Add preface to the main page

doc/source/index.rst

@@ -10,6 +10,16 @@ This project is supported by the AWS Public Data Set Program and the NASA Atmosp
 .. warning::
   *GEOSChem-on-cloud* is at initial development and things are moving very fast. Please use the `GitHub issue tracker <https://github.com/JiaweiZhuang/cloud_GC/issues>`_ to request new funtionalities, report bugs, or just discuss general issues.
 
+How to use this documentation
+-----------------------------
+
+**For GEOS-Chem users**, this website contains everything you need in order to use GEOS-Chem on the cloud. You will be able to finish a complete research workflow, from model simulations to output data analysis and management. **If it is your first time trying GEOS-Chem, this project is perhaps your best starting point**, because :ref:`you don't need to do any initial setup <motivation-label>` and the model is guaranteed to work correctly (see :ref:`quick start guide <quick-start-label>`). Note that this website is not a user guide on the GEOS-Chem model itself. Please refer to our comprehensive `user guide <http://acmg.seas.harvard.edu/geos/doc/man/>`_ and `wiki <http://wiki.seas.harvard.edu/geos-chem/index.php/Main_Page>`_ for all details about GEOS-Chem. To run on the cloud, we only support GEOS-Chem `v11-02 <http://wiki.seas.harvard.edu/geos-chem/index.php/GEOS-Chem_v11-02>`_ or future versions for `GNU-Fortran compatibility <http://wiki.seas.harvard.edu/geos-chem/index.php/GNU_Fortran_compiler>`_ and `NetCDF diagnostics updates <http://wiki.seas.harvard.edu/geos-chem/index.php/List_of_diagnostics_archived_to_netCDF_format>`_.
+
+**For non-GEOS-Chem-users**, this documentation can be used as an introduction to AWS for scientific computing, especially for **Earth science model simulations**. Since all Earth science models are highly similar from a software perspective, it should be quite easy to adapt this guide for you specific use case. More than 90% of this website is about general AWS concepts and tutorials, which doesn't require GEOS-Chem-specific knowledge. Please get a feeling of cloud computing workflow by exploring :doc:`beginner tutorials <../chapter02_beginner-tutorial/index>` and then refer to :doc:`Developer guide <../chapter04_developer-guide/index>` to build your own model. Although cloud computing has a lot of potential in Earth science, it is still significantly under-ultilized due to :doc:`the lack of accessible tutorials <./chapter01_overview/external-resources>` for Earth science researchers. This project tries to fill this gap.
+
+For general reference, GEOS-Chem is a `Chemical Transport Model <https://en.wikipedia.org/wiki/Chemical_transport_model>`_ for simulating atmospheric chemical compositions. It has been developed over 20 years and is used by `more than 100 research groups worldwide <http://acmg.seas.harvard.edu/geos/geos_people.html>`_. The program is mainly written in Fortran 90. Input and output data formats are mostly NetCDF, which can be analyzed easily by most languages such as Python, R and MATLAB. IDL (Interactive Data Language) has historically been the major data analysis tool but now we embrace open-source tools especially Python, `Jupyter <http://jupyter.org>`_ and `xarray <http://xarray.pydata.org>`_. The classic version of GEOS-Chem uses OpenMP parallelization (shared-memory, multi-threading). `The MPI version of GEOS-Chem <https://www.geosci-model-dev-discuss.net/gmd-2018-55/>`_ has also been developed and we are working on making it available on the cloud.
+
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