7 results
Search Results
Now showing 1 - 7 of 7
- ItemLarge-scale modeling of absorbing aerosols and their semi-direct effects(Basel : MDPI AG, 2018) Tegen, I.; Heinold, B.Radiative effects of absorbing black carbon and mineral dust aerosols are estimated from global aerosol climate model simulations with fixed sea surface temperatures as a boundary condition. Semi-direct effects are approximated as the residual between the total direct radiative effect and the instantaneous direct radiative effect of the simulated absorbing aerosol species. No distinction is made for aerosols from natural and anthropogenic sources. Results for global average are highly uncertain due to high model variability, but consistent with previous estimates. The global average results for black carbon aerosol semi-direct effects are small due to cancellation of regionally positive or negative effects, and may be positive or negative overall, depending on the model setup. The presence of mineral dust aerosol above dark surfaces and below a layer containing black carbon aerosol may enhance the reflectivity and act to enhance the positive radiative effect of black carbon aerosol. When mineral dust is absent the semi-direct effect at the top-of-atmosphere of black carbon aerosol from both anthropogenic and natural sources is -0.03 Wm-2, while averaging to +0.09 Wm-2 if dust is included.
- ItemInfluence of volcanic eruptions on midlatitude upper tropospheric aerosol and consequences for cirrus clouds(Malden, Mass. : American Geophysical Union, 2015) Friberg, Johan; Martinsson, Bengt G.; Sporre, Moa K.; Andersson, Sandra M.; Brenninkmeijer, Carl A.M.; Hermann, Markus; van Velthoven, Peter F.J.; Zahn, AndreasThe influence of downwelling stratospheric sulfurous aerosol on the UT (upper troposphere) aerosol concentrations and on cirrus clouds is investigated using CARIBIC (Civil Aircraft for Regular Investigation of the Atmosphere Based on an Instrument Container observations) (between 1999–2002 and 2005–2013) and the cirrus reflectance product from Moderate Resolution Imaging Spectroradiometer (MODIS). The initial period, 1999–2002, was volcanically quiescent after which the sulfurous aerosol in the LMS (lowermost stratosphere) (SLMS) became enhanced by several volcanic eruptions starting 2005. From 2005 to 2008 and in 2013, volcanic aerosol from several tropical eruptions increased SLMS. Due to consequent subsidence, the sulfur loading of the upper troposphere (SUT) was increased by a factor of 2.5 compared to background levels. Comparison of SLMS and SUT during the seasons March–July and August–November shows a close coupling of the UT and LMS. Finally, the relationship between SLMS and the cirrus cloud reflectance (CR) retrieved from MODIS spectrometer (on board the satellites Terra and Aqua) is studied. SLMS and CR show a strong anticorrelation, with a factor of 3.5 increase in SLMS and decrease of CR by 8 ± 2% over the period 2001–2011. We propose that the increase of SLMS due to volcanism has caused the coinciding cirrus CR decrease, which would be associated with a negative radiative forcing in the Northern Hemisphere midlatitudes.
- ItemA neutron diffraction study of crystal and low-temperature magnetic structures within the (Na,Li)FeGe2O6 pyroxene-type solid solution series(Berlin ; Heidelberg : Springer, 2017-5-12) Redhammer, Günther J.; Senyshyn, Anatoliy; Lebernegg, Stefan; Tippelt, Gerold; Dachs, Edgar; Roth, GeorgSolid solution compounds along the Li1–x Na x FeGe2O6 clinopyroxene series have been prepared by solid state ceramic sintering and investigated by bulk magnetic and calorimetric methods; the Na-rich samples with x(Na) > 0.7 were also investigated by low-temperature neutron diffraction experiments in a temperature range of 4–20 K. For samples with x(Na) > 0.76 the crystal structure adopts the C2/c symmetry at all measuring temperatures, while the samples display P21/c symmetry for smaller Na contents. Magnetic ordering is observed for all samples below 20 K with a slight decrease of T N with increasing Na content. The magnetic spin structures change distinctly as a function of chemical composition: up to x(Na) = 0.72 the magnetic structure can be described by a commensurate arrangement of magnetic spins with propagation vector k = (½, 0 0), an antiferromagnetic (AFM) coupling within the Fe3+O6 octahedra zig-zag chains and an alternating AFM and ferromagnetic (FM) interaction between the chains, depending on the nature of the tetrahedral GeO4 chains. The magnetic structure can be described in magnetic space group P a21/c. Close to the structural phase transition for sample with x(Na) = 0.75, magnetic ordering is observed below 15 K; however, it becomes incommensurately modulated with k = (0.344, 0, 0.063). At 4 K, the magnetic spin structure best can be described by a cycloidal arrangement within the M1 chains, the spins are within the a–c plane. Around 12 K the cycloidal structure transforms to a spin density wave (SDW) structure. For the C2/c structures, a coexistence of a simple collinear and an incommensurately modulated structure is observed down to lowest temperatures. For 0.78 ≤ x(Na) ≤ 0.82, a collinear magnetic structure with k = (0 1 0), space group P C21/c and an AFM spin structure within the M1 chains and an FM one between the spins is dominating, while the incommensurately modulated structure becomes dominating the collinear one in the samples with x(Na) = 0.88. Here the magnetic propagation vector is k = (0.28, 1, 0.07) and the spin structure corresponds again to a cycloidal structure within the M1 chains. As for the other samples, a transition from the cycloidal to a SDW structure is observed. Based on the neutron diffraction data, the appearance of two peaks in the heat capacity of Na-rich samples can now be interpreted as a transition from a cycloidal magnetic structure to a spin density wave structure of the magnetically ordered phase for the Na-rich part of the solid solution series.
- ItemCombining cloud radar and radar wind profiler for a value added estimate of vertical air motion and particle terminal velocity within clouds(Göttingen : Copernicus GmbH, 2018) Radenz, M.; Bühl, J.; Lehmann, V.; Görsdorf, U.; Leinweber, R.Vertical-stare observations from a 482MHz radar wind profiler and a 35GHz cloud radar are combined on the level of individual Doppler spectra to measure vertical air motions in clear air, clouds and precipitation. For this purpose, a separation algorithm is proposed to remove the influence of falling particles from the wind profiler Doppler spectra and to calculate the terminal fall velocity of hydrometeors. The remaining error of both vertical air motion and terminal fall velocity is estimated to be better than 0.1ms-1 using numerical simulations. This combination of instruments allows direct measurements of in-cloud vertical air velocity and particle terminal fall velocity by means of ground-based remote sensing. The possibility of providing a profile every 10s with a height resolution of < 100m allows further insight into the process scale of in-cloud dynamics. The results of the separation algorithm are illustrated by two case studies, the first covering a deep frontal cloud and the second featuring a shallow mixed-phase cloud.
- ItemTopology Counts: Force Distributions in Circular Spring Networks(College Park, Md. : APS, 2018) Heidemann, Knut M.; Sageman-Furnas, Andrew O.; Sharma, Abhinav; Rehfeldt, Florian; Schmidt, Christoph F.; Wardetzky, MaxFilamentous polymer networks govern the mechanical properties of many biological materials. Force distributions within these networks are typically highly inhomogeneous, and, although the importance of force distributions for structural properties is well recognized, they are far from being understood quantitatively. Using a combination of probabilistic and graph-theoretical techniques, we derive force distributions in a model system consisting of ensembles of random linear spring networks on a circle. We show that characteristic quantities, such as the mean and variance of the force supported by individual springs, can be derived explicitly in terms of only two parameters: (i) average connectivity and (ii) number of nodes. Our analysis shows that a classical mean-field approach fails to capture these characteristic quantities correctly. In contrast, we demonstrate that network topology is a crucial determinant of force distributions in an elastic spring network. Our results for 1D linear spring networks readily generalize to arbitrary dimensions.
- ItemThree-Dimensional Shapes of Spinning Helium Nanodroplets(College Park, Md. : APS, 2018) Langbehn, Bruno; Sander, Katharina; Ovcharenko, Yevheniy; Peltz, Christian; Clark, Andrew; Coreno, Marcello; Cucini, Riccardo; Drabbels, Marcel; Finetti, Paola; Di Fraia, Michele; Giannessi, Luca; Grazioli, Cesare; Iablonskyi, Denys; LaForge, Aaron C.; Nishiyama, Toshiyuki; Oliver Álvarez de Lara, Verónica; Piseri, Paolo; Plekan, Oksana; Ueda, Kiyoshi; Zimmermann, Julian; Prince, Kevin C.; Stienkemeier, Frank; Callegari, Carlo; Fennel, Thomas; Rupp, Daniela; Möller, ThomasA significant fraction of superfluid helium nanodroplets produced in a free-jet expansion has been observed to gain high angular momentum resulting in large centrifugal deformation. We measured single-shot diffraction patterns of individual rotating helium nanodroplets up to large scattering angles using intense extreme ultraviolet light pulses from the FERMI free-electron laser. Distinct asymmetric features in the wide-angle diffraction patterns enable the unique and systematic identification of the three-dimensional droplet shapes. The analysis of a large data set allows us to follow the evolution from axisymmetric oblate to triaxial prolate and two-lobed droplets. We find that the shapes of spinning superfluid helium droplets exhibit the same stages as classical rotating droplets while the previously reported metastable, oblate shapes of quantum droplets are not observed. Our three-dimensional analysis represents a valuable landmark for clarifying the interrelation between morphology and superfluidity on the nanometer scale.
- ItemRobustness of Light-Transport Processes to Bending Deformations in Graded-Index Multimode Waveguides(College Park, Md. : APS, 2018) Boonzajer Flaes, Dirk E.; Stopka, Jan; Turtaev, Sergey; de Boer, Johannes F.; Tyc, Tomáš; Čižmár, TomášLight transport through a multimode optical waveguide undergoes changes when subjected to bending deformations. We show that optical waveguides with a perfectly parabolic refractive index profile are almost immune to bending, conserving the structure of propagation-invariant modes. Moreover, we show that changes to the transmission matrix of parabolic-index fibers due to bending can be expressed with only two free parameters, regardless of how complex a particular deformation is. We provide detailed analysis of experimentally measured transmission matrices of a commercially available graded-index fiber as well as a gradient-index rod lens featuring a very faithful parabolic refractive index profile. Although parabolic-index fibers with a sufficiently precise refractive index profile are not within our reach, we show that imaging performance with standard commercially available graded-index fibers is significantly less influenced by bending deformations than step-index types under the same conditions. Our work thus predicts that the availability of ultraprecise parabolic-index fibers will make endoscopic applications with flexible probes feasible and free from extremely elaborate computational challenges.