Relationship between aerosol and cloud fraction over Australia

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 ( τ a ), fire counts, and cloud fraction ( f c ) from Aqua‐MODIS, and NCEP Reanalysis vertical velocities at 500 mb ( ω 500 ) as a proxy for dynamic regime. In the NCA we find a monotonic increase f c (35%, absolute f c ) as a function of increasing τ a . In the SEA, we find that f c initially increases by 25% with increasing τ a , followed by a slow systematic decrease (∼18%) with higher τ a . We show that the MRE theory proposed by Koren et al. (2008) adequately represents the variation of f c with τ a in both the NCA and SEA. By conditionally sorting data by ω 500 we investigate the role dynamics plays in controlling the τ a ‐ f c relationship and the rate at which f c changes with τ a . We find that the MRE theory can be used to empirically fit both − ω 500 and + ω 500 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 τ a and f c during biomass burning seasons. However, additional factors such as aerosol type and cloud type may play a role.