Radar observations of snow formation in a warm pre‐frontal snowband     

Gerhard W. Reuter  Raymond Beaubien 《大气与海洋》2013,51(4):605-626

Abstract

Radar reflectivity measurements and sounding data were analyzed to investigate snowfall production in a long‐lasting snowband that formed in advance of a warm surface front moving across Alberta. The sounding data indicated that the band could have been forced by slantwise overturning during the release of moist symmetric instability combined with frontogenesis. The stability analysis presented here is novel in that it includes ice phase thermodynamics, neglected in previous studies of slantwise convection.

Radar reflectivity fields were analyzed to determine the total snow content and the mass outflow rate as factors of time. The peak value of total snow content was 17 kilotons per km of snowband, and the peak mass outflow rate was 10 tons s^‐1 km^‐1. The snowfall rate averaged across the cloud base was about 0.8 cm h^‐1, and the average snow content remained close to 0.2 g m^‐1. The characteristic time (defined as the ratio of total snow content over mass outflow rate) was about 30 minutes, which is approximately the time needed for the growth of snowflakes by aggregation in the observed temperature range. The precipitation efficiency of the snowband, defined as the ratio of snow mass outflow to water vapour inflow was estimated to be 14%. The precipitation production values observed in the Alberta snowband are compared with previous estimates reported for frontal rainbands and Alberta thunderstorms.  相似文献   

The significance of chemical,isotopic, and detrital components in three coeval stalagmites from the superhumid southernmost Andes (53°S) as high-resolution palaeo-climate proxies     

Daniel Schimpf  Rolf Kilian  Andreas Kronz  Klaus Simon  Christoph Spötl  Gerhard Wörner  Michael Deininger  Augusto Mangini 《Quaternary Science Reviews》2011,30(3-4):443-459

Stalagmites are important palaeo-climatic archives since their chemical and isotopic signatures have the potential to record high-resolution changes in temperature and precipitation over thousands of years. We present three U/Th-dated records of stalagmites (MA1–MA3) in the superhumid southern Andes, Chile (53°S). They grew simultaneously during the last five thousand years (ka BP) in a cave that developed in schist and granodiorite. Major and trace elements as well as the C and O isotope compositions of the stalagmites were analysed at high spatial and temporal resolution as proxies for palaeo-temperature and palaeo-precipitation. Calibrations are based on data from five years of monitoring the climate and hydrology inside and outside the cave and on data from 100 years of regional weather station records.Water-insoluble elements such as Y and HREE in the stalagmites indicate the amount of incorporated siliciclastic detritus. Monitoring shows that the quantity of detritus is controlled by the drip water rate once a threshold level has been exceeded. In general, drip rate variations of the stalagmites depend on the amount of rainfall. However, different drip-water pathways above each drip location gave rise to individual drip rate levels. Only one of the three stalagmites (MA1) had sufficiently high drip rates to record detrital proxies over its complete length. Carbonate-compatible element contents (e.g. U, Sr, Mg), which were measured up to sub-annual resolution, document changes in meteoric precipitation and related drip-water dilution. In addition, these soluble elements are controlled by leaching during weathering of the host rock and soils depending on the pH of acidic pore waters in the peaty soils of the cave’s catchment area. In general, higher rainfall resulted in a lower concentration of these elements and vice versa. The Mg/Ca record of stalagmite MA1 was calibrated against meteoric precipitation records for the last 100 years from two regional weather stations. Carbonate-compatible soluble elements show similar patterns in the three stalagmites with generally high values when drip rates and detrital tracers were low and vice versa. δ¹³C and δ¹⁸O values are highly correlated in each stalagmite suggesting a predominantly drip rate dependent kinetic control by evaporation and/or outgassing. Only C and O isotopes from stalagmite MA1 that received the highest drip rates show a good correlation between detrital proxy elements and carbonate-compatible elements. A temperature-related change in rainwater isotope values modified the MA1 record during the Little Ice Age (～0.7–0.1 ka BP) that was ～1.5 °C colder than today. The isotopic composition of the stalagmites MA2 and MA3 that formed at lower drip rates shows a poor correlation with stalagmite MA1 and all other chemical proxies of MA1. ‘Hendy tests’ indicate that the degassing-controlled isotope fractionation of MA2 and MA3 had already started at the cave roof, especially when drip rates were low. Changing pathways and residence times of the seepage water caused a non-climatically controlled isotope fractionation, which may be generally important in ventilated caves during phases of low drip rates. Our proxies indicate that the Neoglacial cold phases from ～3.5 to 2.5 and from ～0.7 to 0.1 ka BP were characterised by 30% lower precipitation compared with the Medieval Warm Period from 1.2 to 0.8 ka BP, which was extremely humid in this region.  相似文献   

The importance of mergers for the origin of intracluster stars in cosmological simulations of galaxy clusters     

Giuseppe Murante  Martina Giovalli  Ortwin Gerhard  Magda Arnaboldi  Stefano Borgani  Klaus Dolag 《Monthly notices of the Royal Astronomical Society》2007,377(1):2-16

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Surface roughness and geological mapping at subhectometer scale from the High Resolution Stereo Camera onboard Mars Express   总被引：2，自引：0，他引：2  

Aurélien Cord  David Baratoux  Patrick Martin  Ronald Greeley  Philippe Masson  Gerhard Neukum 《Icarus》2007,191(1):38-51

The quantitative measurement of surface roughness of planetary surfaces at all scales provides insights into geological processes. A characterization of roughness variations at the scale of a few tens of meters is proposed that complements the analysis of local topographic data of the martian surface at kilometer scale, as achieved from the Mars Orbiter Laser Altimeter (MOLA) data, and at the subcentimeter scale using photometric properties derived from multi-angular observations. Relying on a Gabor filtering process, an algorithm developed in the context of image classification for the purpose of texture analysis has been adapted to handle data from the High Resolution Stereo Camera (HRSC). The derivation of roughness within a wavelength range of tens of meters, combined with analyses at even longer wavelengths, gives an original view of the martian surface. The potential of this approach is evaluated for different examples for which the geological processes are identified and the geological units are mapped and characterized in terms of roughness.  相似文献   

Satellite laser ranging to GPS and GLONASS   总被引：5，自引：4，他引：1  

Krzysztof Sośnica  Daniela Thaller  Rolf Dach  Peter Steigenberger  Gerhard Beutler  Daniel Arnold  Adrian Jäggi 《Journal of Geodesy》2015,89(7):725-743

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The Planetary Nebula Spectrograph elliptical galaxy survey: the dark matter in NGC 4494     

N. R. Napolitano †  A. J. Romanowsky  L. Coccato  M. Capaccioli †  N. G. Douglas  E. Noordermeer  O. Gerhard  M. Arnaboldi  F. De Lorenzi  K. Kuijken  M. R. Merrifield  E. O'Sullivan  A. Cortesi  P. Das  K. C. Freeman 《Monthly notices of the Royal Astronomical Society》2009,393(2):329-353

We present new Planetary Nebula Spectrograph observations of the ordinary elliptical galaxy NGC 4494, resulting in positions and velocities of 255 planetary nebulae out to seven effective radii (25 kpc). We also present new wide-field surface photometry from MMT/Megacam, and long-slit stellar kinematics from VLT/FORS2. The spatial and kinematical distributions of the planetary nebulae agree with the field stars in the region of overlap. The mean rotation is relatively low, with a possible kinematic axis twist outside  1 R _e  . The velocity dispersion profile declines with radius, though not very steeply, down to  ∼70 km s⁻¹  at the last data point.
We have constructed spherical dynamical models of the system, including Jeans analyses with multi-component Λ cold dark matter (CDM) motivated galaxies as well as logarithmic potentials. These models include special attention to orbital anisotropy, which we constrain using fourth-order velocity moments. Given several different sets of modelling methods and assumptions, we find consistent results for the mass profile within the radial range constrained by the data. Some dark matter (DM) is required by the data; our best-fitting solution has a radially anisotropic stellar halo, a plausible stellar mass-to-light ratio and a DM halo with an unexpectedly low central density. We find that this result does not substantially change with a flattened axisymmetric model.
Taken together with other results for galaxy halo masses, we find suggestions for a puzzling pattern wherein most intermediate-luminosity galaxies have very low concentration haloes, while some high-mass ellipticals have very high concentrations. We discuss some possible implications of these results for DM and galaxy formation.  相似文献   

Effects of seed particle size and composition on heterogeneous nucleation of n-nonane     

Paul M. Winkler  Anca Hienola  Gerhard Steiner  Gerard Hill  Aron Vrtala  Georg P. Reischl  Markku Kulmala  Paul E. Wagner 《Atmospheric Research》2008,90(2-4):187-ICNAA07

Heterogeneous nucleation of supersaturated n-nonane vapour on seed particles of different size and composition has been investigated using a fast expansion chamber. Monodisperse seed particle sizes were ranging from about 4 nm up to about 24 nm in diameter. By using different types of particle generators WO_x, Ag and (NH₄)₂SO₄ particles were generated. For direct comparison between different particle compositions overlapping sizes have been generated for WO_x and Ag at about 7 nm particle diameter as well as for Ag and (NH₄)₂SO₄ at about 15 nm. Nucleation temperature was kept constant at about 278 K. Experimental data were compared to Kelvin equation and Fletcher theory including the effect of line tension. It was found that heterogeneous nucleation of n-nonane seems to be independent of seed particle composition and starts well below the Kelvin curve. Good agreement was achieved with Fletcher theory including the effect of line tension.  相似文献   

Experimental and computational study of trace element distribution between orthopyroxene and anhydrous silicate melt: substitution mechanisms and the effect of iron     

Mirjam van Kan Parker  Axel Liebscher  Dirk Frei  Jelle van Sijl  Wim van Westrenen  Jon Blundy  Gerhard Franz 《Contributions to Mineralogy and Petrology》2010,159(4):459-473

Although orthopyroxene (Opx) is present during a wide range of magmatic differentiation processes in the terrestrial and lunar mantle, its effect on melt trace element contents is not well quantified. We present results of a combined experimental and computational study of trace element partitioning between Opx and anhydrous silicate melts. Experiments were performed in air at atmospheric pressure and temperatures ranging from 1,326 to 1,420°C in the system CaO–MgO–Al₂O₃–SiO₂ and subsystem CaO–MgO–SiO₂. We provide experimental partition coefficients for a wide range of trace elements (large ion lithophile: Li, Be, B, K, Rb, Sr, Cs, Ba, Th, U; rare earth elements, REE: La, Ce, Nd, Sm, Y, Yb, Lu; high field strength: Zr, Nb, Hf, Ta, Ti; transition metals: Sc, V, Cr, Co) for use in petrogenetic modelling. REE partition coefficients increase from $ D_{\text{La}}^{{\text{Opx}} {\hbox{-}} {\text{melt}}} \sim 0.0005 Although orthopyroxene (Opx) is present during a wide range of magmatic differentiation processes in the terrestrial and lunar mantle, its effect on melt trace element contents is not well quantified. We present results of a combined experimental and computational study of trace element partitioning between Opx and anhydrous silicate melts. Experiments were performed in air at atmospheric pressure and temperatures ranging from 1,326 to 1,420°C in the system CaO–MgO–Al₂O₃–SiO₂ and subsystem CaO–MgO–SiO₂. We provide experimental partition coefficients for a wide range of trace elements (large ion lithophile: Li, Be, B, K, Rb, Sr, Cs, Ba, Th, U; rare earth elements, REE: La, Ce, Nd, Sm, Y, Yb, Lu; high field strength: Zr, Nb, Hf, Ta, Ti; transition metals: Sc, V, Cr, Co) for use in petrogenetic modelling. REE partition coefficients increase from $ D_{\text{La}}^{{\text{Opx}} {\hbox{-}} {\text{melt}}} \sim 0.0005 $ D_{\text{La}}^{{\text{Opx}} {\hbox{-}} {\text{melt}}} \sim 0.0005 to $ D_{\text{Lu}}^{{{\text{Opx}} {\hbox{-}} {\text{melt}}}} \sim 0.109 $ D_{\text{Lu}}^{{{\text{Opx}} {\hbox{-}} {\text{melt}}}} \sim 0.109 , D values for highly charged elements vary from $ D_{\text{Th}}^{{{\text{Opx}} {\hbox{-}} {\text{melt}}}} \sim 0.0026 $ D_{\text{Th}}^{{{\text{Opx}} {\hbox{-}} {\text{melt}}}} \sim 0.0026 through $ D_{\text{Nb}}^{{{\text{Opx}} {\hbox{-}} {\text{melt}}}} \sim 0.0033 $ D_{\text{Nb}}^{{{\text{Opx}} {\hbox{-}} {\text{melt}}}} \sim 0.0033 and $ D_{\text{U}}^{{{\text{Opx}} {\hbox{-}} {\text{melt}}}} \sim 0.0066 $ D_{\text{U}}^{{{\text{Opx}} {\hbox{-}} {\text{melt}}}} \sim 0.0066 to $ D_{\text{Ti}}^{{\text{Opx}} {\hbox{-}} {\text{melt}}} \sim 0.058 $ D_{\text{Ti}}^{{\text{Opx}} {\hbox{-}} {\text{melt}}} \sim 0.058 , and are all virtually independent of temperature. Cr and Co are the only compatible trace elements at the studied conditions. To elucidate charge-balancing mechanisms for incorporation of REE into Opx and to assess the possible influence of Fe on Opx-melt partitioning, we compare our experimental results with computer simulations. In these simulations, we examine major and minor trace element incorporation into the end-members enstatite (Mg₂Si₂O₆) and ferrosilite (Fe₂Si₂O₆). Calculated solution energies show that R²⁺ cations are more soluble in Opx than R³⁺ cations of similar size, consistent with experimental partitioning data. In addition, simulations show charge balancing of R³⁺ cations by coupled substitution with Li⁺ on the M1 site that is energetically favoured over coupled substitution involving Al–Si exchange on the tetrahedrally coordinated site. We derived best-fit values for ideal ionic radii r ₀, maximum partition coefficients D ₀, and apparent Young’s moduli E for substitutions onto the Opx M1 and M2 sites. Experimental r ₀ values for R³⁺ substitutions are 0.66–0.67 ? for M1 and 0.82–0.87 ? for M2. Simulations for enstatite result in r ₀ = 0.71–0.73 ? for M1 and ~0.79–0.87 ? for M2. Ferrosilite r ₀ values are systematically larger by ~0.05 ? for both M1 and M2. The latter is opposite to experimental literature data, which appear to show a slight decrease in $ r_{0}^{{{\text{M}}2}} $ r_{0}^{{{\text{M}}2}} in the presence of Fe. Additional systematic studies in Fe-bearing systems are required to resolve this inconsistency and to develop predictive Opx-melt partitioning models for use in terrestrial and lunar magmatic differentiation models.  相似文献   

Mafic Late Miocene–Quaternary volcanic rocks in the Kamchatka back arc region: implications for subduction geometry and slab history at the Pacific–Aleutian junction     

Anna O. Volynets  Tatiana G. Churikova  Gerhard Wörner  Boris N. Gordeychik  Paul Layer 《Contributions to Mineralogy and Petrology》2010,159(5):659-687

New ⁴⁰Ar/³⁹Ar and published ¹⁴C ages constrain voluminous mafic volcanism of the Kamchatka back-arc to Miocene (3–6 Ma) and Late Pleistocene to Holocene (<1 Ma) times. Trace elements and isotopic compositions show that older rocks derived from a depleted mantle through subduction fluid-flux melting (>20%). Younger rocks form in a back arc by lower melting degrees involving enriched mantle components. The arc front and Central Kamchatka Depression are also underlain by plateau lavas and shield volcanoes of Late Pleistocene age. The focus of these voluminous eruptions thus migrated in time and may be the result of a high fluid flux in a setting where the Emperor seamount subducts and the slab steepens during rollback during terrain accretions. The northern termination of Holocene volcanism locates the edge of the subducting Pacific plate below Kamchatka, a “slab-edge-effect” is not observed in the back arc region.  相似文献   

Experiments on the photophoretic motion of chondrules and dust aggregates—Indications for the transport of matter in protoplanetary disks     

Gerhard Wurm  Jens Teiser  Henning Haack 《Icarus》2010,208(1):482-491

In a set of 16 drop tower experiments the motion of sub-millimeter to millimeter-sized particles under microgravity was observed. Illumination by a halogen lamp induced acceleration of the particles due to photophoresis. Photophoresis on dust-free chondrules, on chondrules, glass spheres and metal spheres covered with SiC dust and on pure SiC dust aggregates was studied. This is the first time that photophoretic motion of millimeter-sized particles has been studied experimentally. The absolute values for the photophoretic force are consistent with theoretical expectations for spherical particles. The strength of the photophoretic force varies for chondrules, dust covered particles and pure dust from low to strong, respectively. The measurements support the idea that photophoresis in the early Solar System can be efficient to transport solid particles outward.  相似文献