Estimation of winter leaf area index and sky view fraction for snow modelling in boreal coniferous forests: consequences on snow mass and energy balance     

Sirpa Rasmus  David Gustafsson  Harri Koivusalo  Ari Laurén  Achim Grelle  Olli‐Kalle Kauppinen  Ola Lagnvall  Anders Lindroth  Kai Rasmus  Magnus Svensson  Per Weslien 《水文研究》2013,27(20):2876-2891

Leaf area index (LAI) and canopy coverage are important parameters when modelling snow process in coniferous forests, controlling interception and transmitting radiation. Estimates of LAI and sky view factor show large variability depending on the estimation method used, and it is not clear how this is reflected in the calculated snow processes beneath the canopy. In this study, the winter LAI and sky view fraction were estimated using different optical and biomass‐based approximations in several boreal coniferous forest stands in Fennoscandia with different stand density, age and site latitude. The biomass‐based estimate of LAI derived from forest inventory data was close to the values derived from the optical measurements at most sites, suggesting that forest inventory data can be used as input to snow hydrological modelling. Heterogeneity of tree species and site fertility, as well as edge effects between different forest compartments, caused differences in the LAI estimates at some sites. A snow energy and mass balance model (SNOWPACK) was applied to detect how the differences in the estimated values of the winter LAI and sky view fraction were reflected in simulated snow processes. In the simulations, an increase in LAI and a decrease in sky view fraction changed the snow surface energy balance by decreasing shortwave radiation input and increasing longwave radiation input. Changes in LAI and sky view fraction affected directly snow accumulation through altered throughfall fraction and indirectly snowmelt through the changed surface energy balance. Changes in LAI and sky view fraction had a greater impact on mean incoming radiation beneath the canopy than on other energy fluxes. Snowmelt was affected more than snow accumulation. The effect of canopy parameters on evaporation loss from intercepted snow was comparable with the effect of variation in governing meteorological variables such as precipitation intensity and air temperature. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

Application of micro-computed X-ray tomography for improving the hole erosion test analysis on high plastic clay     

Zaid  Bahaeldeen  Vollert  Florian  Gibmeier  Jens  Mengel  Lena  Stelzer  Oliver  Schneider  Achim 《Acta Geotechnica》2023,18(2):865-876

Acta Geotechnica - The hole erosion test (HET) was developed to simulate piping erosion and to study the erosion parameters of cohesive soils. The erosion rate in the HET is evaluated by the...  相似文献   

Tephrochronological dating of varved interglacial lake deposits from Piànico‐Sèllere (Southern Alps,Italy) to around 400 ka     

Achim Brauer  Sabine Wulf  Clara Mangili  Andrea Moscariello 《第四纪科学杂志》2007,22(1):85-96

The sediment record from the Piànico palaeolake in the southern Alps is continuously varved, spans more than 15 500 years, and represents a key archive for interglacial climate variability at seasonal resolution. The stratigraphic position of the Piànico Interglacial has been controversial in the past. The identification of two volcanic ash layers and their microscopic analysis provides distinct marker layers for tephrochronological dating of these interglacial deposits. In addition to micro‐facies analyses reconstructing depositional processes of both tephra layers within the lake environment, their mineralogical and geochemical composition has been determined through major‐element electron probe micro‐analysis on glass shards. Comparison with published tephra data traced the volcanic source regions of the Piànico tephras to the Campanian volcanic complex of Roccamonfina (Italy) and probably the Puy de Sancy volcano in the French Massif Central. Available dating of near‐vent deposits from the Roccamonfina volcano provides a robust tephrochronological anchor point at around 400 ka for the Piànico Interglacial. These deposits correlate with marine oxygen isotope stage (MIS) 11 and thus are younger than Early to Middle Pleistocene previously suggested by K/Ar dating and older than the last interglacial as inferred from macrofloral remains and the geological setting. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

Long-Term Stability Of Geoidal Geopotential FromTopex/Poseidon Satellite Altimetry 1993–1999     

Burša  Milan  Kenyon  Steve  Kouba  Jan  Müller  Achim  Raděj  Karel  Vatrt  Viliam  Vojtíšková  Marie  Vítek  Vojtěch 《Earth, Moon, and Planets》1999,84(3):163-176

The TOPEX/POSEIDON (T/P) satellite altimeter data from January 1, 1993to October 24, 1999 (cycles 11–261) was used for investigating thelong-term variations in the geoidal geopotential W₀ and/orin the geopotential scale factor R₀ = GM/W₀ (GM is theadopted geocentric gravitational constant). The mean valuesdetermined for the whole period covered are: W₀ =(62 636 856.161 ± 0.002) m² s^-2, R₀ =(6 363 672.5448 ± 0.0002) m. The actual accuracy is limited bythe altimeter calibration error (2–3 cm) and it isestimated to be about ± 0.5 m² s^-2 (± 5 cm).The yearly variations of the above mean values are at the formalerror level. No long-term trend in W₀, representing the oceanvolume change, was found for the seven years period 1993–9 on thebasis of T/P altimeter (AVISO) data. No sea surface topography modelwas used in the solution. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

The time dependent magnetization of fine-grained iron in lunar breccias     

Wulf Achim Gose  J.G. Carnes 《Earth and Planetary Science Letters》1973,20(1):100-106

Lunar breccias of low metamorphic grade offer a unique opportunity to investigate the magnetic properties of dispersed fine grained iron. These rocks exhibit a pronounced time dependent magnetization whose acquisition and decay are well explained by Ne´el's single domain theory. The effect is due to iron grains in the range of 120A?to 150A?in diameter which covers the transition from superparamagnetic to stable single domain behavior.  相似文献   

On the momentum forcing of a large-scale sea-ice model     

Achim Stössel  Martin Claussen 《Climate Dynamics》1993,9(2):71-80

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Dendrochronology for the last 1400 years in eastern Tibet   总被引：11，自引：0，他引：11  

Achim Bräuning 《GeoJournal》1994,34(1):75-95

Twentyfour tree-ring chronologies of various tree-species(Picea, Abies, Larix, Juniperus) from sites near the upper timberline in eastern Tibet were established, the longest one dating back to 620 A.D. A new classification of pointer intervals is presented, distinguishing between different kinds of signatures in a tree-ring series and amount of growth change in relation to the previous year. The various types of signatures can be related to meteorological events like cold winters and summers, summer drought or to recreation reactions after years of reduced growth. The distribution pattern and type of pointer intervals combined with ring width variation enables the detection of periods of favourable or unfavourable growth conditions with regard to climate history. Based on far-distance correlations and signature analysis regions of similar tree-growth are mapped out.  相似文献   

Stabilitätsbeziehungen zwischen Chlorit,Cordierit und Almandin bei der Metamorphose     

Dr. Achim Hirschberg  Helmut G. F. Winkler 《Contributions to Mineralogy and Petrology》1968,18(1):17-42

The stability relations between cordierite and almandite in rocks, having a composition of CaO poor argillaceous rocks, were experimentally investigated. The starting material consisted of a mixture of chlorite, muscovite, and quartz. Systems with widely varying Fe²⁺/Fe²⁺+Mg ratios were investigated by using two different chlorites, thuringite or ripidolite, in the starting mixture. Cordierite is formed according to the following reaction: $${\text{Chlorite + muscovite + quartz}} \rightleftharpoons {\text{cordierite + biotite + Al}}_{\text{2}} {\text{SiO}}_{\text{5}} + {\text{H}}_{\text{2}} {\text{O}}$$ . At low pressures this reaction characterizes the facies boundary between the albite-epidotehornfels facies and the hornblende-hornfels facies, at medium pressures the beginning of the cordierite-amphibolite facies. Experiments were carried out reversibly and gave the following equilibrium data: 505±10°C at 500 bars H₂O pressure, 513±10°C at 1000 bars H₂O pressure, 527±10°C at 2000 bars H₂O pressure, and 557±10°C at 4000 bars H₂O pressure. These equilibrium data are valid for the Fe-rich starting material, using thuringite as the chlorite, as well as for the Mg-rich starting mixture with ripidolite. At 6000 bars the equilibrium temperature for the Mg-rich mixture is 587±10°C. In the Fe-rich mixture almandite was formed instead of cordierite at 6000 bars. The following reaction was observed: $${\text{Thuringite + muscovite + quartz}} \rightleftharpoons {\text{almandite + biotite + Al}}_{\text{2}} {\text{SiO}}_{\text{5}} {\text{ + H}}_{\text{2}} {\text{O}}$$ . Experiments with the Fe-rich mixture, containing Fe²⁺/Fe²⁺+Mg in the ratio 8∶10, yielded three stability fields in a P,T-diagram (Fig.1):

1. Above 600°C/5.25 kb and 700°C/6.5 kb almandite+biotite+Al₂SiO₅ coexist stably, cordierite being unstable.
2. The field, in which almandite, biotite and Al₂SiO₅ are stable together with cordierite, is restricted by two curves, passing through the following points:
   1. 625°C/5.5 kb and 700°C/6.5 kb,
   2. 625°C/5.5 kb and 700°C/4.0 kb.
3. At conditions below curves 1 and 2b, cordierite, biotite, and Al₂SiO₅ are formed, but no garnet.

An appreciable MnO-content in the system lowers the pressures needed for the formation of almandite garnet, but the quantitative influence of the spessartite-component on the formation of almandite could not yet be determined. the Mg-rich system with Fe²⁺/Fe²⁺+Mg=0.4 garnet did not form at pressures up to 7 kb in the temperature range investigated. Experiments at unspecified higher pressures (in a simple squeezer-type apparatus) yielded the reaction: $${\text{Ripidolite + muscovite + quartz}} \rightleftharpoons {\text{almandite + biotite + Al}}_{\text{2}} {\text{SiO}}_{\text{5}} {\text{ + H}}_{\text{2}} {\text{O}}$$ . Further experiments are needed to determine the equilibrium data. The occurence of garnet in metamorphic rocks is discussed in the light of the experimental results.  相似文献   

Parametric versus non-parametric modelling? Statistical evidence based on P-value curves     

Nicolai Bissantz  Axel Munk  Achim Scholz 《Monthly notices of the Royal Astronomical Society》2003,340(4):1190-1198

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Reply: Tephrochronological dating of varved interglacial lake deposits from Piànico‐Sèllere (Southern Alps,Italy) to around 400 ka     

Achim Brauer  Sabine Wulf  Clara Mangili  Oona Appelt  Andrea Moscariello 《第四纪科学杂志》2007,22(4):415-418

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