Remote sensing of cirrus cloud properties in the presence of lower clouds: An ATSR-2 case study during the Interhemispheric Differences in Cirrus Properties From Anthropogenic Emissions (INCA) experiment

International audienceThe retrieval of cirrus cloud microphysical properties in a multilayer cloud system is a challenge due to the usually low optical thickness of the ice cloud under investigation. However, it is estimated that about 50% of all cirrus clouds occur in such multilayer systems. Here, we present a method that uses a combination of five observations by the dual-view ATSR-2 radiometer onboard the ERS-2 satellite. Using the channels at 0.87 and 1.6 mm in both nadir and forward views and the nadir view of the 3.7 mm band, the retrieval can clearly separate a thin high cloud from a highly reflective low water cloud and thus determine the optical thickness and effective particle size of the cirrus quantitatively. This is confirmed by a comparison of the retrieved parameters with in situ aircraft observations made on 23 March 2000 near Punta Arenas, Chile, where good agreement was observed. An important finding of this study was that the pixel-to-pixel variability of the low cloud cannot be neglected. In other words, the optical thickness of the low cloud needs to be included as a retrieval parameter, because the assumption of constant optical properties leads to large degree of uncertainties in the derived ice cloud parameters