Building Surface Topography Files

Here we present a simple tutorial to build a new topography map that can be used in the Generic PCM.

Surface topography maps are netcdf files stored in your /datagcm/surface_data/ repository. Such files contain information on terrain elevations, as well as albedo and thermal inertia.

As a reminder, our database of surface topography files is available here: https://web.lmd.jussieu.fr/~lmdz/planets/LMDZ.GENERIC/datagcm/surface_data/

Getting topography and surface data

A few options to do that:

• use existing topographic data for solar system objects. We have already formatted the data for most solar system objects, so please ask someone in the team if your topography map is not already here: https://web.lmd.jussieu.fr/~lmdz/planets/LMDZ.GENERIC/datagcm/surface_data/
• use your own data files

You can also derive data files by digitalizing color maps. See below an example:

HERE WE ADD AN EXAMPLE

Convert the data into a netcdf surface topography map

We have developed Python and fortran routines to easily convert surface data files (elevation, albedo, thermal inertia) into GCM-compatible surface topography files.

Please find below examples:

#!/usr/bin/env python3.0
#-*- coding:Utf-8 -*-

from    numpy import *
import  numpy                 as        np
import  matplotlib.pyplot     as        mpl
import  math
import xarray as xr

# 1. WE GET THE SURFACE TOPOGRAPHY DATA
nc = xr.open_dataset('surface_mars.nc',decode_times=False) # can be any netcdf file (e.g. start/startfi.nc files)

# 2. WE READ THE VARIABLES
lat=nc['latitude']
lon=nc['longitude']
albedo=nc['albedo']
thermal_inertia=nc['thermal']
elevation=nc['zMOL']

# BELOW THE VARIABLES WE WANT TO UPDATE
new_elevation = np.empty((len(lat),len(lon)))
new_thermal_inertia = np.empty((len(lat),len(lon)))
new_albedo = np.empty((len(lat),len(lon)))

# LOOP TO MODIFY THE VARIABLES
for i in range(0,len(lat),1):
    for j in range(0,len(lon),1):
        new_elevation[i,j]=max(elevation[i,j],-3.9) # here you put whatever you want ; in this example, we take the (MOLA) topography of present-day Mars and modify it to fill the topographic depressions with an ocean at an altitude of -3.9km
        new_albedo[i,j]=albedo[i,j] # here you put whatever you want
        new_thermal_inertia[i,j]=thermal_inertia[i,j] # here you put whatever you want

# SANITY CHECK PLOTS

fig1 = mpl.figure(1)
mpl.contourf(lon,lat,elevation)
mpl.xlabel('longitude (deg)')
mpl.ylabel('latitude (deg)')
mpl.show()

fig2 = mpl.figure(2)
mpl.contourf(lon,lat,new_elevation)
mpl.xlabel('longitude (deg)')
mpl.ylabel('latitude (deg)')
mpl.show()

# CREATE THE NEW NETCDF TOPOGRAPHY FILE

nc['albedo'].values = new_albedo
nc['zMOL'].values = new_elevation
nc['thermal'].values = new_thermal_inertia
nc.to_netcdf('new_topography.nc')

Using a new topography in the model

follow the tutorial here: https://lmdz-forge.lmd.jussieu.fr/mediawiki/Planets/index.php/Advanced_Use_of_the_GCM