Bulk microphysical properties of semi-transparent cirrus from AIRS: a six year global climatology and statistical analysis in synergy with geometrical profiling data from CloudSat-CALIPSO
- ISSN: 1680-7324
- DOI: 10.5194/acp-12-503-2012
This article presents a retrieval method and a statistical analysis\nof the bulk microphysical properties of semi-transparent ice clouds\nusing the Atmospheric Infrared Sounder (AIRS). The method relies\non spectral differences of cirrus emissivities in the 8–12 μm range\nand is sensitive to the effective ice crystal diameter (De) and ice\nwater path (IWP) of up to 85 μm and 120 g m−2, respectively. An indication\nof the most frequent ice crystal habit in the cirrus has been obtained\nby using separately single scattering properties of column-like and\naggregate-like ice crystals in the simulations. Uncertainties due\nto hypotheses on atmospheric parameters and ice crystal single scattering\nproperties are discussed and the cirrus emissivity and temperature\nrange for the applicability of the method are determined. To be sure\nthat the cirrus only includes ice crystals, one has to restrict the\ncloud temperature range to Tcld<230 K. On a global scale, these semi-transparent\nice clouds (cirrus) represent about 25% of all high clouds and are\nmainly encountered in the midlatitudes during winter and in the tropics,\nwith an average De and IWP of 52 μm and 27 g m−2, respectively. A\ncomparison with bulk microphysical properties from the TIROS-N Operational\nVertical Sounder (TOVS) shows an agreement on global mean values.\nThe addition of spectral information revealed improvements at the\nlimits of the cirrus emissivity range. Collocated Radar-Lidar Geometrical\nProfiling (GEOPROF) data have been used to study the vertical structure\nof these clouds and to infer average ice water content (IWC) for\ncirrus with a small vertical extent. This allowed us to compare and\ncontrast parameterizations of De as functions of IWC and IWP, respectively.