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Enhanced DMS emissions from a coccolithophore bloom in the Bering Sea and the relationship to possible changes in the regional cloud characteristics
Bernard A. Walter, Northwest Research Associates, Bellevue, WA
In 1997 an extensive coccolithophore bloom formed in the Bering Sea and persisted in varying degrees of intensity into 2002. Coccoliths are copious producers of dimethylsulfide (DMS), which is known to cause increases in cloud condensation nuclei in the atmosphere. Increased CCN can result in the modification of both cloud physical and radiative properties. Possible changes include a decrease in cloud effective droplet radius, and increases in AVHRR channel 3 reflectance, droplet number concentration, cloud albedo, cloud optical depth and cloud liquid water.
Time series of AVHRR Polar Pathfinder (APP) data from 1993-2000, processed with the Cloud and Surface Parameter Retrieval (CASPR) system (Key, 2002), along with similar time series of SSM/I measurements of integrated water vapor, cloud liquid water and rain rate are used to investigate whether there are changes in cloud properties from the pre- to post-bloom years.
Results from the satellite data analysis comparing pre- and post-bloom cloud characteristics do indeed show significant changes. The average value of cloud effective droplet radius decreases by 7.3 % from the pre- to post-bloom time period, channel 3 reflectance decreases slightly (2.9 %), droplet concentration and cloud optical depth increase significantly (22 % and 16.5 % respectively). The broadband albedo also increases (6.9 %). The values of the AVHRR-derived liquid water path and the SSM/I cloud liquid water both increase (3.4 % and 7.6% respectively). Also, as expected, there was a shift in the shape of the droplet size distribution as shown in the droplet radius histograms.
The results from this study show quantitatively that indeed there were changes in cloud physical and radiative properties from the pre- to post-bloom time period. Whether this in fact means that marine biological activity can impact regional cloud physical and radiative properties is a question for further investigation, but the results are intriguing.
Session 6, The Polar Atmosphere - Clouds, Radiation, Aerosols
Tuesday, 11 January 2005, 3:30 PM-5:45 PM
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