation, (6) downward longwave
radiation and (7) air pressure are unaffected by the land model coupling
and can be downloaded at the surface level. This is beneficial because
surface-level downloads are substantially faster than pressure-level
downloads. [^1]” [^1]: from:
https://github.com/CH-Earth/CWARHM/tree/main/3a_forcing#forcing-needed-to-run-summa