Abstract

Anthropogenic aerosols may alter clouds by acting as cloud condensation nuclei (CCN) or as ice nuclei (IN). In warm (liquid phase clouds) an increase in CCN is known to cool the climate, but in cirrus clouds, since the absorption of long-wave radiation is larger than the reflection of solar radiation, the increase in IN may act to warm the climate. Here, we examine the possible effects of anthropogenic aerosols on cirrus clouds. The effect of aerosols on cirrus clouds depends on the natural aerosols present in the upper troposphere as well as the relative humidity distribution and updraft velocity distribution. We apply two different parameterizations for the impact of aerosols on cirrus clouds using the NCAR CAM3 model and compare the resulting ice particle sizes and number concentrations with observations. The forcing by anthropogenic aerosols depends on the mode of nucleation in the absence of anthropogenic aerosols and is highly uncertain.

Biography