Browsing by Author "Trautmann, Thomas"
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- ItemSolar radiative effects of a Saharan dust plume observed during SAMUM assuming spheroidal model particles(Milton Park : Taylor & Francis, 2017) Otto, Sebastian; Bierwirth, Eike; Weinzierl, Bernadett; Kandler, Konrad; Esselborn, Michael; Tesche, Matthias; Schladitz, Alexander; Wendisch, Manfred; Trautmann, ThomasThe 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 ωo 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: ωo 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.
- ItemThermal IR radiative properties of mixed mineral dust and biomass aerosol during SAMUM-2(Milton Park : Taylor & Francis, 2011) Köhler, Claas H.; Trautmann, Thomas; Lindermeir, Erwin; Vreeling, Willem; Lieke, Kirsten; Kandler, Konrad; Weinzierl, Bernadett; Groß, Silke; Tesche, Matthias; Wendisch, ManfredGround-based high spectral resolution measurements of downwelling radiances from 800 to 1200 cm−1 were conducted between 20 January and 6 February 2008 within the scope of the SAMUM-2 field experiment. We infer the spectral signature of mixed biomass burning/mineral dust aerosols at the surface from these measurements and at top of the atmosphere from IASI observations. In a case study for a day characterized by the presence of high loads of both dust and biomass we attempt a closure with radiative transfer simulations assuming spherical particles. A detailed sensitivity analysis is performed to investigate the effect of uncertainties in the measurements ingested into the simulation on the simulated radiances. Distinct deviations between modelled and observed radiances are limited to a spectral region characterized by resonance bands in the refractive index. A comparison with results obtained during recent laboratory studies and field experiments reveals, that the deviations could be caused by the aerosol particles’ non-sphericity, although an unequivocal discrimination from measurement uncertainties is not possible. Based on radiative transfer simulations we estimate the aerosol’s direct radiative effect in the atmospheric window region to be 8 W m−2 at the surface and 1 W m−2 at top of the atmosphere.