Global ozone distribution as seen in Airmass RGB

By HansPeter Roesli (EUMETSAT)

The Meteosat-8 Airmass RGB composite below displays distinctly paler colour shades towards the South Pole. This is due to the low values of total ozone (TOZ) over the south-polar region. This is most prominent during the southern-hemisphere spring as illustrated by the TOZ analysis of October 2006 from the ECMWF model (see monthly average).

The day of 9 October 2006, being very close to the 2006 TOZ minimum, illustrates the situation very well (see reprojected Airmass RGB with ECMWF TOZ analysis (view from South Pole)). The white to grey contours of ECMWF TOZ analysis show the steep TOZ gradient towards the South Pole rather well.

Even the limb effect (increasing ozone path because of high viewing angle) does not compensate for the low TOZ content, i.e. the brightness temperature difference between the IR10.8 and IR9.7 channels is relatively small considering the high viewing angle (see difference image).

Over the north-polar region TOZ never decreases to values observed over, and close to Antarctica. Therefore, the colour shades of the Airmass RGB over this area remain stronger (dark blue) throughout the whole year, except over land in situations of very low surface temperatures (see unusual colours of Airmass RGB).

With the high viewing angles on the western and eastern parts of the edge of the field of view of Meteosat, the increasing ozone path amplifies the ozone signal considerably, with a tendency to add to the Airmass RGB a magenta hue around the limb.

With Meteosat-8 on 10°W and with its imagery rectified to 0° longitude, this effect is slightly asymmetric, being stronger on the eastern side than on the western side. The asymmetry is best seen on a image of the IR10.8-IR9.7 difference (see difference image). Note, full-disc images from Meteosat-9 positioned at 0° longitude show east-west symmetry.