Response of marine stratus to successive polluting events by ships Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/kp78gk74h

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  • Ship tracks have proven to be an ideal laboratory for studying the response of marine stratocumulus to an increase in aerosol pollution. Here the response of already polluted marine stratocumulus to further pollution was examined by studying the clouds where two ship tracks cross. 78 crossings of ship tracks were collected and analyzed using Terra and Aqua MODIS multispectral satellite daytime imagery for summertime passes off the west coast of the U.S. Partly cloudy pixels, though collected and compared to overcast pixels, were excluded from the analysis of the crossings in order to reduce biases in the retrieved cloud properties caused by subpixel spatial variation among the clouds. When clouds were polluted by the plume of a ship, the visible optical depth and the column droplet number concentration increased significantly. The cloud droplet radius decreased significantly, while the cloud liquid water path decreased slightly. The cloud temperature was unchanged. Of any two ship tracks that crossed, one of the tracks exhibited a much larger change in droplet radius compared to the control clouds than did the other track compared to the control clouds. This dominant ship track typically displayed properties closer to those of the clouds at the crossing than did the subordinate ship track. Local gradients in the retrieved cloud properties were determined for both the dominant and subordinate ship tracks. The values of the gradients at the crossing were used as proxies for what the cloud properties would have been if the clouds had been polluted by only one ship. The differences between the retrieved properties of the clouds in the crossing pixels and the values established by the gradients represented the response of the clouds in one ship track to pollution by a second ship. The responses to pollution by the dominant ship were compared for the uncontaminated control clouds and the clouds that had already been polluted by the subordinate ship. The same analysis was repeated with the subordinate ship polluting both the uncontaminated control clouds and the clouds that had already been polluted by the dominant ship. These comparisons revealed that the response to additional aerosol loading was diminished for clouds that had already been polluted. The decreased response at higher aerosol concentrations was observed for the optical depth, the droplet radius, and the column droplet number concentration. Cloud susceptibility was also examined by comparing the response to pollution of optically thin clouds with that of optically thick clouds. The visible reflectivity of the optically thin clouds was observed to be highly susceptible to an increase in column droplet number concentration, while the reflectivity of the optically thick clouds proved to be insensitive.
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