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doc/source/contrib.rst

@@ -56,8 +56,90 @@ In order to submit changes, you must use GitHub to issue a pull request.
 Overview of WRF-Python Internals
 ----------------------------------
 
-WRF-Python is a collection of diagnostic and interpolation routines for WRF-ARW
+WRF-Python is a collection of diagnostic and interpolation routines for 
-data. The API consists of a handful of functions 
+WRF-ARW data. The API is kept to a minimal set of functions, since we've found
-
+this to be the easiest to teach to new programmers, students, and scientists. 
+Future plans do include adopting the Pangeo xarray/dask model for more 
+advanced programmers, but is not currently supported as of this user guide.
+
+A typical use case for a WRF-Python user is to:
+
+1) Open a WRF data file (or sequence of files) using NetCDF4-python or PyNIO.
+2) Compute a WRF diagnostic using :meth:`wrf.getvar`.
+3) Performing other computations using methods outside of WRF-Python.
+4) Creating a plot of the output using matplotlib (basemap or cartopy) or 
+   PyNGL.
+   
+The purpose of this guide is to explain the internals of item 2 so that 
+users can help contribute or support the computational diagnostics.
+
+
+Overview of a :meth:`wrf.getvar` Diagnostic Computation
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+A diagnostic computed using the :meth:`wrf.getvar` function consists of the 
+following steps:
+
+1) Using the diagnostic string, call the appropriate 'get' function. This 
+   step occurs in the :met:`wrf.getvar` routine in routines.py. 
+2) Extract the required variables from the NetCDF data file (or files).
+3) Compute the diagnostic using a wrapped Fortran, C, or Python routine.
+4) Convert to the desired units if applicable.
+5) If desired, set the metadata and return the result as an 
+   :class:`xarray.DataArray`, or return a :class:`numpy.ndarray` if no 
+   metadata is desired.
+   
+In the source directory, the :meth:`wrf.getvar` 'get' routines have a 
+"g_" prefix for the naming convention (the "g" stands for "get", but didn't 
+want to cause namespace conflicts with functions already named with "get" in 
+the title). 
+
+The unit conversion is handled by a wrapt decorator that can be found in 
+decorators.py. The setting of the metadata is handled using a wrapt decorator, 
+which can be found in the metadecorators.py file.
+
+
+Overview of Compiled Computational Routines
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Currently, the compiled computational routines are written in Fortran 
+90 and exposed the Python using f2py. The routines have been aquired over 
+decades, originated from NCL's Fortran77 codebase, and do not necessarily 
+conform to a common mindset (e.g. some use 1D arrays, 2D arrays, etc).
+
+The raw Fortran routines are compiled in to the :mod:`wrf._wrffortran`, but 
+are not particularly useful for applications in that raw form. These 
+routines are imported in the extention.py module, where additional
+functionality is added to make the routines more user friendly.
+
+The common behavior for the fully exported Fortran routine in extension.py 
+is:
+
+1) Verify that the arguments passed in are valid in shape. While f2py does this 
+   as well, the errors thrown by f2py are confusing to users, so this step 
+   helps provide better error messages.
+
+2) Allocate the ouput array based on the output shape of the algorithm, 
+   number of "leftmost" dimensions, and size of the data.
+   
+3) Iterate over the leftmost dimensions and compute output for argument 
+   data slices that are of the same dimensionality as the compiled algorithm. 
+   For example, if the compiled algorithm is written for two dimensional data, 
+   but your data is four dimensional, you have two leftmost dimensions.
+   
+4) Cast the argument arrays in to the type used in the 
+   compiled routine (usually for WRF data, the conversion is from 4-byte float 
+   to 8-byte double).
+   
+5) Extract the argument arrays out of xarray in to numpy arrays 
+   (if applicable) and transpose them in to Fortran ordering. Note that this 
+   does not actually do any copying of the data, it simply reorders the shape 
+   tuple for the data and sets the Fortran ordering flag. This allows data
+   pointers from the output array to be directly passed through f2py so that 
+   copying is not required in to the output array.
+   
+The steps described above are handled in wrapt decorators that can be found in 
+decorators.py. For some routines that produce multiple outputs or have 
+atypical behavior, the special case decorators are located in specialdec.py.