US EPA

1607260 

Journal Article 

Introduction to Special Section: An experimental study of the aerosol indirect effect for validation of climate model parameterizations 

Brenguier, JL 

2003 

108 

D15 

10.1029/2003JD003849 

WOS:000184826000001 

[ 1] In 1995, experts of the Intergovernmental Panel on Climate Change (IPCC) evaluated the uncertainty range in the prediction of the anthropogenic aerosol impact on climate via modification of cloud radiative properties, the so-called aerosol indirect effect (AIE), from 0 to -1.5W m(-2), though they did not conclude on any best estimate of that forcing [Houghton et al., 1995]. Six years later, a best estimate was still missing in the new IPCC report [Penner et al., 2001], and the uncertainty range was even larger, from 0 to -4.8 W m(-2), hence leading readers to the paradoxical conclusion that our scientific knowledge had declined during that period. In fact, most of the physical processes involved in the indirect effect were known when Twomey [1977] and Albrecht [1989] published their pioneering papers, briefly, that cloud droplets are formed on cloud condensation nuclei (CCN), that the droplet number concentration depends on the size distribution and chemical composition of the CCN at cloud base, that continental clouds are characterized by a higher number concentration and smaller droplets than similar maritime clouds, that more numerous but smaller droplets have a greater extinction at constant liquid water content, and that the onset of precipitation is sensitive to the formation of a few big droplets, also referred to as precipitation embryos. The challenge for the IPCC participants was rather to implement all these processes interactively in a climate model. The increase of the uncertainty range between the 1995 and 2001 IPCC exercises thus reflects the variability of the model predictions when additional processes were implemented, in particular, the parameterization of drizzle formation in boundary layer clouds. 

climate; aerosol; cloud