Influence of aerosol-cloud interactions on climate

Indirect aerosol effects

Indirect aerosol effects, i.e. the influence of aerosols on cloud microphysical properties, are the largest source of uncertainty when estimating climate sensitivity. To reduce this uncertainty, much basic research is needed.

I am working on ways to investigate aerosol indirect effects stemming from ship emissions. This is done by means of satellite and model data.

• Concerning the modelling aspect, I use the aerosol climate model ECHAM5-HAM in which I implement a state-of-the-art, high-resolution ship emission inventory covering the shipping emissions of the year 2000. Our simulations show that knowledge of the correct emission particle size distribution is of utmost importance when quantifying aerosol indirect effects. Furthermore, we find that the aerosol indirect effect from shipping emissions may be substantially lower than previously estimated. Results are presented in my PhD-Thesis.

• The analysis of satellite data is based on the idea of separating "polluted"- from "clean"  marine environments. This facilitates the statistical analysis and comparison of cloud properties in both environments. For this, I analyze wind trajectories in regions where shipping emissions are thought to substantially disturb the pristine marine boundary layer composition. Satellite-instrument derived cloud microphysical properties are then analyzed along suited wind trajectories to detect possible differences between "clean" and "polluted" clouds. We find no statistical significant influence of shipping emissions on cloud fields in the regions of interest. The accompanying paper is found here.
  • This work was chosen as a research highlight by Nature Climate Change.

For most atmospheric aerosol species, scattering dominates over absorption of radiation in the visible spectral range. The direct radiative forcing (DRF) at the top of the atmosphere (TOA) exerted by a layer of aerosol depends on the albedo of the underlying surface and the aerosol single scattering albedo (SSA). In cloud-free scenes, even strongly (but still also scattering) absorbing aerosols impose a negative TOA DRF over ice-free oceanic regions due to the dark underlying surface -- the local planetary albedo α is increased. If the surface albedo is increased, such as in case of clouds residing below aerosols, absorbing aerosols can exert a positive TOA DRF. In this case, the absorption of the particles dominates the scattering for the net TOA effect -- α is decreased.

I am interested in quantifying this warming effect of absorbing aerosols from satellite data only; a peer-reviewed publication which resulted from this work can be found here.