Improved comments and one error message in create_xgrid.c based on

input from r.benson and c.blanton.

README.md

@@ -58,8 +58,9 @@ The tools available in FRE-NCtools are:
 
 
 ### Other Tools
-There are several tools that have parallel versions and can overcome memory and cpu constrains of the serial conterpart.
-E.g. fregrid_parallel is used to generate remapping weights for high resolution grids (for further information, see the
+There are several tools that have parallel versions and can overcome memory and cpu constrains of the serial
+conterpart. E.g. fregrid_parallel reproduces the functionality of fregrid, and among other things it is
+commonly used to generate the remapping weights for high resolution grids.  (for further information, see the
 "extreme fregrid" document).
 The [Ocean Model Grid Generator](https://github.com/NOAA-GFDL/ocean_model_grid_generator) can be copied or
 cloned from its GFDL homepage.

docs/extreme_fregrid_sample_runscript.txt

@@ -3,17 +3,21 @@
 # generate this message when the remapping weights file is not available and
 # grids are too large for it to process:
 #
-#   FATAL Error: nxgrid is greater than MAXXGRID/nthreads, increase MAXXGRID, decrease
-#       nthreads, or increase number of MPI ranks
+#   FATAL Error: The xgrid size is too large for resources.
+#    nxgrid is greater than MAXXGRID/nthreads; increase MAXXGRID,
+#    decrease nthreads, or increase number of MPI ranks.
 #
-# fregrid_parallel should instead be used if such an error is encountered. fregrid_parallel
-# can be used to generate the weights file, and the file is subsequently reused (usually
-# as an input file to the serial fregrid) for regridding the fields. However, even
-# fregrid_parallel can generate the same error if run with insufficient computational
-# resources. Below (after the dashed line) is an example runscript configured for using
-# fregrid_parallel with a sufficiently large number of ranks and cores to avoid the fatal
-# error for a common "extreme fregrid" case. This configuration runs in about 43 minutes
-# on Gaea . The tail end of the runs output follows:
+# fregrid_parallel should instead be used if such an error is encountered. Like the
+# serial version, it can be used to perform the remapping; but commonly it is used
+# to perform only the compute intensive operations - the generation of the remapping
+# weights file. The weights file is saved and in turn it is then used (and re-used)
+# as an input file to the serial fregrid to quickly perform the remapping. However,
+# even fregrid_parallel can generate the same error if run with insufficient
+# computational resources. Below (after the dashed line) is an example runscript
+# configured for running fregrid_parallel with a sufficiently large number of ranks
+# and cores to avoid the fatal error for a common "extreme fregrid" case. This
+# configuration runs in about 43 minutes on Gaea C5. The tail end of the runs output
+# follows:
 #
 #> NOTE: done calculating index and weight for conservative interpolation
 #> Memuse(MB) at After setup interp, min=4814.17, max=4881.2, avg=4834.05

tools/libfrencutils/create_xgrid.c

@@ -807,7 +807,9 @@ nxgrid = 0;
 	  if( xarea/min_area > AREA_RATIO_THRESH ) {
 	    pnxgrid[m]++;
             if(pnxgrid[m]>= MAXXGRID/nthreads)
-	      error_handler("nxgrid is greater than MAXXGRID/nthreads, increase MAXXGRID, decrease nthreads, or increase number of MPI ranks");
+	      error_handler("The xgrid size is too large for resources.\n"
+        " nxgrid is greater than MAXXGRID/nthreads; increase MAXXGRID,\n"
+        " decrease nthreads, or increase number of MPI ranks.");
 	    nn = pstart[m] + pnxgrid[m]-1;
 
 	    pxgrid_area[nn] = xarea;