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Subvisual cirrus clouds in the midlatitude and tropical tropopause region observed by lidar

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Citation:
Immler, F. and Schrems, O. (2003): Subvisual cirrus clouds in the midlatitude and tropical tropopause region observed by lidar , EGU General assembly, Nice 2003. .
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Abstract:

The Mobile Aerosol Raman Lidar MARL performed measurements in Punta Arenas/Chile (53.12°S, 70.88°W), in Prestwick /Scotland (55.5°N, 4.60°W), and aboard the German research vessel Polarstern in 2000. The lidar is able to measure backscatter profiles at 532 nm and 355 nm wavelength with a vertical resolution of 7.5 m and a time resoltion of 140 s. Additionally, the depolarization is detected at both wavelength. A newly developed algorithm allows for a very sensitive detection of clouds by making use of the backscatter and the depolarization created by the ice particles. We are therefore able to detect clouds with an estimated optical depth down to 10^-4.The northern and southern hemispheric campaigns were part of the European INCA project. Around 60 hours of measurements were perfomed at each site. In about 30 % of these data subvisual cirrus (Optical depth (OD) <0.03) were detected. An impact of the strong air traffic on the abundance of thin clouds in the northern hemisphere could not be proven, however, an anlysis of the depolarization and the color index suggests differences in the microphysical properties of the cloud particles.The measurements in the tropics aboard Polarstern revealed the great abundance of tropical subvisual cirrus (TSC) in the equatorial Atlantic region. The TSCs differed significantly from their midlatitude counterparts in respect to their livetime as well as in their optical and microphysical properties. Simultaneaous radiosonde soundings performed aboard Polarstern allowed the determination of the temperature and other meteorological parameters of the tropical tropopause layer.

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