Ship Trails over the southern Atlantic

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A series of ship trails were seen in stratocumulus clouds around the centre of a low-level high pressure system on different types of MSG images.

Date & Time
26 June 2015 00:00–12:00 UTC
Day Microphysics RGB, Night Microphysics RGB

By Edwin Thema

The Atlantic high pressure system was situated over the southern Atlantic, west of Southern Africa. There was also a cut-off low system developing over the north-western parts of South Africa.

Figure 1
Figure 1: Meteosat-10 Day Microphysics RGB, 26 June 12:00 UTC
Full resolution

Ship trails, which are formed by aerosols emitted from the ship funnels, are trapped in small water drop usually in a stratocumulus clouds in a stable atmosphere.

During the day, small water droplets have a higher reflectivity in the NIR1.6 and the IR3.9 channels, so bright trails are visible on the Meteosat-10 Day Microphysics RGB image (Figure 1).

The Meteosat-10 Day Microphysics RGB animation, 26 June 08:00–13:00 UTC (MP4, 2 MB) shows the ships trails rotating anti-clockwise around the Atlantic high pressure system.

The ship trails can also be tracked at night using the Night Microphysics RGB or the 24-hour Microphysics RGB.

Small droplets emit less radiation (lower emissivity) in the IR3.9 and IR8.7 bands (compared to large droplets), so the brightness temperature difference (BTD) IR10.8-IR3.9 (or IR10.8-IR8.7) of ship trails is more positive than the brightness temperature difference of stratocumulus clouds with large droplets.

This can be seen in the Night Microphysics RGB image (Figure 2, left) and the IR10.8-IR3.9 BTD (green component of the Night Microphysics RGB) (Figure 3, right) where the ship trails appear brighter compared to the 'normal' stratocumulus around.

Image comparison
Night Microphysics RGB with isolines of 850 hPa overlaid, 26 June 00:00 UTC IR10.8-IR3.9 BTD, 26 June 00:00 UTC
Figure 2: Comparison of Meteosat-10 Night Microphysics RGB images showing BTD.

Previous case studies

Ship trails in the region off the Bay of Biscay (27 Jan 2003)
Atlantic ship trails (30 Sept 2010)
Night-time ship trails (28 Aug 2006)
Ship trails over the Northern Atlantic (25 Jan 2007)

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