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Johannes Bühl, Ronny Engelmann, and Albert Ansmann

Abstract

A chirped laser pulse can introduce artifacts into datasets of coherent Doppler wind lidars. At close vicinity of strong signal peaks undesired artificial velocities can be measured and continuous signals can be shifted by a constant factor. It is shown how to remove these artifacts and how to retrieve accurate velocity estimations from both clouds and the planetary boundary layer. Therefore, a two-dimensional deconvolution technique is applied to the wind lidar datasets in order to correct the chirp effect in the range and frequency space. The chirp correction for a 1-h measurement of vertical velocities in the atmosphere is presented. The method is applied to the averaged Doppler spectra. Therefore, no access to the raw heterodyne signal is necessary. The complexity of the data acquisition software and the amount of data to be stored is hereby significantly reduced. Simulations suggest that the remaining velocity error resulting from the laser pulse chirp is smaller than 0.02 m s−1 and chirp-induced artifacts are removed reliably. The method also increases the signal resolution in the range and frequency dimension and can be applied for this intent even if there is no chirp.

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Dietrich Althausen, Detlef Müller, Albert Ansmann, Ulla Wandinger, Helgard Hube, Ernst Clauder, and Steffen Zörner

Abstract

A transportable multiple-wavelength lidar is presented, which is used for the profiling of optical and physical aerosol properties. Two Nd:YAG and two dye lasers in combination with frequency-doubling crystals emit simultaneously at 355, 400, 532, 710, 800, and 1064 nm. A beam-combination unit aligns all six laser beams onto one optical axis. Hence the same air volume is observed by all six beams. The combined beam can be directed into the atmosphere from −90° to +90° zenith angle by means of a turnable mirror unit. From the simultaneous detection of the elastic-backscatter signals and of the Raman signals backscattered by nitrogen molecules at 387 and 607 nm and by water vapor molecules at 660 nm, vertical profiles of the six backscatter coefficients between 355 and 1064 nm, of the extinction coefficients, and of the extinction-to-backscatter ratio at 355 and 532 nm, as well as of the water vapor mixing ratio, are determined. The optical thickness between the lidar and a given height can be retrieved for all six transmitted wavelengths from measurements under two different zenith angles. In contrast to sun-radiometer observations, this option allows the resolution of spectral extinction information of each of the aerosol layers present in the vertical. The profile of the depolarization ratio is determined at 710 nm and used to investigate particle shape. A few measurement cases are presented to demonstrate the capabilities of the new lidar.

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Ronny Engelmann, Ulla Wandinger, Albert Ansmann, Detlef Müller, Egidijus Žeromskis, Dietrich Althausen, and Birgit Wehner

Abstract

The vertical aerosol transport in the planetary boundary layer (PBL) is investigated with lidars. Profiles of the vertical wind velocity are measured with a 2-μm Doppler wind lidar. Aerosol parameters are derived from observations with an aerosol Raman lidar. Both instruments were operated next to each other at the Institute for Tropospheric Research (IfT) in Leipzig, Germany. The eddy correlation technique is applied to calculate turbulent particle mass fluxes on the basis of aerosol backscatter and vertical wind data obtained with a resolution of 75 m and 5 s throughout the PBL. A conversion of particle backscatter to particle mass is performed by applying the IfT inversion scheme to three-wavelength Raman lidar observations. The method, so far, is restricted to stationary and dry atmospheric conditions under which hygroscopic particle growth can be neglected. In a case study, particle mass fluxes of 0.5–2.5 μg m−2 s−1 were found in the upper part of a convective PBL on 12 September 2006.

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Dietrich Althausen, Ronny Engelmann, Holger Baars, Birgit Heese, Albert Ansmann, Detlef Müller, and Mika Komppula

Abstract

Two versions of the portable aerosol Raman lidar system (Polly) are presented. First, the two-channel prototype is depicted. It has been developed for the independent and simultaneous determination of particle backscatter and extinction coefficient profiles at 532 nm. Second, the 3 + 2 Raman lidar PollyXT (3 + 2: three backscatter and two extinction coefficients), the second generation of Polly, is described. The extended capabilities of PollyXT are due to the simultaneous emission of light with three wavelengths, more laser power, a larger main receiver mirror, and seven receiver channels. These systems are completely remotely controlled and all measurements are performed automatically. The collected data are transferred to a home server via the Internet and are displayed on a Web page. This paper describes the details of the optical setup, the housekeeping of the systems, and the used data retrieval routines. A measurement example taken close to Manaus, Brazil, on 15 August 2008 shows the capabilities of PollyXT.

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Albert Ansmann, Jens Bösenberg, Gérard Brogniez, Salem Elouragini, Pierre H. Flamant, Karlheinz Klapheck, Holger Linn, Louis Menenger, Walfried Michaelis, Maren Riebesell, Christoph Senff, Pierre-Yves Thro, Ulla Wandinger, and Claus Weitkamp

Abstract

Four lidars located roughly 75 km from each other in the inner German Bight of the North Sea, were used to measure geometrical and optical properties of cirrus clouds during the International Cirrus Experiment 1989 (ICE '89). A complete cirrus life cycle was observed simultaneously with three lidan during a case study on 18 October 1989. Time series of particle backscatter, depolarization-ratio height profiles, cloud depth, optical thickness, and of the cirrus extinction-to-backscatter, or lidar, ratio describe the evolution of the cloud system. A two-wavelength lidar measurement was performed and indicates wavelength independence of ice-crystal scattering. The optical and geometrical depths of the cirrus were well correlated and varied between 0.01 and 0.5 and 100 m and 4.5 km, respectively. Although the evolution of the cloud deck was similar over the different observation sites, cirrus geometrical, scattering, and microphysical properties were found to vary considerably within the lidar network. A statistical analysis of ice-cloud properties is performed based on 38 different cirrus cases sampled during ICE '89. Cirrus formation was found to start at the tropopause in most cases. Ice clouds, measured at high midlatitudes (around 54°N), were thin with mean optical and geometrical depths mainly below 0.4 and 2 km, respectively. A good correlation between mean cloud optical and geometrical thickness, and a weak decrease of the mean optical depths with temperature was observed.

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