Abstract
Abstract. Aircraft at cruise levels can cause two kinds of contrails, the well known exhaust contrails and the less well-known aerodynamic contrails. While the possible climate impact of exhaust contrails has been studied for many years, research on aerodynamic contrails began only a few years ago and nothing is known about a possible contribution of these ice clouds to climate impact. In order to make progress in this respect, we first need a climatology of their formation conditions and this is given in the present paper. Aerodynamic contrails are defined here as line shaped ice clouds caused by aerodynamically triggered cooling over the wings of an aircraft in cruise which become visible immediately at the trailing edge of the wing or close to it. Effects at low altitudes like condensation to liquid droplets and their potential heterogeneous freezing are excluded from our definition. We study atmospheric conditions that allow formation of aerodynamic contrails. These conditions are stated and then applied to atmospheric data: first to a special case where an aerodynamic contrail was actually observed and then to a full year of global reanalysis data. We show where, when (seasonal variation), and how frequently (probability) aerodynamic contrails can form, and how this relates to actual patterns of air traffic. We study the formation of persistent aerodynamic contrails as well. Furthermore, we check whether aerodynamic and exhaust contrails can coexist in the atmosphere. We show that visible aerodynamic contrails are possible only in an altitude range between roughly 540 and 250 hPa, and that the ambient temperature is the most important parameter, not the relative humidity. Finally, we argue that currently aerodynamic contrails have a much smaller climate effect than exhaust contrails, which may however change in future with more air traffic in the tropics.
Highlights
It is well known that aviation contributes to climate change and that a significant share of this contribution stems from persistent contrails and contrail cirrus (Lee et al, 2009)
When air flows around the wings of an aircraft it is accelerated, and because of conservation of total enthalpy it cools as the flow gets faster. This cooling implies the rising of the relative humidity of the air, an effect that is quite strong at aviation cruise levels, such that condensation and freezing can occur even in relatively dry ambient air (Gierens et al, 2009)
In contrast to aerodynamic contrails, exhaust contrails have no visibility threshold at this temperature because under these conditions about 99 % of the water vapour in the exhaust plume at engine exit are contributed by the water that results from kerosene
Summary
It is well known that aviation contributes to climate change and that a significant share of this contribution stems from persistent contrails and contrail cirrus (Lee et al, 2009). When air flows around the wings of an aircraft it is accelerated, and because of conservation of total enthalpy (the sum of kinetic energy and enthalpy) it cools as the flow gets faster This cooling implies the rising of the relative humidity of the air, an effect that is quite strong at aviation cruise levels, such that condensation and freezing can occur even in relatively dry (say RHi ≈ 20 %) ambient air (Gierens et al, 2009). Ice crystals once formed must grow to a size comparable to the wavelength of light (350 to 700 nm) and this requires that the air has sufficient water vapour concentration Since the latter decreases roughly exponentially with altitude throughout the troposphere it is often too dry at the higher cruise levels to form a visible aerodynamic contrail (Kärcher et al, 2009). This is a necessary step towards assessment of the role of aerodynamic contrails in climate
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