Mark Schoeberl

and 7 more

We describe our Solar Aerosol and Gas Experiment (SAGE) III/ISS cloud detection algorithm. As in previous SAGE II/III studies this algorithm uses the extinction at 1022 nm and the extinction color ratio 520nm/1022nm to separate aerosols and clouds. We identify three types of clouds: visible cirrus (extinction coefficient > 3x10-2 km-1, subvisible cirrus (extinction < 3x10-2 km-1 and >10-3 km-1), and very low extinction cloud-aerosol mixtures (extinction < 10-3 km-1). Visible cirrus cannot be quantitatively measured by SAGE because of its high extinction, but we infer the presence of cirrus through the solar attenuation of the SAGE vertical scan. We then assume that cirrus layers extend 0.5 km below the scan termination height. SAGE cirrus cloud fraction estimated this way is in qualitative agreement with CALIPSOmeasurements. Analyzing three years of SAGE III/ISS data, we find that visible cirrus and subvisible cirrus have nearly equal abundance in the tropical upper troposphere and the average cloud fraction is about 25%. At 16 km, the highest concentration visible cirrus and subvisible cirrus is over the Tropical West Pacific, central Africa and central South America during winter. Latitudinal gaps in zonal mean cloud fraction and average aerosol extinction apparent in the subtropical transition region are aligned with descending branch of the residual mean circulation. We also identify four anomalous aerosol extinction periods that can be tentatively assigned to significant volcanic or fire events. Using tropopause relative coordinates, we show that maximum cloud top heights are consistently restricted to a narrow region near the tropopause.