Measurements of short-lived contrails embedded in thin/subvisible cirrus clouds

Aircraft contrails frequently occur in the upper troposphere. They consist of ice particles having the potential to directly affect the Earth's climate. The frequency, life time, ice crystal size spectra and thus radiative properties of contrails depend strongly on the ambient distribution of the relative humidity with respect to ice (RHi). In air with RHi below 100% contrails are believed to be short-lived, while persistent contrails require an ambient RHi of at least 100%.

During the mid-latitude aircraft experiment CONCERT 2008 (CONtrail and Cirrus ExpeRimenT, out of Oberpfaffenhofen, Germany), RHi inside of contrails were measured using the high precision 'Fast In situ Stratospheric Lyman-Alpha Hygrometer' FISH. The ice crystal size distribution in the size range Dice = 2 - 1000 µm diameter is recorded simultaneously using a FSSP ('Forward Scattering Spectrometer Probe'), a CPI ('Cloud Particle Imager) and a 2D-C (2D-Cloudprobe). Here, we present results from about 1.7 hours of observation time during 6 flights.

In most cases the ambient air was slightly subsaturated (RHi 80-100%), i.e. the observed contrails were short-lived. Nevertheless, ice crystals > 200 µm were detected in several cases in addition to a high number of small ice crystals typical for contrails. Analysis of the vertical structure of the atmosphere indicate that the contrails were embedded in a layer of thin/subvisible cirrus clouds.

Model simulations with the kinetical microphysical model MAID (Bunz et al., 2008) are performed along air mass trajectories ending in the region of the measurements -Ireland- during a flight on November 17. They show that the large ice crystals formed south-westerly of Iceland, about 18 hours before detection, at an altitude of about 1.5 km above the level of the measurements. The origin of the air masses is the middle Atlantic between Florida and the Canary Islands. After formation, the air rises further with a very slow updraft, ideal conditions for the crystals to grow to larger sizes. We suggest that they sediment to lower altitiudes while growing in a supersaturated environment until they reach the slightly subsaturated region where they were detected after mixing in the aircaft contrail.

Bunz, H., Benz, S., Gensch, I., & Krämer, M. (2008): MAID: a model to simulate UT/LS aerosols and ice clouds, Envir. Res. Lett., 3, doi10.1088/17489326/3/3/035001.