Atmos. Chem. Phys. Discuss., 6, 1023-1071, 2006
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Impact of ship emissions on the microphysical, optical and radiative properties of marine stratus: a case study
M. Schreier1, A. A. Kokhanovsky1, V. Eyring2, L. Bugliaro2, H. Mannstein2, B. Mayer2, H. Bovensmann1, and J. P. Burrows1
1Institute for Environmental Physics (IUP), University of Bremen, Germany
2Institut für Physik der Atmosphäre, DLR-Oberpfaffenhofen, Wessling, Germany

Abstract. Modifications of existing clouds by the exhaust of ships are well-known but poorly studied atmospheric effects which could contribute to climate change. The perturbation of a cloud layer by ship-generated aerosol changes the cloud reflectivity and is identified by long curves in satellite images, known as ship tracks. As ship tracks indicate a pollution of a very clean marine environment and also affect the radiation budget below and above the cloud, it is important to investigate their radiative and climatic effects. Satellite-data from MODIS on Terra are used to examine a scene from 10 February 2003 where ship tracks were detected close to the North American West-Coast. The cloud optical and microphysical properties are derived using a semi-analytical retrieval technique combined with a look-up-table approach. Ship-track-pixels are distinguished from the unperturbed cloud pixels and the optical properties of the former are compared to those of the latter. Within the ship tracks a significant change in the droplet number concentration, the effective radius and the optical thickness are found compared to the unaffected cloud. Significant increase of liquid water could not be confirmed. The resulting cloud properties are used to calculate the radiation budget below and above the cloud. Assuming a mean solar zenith angle, the mean surface radiation below the ship track is decreased by 43.25 Wm−2 and the mean reflectance at TOA is increased by 40.73 Wm−2. For the selected scene the ship emission decreases the solar radiation at the surface by 2.10 Wm−2 and increases the backscattered solar radiation at top of the atmosphere (TOA) by 2.00 Wm−2. Increased backscattered radiation is partly compensated by a decrease of the thermal radiation of 0.43 Wm−2. The resulting net-effect at TOA is an increase of 1.57 Wm−2 corresponding to a negative radiative forcing and a cooling.

Citation: Schreier, M., Kokhanovsky, A. A., Eyring, V., Bugliaro, L., Mannstein, H., Mayer, B., Bovensmann, H., and Burrows, J. P.: Impact of ship emissions on the microphysical, optical and radiative properties of marine stratus: a case study, Atmos. Chem. Phys. Discuss., 6, 1023-1071, doi:10.5194/acpd-6-1023-2006, 2006.
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