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ABSTRACT
Aerodynamic contrails have been recognized for a long time although they appear sporadically. Usually one observes them under humid conditions near the ground, where they are short-lived phenomena. Aerodynamic contrails appear also at cruise levels where they may persist when the ambient atmosphere is ice-supersaturated. The present paper presents a theoretical investigation of aerodynamic contrails in the upper troposphere. The required flow physics are explained and applied to a case study. Results show that the flow over aircraft wings leads to large variations of pressure and temperature. Average pressure differences between the upper and lower sides of a wing are on the order of 50 hPa, which is a quite substantial fraction of cruise-level atmospheric pressures. Adiabatic cooling exceeds 20 K about 2 m above the wing in a case study shown here. Accordingly, extremely high supersaturations (exceeding 1000%) occur for a fraction of a second. The potential consequences for the ice microphysics are discussed. Because aerodynamic contrails are independent of the formation conditions of jet contrails, they form an additional class of contrails that might be complementary because they form predominantly in layers that are too warm for jet contrail formation.
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1. Introduction
The most obvious effect of aviation in the atmosphere is the production of condensation trails (contrails). The contrails that one often observes in the upper troposphere (cruise altitudes of international flights, 8-13 km) are produced from the water vapor in the exhaust gas and from the exhaust particles that serve as condensation nuclei when the mixing of the hot and moist exhaust gases with the cold and dry ambient air leads temporarily to a state of water saturation in the mixture. Liquid droplets form and quickly freeze because of the low temperatures in the upper troposphere. The condition that water saturation (instead of ice saturation) must be reached for contrail formation is known as the SchmidtAppleman criterion (Schmidt 1941; Appleman 1953). A complete derivation of that criterion has been given by Schumann (1996). Because contrails, when they are produced in ice-supersaturated air, persist and spread in a sheared wind field, they sometimes lead to extended decks of ice clouds (Kuhn 1970; Schumann 1994; Minnis et al. 1998), called contrail cirrus. There is concern that...