Relationship between aerosol and cloud fraction over Australia

[1] We study the relationships between aerosols, clouds, and large scale dynamics over a north coastal Australia (NCA) region and a southeast Australia (SEA) region during the period 2002–2009 to evaluate the applicability of the aerosol microphysics-radiation-effect (MRE) theory proposed by Koren et al. (2008) in a low aerosol environment. We use aerosol optical depth (τa), fire counts, and cloud fraction (fc) from Aqua-MODIS, and NCEP Reanalysis vertical velocities at 500 mb (ω500) as a proxy for dynamic regime. In the NCA we find a monotonic increase fc (35%, absolute fc) as a function of increasing τa. In the SEA, we find that fc initially increases by 25% with increasing τa, followed by a slow systematic decrease (∼18%) with higher τa. We show that the MRE theory proposed by Koren et al. (2008) adequately represents the variation of fc with τa in both the NCA and SEA. By conditionally sorting data by ω500 we investigate the role dynamics plays in controlling the τa-fc relationship and the rate at which fc changes with τa. We find that the MRE theory can be used to empirically fit both −ω500 and +ω500 observations. By analyzing meteorological parameters from the NCEP Reanalysis, we find that variations in local meteorology are not likely the cause of the observed relationships of τa and fc during biomass burning seasons. However, additional factors such as aerosol type and cloud type may play a role.