ESRL/PSD Seminar Series

Abstract

Airborne aerosol particles cool the Earth's surface on average, both directly by absorption and scattering of sunlight, and indirectly by altering cloud properties. Clouds both cool the surface by reflecting sunlight (shortwave) and warm it by absorbing outgoing infrared radiation (longwave) and re-emitting a part of it back towards the surface. Cloud formation relies on the presence of both sufficient water vapour and cloud condensation nuclei (CCN), which is the subset of the aerosol particle population that activate at a given water vapour supersaturation. Therefore, clouds cannot form in the absence of CCN. We suggest that at low CCN concentrations cloud formation is limited by the available CCN, and therefore both the longwave and shortwave components of the cloud forcing. The reason is that droplets grow large enough to form drizzle when CCN are few, thereby limiting cloud liquid content. We demonstrate that this effect may cause a net surface warming aerosol indirect effect for typical central Arctic conditions. The warming effect occurs because the change in longwave cloud forcing is larger than the change in shortwave forcing when CCN are increased. The radiative transfer calculations are found to be in good agreement with observations obtained during the Arctic Summer Cloud Ocean Study 2008 (ASCOS) at approximately 87N. The results suggest that the change in longwave cloud surface forcing may be as large as 60-80 W/m2 across the observed range from 0.2 to 10 or more CCN per cubic centimeter. In the Arctic region it is possible, therefore, that an increased aerosol loading leads to regional warming, rather than cooling.