Advances in CALIPSO (IIR) cirrus cloud property retrievals – Part 1: Methods and testing

Abstract. In this study, we describe an improved Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite retrieval which uses the CALIPSO Imaging Infrared Radiometer (IIR) and the CALIPSO lidar for retrievals of ice particle number concentration Ni, effective diameter De and ice water content (IWC). By exploiting two IIR channels, this approach is fundamentally different from another satellite retrieval based on cloud radar and lidar that retrieves all three properties. A global retrieval scheme was developed using in situ observations from several field campaigns. The Ni retrieval is formulated in terms of Ni/APSD ratios, where APSD is the directly measured area concentration of the ice particle size distribution (PSD), along with the absorption optical depth in two IIR channels and the equivalent cloud thickness seen by IIR. It is sensitive to the shape of the PSD, which is accounted for, and uses a more accurate mass-dimension relationship relative to earlier work. The new retrieval is tested against corresponding cloud properties from the field campaigns used to develop this retrieval, as well as a recent cirrus cloud property climatology based on numerous field campaigns from around the world. In all cases, favorable agreement was found. This analysis indicated that Ni varies as a function of τ. By providing near closure to the ice PSD, the natural atmosphere may be used more like a laboratory for studying key processes responsible for the evolution and life cycle of cirrus clouds and their impact on climate.