Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 7 3 2007 10.5194/acpd-7-6255-2007 http://www.atmos-chem-phys-discuss.net/7/6255/2007/ http://www.atmos-chem-phys-discuss.net/7/6255/2007/acpd-7-6255-2007.html http://www.atmos-chem-phys-discuss.net/7/6255/2007/acpd-7-6255-2007.pdf 6255 6292 2007-05-10 Aircraft measurements of microphysical properties of subvisible cirrus in the tropical tropopause layer R. P. Lawson plawson@specinc.com B. Pilson B. Baker Q. Mo E. Jensen L. Pfister P. Bui SPEC Incorporated, Boulder, CO, USA NASA Ames Research Center, Moffett Field, CA, USA Subvisible cirrus (SVC) clouds are often observed within the tropical tropopause layer (TTL) and have been shown to have a significant impact on the earth radiation budget. The Costa Rica Aura Validation Experiment (CR-AVE) sponsored by the National Aeronautics and Space Administration (NASA) took place near San Jose, Costa Rica from 14 January–15 February 2006. The NASA WB-57F sampled SVC in the TTL from −75°C to −90°C with an improved set of cloud particle probes. The first digital images of ice particles in the TTL are compared with replicator images of ice particles collected in 1973 by a WB-57F in the TTL. The newer measurements reveal larger particles, on the order of 100 μm compared with <50 μm from the earlier measurements, and also different particle shapes. The 1973 particles were mainly columnar and trigonal, whereas the newer measurements are quasi-spherical and hexagonal plates. The WB-57F also measured very high water vapor contents with some instruments, up to 4 ppmv, and aerosols with mixed organics and sulfates. It is unknown whether these ambient conditions were present in the 1973 studies, and whether such conditions have an influence on particle shape and the development of the large particles. 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