Joint Session J1.3 Global impacts of anthropogenic aerosols on convective clouds and precipitation

Tuesday, 11 July 2006: 2:00 PM
Ballroom AD (Monona Terrace Community and Convention Center)
Ulrike Lohmann, ETH, Zurich, Switzerland

Presentation PDF (238.7 kB)

Aerosols are an integral part of the atmospheric hydrological cycle and the atmosphere's radiation budget, with many possible feedback mechanisms that are not fully understood yet. Human activities modify – through direct emission and secondary formation processes – aerosol parameters and cloud properties in warm, mixed-phase and ice clouds. The easiest understood interaction between aerosols and climate is the direct effect (scattering and absorption of shortwave and thermal radiation). In addition, interactions of aerosols with the hydrological cycle, and additional impacts on the radiation budget, occur through the role of aerosols in cloud microphysical processes, as aerosol particles act as cloud condensation nuclei and ice nuclei. Because clouds in mid-latitudes originate predominately via the ice phase, changes of the properties of ice nuclei are of crucial importance for the hydrological cycle. An increase in ice nuclei can result in a rapid glaciation of a supercooled liquid water cloud due to the difference in vapour pressure over ice and water. Unlike cloud droplets, these ice crystals grow in an environment of high supersaturation with respect to ice, quickly reaching precipitation size, and with that can turn a non-precipitating into a precipitating cloud (glaciation effect).

The impact of aerosols on convective clouds is not known yet. Previous estimates of changes in convective precipitation from individual cloud systems due to anthropogenic aerosols are inconclusive, with suggestions for precipitation enhancement or suppression.

In this presentation, I am going to evaluate the impact of anthropogenic aerosols on convective clouds, precipitation and the radiation balance globally using the ECHAM5 general circulation model. ECHAM5 includes a double moment aerosol microphysics scheme ECHAM5-HAM that predicts the evolution of an ensemble of microphysically interacting internally- and externally-mixed aerosol populations as well as their size distribution and composition. The size-distribution is represented by a superposition of log-normal modes. In the current setup, the major global aerosol compounds sulfate, black carbon, particulate organic matter, sea salt, and mineral dust are included. The cloud scheme, originally developed for stratiform clouds and recently extended also to convective clouds, predicts the number and mass mixing ratios of cloud droplets and ice crystals.

Results from the coupled ECHAM5-HAM - cloud microphysics scheme will be compared with different present-day observations. Thereafter sensitivity simulations with present-day aerosol concentrations will be compared with simulations with pre-industrial aerosol concentrations in order to evaluate the impact of anthropogenic aerosols on convective clouds and precipitation globally and in different geographical regions and seasons.

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