Tropical thin cirrus and relative humidity observed by the Atmospheric Infrared Sounder

[摘要] Global observations of cloud and humidity distributions in the uppertroposphere within all geophysical conditions are critically important inorder to monitor the present climate and to provide necessary data forvalidation of climate models to project future climate change. Towards thisend, tropical oceanic distributions of thin cirrus optical depth (τ),effective diameter (D_e), and relative humidity with respect to ice(RH_i) within cirrus (RH_ic) are simultaneously derived from theAtmospheric Infrared Sounder (AIRS). Corresponding increases in D_e andcloud temperature are shown for cirrus with τ>0.25 thatdemonstrate quantitative consistency to other surface-based, in situ andsatellite retrievals. However, inferred cirrus properties areshown to be less certain for increasingly tenuous cirrus. In-cloudsupersaturation is observed for 8–12% of thin cirrus and is severalfactors higher than all-sky conditions; even higher frequencies are shownfor the coldest and thinnest cirrus. Spatial and temporal variations inRH_ic correspond to cloud frequency while regional variability inRH_ic is observed to be most prominent over the N. Indian Ocean basin.The largest cloud/clear sky RH_i anomalies tend to occur in dry regionsassociated with vertical descent in the sub-tropics, while the smallestoccur in moist ascending regions in the tropics. The characteristics ofRH_ic frequency distributions depend on τ and a peak frequency islocated between 60–80% that illustrates RH_ic is on average biaseddry. The geometrical thickness of cirrus is typically less than the verticalresolution of AIRS temperature and specific humidity profiles and thus leadsto the observed dry bias, shown with coincident cloud vertical structureobtained from the Cloud-Aerosol Lidar and Infrared Pathfinder SatelliteObservation (CALIPSO). The joint distributions of thin cirrus microphysicsand humidity derived from AIRS provide unique and important regional andglobal-scale insights on upper tropospheric processes not available fromsurface, in situ, and other contemporary satellite observing platforms.

[学科分类] 大气科学