Perturbation of Clouds due to Contrail Formation

Air traffic is a large source of anthropogenic aerosols. The formation of contrails impacts the upper tropospheric water budget and radiative transfer significantly. Contrails form when the hot and moist air of the exhaust plume mixes with cold ambient air. During mixing, if supersaturation with respect to water is reached many emitted soot particles and ambient aerosols that get entrained into the plume activate into water droplets and subsequently freeze to form contrail ice crystals. The number of ice crystals, forming and surviving the early stages of contrail formation, depend on atmospheric conditions. Contrails have been shown to increase the cirrus coverage significantly and exert a warming on the atmosphere. Modifications of cloud properties can also be observed when air traffic happens within natural clouds. In particular the ice crystal number concentration is increased by air traffic and, therefore, microphysical process rates and radiative properties of clouds are changed.

We have implemented a parameterization for contrail cirrus in the high resolution ICON-LEM model (resolution of 600m) to study the impact of contrail formation on cloud properties. In particular we study the modification of natural clouds due to contrail ice crystal formation within clouds. The number of ice crystals nucleating within the plume depends on soot number emissions and the atmospheric state. After ice crystal formation most of the contrail ice crystals are trapped within downward propagating vortices and depending on the atmospheric conditions a fraction of ice crystals sublimate due to adiabatic heating of the air volume. Contrail formation within cirrus introduces a large increase in the ice crystal number concentration of clouds. Ice water content, on the other hand, is not strongly disturbed since water vapor aircraft emissions are low relative to the atmospheric water vapor. The many small ice crystals lead to an increase in cloud optical depth. The modification in ice crystal numbers is especially pronounced in natural clouds that have low number concentrations and relative high in-cloud relative humidity. We study the modification of natural cloud properties induced by contrail formation within clouds, the resulting changes in microphysical process rates and the life time of those cloud perturbations.