Explanation of discrepancies among satellite observations of the aerosol indirect effects

Satellite-based remote sensing instruments for measuring the aerosol indirect effect (IE = -d ln r_e/d ln T_a where r_e is the cloud drop effective radius and T_a is the aerosol optical depth) show large disparities in the magnitude of the effect for similar regions of the globe. Over the oceans, the Advanced Very High Resolution Radiometer (AVHRR) measures an indirect effect twice that measured by the POLarization and Directionality of the Earth Reflectances (POLDER) (0.17 vs. 0.085). We address possible reasons for these disparities. It is argued that AVHRR misses the optically thin and broken clouds, especially over land, while POLDER misses clouds with variable top heights in its field of view. POLDER is also biased to thinner, less turbulent clouds. The sensitivity of the indirect effect to cloud turbulence therefore biases POLDER to lower values. POLDER measures an indirect effect over the ocean that is about twice that over the land (0.085 vs. 0.04). By considering factors such as dynamics, variability in cloud liquid water path, decoupling of the boundary layer, and the effect of salt particles, we argue that this could be an artifact, and that the indirect effect on cloud microstructure may be stronger over land than over the ocean.