A 'natural observatory' for aerosol cloud interaction studies

Anthropogenic aerosols are suspected to contribute to regional scale climate change via aerosol cloud interactions. The first step (first indirect effect) changes the cloud microphysics towards smaller droplet sizes resulting in changes in the cloud albedo, the second indirect effect modifies the time scale of raindrop formation and subsequently the spatial, temporal and intensity rainfall distribution. While a regional scale change in cloud albedo and a possible change of cloud type or lifetime is difficult to observe on a long term (climate relevant) time scale due to the high variability of clouds and missing long term observations rainfall data are available from meteorological observations over long enough time periods to derive statistically significant trends. For Western Australia (WA) the rainfall data show a significant decline within a few years in the early 1970ies, for Eastern Australia (Queensland, Qld) a constant decline of rainfall is reported since 1970, while in both areas no positive or negative trend was observed over 80 years from 1890 to 1970. Fortunately experimental observations on aerosol concentrations and ... mehraerosol sources are available already from a 1974/1976 survey allowing a reconstruction of the historic development of anthropogenic aerosol emissions and a comparison to present day measurements of aerosol abundance. Aerosol emissions are anti-correlated to the rainfall trends. Including these anthropogenic aerosol emissions into regional scale rainfall calculations for Western Australia using a high resolution WRF CHEM model with an aerosol-aware microphysics scheme matches both, rainfall rate and timeline of the rainfall decline.