Clouds and Climate

Anthropogenic changes in atmospheric aerosols can alter the global radiation balance and climate directly through their extinction of incoming solar radiation, and indirectly through their impact on clouds. The direct effect can be estimated based on aerosol extinction measurements, but the sensitivity to clouds is very complicated, given the wide variety of cloud types and properties.

In this paper, we address the potential for changes in cirrus cloud properties and climatic impact as a result of changes in upper tropospheric aerosol size distributions. Cirrus clouds have a strong potential to alter the earth's radiation budget, but the sign and magnitude of their net radiative effect is not clear and depends sensitively upon cloud properties such as optical depth and particle size. Using a detailed l-D microphysical cirrus model, we have investigated the sensitivity of cirrus microphysical and optical properties to the aerosol size distribution. Unlike water clouds lower in the atmosphere, ice crystal concentrations in cirrus are nearly always more than a factor of 100 smaller than the total number of aerosols typically present in the upper troposphere. As a result, the number of ice crystals which nucleate is independent of the total number of aerosols. The number of ice crystals which nucleate is determined by the shape of the aerosol size distribution, the mode of nucleation, and the cooling rate. For most of the environmental conditions considered, the cirrus radiative properties are relatively insensitive to the aerosol size distribution. However, under certain conditions, the size distribution can have a dramatic eftect. This study suggests that increasing the aerosol mass loading in the upper troposphere may have very little impact on cirrus clouds or could have a significant impact, depending upon the details of how the aerosol size distribution changes with time and what ice nucleation processes are active in the upper troposphere.