Der indirekte Einfluss von Aerosol aus Schiffsemissionen auf den Strahlungshaushalt der Atmosphäre: Untersuchung mittels Fernerkundungsdaten

Abstract

The emissions from anthropogenic sources are able to change cloud optical properties and can affect the cloud backscattering, known as "indirect aerosol-effect". Especially the particulate and sulfuric emissions from international shipping, which contribute significantly to the total budget of anthropogenic emissions from the transportation sector, can result in changes of cloud reflectance. In the clean marine environment, the amount of cloud condensation nuclei (CCN) is small and low marine clouds usually have large droplet radii combined with low droplet numbers compared to continental clouds. Emissions of aerosols and their precursors by ships result in a high amount of additional CCNs and can possibly lead to a change of the optical and microphysical properties of clouds. This can affect the radiation budget below and above the cloud. Obvious evidence can be seen in satellite data, known as Ship Tracks. The aim of this work is to investigate their radiative and climatic effects on a local and global scale. In the first part, local radiative effects of ship tracks are examined. Here, satellite data are used to examine a scene where ship tracks were detected close to the North American West-Coast. The cloud optical and microphysical properties are derived and an algorithm is examined to distinguish ship-track-pixels from the unperturbed cloud pixels. The resulting cloud properties are used to calculate the radiation budget below and above the cloud. Assuming a mean solar zenith angle of 63 degrees for the selected scene, the mean reflectance at top of atmosphere (TOA) is increased by 41 Wm^-2. For the entire analyzed scene, the increased backscattered solar radiation at TOA results in a loss of -2.0 Wm^-2. The first part showed that ship tracks change the radiation budget on a local scale. Therefore, in a second step the radiative impact on global scale is estimated by one year of satellite data. An algorithm is developed to extract scenes dominated by marine low clouds and these scenes are examined for ship tracks. The results show a high temporal variability of ship track occurrence with peak values from July to September. They also show a high spatial variability with peak values in the North Pacific Ocean and off the west coast of southern Africa. The analysis of backscattered radiation at top of the atmosphere (TOA) compared to the surrounding area reveals enhanced backscattering with values between 0 and 100 Wm^-2. For particular regions off the west coast of North America, the annual mean radiative forcing due to ship tracks can be up to -50mWm^-2. The global annual mean RF due to ship tracks is small (-0.4 to -0.6mWm^-2) and negligible compared to other anthropogenic RF contributions and model results. The results of the second part differ from global model studies. Therefore, the third part of the analyis is concerned with the possible long-term influence of ship emissions on cloud properties in the marine boundary layer. Cloud optical properties and radiative transfer estimations are used for different global areas, to find indications of influence of ship emissions on clouds. The results for regions far away from the coast indicate an influence of ship emissions on clouds. They also indicate possible different reactions of cloud properties in different regions, when disturbed by ship emissions.