Limits to Cloud Susceptibility

James A. Coakley, Jr.

Abstract: 1-km AVHRR, VIRS, and MODIS observations of ship tracks in low- level clouds off the west coast of the U.S. will be used to derive limits for the degree to which clouds might be altered by increases in anthropogenic aerosols. The goal is to analyze several thousand ship tracks to determine whether changes in droplet radii, visible optical depths, and cloud top altitudes are consistent with the pre-existing optical depths, radii, and altitudes as anticipated for fixed liquid water paths. A unique feature of this study is that it will rely only on observations for which the ~ 1-km field of view of the satellite radiometer is overcast for both the portions of the cloud affected by the ship and the nearby portions of the cloud that are unaffected. Use of only the overcast pixels will greatly reduce the variability in the observed radiances through which the effects due to the ships must be extracted. Changes due to ships will be examined as functions of distance from the start of the track to distinguish between freshly contaminated clouds and clouds that have been affected for several hours. By allowing for effects due to dispersion, the portions of the tracks far from the ship should reflect influences, if any, of the changes in droplet sizes and numbers on the interaction of the cloud with its environment. The frequency with which ship tracks are found in pixels overcast by marine stratus but the layer is broken in the nearby surrounding region will be used as a measure of effects due to the changes in droplet sizes. Regions where ship tracks intersect will be analyzed to determine whether the changes in droplet sizes and numbers reach saturation, based on the changes expected from analysis of the changes in the separate tracks comprising the intersection. Previous work has indicated that the retrieved optical depths and effective droplet radii are likely to be sensitive to viewing geometry; consequently, the sensitivity of the results to viewing geometry will also be studied. Likewise, previous work has indicated that retrievals of effective droplet radii may be sensitive to the near infrared wavelengths used in the retrieval. Observations from VIRS and MODIS will be analyzed to determine how the findings change with the wavelengths of the near infrared radiances used in the retrievals.

For this project, desired input would be observations that could assess the cloud properties retrieved from multispectral imagery data, namely, in situ observations of droplet sizes, cloud liquid water amount, and cloud top altitudes coincident with multispectral imagery data. As output, the analysis of thousands of ship tracks is expected to address issues such as: