Ocean, Ice, Atmosphere Seminar

Anthropogenic aerosols, such as sulfates and carbonaceous particles, exert an indirect climate effect by acting as cloud condensation nuclei and thereby affecting the initial cloud droplet number concentration, albedo, precipitation formation, and lifetime of warm clouds. For a constant liquid water path, a higher cloud droplet number causes an increase in cloud albedo (cloud albedo effect). Reductions in precipitation efficiency due to more but smaller cloud droplets slow down the precipitation formation and increase cloud lifetime (cloud lifetime effect). The cooling from both indirect effects has been estimated by climate models to be -1 to -4.4 W/m2 in the global mean, whereas it is smaller when estimated from inverse simulations that take the past oceanic and atmospheric warming into account. One new approach is to extend the purely modelling approach of estimating these two indirect aerosol effects by combining satellite data and climate model simulations, which reduces the global mean anthropogenic aerosol effect.

A subset of aerosols act as ice nuclei (IN) and thus control cold cloud formation in addition to or in competition with homogeneous freezing of supercooled aqueous phase aerosols. Furthermore, recent observations suggest that snowfall rates are smaller because anthropogenic pollution decreases the riming rate. In this talk, I will highlight some of these indirect aerosol effects including the ability of anthropogenic aerosols to enhance or reduce precipitation.