A critical look at spatial scale choices in satellite-based aerosol indirect effect studies

Abstract. Analysing satellite datasets over large regions may introduce spurious relationships between aerosol and cloud properties due to spatial variations in aerosol type, cloud regime and synoptic regime climatologies. Using MODerate resolution Imaging Spectroradiometer data, we calculate relationships between aerosol optical depth τ_a derived liquid cloud droplet effective number concentration N_e and liquid cloud droplet effective radius r_e at different spatial scales. Generally, positive values of dlnN_edlnτ_a are found for ocean regions, whilst negative values occur for many land regions. The spatial distribution of dlnr_edlnτ_a shows approximately the opposite pattern, with generally postive values for land regions and negative values for ocean regions. We find that for region sizes larger than 4° × 4°, spurious spatial variations in retrieved cloud and aerosol properties can introduce widespread significant errors to calculations of dlnN_edlnτ_a and dlnr_edlnτ_a. For regions on the scale of 60° × 60°, these methodological errors may lead to an overestimate in global cloud albedo effect radiative forcing of order 80% relative to that calculated for regions on the scale of 1° × 1°.