Limits to the Aerosol Indirect Radiative Effect Derived from Observations of Ship Tracks

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Abstract
One-kilometer Advanced Very High Resolution Radiometer (AVHRR) observations of the effects of ships on low-level clouds off the west coast of the United States are used to derive limits for the degree to which clouds might be altered by increases in anthropogenic aerosols. As ships pass beneath low-level clouds, particles from their plumes serve as condensation nuclei around which new cloud droplets form. The increased droplet concentrations lead to a decrease in droplet sizes. The change in sizes is manifested as an increase in the reflected sunlight observed at 3.7 μm in satellite imagery data. Images at 3.7 μm are used in a semiautomated procedure for identifying polluted portions of clouds and distinguishing them from the nearby unaffected portions. Radiances at 0.64, 3.7, and 11 μm are used to determine visible optical depths, droplet effective radii, and cloud emission temperatures for both the polluted and unpolluted portions. The analysis of several hundred 30-km segments of ship tracks reveals that changes in visible optical depths are about half the values expected, given the changes observed for the droplet radii and assuming cloud liquid water amount remains constant. Simple radiative transfer calculations indicate that the shortfall in the optical depth change is unlikely to be due solely to the absorption by the polluting particles. It is likely that polluted clouds lose liquid water. The equivalent loss is approximately 15%–20% of the initial cloud liquid water.