Aircraft measurements of microphysical properties of subvisible cirrus in the tropical tropopause layer R. P. Lawson1, B. Pilson1, B. Baker1, Q. Mo1, E. Jensen2, L. Pfister2, and P. Bui2 1SPEC Incorporated, Boulder, CO, USA 2NASA Ames Research Center, Moffett Field, CA, USA
Abstract. Subvisible cirrus (SVC) clouds are often observed within
the tropical tropopause layer (TTL). Some studies suggest that SVC has 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.
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. A companion paper (Jensen et al.,
2008) presents crystal growth calculations that suggest that the high water
vapor measurements are required to grow ice particles to the observed sizes
of 100 μm and larger.
Citation: Lawson, R. P., Pilson, B., Baker, B., Mo, Q., Jensen, E., Pfister, L., and Bui, P.: Aircraft measurements of microphysical properties of subvisible cirrus in the tropical tropopause layer, Atmos. Chem. Phys., 8, 1609-1620, doi:10.5194/acp-8-1609-2008, 2008.