08 Sep 2022
08 Sep 2022
Status: this preprint is currently under review for the journal ACP.

Upper tropospheric slightly ice-subsaturated regions: Frequency of occurrence and statistical evidence for the appearance of contrail cirrus

Yun Li1,2, Christoph Mahnke1, Susanne Rohs1, Ulrich Bundke1, Nicole Spelten2, Georgios Dekoutsidis3, Silke Groß3, Christiane Voigt3,4, Ulrich Schumann3, Andreas Petzold1, and Martina Krämer2,4 Yun Li et al.
  • 1Forschungszentrum Jülich, Institute of Energy and Climate Research – Troposphere (IEK-8), Jülich, Germany
  • 2Forschungszentrum Jülich, Institute of Energy and Climate Research – Stratosphere (IEK-7), Jülich, Germany
  • 3Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Physik der Atmosphäre, Oberpfaffenhofen, Germany
  • 4Johannes Gutenberg-Universität, Institute of Atmospheric Physics, Mainz, Germany

Abstract. Microphysical, optical, and environmental properties of contrail cirrus and natural cirrus were investigated by applying a new, statistically based contrail–cirrus separation method to 14.7 hours of cirrus cloud measurements during the airborne campaign ML-CIRRUS in Central Europe and the Northeast Atlantic flight corridor in Spring 2014. We find that pure contrail cirrus appears frequently at the aircraft cruising altitude (CA) range with ambient pressure varying from 200 to 245 hPa. They exhibit a higher median ice particle number concentration (Nice), a smaller median mass mean radius (Rice), and lower median ice water content (IWC) (median: Nice = 0.045 cm-3, Rice = 16.6 µm, IWC = 3.5 ppmv), and they are optically thinner (median extinction coefficient Ext = ~ 0.056 km-1) than the cirrus mixture of contrail cirrus, natural in situ-origin and liquid-origin cirrus found around the CA range (median: Nice = 0.038 cm-3, Rice = 24.1 µm, IWC = 8.3 ppmv, Ext = ~ 0.096 km-1). The lowest and thickest cirrus, consisting of a few large ice particles, are identified as pure natural liquid-origin cirrus (median: Nice = 0.018 cm-3, Rice = 42.4 µm, IWC = 21.7 ppmv, Ext = ~ 0.137 km-1). Furthermore, we observe that, in particular, contrail cirrus occurs more often in slightly ice-subsaturated instead of merely ice saturated to supersaturated air as often assumed, thus indicating the possibility of enlarged contrail cirrus existence regions. The enlargement is estimated, based on IAGOS long-term observations of relative humidity with respect to ice (RHice) aboard passenger aircraft, to be approximately 10 % for Europe and the North Atlantic region with the RHice threshold for contrail cirrus existence decreased from 100 % to 90 % RHice and a 4-hour lifetime of contrail cirrus in slight ice-subsaturation assumed. This increase may not only lead to a non-negligible change in contrail cirrus coverage and radiative forcing but also affect the mitigation strategies of reducing contrails by rerouting flights.

Yun Li et al.

Status: open (until 26 Oct 2022)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2022-632', Minghui Diao, 28 Sep 2022 reply

Yun Li et al.


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Short summary
The radiative effect of aviation-induced cirrus is closely related to ambient conditions and their microphysical properties. Our study investigated the occurrence of contrail and natural cirrus measured above Central Europe in 2014. It finds that contrail cirrus appears frequently in the pressure range 200 to 245 hPa and occurs more often in slightly ice-subsaturated environments than expected. The avoidance of slightly ice-subsaturated regions by aviation might thus mitigate contrail cirrus.