Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 7 6 2007 10.5194/acpd-7-16185-2007 http://www.atmos-chem-phys-discuss.net/7/16185/2007/ http://www.atmos-chem-phys-discuss.net/7/16185/2007/acpd-7-16185-2007.html http://www.atmos-chem-phys-discuss.net/7/16185/2007/acpd-7-16185-2007.pdf 16185 16225 2007-11-20 Tropical thin cirrus and relative humidity observed by the Atmospheric Infrared Sounder B. H. Kahn brian.h.kahn@jpl.nasa.gov C. K. Liang A. Eldering A. Gettelman Q. Yue K. N. Liou Jet Propulsion Laboratory, California Inst. of Technology, Pasadena, CA, USA Joint Inst. for Regional Earth System Science and Engineering, Univ. of California, Los Angeles, CA, USA Dept. of Atmospheric and Oceanic Sciences, University of California, Los Angeles, CA, USA National Center for Atmospheric Research, Boulder, CO, USA Global observations of cloud and humidity distributions in the upper troposphere within all geophysical conditions are critically important in order to monitor the present climate and to provide necessary data for validation of climate models to project future climate change. Towards this end, tropical oceanic distributions of thin cirrus optical depth (τ), effective diameter (De), and relative humidity with respect to ice (RHi) within cirrus (RHic) are simultaneously derived from the Atmospheric Infrared Sounder (AIRS). Corresponding increases in De and cloud temperature are shown for cirrus with τ>0.25 that demonstrate quantitative consistency to other surface-based, in situ and satellite retrievals of cirrus. However, inferred cirrus properties are shown to be less certain for increasingly tenuous cirrus. In-cloud supersaturation is observed for 8–12% of thin cirrus and is several factors higher than all-sky conditions; even higher frequencies are shown for the coldest and thinnest cirrus. Spatial and temporal variations in RHic correspond to cloud frequency while regional variability in RHic is observed to be most prominent over the N. Indian Ocean basin. The largest cloud/clear sky RHi anomalies tend to occur in dry regions associated with vertical descent in the sub-tropics, while the smallest occur in moist ascending regions in the tropics. 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