Ship trails over the Eastern Atlantic (17-18 January 2006)

by Nuno Moreira (Instituto de Meteorologia, Portugal) and Jochen Kerkmann (EUMETSAT)

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Ship trails can be considered man-made/modified clouds as they result from enhanced droplet nucleation on particles that are emitted from ship funnels.

A large number of ship trails were observed over the north-eastern Atlantic on 17-18 January 2006. They were embedded within a north-eastern flow at low and medium-levels associated with a high pressure system that persisted over the Azores for some days. On the following day (19th), as a low pressure area replaced the high pressure system that weakened and moved northeast, ship trails could no longer be seen on MSG images (see image from 19 January under "See also").

As shown below, ship trails can be easily detected in Meteosat-8 images during day-time using the high-resolution visible channel (lowest image) or by using RGB combinations that make use of the IR3.9 channel (top image). As regards the visible channels, the detectability is due to the higher reflectivity of water clouds formed of small particles (ship trails) as compared to maritime St and Sc clouds that are usually formed of larger water droplets. To put this into numbers, assuming for example a water cloud with a water content of 1 g/m-3 and a geometrical cloud depth of 100 m, the reflectivity in the VIS0.6 channel varies from about 60% (small droplets of about 12 microns) to 30% (large droplets of about 30 microns). As regards the RGB composite shown in the top image, the reflectivity of the IR3.9 channel (solar, reflected component) strongly depends on the droplet size, with small droplets reflecting more than large droplets. Thus, the ship trails appear much brighter than the maritime Stratus (St) and Stratocumulus (Sc) clouds.

Ship trails may also be tracked during night-time using the so-called "fog" RGB composite (IR12.0-IR10.8, IR10.8-IR3.9, IR10.8). This RGB takes advantage of the fact that the brightness temperature of water droplets in ship trails is lower in IR3.9 than in IR10.8 (as water droplets emissivity is lower in IR3.9), therefore yielding positive values of the difference IR10.8-IR3.9. Furthermore, this difference depends on the droplet size and is generally larger for small water droplets. As this difference is allocated to the green colour beam, ship trails can be traced during the night as greenish stripes over the ocean (see middle image).

Fonte: eumetsat.org