How do contrail lobes form?
Scorer (
1955;
1972) and Scorer and Davenport (
1970) provide an explanation for the formation of the contrail lobes from the interaction between two counter-rotating vortices cast by the aircraft. They hypothesised that, where these vortices interact, they produce descending lobes due to mutual amplification. This explanation was shown to be incorrect by Lewellen and Lewellen (
1996), who later modelled the evolution of the contrail lobes using a three-dimensional large-eddy simulation model with a passive tracer representing the cloudy exhaust. Their simulations showed that the two counter-rotating vortex tubes formed by the aircraft jet are subject to an instability identified by Crow (
1970), in a manner similar to that proposed by Scorer and Davenport (
1970).
This instability causes the two vortices to bend towards each other at quasi regularly spaced intervals, tens to a few hundred metres apart. Eventually, these bending vortices merge at these points, creating a series of ring vortices. Once formed, the vorticity in these rings advects the rings downward relative to the flight level (similar to smoke rings). Eventually, the descent rate slows as the rings weaken, terminating tens to a few hundred metres below the aircraft flight level. The descended cloud remains visible as the condensate is trapped within the vortical circulations.
Later experiments with increasing sophistication of ice microphysics confirmed these initial simulations (Lewellen and Lewellen,
2001; Lewellen,
2014; Lewellen
et al.,
2014), and showed that the dynamics of the interacting vortices was the dominant effect that produces the contrail lobes, with ice microphysics being of secondary importance. As an example, Figure
7 shows a drift plot that captures the space and time structure of a 3.6 km long segment of contrail created by a three-dimensional large-eddy numerical model (Lewellen,
2014). The quantity plotted – integrated ice surface area – is a measure of the brightness of the contrail cloud and represents an easy way to visualise the cloud that surrounds the vorticity structures. The lobes form underneath the contrail after about 200 s (2.4 km behind the plane). Other researchers have also simulated contrail lobes, confirming the essence of these results (e.g. Paugam
et al.,
2010; Naiman
et al.,
2011; Unterstrasser,
2014; Unterstrasser
et al.,
2014; Picot
et al.,
2015). With this large body of literature that has simulated and explained contrail lobe formation, we find it confusing that Paoli and Shariff (
2016, p. 419) have subsequently asked,
What is the mechanism of the intriguing and often-observed mamma structures…? Are they … the result of vortex loops formed after vortex reconnection? Indeed, they are. Thus, the contrail lobes are a result of the vorticity generated by the aircraft and the subsequent evolution of that vorticity.
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)
Dreamliner Overhead
