Effects of solar heating on the indirect effect of aerosols as deduced from observations of ship tracks

Graduation date: 2009One kilometer Moderate Resolution Imaging Spectroradiometer (MODIS) observations for Terra (morning) and Aqua (afternoon) satellites were used to follow the morning to afternoon evolution of marine stratocumulus clouds that were affected by ship stack exhaust. The observations covered the summer months of 2002-2003 and August 2007 for marine layers off the west coast of the U.S. Low-level winds from the National Center for Environmental Prediction (NCEP) re-analyses were used to identify the clouds common to both the Terra and Aqua observations. The data contained 83 ship track pairs in which the polluted clouds observed by the Terra MODIS were also observed by the Aqua MODIS. The protocols for logging ship tracks and identifying polluted and unpolluted pixels were the same as those described in Segrin et al. (2007). The results presented here are restricted to 30-km segments\ud along the ship tracks in which the mean cloud cover fraction of the segment, taken from all pixels identified as polluted and unpolluted, was greater than 95%. Comparisons were made between this study, which monitored the morning to afternoon evolution of both polluted and unpolluted clouds, and Segrin et al. (2007), which averaged the properties of the clouds for both the morning and afternoon observations. Both studies showed that pollution caused increased optical depths, decreased droplet radii, and decreased liquid water paths. While changes in droplet radii remained constant for polluted and unpolluted clouds from morning to afternoon in the study by Segrin et al. (2007), results from this study showed that droplet radii diminished more than twice as rapidly from morning to afternoon for the unpolluted clouds. Optical depths were reduced by 11%, droplet radii by 7%, and liquid water paths by 15% for unpolluted clouds, while optical depths were reduced by 11%, droplet radii by 3%, and liquid water paths by 12% for the polluted clouds. These reductions were expected as a result of cloud thinning due to prolonged daytime solar heating. By choosing tracks that survived from morning to afternoon, the morning clouds selected in this study were more susceptible to the ship pollution plumes than were the clouds studied by Segrin et al. (2007). Evidently, the more susceptible clouds, those with larger droplets but smaller droplet numbers, stand a better chance of showing up in the afternoon pass than the less susceptible clouds