.. _dry-run-run:

############################
Execute a dry-run simulation
############################

Follow the steps below to perform a GEOS-Chem Classic dry-run simulation:

==========================
Complete preliminary setup
==========================

Make sure that you have done the following steps;

#. :ref:`Downloaded GEOS-Chem Classic source code <get-code>`
#. :ref:`Compiled the source code <compile>`
#. :ref:`Configured your simulation <cfg>`

Then doublecheck these settings in the following :ref:`configuration
files <cfg>`:

.. option:: geoschem_config.yml

   #. :option:`start_date`: Set the start date and time for your simulation.
   #. :option:`end_date`: Set the end date and time for your simulation.
   #. :option:`met_field`: Check if the meteorology setting
      (:option:`GEOS-FP`, :option:`MERRA2`, :option:`GCAP2`) is
      correct for your simulation.

      .. attention::

         The convection scheme used to generate archived GEOS-FP
	 meteorology files changed from RAS to Grell-Freitas starting
	 01 June 2020 with impact on vertical transport. Discussion
	 and analysis of the impact is available at
         https://github.com/geoschem/geos-chem/issues/1409.

         To fix this issue, different GEOS-Chem convection schemes are
	 called based on simulation start time. This ensures
	 comparability in GEOS-Chem runs using GEOS-FP fields
	 generated using the RAS convection scheme and fields
	 generated using Grell-Freitas, but only if the simulation
	 does not cross the 01 June 2020 boundary. We therefore
	 recommend splitting up GEOS-FP runs in time such that a
	 single simulation does not span this date. For example,
	 configure one run to end on 01 June 2020 and then use  its
	 output restart to start another run on 01 June
	 2020.. Alternatively consider using MERRA2 which was entirely
	 generated with RAS, or GEOS-IT which was entirely generated
	 with Grell-Freitas. If you wish to use a GEOS-FP meteorology
	 year different from your simulation year please create a
	 GEOS-Chem GitHub issue for assistance to avoid accidentally
	 using zero convective precipitation flux.

   #. :option:`root_data_dir`: Make sure that the path to
      :file:`ExtData` is correct.

.. option:: HISTORY.rc

   #. Set the frequency and duration for each :ref:`diagnostic
      collection <histguide-collections>` to be consistent with the
      settings in :option:`geoschem_config.yml`.

.. option:: HEMCO_Config.rc

   #. Check the `Settings section
      <https://hemco.readthedocs.io/en/latest/hco-ref-guide/hemco-config.html#settings>`_
      to make sure that diagnostic frequency :envvar:`DiagnFreq`: is
      set to the interval that you wish (e.g. :envvar:`Monthly`,
      :envvar:`Daily`, :envvar:`YYYYMMDD hhmmss`, etc).
   #. Check the `Extension Settings section
      <https://hemco.readthedocs.io/en/latest/hco-ref-guide/hemco-config.html#extension-switches>`_,
      to make sure all of the required emissions inventories and data
      sets for your simulation have been switched on.

   .. tip::

      You can reduce the amount of data that needs to be downloaded for
      your simulation by turning off inventories that you don't need.

.. _dry-run-run-flag:

=============================================
Run the executable with the ``--dryrun`` flag
=============================================

Run the GEOS-Chem Classic executable file at the command line with the
:command:`--dryrun` command-line argument as shown below:

.. code-block:: console

   $ ./gcclassic --dryrun | tee log.dryrun

The :program:`tee` command will send the output of the dryrun to the
screen as well as to a file named :file:`log.dryrun`.

The :file:`log.dryrun` file will look somewhat like a regular
GEOS-Chem log file but will also contain a list of data files and
whether each file was found on disk or not.  This information will be
used by the :file:`download_data.py` script in the next step.

You may use whatever name you like for the dry-run output
log file (but we prefer :file:`log.dryrun`).  You will need this file
to download data (:ref:`see the next chapter <dry-run-download>`).