Alexander Geiß

Publications (2)4.38 Total impact

• Article: Characterization of the planetary boundary layer during SAMUM‐2 by means of lidar measurements

  SILKE GROß, JOSEF GASTEIGER, VOLKER FREUDENTHALER, MATTHIAS WIEGNER, ALEXANDER GEIß, ALEXANDER SCHLADITZ, CARLOS TOLEDANO, KONRAD KANDLER, MATTHIAS TESCHE, ALBERT ANSMANN, ALFRED WIEDENSOHLER
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  ABSTRACT: Measurements with two Raman-depolarization lidars of the Meteorological Institute of the Ludwig-Maximilians-Universität, München, Germany, performed during SAMUM-2, were used to characterize the planetary boundary layer (PBL) over Praia, Cape Verde. A novel approach was used to determine the volume fraction of dust υd in the PBL. This approach primarily relies on accurate measurements of the linear depolarization ratio. Comparisons with independent in situ measurements showed the reliability of this approach. Based on our retrievals, two different phases could be distinguished within the measurement period of almost one month. The first (22–31 January 2008) was characterized by high aerosol optical depth (AOD) in the PBL and large υd > 95%. During the second phase, the AOD in the PBL was considerably lower and υd less than ∼40%. These findings were in very good agreement with ground based in situ measurements, when ambient volume fractions are considered that were calculated from the actual measurements of the dry volume fraction. Only in cases when dust was not the dominating aerosol component (second phase), effects due to hygroscopic growth became important.
  Tellus B 08/2011; 63(4):695 - 705. · 4.38 Impact Factor
• Article: Dual-wavelength linear depolarization ratio of volcanic aerosols: Lidar measurements of the Eyjafjallajökull plume over Maisach, Germany

  Silke Groß, Volker Freudenthaler, Matthias Wiegner, Josef Gasteiger, Alexander Geiß, Franziska Schnell
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  ABSTRACT: The ash plume of the Eyjafjallajökull eruption in April 2010 offered an exceptional opportunity to assess the potential of advanced lidar systems to characterize the volcanic aerosols. Consequently, the plume was continuously observed in the framework of EARLINET. In this paper we focus on the EARLINETRaman- depolarization-lidar measurements at Maisach near Munich, Germany. From these data sets the lidar ratio Sp and the particle linear depolarization ratio dp at two wavelengths (355 nm and 532 nm) were retrieved. These quantities can be used to characterize volcanic aerosols and to establish criteria for the discrimination from other aerosol types. In the pure volcanic ash plume, observed until noon of 17 April, wavelength independent values of dp as high as 0.35 < dp < 0.38, indicating non-spherical particles, were found, and lidar ratios of 50 < Sp < 60 sr at 355 nm and 45 < Sp < 55 sr at 532 nm. Later, volcanic aerosols were mixed into the boundary layer. This mixture showed in general lower values of dp as expected from the contribution of boundary layer aerosols. Especially noteworthy is the increase of dp with wavelength, when volcanic ash was mixed with small spherical particles.
  Atmospheric Environment.