Ship trails & industrial plumes
Particles that are emitted from ships and factories are often visible, with a similar appearance, in satellite images.
15 April 2021
25 March 2021
By Maria Putsay and Dóra Cséke (Hungarian Meteorological Service), Natasa Strelec Mahovic (EUMETSAT), Ivan Smiljanic (CGI), HansPeter Roesli (Switzerland)
The emmited particles act as condensation nuclei, forming small-droplet clouds which appear as linear structures within low-level clouds that are made of larger droplets.
Somewhat similar in shape to contrails, when seen in the satellite imagery (as long, thin cloud structures), ship trails form when ship exhaust interacts with clouds in the lower troposphere. Ship trails or tracks are particularly visible in the environment of low water clouds (stratocumulus). This is because of the difference between the particle (droplet) size of the clouds and the size of droplets (re-)formed under the influence of the ship exhaust. Low water clouds over the ocean usually consist of large droplets due to the limited number of condensation nuclei (i.e. less air pollution). As the ships emit large amounts of aerosols that act as condensation nuclei, the droplets (re-)forming on these nuclei are smaller than the surrounding cloud droplets, making them more reflective in near-infrared (NIR) satellite channels. Therefore, ship trails can be best seen in the RGBs composed with channels sensitive to particle sizes, such as the Day Microphysics RGB (see example in Figure 1), the Night Microphysics RGB, and the Cloud Phase RGBs (see the RGB Quick Guides for more information).
By comparing with higher resolution VIIRS imagery (pixel size up to 750x750 m) we know that for SEVIRI (pixel size nominally 3x3 km) one pixel may have mixed contributions — from clouds and from underlying water body/sea. So, in such a scenario for SEVIRI data, the colour contrast between a ship track and its surroundings is affected by both the droplet size difference and the cloud/water ratio. Adding to that, due to reduced spatial resolution, in the SEVIRI image the cellular structure of ship track cloud lines is not visible.
The example in Figure 2 shows ship trails as seen in different VIIRS and SEVIRI RGB combinations. The different structure of the ship trails compared to the neighbouring clouds is clearly seen as relatively broad continuous lines in the cellular structured background. The line-shaped ship trail clouds consist of smaller particles than the surrounding cellular structured water clouds, as can be seen from the colour shades in the images.
Water clouds with small and large droplets appear in different colours, for instance light yellow against pink in the Cloud Phase RGB. The highest colour contrast can be found in the Cloud Phase RGB image, which is more sensitive to the cloud top particle sizes. Note that the VIIRS Day Microphysics RGB, shown in Figure 2, was created with the NIR1.6 channel instead of the IR3.7 channel. The VIIRS Day Microphysics and Cloud Type RGBs use one microphysical channel, the NIR1.6. Cloud Phase RGB uses two microphysical channels, one of which is NIR2.25 which is more sensitive to the particle size than NIR1.6, and will be a new channel with FCI on board MTG-I satellites.
NWC SAF Cloud Microphysics products (CMIC), in particular Cloud effective radius (CMIC 2) and Cloud Optical thickness (CMIC 3), also reveal the microphysical characteristics of the ship trail clouds. As shown in the animation in Figure 3, ship trails have smaller cloud drop effective radius, i.e. they are made of smaller droplets, than the neighbouring water clouds (Figure 3, left). The cloud optical thickness product (Figure 3, right), on the other hand, shows that the clouds created through the interaction of the ship’s exhaust and the existing low-level water cloud have larger optical thickness, meaning they are more dense then the neighbouring clouds.
Similar to the trails formed from the exhaust of the ships, clouds with small droplets can also form on the aerosols emitted from industrial hot spots/chimneys. The difference is that the source of the emission is not moving, but the wind causes elongated cloud features, very similar to that of the ship trail, with similar microphysical characteristics. A very good example of that is this case from 22-23 November 2018 over Belarus.
The VIIRS Night Microphysics RGB image taken on 22 November 2018 (Figure 4) shows that the northern and north-eastern region of Belarus was covered by fog or low cloud, consisting of relatively large droplets, appearing in pink in the Night Microphysics RGB. According to the synop observations these clouds were stratus. Within this region several bright yellow-green plumes indicating low clouds with small droplets can be seen spreading southwards. In the southern and south-western region there are water clouds with small droplets, appearing in bright yellow-green. The most probable reason for the largest plume (indicated by the large black arrow) was the factories of Navapolatsk city, the leading producer in the refining and chemical industry business. Besides the largest 'plume', several smaller plumes are seen. The whole region has many such (smaller), probable industry- or city-related hot spots, some of them marked by arrows in Figure 4. The plumes are elongated due to the cloud advection above the sources of nuclei particles.
The main plume is even visible in the (strongly enhanced) Day-Night Band image on 22 November (Figure 5). It is slightly brighter than the surrounding cloud mass due to a smaller droplets.
The Optimal Cloud Analysis – OCA Cloud Effective Radius product reveals a plume of droplets with a smaller effective radius in darker blue shades, (see the white arrow in Figure 6). Note that this product is retrieved from SEVIRI data.
During the day, as the wind changed the direction the main plume of smaller water droplets in Day Microphysics RGB (created with NIR 1.6 instead of IR 3.7 micron channel) spread eastwards, see the bright green plume indicated by the black arrow in Figure 7.
The main plume is still visible the next night on 23 November (Figure 8). During the day the main plume could still be seen (with a hook shape) in the comparison of Day Microphysics and Cloud Phase RGBs (Figure 8).
The cloud top droplet size difference is better seen in the Cloud Phase RGB (light yellow against pink). However, it is still recognisable (albeit dimly) in the Day Microphysics RGB (brighter green colour against a background with slightly pink shades).
In the imagery in this study the industrial plumes were better seen during the night than during the day. The colour contrast between water clouds with larger and smaller droplets were higher during night in the Night Microphysics RGB (see Figures 4, 5, and 8) than during the day in the Cloud Phase RGB (see Figure 9). The reason might be the low sun angle and high satellite scanning angle (causing generally weaker reflection towards satellite during the day), but also the fact that during night the mixing in the boundary layer is weaker, so the size difference might be larger.
Other sources of ‘microphysical trails’
Trails of altered microphysical signature, most vividly seen in the low laying stratiform clouds, can, likely, have other persistent sources of condensation nuclei (aside from ships and industry hot-spots). One such example is fires.
Fires (if persistent, and with enough intensity and spread) are also a large and a steady source of small particles (smoke, and ash) that can change the microphysical structure of the clouds above. Again, with injection of the bigger concentration of condensation nuclei, elongated linear shapes start to appear in stratiform cloudiness, showing different a spectral signature, i.e. signature of relatively small cloud particles (Figure 10).
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