Atmospheric Toolbox

2019 Ozone Hole using GOME-2 L4 data

The ozone hole in 2019 was the smallest on record since its discovery. In this exercise we will compare the ozone hole in 2018 and 2019 using data from the GOME-2 instrument that is flying on METOP-B.

The ozone hole is most prominent in late September and early October so to perform the comparison we will look at daily assimlated ozone fields from 15 September to 15 October for both years.

The VISAN world plot window.

Downloading the data

The daily global measurements for GOME-2 can be downloaded as HDF files from the Tropospheric Emission Monitoring Internet Service (TEMIS) website. To retrieve full set, download each of the daily HDF files for 2018-09-15 - 2018-10-15 and for 2019-09-15 - 2019-10-15 starting from this page.

Put all files into a single directory on your local hard disk.

Starting VISAN

We will use VISAN to inspect the ozone whole for the two years.

Make sure you have installed and started the VISAN application. Within the main VISAN window use the os.chdir() function to change your current directory to the directory in which you have put the HDF files. For instance, if this was in /data you would run:

>>> os.chdir('/data')

Reading the data

We will read the data using the HARP software that is included with VISAN. With this software you can read the full 15 September - 15 October period of files for a single year (which are 31 files) into single data structure in Python. Run the following commands to read the data for 2018 and 2019:

>>> gome2018 = harp.import_product('o3col_gome2b_2018*.hdf')
>>> gome2019 = harp.import_product('o3col_gome2b_2019*.hdf')

You can inspect the contents of these structures with the python print() function:

>>> print(gome2018)
double datetime {time=31} [days since 2000-01-01]
double latitude {time=31, latitude=181} [degree_north]
double longitude {time=31, longitude=240} [degree_east]
double O3_column_number_density {time=31, latitude=181, longitude=240} [DU]
double O3_column_number_density_uncertainty {time=31, latitude=181, longitude=240} [DU]

>>> print(
[[[199. 199. 199. ... 199. 199. 199.]
  [199. 199. 199. ... 199. 199. 199.]
  [196. 196. 196. ... 196. 196. 196.]

Visualizing the data

Creating visualisations of the data only takes one additional command, which is the wplot() command. This command creates a worldmap plot of the data. When you use the result of a HARP import as parameter to the wplot() function it will automatically find the right variables to use for the plot. We will pass a few extra arguments to the function call to align the color range (in Dobson Units) between both plots, to use the right color table for Ozone, and to set a title:

>>> wplot(gome2018, colorrange=[150,500], colortable="Ozone", title="2018")
>>> wplot(gome2019, colorrange=[150,500], colortable="Ozone", title="2019")


GOME-2 data for 2018

The plot window that appears has an animation toolbar at the bottom that allows you to scroll through the different days in the period.

If you keep your left mouse button pressed in the plot window you can move your mouse to rotate the earth. With your right mouse button pressed, you can zoom in and out.

To show the ozone hole, rotate the earth to bring the South Pole into view.

For each plot window, you can select 'Properties' from the 'View' menu to show a property panel on the right. This panel will allow you to set various plot settings. But the window will in this case also show you the date for the day that you selected in the animation bar.

Ozone Hole at South Pole for 30 September 2018
Ozone Hole at South Pole for 30 September 2019