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Clouds and Aerosol Measurements aboard POLARSTERN with a mobile Raman Lidar (MARL)

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Citation:
Immler, F. (2002): Clouds and Aerosol Measurements aboard POLARSTERN with a mobile Raman Lidar (MARL) , Seminar Umweltphysik, Universität Bremen.12.2002. .
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Abstract:

Extended high altitude cirrus were observed during a cruise of the German research vessel POLARSTERN by a Mobile Aerosol Raman Lidar (MARL). The clouds occurred between 8°S and 12°N along 22°W during 34 h of observations from May 30th to June 3rd 2000. The altitude, vertical and optical depth at 532 nm and 355 nm of the cloud are determined from the Lidar data, as well as depolarisation characteristics at both this wavelength. The clouds appear in an altitude around 16 km and are between 0.5 and 2 km thick. The optical depth at 532 nm lies between 0.01 an 0.1. The data of radiosondes launched aboard POLARSTERN on a daily basis allow the determination of the temperatures, humidity and airmass of the clouds. Obviously the high-altitude cirrus appear in two distinct layers of purely tropospheric air separeted by wind direction and speed. The poster will discuss the lidar observations of the tropical cirrus in respect to this dynamical situation. Comparison of the depolarisation behavior measured for the tropical clouds with those of midlatitudinal cirrus allow some insight in the particle characterisitics.Between 10N and 30N extended Saharan dust layers were observed in an altituderange between 2 and 6 km. The continental, North African origin of the correspondingair masses is confirmed by backward trajectories. The Saharan dust is characterizedby an optical depth between 0.1 and 0.3, a depolarization around 10% and high lidarratios of 45 sr at 532 nm and 75 sr at 355 nm. The backscattering in the UV is relativelyinefficient, resulting in a negative color index.The lidar data are analyzed using a new approach based on Mie Theory and nonsphericalscattering calculations. The measured optical properties are consistent withan imaginary part of the refractive index of 0.002 and 0.004 at 532 nm and 355 nm,respectively. This increasing absorption with decreasing wavelength may be attributed to the content of hematite in the Saharan dust. It accounts for the strong wavelength dependence of the lidar ratio and the negative color index. The non-spherical nature of the dust articles is responsible for the high values of the lidar ratios. The effectiveradius retrieved on this bases was found to decrease with increasing altitude fromabout 3 µm to 0.6 µm.

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