Distrails (also called fallstreak holes or punch holes) formed over Michigan in late November 2020.
04 February 2021
03 February 2021
By Jochen Kerkmann (EUMETSAT) and Daniel Lindsey (NOAA)
A distrail, short for dissipation trail, forms when an aircraft flies through a supercooled cloud. 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 or a short line), since they 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). If an aircraft flies a longer path through a supercooled cloud, which can happen during the landing phase, a longer distrail can be observed.
In November 2020, CIRA posted an exceptional case of 'fallstreak holes form over Michigan' (probably caused by aircraft approaching or coming from Chicago) on their GOES loop of the day website.
The loop, Figure 1, is the best case of distrails observed by GEO satellites that we have seen so far. It is extraordinary because a) it was captured by one of the GOES Mesoscale areas (i.e. rapid scan imagery at 1-min intervals), b) it shows the high-resolution VIS0.6 channel (500 m resolution) and c) the animation is centred on the distrail, i.e. it follows the mid-level flow. This allows to monitor the development of the distrails from the first signs to a complete hole in the clouds. It seems like one can see how the larger ice particles fall out of the cloud.
Figure 2 shows the same animation, but not centred on the distrail and not rapid scan, i.e. normal scan at 5-minute intervals (for the so-called CONUS area). Since the loop is at 5-minute intervals, it does not capture the rapid development of the punch holes and it is difficult to track individual distrails because they move so quickly.
Physically speaking, the cloud band that stretches over southern Michigan represents a mid-to-high level supercooled cloud with an IR brightness temperature of around -30 °C, which indicates an unusually high altitude for a water cloud (probably 7-8 km high, see Figures 3, 4 and 5). The colours in the RGB products, green to a bit yellowish in the Day Microphysics RGB and grey-yellow in the Convection RGB, are very typical for high-level water clouds (supercooled).
As already shown in this distrail case over Central Europe, distrails are best seen in the Convection RGB (also called ice particle size RGB). The supercooled cloud is very cold and has small droplets which creates the strong grey-yellow colour (large IR3.9–IR10.3 differences), in contrast to the distrails/punch holes which assume either purple colour (ice particles) or blue (colour of the surface background).
Aqua MODIS RGB 7-2-1 (Natural Color) RGB image from 20 Nov 2020
Hole Punch Clouds in Acadiana (NASA Earth Observatory)
Punching Holes in the Sky (NASA)
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