# Output Along a Track¶

HISTORY collections can define a track_file that specifies a 1D timeseries of coordinates that the model is sampled at. The collection output has the same coordinates as the track file. This feature can be used to sample GCHP along a satellite track or a flight path. A track file is a NetCDF file with the following format

$ncdump -h example_track.nc netcdf example_track.nc { dimensions: time = 1234 ; variables: float time(time) ; time:_FillValue = NaNf ; time:long_name = "time" ; time:units = "hours since 2020-06-01 00:00:00" ; float longitude(time) ; longitude:_FillValue = NaNf ; longitude:long_name = "longitude" ; longitude:units = "degrees_east" ; float latitude(time) ; latitude:_FillValue = NaNf ; latitude:long_name = "latitude" ; latitude:units = "degrees_north" ; }  Important Longitudes must be between 0 and 360. Important When using recycle_track, the time offsets must be between 0 and 24 hours. To configure 1D output, you can add the following attributes to any collection in HISTORY.rc. track_file Path to a track file. The associated collection will be sampled from the model along this track. A track file is a 1-dimensional timeseries of latitudes and longitudes that the model is be sampled at (nearest neighbor). recycle_track Either .false. (default) or .true.. When enabled, HISTORY replaces the date of the time coordinate in the track file with the simulation’s current day. This lets you use the same track file for every day of your simulation. Note 1D output only works for instantaneous sampling. The frequency attribute is ignored when track_file is used. ## Creating a satellite track file¶ GCPy includes a command line tool, gcpy.raveller_1D, for generating track files for polar orbiting satellites. These track files will sample model grid-boxes at the times that correspond to the satellite’s overpass time. You can also use this tool to “unravel” the resulting 1D output back to a cubed-sphere grid. Below is an example of using gcpy.raveller_1D to create a track file for a C180 simulation for TROPOMI, which is in ascending sun-synchronous orbit with 14 orbits per day and an overpass time of 13:30. Please see the GCPy documentation for this program’s exact usage, and for installation instructions. $ python -m gcpy.raveller_1D create_track --cs_res 24 --overpass_time 13:30 --direction ascending --orbits_per_day 14 -o tropomi_overpass_c24.nc


The resulting track file, tropomi_overpass_c24.nc, looks like so

$ncdump -h tropomi_overpass_c24.nc netcdf tropomi_overpass_c24 { dimensions: time = 3456 ; variables: float time(time) ; time:_FillValue = NaNf ; time:long_name = "time" ; time:units = "hours since 1900-01-01 00:00:00" ; float longitude(time) ; longitude:_FillValue = NaNf ; longitude:long_name = "longitude" ; longitude:units = "degrees_east" ; float latitude(time) ; latitude:_FillValue = NaNf ; latitude:long_name = "latitude" ; latitude:units = "degrees_north" ; float nf(time) ; nf:_FillValue = NaNf ; float Ydim(time) ; Ydim:_FillValue = NaNf ; float Xdim(time) ; Xdim:_FillValue = NaNf ; }  Note Track files do not require the nf, Ydim, Xdim variables. The are used for post-process “ravelling” with gcpy.raveller_1D (changing the 1D output’s coordinates to a cubed-sphere grid). Note With recycle_track, HISTORY replaces the reference date (e.g., 1900-01-01) with the simulation’s current date, so you can use any reference date. ## Updating HISTORY¶ Open HISTORY.rc and add the track_file and recycle_track attributes to your desired colleciton. For example, the following is a custom collection that samples NO2 along the tropomi_overpass_c24.nc.  TROPOMI_NO2.template: '%y4%m2%d2_%h2%n2z.nc4', TROPOMI_NO2.format: 'CFIO', TROPOMI_NO2.duration: 240000 TROPOMI_NO2.track_file: tropomi_overpass_c24.nc TROPOMI_NO2.recycle_track: .true. TROPOMI_NO2.mode: 'instantaneous' TROPOMI_NO2.fields: 'SpeciesConc_NO2 ', 'GCHPchem', ::  ## Unravelling 1D overpass timeseries¶ To covert the 1D timeseries back to a cubed-sphere grid, you can use gcpy.raveller_1D. Below is an example of changing the 1D output back to model grid. Again, see the GCPy documentation for this program’s exact usage, and for installation instructions. $ python -m gcpy.raveller_1D unravel --track tropomi_overpass_c24.nc -i OutputDir/GCHP.TROPOMI_NO2.20180101_1330z.nc4 -o OutputDir/GCHP.TROPOMI_NO2.20180101_1330z.OVERPASS.nc4


The resulting dataset, GCHP.TROPOMI_NO2.20180101_1330z.OVERPASS.nc4, are simulated concentration on the model grid, sampled at the times that correspond to TROPOMI’s overpass.