Colloquium, Friday, 23 January, 3:00 pm, EN6085A

The aerosol impacts on climate via modifications of cloud radiative properties, or "aerosol indirect effects", have been extensively explored during the past decade. Like green house gases, aerosols interact with radiation (direct and semi direct effects) and as such they constitute an anthropogenic forcing. Aerosols also affect the climate system at the microscale via their ability to act as droplet embryos. Since the fifties, there are numerous observational evidences of enhanced cloud droplet number concentrations in polluted clouds, smaller droplet sizes at constant liquid water content, and more recently with the ship tracks studies and column closure experiments, evidence of an enhanced cloud albedo (Twomey effect).

The key question is now to assess if such microphysical modifications are likely to affect the climate system by scaling up from the droplet to the cloud system scale (the second aerosol indirect effect). To better understand why it has not been yet possible to show observational evidence of such impacts, cloud controlling factors will be discussed and quantified with respect to the expected aerosol impacts. The cloud susceptibilities to the meteorology and to the aerosol will be compared to provide a framework for the design of future field experiments.

Further info can be found at: http://mitpress.mit.edu/catalog/item/default.asp?ttype=2&tid=11796