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Poster Session:
Aircraft Emissions and Near-field
It is interesting to note that the mere disturbance of the air by the aircraft is able to form cloud particles. Aerodynamic cloud formation might not be restricted to temperatures warmer than -30~\Celsius, for which the APIP phenomenon has been studied so far. In the temperature range between -30 and -60~\Celsius\ which corresponds to the crusing altitudes of commercial airtraffic, the most important ice nucleation process seems to be freezing of haze droplets at relative humidities near water saturation (i.e. above ice saturation). Thus, is it possible that the turbulence alone from an aircraft flying through a metastable atmosphere, with reference to humidity, can form an APIP trail? The magnitude of the vertical air motion caused by the trailing vortices behind a big airliner can be on the order of meters per second, which results in very strong turbulence (evident to anyone who have been flying close behind an other aircraft). If an airparcel in the wake of the aircraft is lifted to the point where the humidity is increased enough for homogeneous freezing to occur, a cloud would rapidly form.
To test the hypothesis: that it is possible for an aircraft to form a contrail by the aerodynamic effects from the aircraft alone, we have performed numerical simulations of ice cloud formation in the wake of an aircraft flying at cruise altitude. The engine exhaust has been excluded from the simulations in order to study cloud formation due to aerodynamic effects. The ice is formed via homogeneous freezing nucleation of ambient haze droplets in the upwelling limbs of the vortex pair behind the aircraft. Properties of wake ice clouds are compared with properties of contrails obtained with in situ measurements and recent simulations. In particular, we find that aerodynamically induced ice clouds are similar in microphysical and radiative respects to contrails that are formed from the nucleation of exhaust particles. The results show that significant fractions of contrails as young as 2 to 5 minutes may originate from aerodynamic effects and not only from nucleation of the exhaust particles.