J1.15
Limits to the Aerosol Indirect Radiative Forcing Derived from Observations of Ship Tracks
James A. Coakley Jr., Oregon State University, Corvallis, OR; and C. D. Walsh
The effect of particle pollution on the numbers and sizes of droplets in low-level clouds and the consequent effects on cloud lifetimes and the amount of sunlight reflected by clouds ranks among the largest sources of uncertainty in determining man's impact on climate. 1-km Advanced Very High Resolution Radiometer observations of the effects of ships on low-level clouds off the west coast of the U.S. 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 microns in satellite imagery data. Images at 3.7 microns show tracks in clouds that reveal the presence of the underlying ships. The enhanced reflectivities at 3.7 microns are used in an automated procedure for identifying polluted portions of clouds and distinguishing them from the nearby unaffected portions. Radiances at 0.63, 3.7, and 11 microns are used to determine effective droplet radii, liquid water amounts, and cloud top altitudes for both the polluted and unpolluted portions. A unique feature of this study is that it relies only on observations for which the ~ 1-km field of view of the satellite radiometer is overcast. Use of only the overcast pixels greatly reduces the variability in the observed radiances through which the effects due to the ships must be extracted. The analysis of several hundred ship tracks reveals that changes in visible optical depths are somewhat smaller than would be expected for the observed changes in droplet radii and fixed cloud liquid water. Based on these results, estimates of the indirect radiative forcing due to aerosols in which cloud liquid water is held constant would overestimate the radiative forcing.
Joint Session 1, Global Climatology of Aerosols (Joint with the Millennium Symposium on Atmospheric Chemistry and the 12th Symposium on Global Change and Climate Variations)
Thursday, 18 January 2001, 8:15 AM-2:15 PM
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