Aerosol Impacts on Mixedphase Clouds

GCM studies suggest that if, in addition to mineral dust, hydrophilic black carbon particles are assumed to act as ice nuclei at temperatures between 0° and -35°C, then increases in aerosol number concentration from preindus-trial to present times may have resulted in greater glaciation of supercooled stratiform clouds and an increase in the amount of precipitation via the ice phase. This process could decrease the global mean cloud cover, leading to enhanced absorption of solar radiation. Whether the glaciation effect or the warm cloud lifetime effect is larger depends on the chemical nature of the particles (Lohmann and Diehl 2006).

Simulations of precipitation from single-cell mixed-phase convective clouds suggest a reduction for various background aerosol concentrations when particle concentrations are increased. Khain et al. (2005) postulated that smaller cloud droplets, such as those affected by human activities, would change the thermodynamics of convective clouds. More but smaller droplets would reduce the production of rain in convective clouds. When these droplets freeze, the associated latent heat release would then result in more vigorous convection and more precipitation. In a clean cloud, on the other hand, rain would have depleted the cloud so that less latent heat is released when the cloud glaciates, resulting in less vigorous convection and less precipitation. For a thunderstorm in Florida, in the presence of Saharan dust, the simulated precipitation enhancement lasted only two hours, after which precipitation decreased as compared with clean conditions. This highlights the complexity of the system and indicates that the sign of the global change in precipitation attributable to aerosols is not yet known. Note that microphysical processes can only change the temporal and spatial distribution of precipitation, whereas the total amount of precipitation can only change if evaporation from the surface changes.

0 0

Post a comment