Graduate Thesis Or Dissertation
 

Aerosol indirect radiative forcing over the northeastern Atlantic from AVHRR observations

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/6d570005w

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  • Advanced Very-High Resolution Radiometer 4-km data were collected over the northeast Atlantic for May-August, 1995-1999. Aerosol optical depth was retrieved in cloud-free pixels. In pixels containing clouds from only single-layered, low-level systems, a retrieval scheme that accounts for partly-cloudy pixels was used to retrieve: cloud optical depth, droplet effective radius, cloud altitude, pixel-scale fractional cloud cover, liquid water path and column droplet concentration. Mean aerosol optical depths from 1° x 1° latitude-longitude regions were associated with mean cloud properties in the same region for the same satellite overpass. Results were composited for 5° latitude-longitude regions. As aerosol optical depth increased, droplet radius decreased and column droplet number concentration and cloud optical depth increased, consistent with the aerosol indirect effect. In many regions, liquid water path decreased as aerosol optical depth increased, contrary to the trends expected for drizzle suppression. The simultaneous increase in aerosol and cloud optical depths with increasing fractional cloud cover might be mistaken for the aerosol indirect effect. The five-year data set was sorted into clean and polluted cases. Clouds in clean air had larger droplets and smaller cloud optical depths than clouds in polluted air, consistent with the aerosol indirect effect. Liquid water path increased as fractional cloud cover increased but no difference was found between clouds in clean and polluted air. Influences other than changes in aerosol burden may be controlling changes in liquid water. Observed changes in aerosol and cloud properties were used to estimate the aerosol indirect radiative forcing. Aerosol indirect forcing for overcast conditions was 1.4-2.2 times larger than the aerosol direct radiative forcing for cloud-free conditions. To simulate threshold retrievals, which do not account for partial cloud cover in partly-cloudy pixels, radiances in pixels that had a fractional cloud cover greater than 0.20 were used to recalculate cloud properties assuming the pixel was overcast. The decrease in droplet radii and increase in cloud optical depths for a given change in aerosol optical depth were larger using threshold retrievals than when using partly-cloudy retrievals. Threshold retrievals lead to a significant overestimation of the aerosol indirect radiative forcing.
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