CESM1 Paleo Users Guide

Runoff directional dataset for RTM

Deep time experiments typically require significant modification of the land/sea mask, and therefore must remap river networks across the paleo topography by creating a new runoff directional dataset.  This dataset is used by the River Transport Model (RTM) to route river runoff to the ocean.

RTM now runs outside the land model, using a fixed regular grid that is different from the parent CLM grid (the CLM and CAM models use the same grid). The runoff directional forcing file required for RTM describes  vector (direction) for runoff flow at each RTM grid point. The integer values are numbered from 1 to 8: 1=N, 2=NE (45o), 3=E (90o), etc.   

  8    1 
ref   3 


The CESM1 default RTM grid is at 0.5 degree resolution.  However, deep time modelers typically use a 2x2 degree grid.

Creating RTM forcing files [rdirc.csh/topo2rdirc.F90]

The script rdirc.csh computes the runoff direction at each land grid point based on the user-provided topobathy.nc file containing topography and ocean bathymetry.  An ASCII file of runoff vectors is created which is used as input to the RTM at runtime. This tool uses topography to compute the direction of runoff flow.  The output filename from rdirc.csh for your RTM forcing file is simply fort.10_$CASE (Fortran output file), so you may wish to rename your RTM forcing file to something more descriptive. For example, a filename for a paleo run could be: rdirc_myrun.resolution.date.nc.

shell script   rdirc.csh
source code  topo2rdirc.F90
inputs topobathy.nc

fort.10_$CASE (RTM forcing file used in model)
fort.11_$CASE (infinite loop vectors)
fort.13_$CASE (redirected runoff vectors)
fort.14_$CASE (error file)


CAVEAT:  Redirecting runoff in RTM forcing file

If your surface topography has internal basins or large flat regions, infinite loops will result, and rdirc.csh will produce another ASCII file with these loops (fort.11_$CASE).  An infinite loop is a region from which runoff will never flow out to the coastline, but circulate back to the starting point.  If infinite loops are not removed, global freshwater will not be conserved, and undesirable trends in global volume averaged ocean salinity may result. topo2dirc.F90 automatically checks for internal basins and reroutes the resulting drainage to the nearest coastline, irrespective of topography.  While the result will conserve freshwater, it may also produce unnatural drainage features that affect the science outcome of the simulation so check the output maps carefully by plotting the uncorrected and redirected runoff maps using the tool provide (plotrdirc.csh).


Plotting the vectors on a map [plotrdirc.csh/plot_rdirc.ncl]

An NCL script to plot your RTM forcing file vectors onto a latitude/longitude map is included in the setup_tools.  (Note that the code may report errors and pause, waiting for input, before it completes.  Press the return key to continue to the end of the program).

shell script plotrdirc.csh
source code plot_rdirc.ncl


output postcript file with maps of uncorrected and redirected runoff vectors


Creating netCDF file for CESM1/CLM4 [rtm_ncdf.pro]

RTM now requires netcdf.  We provide an IDL tool to convert the ASCII output into netCDF for RTM.

Source Code:  rtm_ncdf.pro

Usage: Edit rtm_ncdf.pro to point to your fort.13_$CASE file (your renamed file)

Start IDL.

IDL> .rn rtm_ncdf                       ; compile rtm module

IDL> rtm                                   ; execute code