Increases in aerosol optical depths in the vicinity of marine stratocumulus

Aerosols not only affect droplet sizes and number concentrations in marine stratocumulus but in turn the moist cloud environment causes the aerosol particles to grow. In addition, the enhanced illumination of the cloud-free air in the vicinity of clouds leads to overestimates of aerosol optical depths and fine mode fractions retrieved from multispectral satellite imagery data. Collocated MODIS and CALIPSO lidar observations were used to deduce the effects of the clouds on the scattering properties of nearby aerosols in large cloud-free ocean regions bounded on both ends, or if sufficiently large (>100 km), at least on one end by an extensive layer (≥ 25 km in length) of marine stratocumulus. The CALIPSO lidar returns were used to identify both the marine stratocumulus layers and adjacent cloud-free oceans. The combined observations covered the global oceans, 60°S – 60°N, for a period of 15 months. The CALIPSO 5-km and MODIS 10-km aerosol optical depths both increase as marine stratus is approached. The variability in the MODIS aerosol optical depths is sufficiently small to discern that the increases grow exponentially as clouds are approached from a distance of 35 km. The distance for doubling the increase in aerosol optical depth is equivalent to the size of the sample, 10 km. For large cloud-free oceans, in going from 35 km to within 5 km from the stratus the 10-km average optical depth increases 5%. While small, this result is consistent with estimates for the increases in aerosol optical depth deduced for the higher relative humidity found in aircraft observations near clouds. Changes in the optical depths derived using CALIPSO lidar returns are hampered by the noise of the detector which is further amplified through the retrievals. Consequently, the changes in the CALIPSO aerosol optical depths place no constraints on the collocated MODIS optical depths. The associated 550/870 Ånsgtröm exponent deduced from the MODIS optical depths is consistent with larger particles near clouds, which in turn is consistent with the reduction in the fine mode fraction found near clouds, but the changes are statistically significant at the 95% confidence level only for the 10-km sample region adjacent to the clouds. Changes in the MODIS derived Ånsgtröm exponent and fine mode fractions are not statistically significant at distances greater than 10 km from the clouds. Owing to the variability in the CALIPSO optical depths, no statistically significant change in the 532/1064 Ånsgtröm exponent is found for the daytime observations and only that for the 5-km region adjacent the clouds is statistically significant for the nighttime observations. The change in the Ånsgtröm exponent for the nighttime observations suggests that the aerosol particles are larger adjacent to the cloud. The changes in both CALIPSO and MODIS aerosol optical depths could be caused by cloud contamination of the aerosol retrievals. The finding for the MODIS aerosol optical depths that particles appear to grow as clouds are approached rather than shrink, as has been reported by others, may be the result of the tenuous marine stratocumulus layers that sit adjacent large cloud-free oceans. The CALIPSO lidar is able to penetrate nearly all of the stratus layers observed within 25 km of the cloud-free regions which indicates that the clouds are either broken or optically thin, visible optical depths of 5 or less. When all MODIS aerosol optical depths within marine stratus layers are considered, the fine mode fraction increases with increasing cloud fraction, as has been reported by others.