Impact of ship emissions on the microphysical, optical and radiative properties of marine stratus: a case study M. Schreier1,2, 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
inadequately quantified impacts, 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. An algorithm is presented to distinguish ship-track-pixels from the
unperturbed cloud pixels in the scene and from this 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. The resulting cloud properties are used to calculate the
radiation budget below and above the cloud. Assuming a mean solar zenith
angle of 63° for the selected scene, the mean solar surface radiation below
the ship track is
decreased by 43.2 Wm−2
and the mean reflectance at top of atmosphere (TOA) is increased by
For the entire analyzed scene the ship emission decreases the solar radiation
the surface by 2.1 Wm−2 and increases the backscattered solar
radiation at TOA by 2.0 Wm−2, whereas no significant effect on thermal
radiation was detected.
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., 6, 4925-4942, doi:10.5194/acp-6-4925-2006, 2006.