How Are Clouds Responding to Anthropogenic Aerosols

A second key question is how clouds and CRF have been altered in response to changes in the concentration and composition of anthropogenic aerosol particles. Without a dynamic feedback on cloud macrophysics, an increase in the number of hygroscopic particles will produce a larger number of smaller cloud droplets and hence increase cloud albedo. Changes in cloud particle size, phase, and number concentration have furthermore been hypothesized to have a substantial influence on cloud microphysical and precipitation processes, thus potentially affecting cloud condensate, cloud lifetime, and cloud fraction. Although local changes in cloud albedo and regional changes in cloud droplet radius have been observed, it has been difficult to demonstrate unambiguously a significant large-scale influence of anthropogenic aerosol on cloud albedo and cloud fraction due to the very large confounding impact of meteorology. Since aerosol transport is highly correlated with atmospheric dynamics, it is no simple matter to distinguish the effects of anthropogenic aerosol from natural meteorological inf uences on cloud properties. Another complication is that strong atmospheric radiative heating and surface radiative cooling by aerosols may possibly change regional circulation patterns and thus affect cloudiness through a non-microphysical mechanism (e.g., Menon et al. 2002). Investigations of long-term trends in cloud properties in regions that have experienced trends in aerosols may reveal the true aerosol inf uence on cloudiness and provide constraints for GCM simulations. It is necessary, however, to control carefully for changes in meteorological conditions and to determine whether aerosol radiative heating may have modified regional circulation4. The specific radiative impacts of aerosols, cloudiness, or aerosol-cloud interactions must also be correctly identified and distinguished in long-term satellite measurements, which is difficult because thin cloudiness appears optically similar to aerosol haze (e.g., Charlson et al. 2007; Koren et al. 2007).

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