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Solar radiative effects of a Saharan dust plume observed during SAMUM assuming spheroidal model particles

Otto, Sebastian and Bierwirth, Eike and Weinzierl, Bernadett and Kandler, Konrad and Esselborn, Michael and Tesche, Matthias and Schladitz, Alexander and Wendisch, Manfred and Trautmann, Thomas (2009) Solar radiative effects of a Saharan dust plume observed during SAMUM assuming spheroidal model particles. Tellus B - Chemical and Physical Meteorology, 61B, pp. 270-296. DOI: 10.1111/j.1600-0889.2008.00389.x.

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Official URL: http://www3.interscience.wiley.com/journal/118533053/home

Abstract

The solar optical properties of Saharan mineral dust observed during the Saharan Mineral Dust Experiment (SAMUM) were explored based on measured size-number distributions and chemical composition. The size-resolved complex refractive index of the dust was derived with real parts of 1.51�1.55 and imaginary parts of 0.0008�0.006 at 550 nm wavelength. At this spectral range a single scattering albedo ��<sub>o</sub> and an asymmetry parameter g of about 0.8 were derived. These values were largely determined by the presence of coarse particles. Backscatter coefficients and lidar ratios calculated with Mie theory (spherical particles) were not found to be in agreement with independently measured lidar data. Obviously the measured Saharan mineral dust particles were of non-spherical shape. With the help of these lidar and sun photometer measurements the particle shape as well as the spherical equivalence were estimated. It turned out that volume equivalent oblate spheroids with an effective axis ratio of 1:1.6 matched these data best. This aspect ratio was also confirmed by independent single particle analyses using a scanning electron microscope. In order to perform the non-spherical computations, a database of single particle optical properties was assembled for oblate and prolate spheroidal particles. These data were also the basis for simulating the non-sphericity effects on the dust optical properties: ��<sub>o</sub> is influenced by up to a magnitude of only 1% and g is diminished by up to 4% assuming volume equivalent oblate spheroids with an axis ratio of 1:1.6 instead of spheres. Changes in the extinction optical depth are within 3.5%. Non-spherical particles affect the downwelling radiative transfer close to the bottom of the atmosphere, however, they significantly enhance the backscattering towards the top of the atmosphere: Compared to Mie theory the particle non-sphericity leads to forced cooling of the Earth-atmosphere system in the solar spectral range for both dust over ocean and desert.

Document Type:Article
Title:Solar radiative effects of a Saharan dust plume observed during SAMUM assuming spheroidal model particles
Authors:
AuthorsInstitution or Email of Authors
Otto, SebastianUNSPECIFIED
Bierwirth, EikeJohannes Gutenberg Univ., Mainz
Weinzierl, BernadettUNSPECIFIED
Kandler, KonradDarmstadt Univ. of Technology, Darmstadt
Esselborn, MichaelUNSPECIFIED
Tesche, MatthiasLeibniz Institute for Tropospheric Research, Leipzig
Schladitz, AlexanderLeibniz Institute for Tropospheric Research, Leipzig
Wendisch, ManfredJohannes Gutenberg Univ., Mainz
Trautmann, ThomasUNSPECIFIED
Date:2009
Journal or Publication Title:Tellus B - Chemical and Physical Meteorology
Refereed publication:Yes
In Open Access:Yes
In SCOPUS:Yes
In ISI Web of Science:Yes
Volume:61B
DOI:10.1111/j.1600-0889.2008.00389.x
Page Range:pp. 270-296
Status:Published
Keywords:Saharan mineral dust, optical properties, spheroidal aerosol particles, scattering database, radiative transfer simulations, spectral radiative flux densities
HGF - Research field:Aeronautics, Space and Transport (old)
HGF - Program:Space (old)
HGF - Program Themes:W EO - Erdbeobachtung
DLR - Research area:Space
DLR - Program:W EO - Erdbeobachtung
DLR - Research theme (Project):W - Vorhaben Entwicklung von Atmosphärenprozessoren (old)
Location: Oberpfaffenhofen
Institutes and Institutions:Institute of Atmospheric Physics > Atmospheric Trace Species
Remote Sensing Technology Institute > Atmospheric Processors
Deposited By: Prof.Dr. Thomas Trautmann
Deposited On:21 Jan 2009
Last Modified:12 Dec 2013 20:32

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