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Lidar measurements of cirrus clouds in the northern and southern hemisphere (INCA): An interhemispheric comparison.

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Citation:
Immler, F. , Schrems, O. and Ström, J. (2002): Lidar measurements of cirrus clouds in the northern and southern hemisphere (INCA): An interhemispheric comparison. , EGS, General Assembly, Nice .
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Abstract:

Cirrus cloud measurements were performed during the INCA field campaigns in Punta Arenas/Chile (53.12°S, 70.88°W) and in Prestwick/Scotland (55.51°N, 4.60°W) in 2000. From lidar backscatter profiles at 532 nm and 355 nm wavelength the optical depth of the clouds are retrieved as well as base and top altitude and the particle phase. The range of detection covers clouds with optical depth (OD) of 3 down to below 10-3 setting the focus on thin and subvisual cirrus. Comparisons with climatologies of cloud properties suggest that the data obtained within a period of 3 weeks are to some extend representative.One main difference found is the observation of very faint layers of particles in the northern hemisphere in an altitude range of 5 to 8 km. Comparable structures in the SH have not been observed.For cirrus itself the distributions of the occurrence frequencies versus the optical depth (dt / dlogOD) look very similar. In Punta Arenas about 38 % of all detected ice clouds are found in the subvisible range (OD<.03) while 35% were characterized as thin cirrus (.03<OD<.3) and 25 % as thick ones (OD > .3) In Prestwick these fractions were 32%, 41% and 27%, respectively.Differences in the results from the southern and the northern hemisphere are found in the wavelength dependence of the backscatter coefficient and the depolarization behaviour. These results suggest, that there are clouds consisting of rather large particles in Punta Arenas, which have not been detected in Prestwick. In summary our data suggest that the higher concentration of aerosol (including anthropogenic aerosols) in the northern hemisphere does not have an impact on the abundance of cirrus, including those in the subvisible range. However, there are indications for an influence on the microphysical properties of all high altitude clouds.

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