Anthropogenic Aerosol Emissions and Rainfall Decline in South-West Australia

It is commonly understood that the observed decline in precipitation in South-West Australia during the twentieth century is caused by anthropogenic factors. In our project wa-aero on ForHLR1, we focus on the role of rapidly rising aerosol emissions from anthropogenic sources in South-West Australia around 1970. An analysis of historical longterm rainfall data of the Bureau of Meteorology shows that South-West Australia as a whole experienced a gradual decline in precipitation over the twentieth century. However, on smaller scales and for the particular example of the Perth catchment area, a sudden drop in precipitation around 1970 is apparent. Modelling experiments at a convection-resolving resolution of 3.3 km using the Weather and Research Forecasting (WRF) model version 3.6.1 with the aerosol-aware Thompson-Eidhammer microphysics scheme are conducted for the period 1970–1974. A comparison of four runs with different prescribed aerosol emissions and without aerosol effects demonstrates that tripling the pre-1960s atmospheric CCN and IN concentrations, as suggested by air-borne measurements, can suppress precipitation by 2–9 %, depending on the area and the season. An extended version of the results presented here was accepted for publication in the Journal of Climate in June 2016.