I Koren, L. A Remer, O Altaratz, J. V Martins, A Davidi
Atmos. Chem. Phys. (2010) vol. 10 (10) pp. 5001-5010 (Open access!)
This paper examines the possibility of an aerosol/cloud interaction which may lead to atmospheric warming. Such an interaction may be a source of large uncertainty in estimates of anthropogenic climate change.
Aerosols are small particles that remain in the air for long periods – from a variety of sources (human caused and natural), such as biomass burning, urban+industrial pollution, volcanic eruptions and desert dust.
In the regions of deep convection such as the Pacific and Atlantic ITCZs, the convective storms consist of a cloud tower where most of the convection occurs and an anvil – a thinner layer of mostly ice extending away from the cloud towers. The anvils form when the convection butts up against a stable layer (or inversion) in the atmosphere – like the tropopause, which forms a ceiling preventing the convective parcels from moving upwards. The anvils are then carried and spread by the winds in the upper levels of the atmosphere.
The authors of this paper find that the effect of increases in aerosol amounts in both tropical regions is to create taller clouds and anvils. Aerosols are thought to do this by providing nuclei for the water vapor to condense around, leading to more, smaller droplets when the aerosol concentration is higher. This has many possible consequences on the clouds evolution, by changing (for example) the updraft winds, the droplet growth rates, or the precipitation rates/types int eh cloud.
The taller clouds tend to reach higher levels in the tropopause region, where the winds are stronger. This causes the anvils to cover a larger area, but it also makes them thinner. The net effect of this is to decrease the amount of heat radiated to space more strongly than it increases the amount of incoming heat reflected away by the clouds), leading to a net warming of the atmopshere. The small perturbations of the cloud characteristics via aerosol changes can create large changes in the radiation balance at the top of the atmosphere up to hundreds of W/m2.
Analysis by the authors supports this mechanism, but “data quality” is insufficient to calculate a quantitative measure of the effect.