Contrails over Benelux, North Sea & Baltic Sea

Experience with AVHRR and MODIS imagery has shown that the split window channels (IR10.8 and IR12.0 on Meteosat-8) are very useful for the following applications: surface and cloud top temperatures, lower tropospheric humidity, cloud classification, cloud tracking, clear sky radiances (for NWP), precipitation estimation, dust storm detection and detection of volcanic ash clouds.

This case study shows that the split window channels on Meteosat-8 are also very useful for the detection and monitoring of very thin clouds, including condensation trails from aircraft (called contrails). Contrails, being nearly transparent in the solar channels (see e.g. the RGB composite NIR1.6, VIS0.8, VIS0.6 below), can frequently be detected in IR10.8–IR12.0 brightness temperature difference images.

Indeed, thin clouds appear colder in the IR12.0 channel than in the IR10.8 channel because thin clouds are less transparent at this wavelength (i.e. there is less radiation from the warmer layers below the thin cloud reaching the satellite sensor). Typical brightness temperature differences (IR10.8–IR12.0) for thin contrails range from about +3K to +7K.

The images below show a high pressure situation over Central Europe and a cyclonic circulation over the Iberian Peninsula. Looking at the RGB composite of solar channels, Germany and the Benelux seem to be cloud-free, but the split window channel difference shows a large part of the Benelux and Northern Germany covered by thin cirrus clouds.

Most of these clouds are probably contrails, which can also be identified over the North Sea and the Baltic Sea. Animation of the split window channel difference shows nicely how the thin cirrus clouds rotate around the centre of the high pressure in a clockwise (anticyclonic) sense.

The two images at the bottom of this page show the meteorological situation in the afternoon (15:00 UTC) of the same day. Other interesting features observed are ship trails (within the area of stratocumulus clouds over the Atlantic), high-level lee clouds (Sierra Nevada, Spain), desert dust (over the Western Mediterranean Sea and the Bay of Biscay), convection (Northern Spain) and supercooled clouds (sausage-shaped cloud to the north of the UK).