Distrails over Central Europe

By Jochen Kerkmann, Vesa Nietosvaara and Zanita Avotniece (EUMETSAT); Tom Goren and Daniel Rosenfeld (HUJ, Hebrew University of Jerusalem), HansPeter Roesl (Switzerland)

The supercooled cloud glaciates due to the disturbing airflow created by the aircraft and the addition of plenty of freezing nuclei due to the aircraft's exhaust gases. The ice crystals collide and grow big enough to fall out of the cloud (virga) leaving a kind of 'punch hole' in the cloud shield.

Distrails are usually not very long (often the form of an ellipse), since they usually show in altocumulus or altostratus clouds which is not at a cruising altitude of the aircraft (i.e. the aircraft is either descending or ascending through the cloud). In case an aircraft flies a longer path through a supercooled cloud (not recommended), which is common during the landing phase, a longer distrail can be observed.

NASA has published several interesting cases of distrails on the Earth Observatory web page A very interesting case occurred over Texas on 29 January 2007 . Pictures of distrails from ground have been published on many web pages.

A nice case of distrails over Central Europe was observed by Meteosat-10, Metop-A/B and Suomi NPP on 6 November. A large supercooled cloud extended from eastern UK, across Germany to the eastern Alps (see Figure 1). In terms of conceptual models, the cloud represents a typical warm front with strong positive temperature advection at 700hPa and a clear signal in the Thermal Front Parameter (TFP) and in the 1000hPa divergence field (which indicates the shift of the wind and marks the surface front). The warm front also shows up very beautifully in vertical cross sections through the front, with slant isentrops from the surface right to 300hPa.

The IR brightness temperatures of the cloud range from -30 to -33°C, which indicates an unusually high altitude for a water cloud. According to the Nowcasting SAF Cloud Top Height product (see Figure 2) the cloud is at 7–8km height, also confirmed by the KNMI cloud products (see Figure 3).

Furthermore, the cloud is composed of rather small cloud droplets (effective radii of 10–15 micron), not very thick (around 500m geometrical thickness) and, thus, not producing any precipitation.

Zooming-in on the cloud over Germany, one can see that there is actually another cloud layer below the supercooled cloud, namely a layer of low-level stratocumulus clouds with large droplets.

Finally, the distrails (punch holes) in the cloud deck (caused by airplanes) are shown in the Meteosat-10 image collections from 13:00 UTC and 14:00 UTC. Surprisingly, the distrails are best visible in the Convection RGB (also called ice particle size RGB). The supercooled cloud is very cold and has small droplets which creates the strong yellow colour (large IR3.9–IR10.8 differences), in contrast to the distrails which assume the purple colour of the lower clouds or ground.

Note: Because of the strong high-level winds (upper level jet), the punch holes move so quickly that it is difficult to follow them in the animation of HRV images

The best view of the distrails is given in these high-resolution VIIRS images (see Figures 4–6). Figure 4 shows the Day-Natural RGB scheme, in which the supercooled cloud appears with a light pink color, while the distrails and the punch-holes are shown by the cyan color (indicating the ice crystals). The longest linear distrail is about 200km and probably caused by an aircraft that flew a long path within the supercooled cloud. On the upper right part of the image clear holes within the cloud field can be seen. These may be mature punch-holes in which the ice crystals fell down and sublimated in warmer layers of the atmosphere, leaving holes in the cloud field.

Figure 5 shows Day-Microphysical RGB image focusing on other part of the cloud that has interesting features. In this RGB scheme the supercooled clouds appear in yellow/green because of the small geometrical depth of the clouds, the very small droplets size and the low temperatures. The distrail appears in red due to the ice crystals. Note the stitches feature within the cloud field that gives the supercooled cloud a kind of hexagonal structure. Such cloud features form when radiative cooling from the cloud tops causes an organized dynamics in the form of Rayleigh-Benard convection.

Once a supercooled cloud is glaciated, the falling ice crystals continue to grow as they fall down below the cloud. This is so because in such conditions the super-saturation over ice is larger than the super-saturation over water. As a result, in cases where the supercooled cloud is relatively thin, the amount of ice that fall within the punch-holes can be larger than the amount of supercooled water that was originally in the cloud. Figure 6 that shows the visible reflectance demonstrates this phenomena. It can be seen that in areas where the supercooled cloud is very thin, the distrail is more reflective than the clouds that surround it. This is in opposite to most cases in which water clouds are more reflective than ice clouds.

Additional content

Metop-A, AVHRR, Natural Colour RGB, 10:30 UTC
Suomi NPP, VIIRS, Natural Colour RGB, 11:00 UTC