Global modeling of mixed-phase clouds - The albedo and lifetime effects of aerosols

Artikel i vetenskaplig tidskrift, 2011

We present a global modeling study of mixed-phase clouds and have performed sensitivity simulations to explore the ways in which aerosol particles can affect this type of cloud. This study extends previous similar studies in that it takes into account not only the so-called aerosol lifetime effects on mixed-phase clouds but also aerosol effects on their albedo. Our findings generally agree with previous studies in that an increase in ice-nucleating aerosol particles (IN) leads to a decreased cloud lifetime and therefore a warming of the Earth-atmosphere system. However, an increase in IN will also generally decrease ice crystal sizes, thereby increasing the cloud albedo, which is analogous to the well-established Twomey effect on liquid clouds. This decrease in ice crystal effective radii leads to an increase in cloud albedo and hence to a cooling that counteracts the lifetime effect of mixed-phase clouds. Taking both the albedo and lifetime effects of mixed-phase clouds into account, we find the net radiative forcing effect of an IN increase to be positive but small, which is in contrast to a much stronger warming that is found if the albedo effect is not taken into account. The latter has been the common approach in global studies of aerosol effects on mixed-phase clouds so far. Results were found to be extremely sensitive to the choice of heterogeneous freezing parameterization and the maximum fraction of black carbon particles available for ice nucleation. Copyright 2011 by the American Geophysical Union.