per #1835, add documentation to describe how to use PCPCombine wrappe…

…r, including how to use [FCST/OBS]_PCP_COMBINE_CONSTANT_INIT to find files with the same initialization time

docs/Users_Guide/wrappers.rst

@@ -5123,6 +5123,61 @@ The PCPCombine wrapper is a Python script that encapsulates the MET
 PCPCombine tool. It provides the infrastructure to combine or extract
 from files to build desired accumulations.
 
+PCPCombine wrapper can be configured to process forecast and/or observation
+data. Setting :term:`FCST_PCP_COMBINE_RUN` = True will process forecast data
+and setting :term:`OBS_PCP_COMBINE_RUN` = True will process observation data.
+
+PCPCombine wrapper can be configured to build a command for sum, add, subtract,
+and derive methods using :term:`FCST_PCP_COMBINE_METHOD` and/or
+:term:`OBS_PCP_COMBINE_METHOD`. Each method executes logic to gather the
+desired input files to build the command based on specific examples.
+
+Accumulations
+^^^^^^^^^^^^^
+
+The desired accumulation to build is defined using
+:term:`FCST_PCP_COMBINE_OUTPUT_ACCUM` or :term:`OBS_PCP_COMBINE_OUTPUT_ACCUM`.
+The default units are hours unless otherwise specified.
+The output field name can be set explicitly using
+:term:`FCST_PCP_COMBINE_OUTPUT_NAME` or :term:`OBS_PCP_COMBINE_OUTPUT_NAME`.
+
+For ADD and DERIVE methods, the input accumulation(s) can be specified using
+:term:`FCST_PCP_COMBINE_INPUT_ACCUMS` or :term:`OBS_PCP_COMBINE_INPUT_ACCUMS`.
+The default units are hours unless otherwise specified.
+This can be a list of accumulation amounts in order of preference.
+If the remaining accumulation needed to build the desired accumulation is
+less than the first accumulation, then the next value in the list will be used.
+The name and level of the field to read for each input accumulation can be
+specified with
+:term:`FCST_PCP_COMBINE_INPUT_NAMES`/:term:`FCST_PCP_COMBINE_INPUT_LEVELS` or
+:term:`OBS_PCP_COMBINE_INPUT_NAMES`/:term:`OBS_PCP_COMBINE_INPUT_LEVELS`.
+These lists must be the same length as
+:term:`FCST_PCP_COMBINE_INPUT_ACCUMS` or :term:`OBS_PCP_COMBINE_INPUT_ACCUMS`.
+
+Constant Initialization Time
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+For ADD and DERIVE methods, :term:`FCST_PCP_COMBINE_CONSTANT_INIT` or
+:term:`OBS_PCP_COMBINE_CONSTANT_INIT` can be set to **True** to gather input
+files that all contain the same initialization time.
+
+
+User-Defined Commands
+^^^^^^^^^^^^^^^^^^^^^
+
+There are many ways to utilize PCPCombine that may not align with the logic
+used to gather files. If this is the case, then the method can be set to
+**USER_DEFINED** and the explicit command arguments can be specified using
+:term:`FCST_PCP_COMBINE_COMMAND` or :term:`OBS_PCP_COMBINE_COMMAND`.
+Other METplus configuration variables and filename template tags can be
+referenced in the explicit command. Note that the path to the pcp_combine
+executable and the output path should not be included in the command value.
+The output path is controlled by
+:term:`FCST_PCP_COMBINE_INPUT_TEMPLATE`/:term:`FCST_PCP_COMBINE_INPUT_DIR` or
+:term:`OBS_PCP_COMBINE_INPUT_TEMPLATE`/:term:`OBS_PCP_COMBINE_INPUT_DIR` and
+will automatically be added to the end of the command.
+
+
 METplus Configuration
 ---------------------