Optical properties of marine stratocumulus clouds modified by ships

Abstract

The angular distribution of scattered radiation deep within a cloud layer was measured in marine stratocumulus clouds modified by the emissions from ships. These observations, obtained at 13 discrete wavelengths between 0.5 and 2.3 μm, were acquired as the University of Washington C‐131A aircraft flew through a pair of roughly parallel ship track signatures produced in clouds off the coast of southern California on July 10, 1987. In the first of these ship tracks, the nadir (upwelling) intensity increased from 40 to 110 W m−2 μm−1 sr−1 at 0.744 μm. The second ship track produced a less dramatic, but more uniform, increase in the upwelling intensity. In contrast, the nadir intensity at 2.20 μm decreased from 1 to 0.13 W m−2 μm−1 sr−1 in the first ship track and to 0.6 W m−2 μm−1 sr−1 in the second ship track. The relative angular distribution of the intensity field at each wavelength was used to determine the similarity parameter, and hence single scattering albedo, of the cloud using the diffusion domain method. Besides the spectral similarity parameter, these measurements provide a good estimate of the optical depth of the cloud layer both above and below the aircraft. Results of this analysis are presented for a 120‐km section of marine stratocumulus cloud including both ship tracks. This analysis shows that the total optical thickness of the cloud layer increased in the ship tracks, in contrast to the similarity parameter which decreased. The decrease in absorption was a direct consequence of the reduction in cloud droplet size that occurred within the ship tracks.