384 research outputs found

    Observations of extremely thin clouds in the tropical tropopause region by a ship based lidar.

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    During the cruises ANT XVII/4 (Punta Arenas Bremerhaven, 2000) and ANT XX1/1 (Bremerhaven - Cape Town, 2003) continuous measurements of clouds and aerosol have been performed by a Mobile Aerosol Raman Lidar (MARL). This system measures backscatter of light at 355 nm and 532 nm and is able to detect even extremely thin layers of particles in the atmosphere. Cloud base and top height, depolarisation optical depth and color index can be deduced from this data. During the Polarstern cruises very thin layers of ice particles have been observed frequently in the tropical tropopause layer (TTL). Cases of persistent cloud layers with an optical depth below 10^-3 are reported. Based on the data of radiosondes which were launched daily aboard the vessel, the conditions at which these type of clouds form are analysed. It is shown, that these clouds form in a layer below the cold point, but above the lapse rate tropopause. The relation between cloud occurrence and temperature and wind in the TTL as well as state of the QBO are discussed

    Cirrus clouds, contrails, and ice supersaturated regions: observations by lidar and radiosonde in Lindenberg/Germany

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    From April to October 2003 measurements have been performed with a mobile Aerosol Raman Lidar (MARL) at the Meteorological Observatory in Lindenberg(14.5 E, 52.5 N) . The aim of this extensive campaign was the investigation of tropospheric water vapour, cirrus clouds and contrails over a longer period of time. The lidar system detects aerosols and clouds in the troposphere and lower stratosphere. Cirrus clouds are detected by the large depolarisation of the backscattered laser beam. The lidar operates day and night, during daytime contrails may be identified with the help of a video camera. In the upper troposphere cirrus has been detected in 55% of the measured time. Radiosonde (Vaisala RS 80) data are available four times a day form the routine observational program at the site. They frequently detect ice supersaturatedregions (ISSR) in the upper troposphere. The comparison of the RS 80 andthe MARL data shows that the correlation between ISSR and the occurrence of cirrus and contrails is rather week. On the other hand there is a strong link between the occurrence of subvisual cirrus and contrails in the upper troposphere

    Lidar observations of extremely thin clouds at the tropical tropopause

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    Our two lidar systems MARL (Mobile Aerosol Raman Lidar) and ComCAL (Compact Cloud and Aerosol Lidar) have been operated during measurement campaignsaboard the research vessel Polarstern (2003 and 2005) and at Paramaribo/Suriname (5.8°N, 55°W) in 2004/2005. The lidar systems detect cirrus in the upper troposphere with a very high sensitivity by the depolarization measurement. Cirrus properties like altitude, optical depth, particle phase are derived from the lidar signals. Using scattering theory and an estimate of the particle size, the number concentration of ice particles can be calculated. In almost 90% of the measurements performed at Paramaribo thin or subvisible cirrostratus were detected in the tropical transition layer (TTL). Occasionally, extremely thin clouds with optical depths below 10-4 were observed at the cold point tropopause (CPT). The condensed mass concentration was 0.5 ŠÌg/m3. If we assume a particle size of 5 ŠÌm (effective radius) this corresponds to a number density of only a few particle per liter. The extremely thin clouds which were observed at - or even slightly above - the cold point tropopause seem to dwell in subsaturated air. Our findings indicate that these thin layers of particles, are not composed of pure water ice

    Vertical profiles, optical and microphysical properties of Saharan dust layers determined by a ship-borne lidar.

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    A unique data set of ship-borne lidar measurements of Saharan dust layers above the Atlantic ocean has been collected aboard the research vessel Polarstern with a mobile Aerosol Raman Lidar (MARL) during the LIMPIDO-campaign in June 2000. Extended Saharan dust layers have been observed in the region between 8.5° N and 34° N in an altitude range between 2 and 6 km. The continental, North African origin of the probed air masses is confirmed by 8-day backward trajectories. The Saharan dust is characterized by an optical depth in the range of 0.1 and 0.3, a depolarization around 10% and high lidar ratios of 45 sr at 532 nm and 75 sr at 355 nm. The backscattering by the dust particles at the UV-wavelength is relatively weak, resulting in a negative color index.From the measured optical properties the effective radius and the refractive index of the dust particles are derived using a new approach based on Mie Theory and non-spherical scattering calculations. The low backscatter coefficient observed at 355 nm is due to significant absorption which increases with decreasing wavelength. This finding agrees very well with results from satellite and sun photometer measurements. The effective radii decrease from about 3um base to 0.6 um at the top of the dust plumes. The non-spherical shapes of the dust particles are responsible for the high values of the lidar ratios

    Fluorescing aerosol from Siberian forest fires in the lowermost stratosphere.

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    During a measuring campaign in Lindenberg/Germany (14.5 E, 52.5 N), we observed in June 2003 an extended aerosol layer at 13 km altitude in the lowermost stratosphere with a mobile Aerosol Raman Lidar (MARL). The stratospheric layer created an inelastic backscatter signal that we detected with a water vapour Raman channel, but that was not produced by Raman scattering. Also, we find evidence for inelastic scattering by a smoke plume from a forest fire that we observed in the troposphere. We interpret the unexpected properties of these aerosols as fluorescence induced by the laser beam at organic components of the aerosol particles. Fluorescence from ambient aerosol had not yet been considered detectable by lidar systems. However, organic compounds such as polycyclic aromatic hydrocarbons sticking to the aerosol particles, or bioaerosol such as bacteria, spores or pollen fluoresce when excited with UV-radiation in a way that is detectable by our lidar system. Therefore, we conclude that fluorescence from organic material released by biomass burning creates, inelastic backscatter signals that we measured with our instrument and thus demonstratea new and powerful way to characterize aerosols by a remote sensing technique.In spring 2003 heavy fires burned around 18 million hectares of boreal forest inSiberia, the smoke plume from these fires travelled eastwards around the globe and was detected in Europe in late May 2003. The stratospheric aerosol layer that we have observed in Lindenberg for three consecutive days is likely to be a remnant from theseSiberian forest fire plumes that was lifted across the tropopause
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