Add vector regridding section to general regridding description section.

src/Infrastructure/Field/src/ESMF_FieldRegrid.F90

@@ -550,7 +550,8 @@ subroutine ESMF_FieldRegridStoreNX(srcField, dstField, keywordEnforcer, &
 !           as the components of a vector. If true and there is more than one ungridded dimension in either
 !           the source or destination, then an error will be returned. Currently this functionality only
 !           works when both the source and destination Fields are build on a geometry (e.g. an ESMF Grid) with
-!           a spherical coordinate sytem (e.g. ESMF\_COORDSYS\_SPH\_DEG). If not specified, this argument defaults to false.
+!           a spherical coordinate system (e.g. ESMF\_COORDSYS\_SPH\_DEG). See section~\ref{sec::vectorRegrid} for further
+!           information on this functionality. If not specified, this argument defaults to false.
 !     \item [{[extrapMethod]}]
 !           The type of extrapolation. Please see Section~\ref{opt:extrapmethod} 
 !           for a list of valid options. If not specified, defaults to 

src/doc/ESMF_infradataoverview.tex

@@ -768,7 +768,29 @@ \subsubsection{Spherical grids and poles}
 Distances are measured along the great circle path. Under this option cells are on the sphere surface, and their boundaries 
 are great circle paths between their corner points. 
 
-Figure~\ref{line_type_support} shows which line types are supported for each regrid method as well as the defaults (indicated by *). 
+Figure~\ref{line_type_support} shows which line types are supported for each regrid method as well as the defaults (indicated by *).
+
+
+\subsubsection{Vector regridding}\label{sec::vectorRegrid}
+
+ESMF's initial vector regridding capability is intended to give cleaner results for spherical vectors expressed in
+terms of local directions (e.g. east and north) than just regridding each vector component separately. To do this, it
+converts the vectors to 3D Cartesian space and does the regridding there. This allows all the vectors participating in
+the regridding to have a consistent representation. After regridding, the resulting vectors are then converted
+back to the local direction form. The entire process is expressed in the usual weight matrix and/or routeHandle form and so
+the typical {\tt ESMF\_FieldRegridStore()/ESMF\_FieldRegrid()/ESMF\_FieldRegridRelease()} regridding paradigm can be used.
+However, the weight matrix will be in the format that allows it to contain tensor dimension indices (i.e. the leading
+dimension of the {\tt factorIndexList} will be of size 4). 
+
+In this initial version the meaning of the different components in the vector is fixed. They will be interpreted as:
+\begin{description}
+  \itemsep0em
+  \item[1st component] the east component.
+  \item[2nd component] the north component.
+\end{description}
+
+Note that because the different components are mixed, using vector regridding with a conservative regrid method will {\bf NOT} necessarily be conservative. 
+
 
 \subsubsection{Troubleshooting guide}