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31.
Karl-Heinz Böhm 《Astrophysics and Space Science》1968,2(3):375-383
The outer convection zone of the low-temperature white dwarf Van Maanen 2 has been studied for two different atmospheric models given byWeidemann (1960). A slight modification of the standard mixing length theory and the abundances derived by Weidemann have been used.The thickness of the convection zone is about 8 km for the atmospheric model withT
eff=5780 K,g=108 cm sec–2 and about 23 km forT
eff=5040,g=3.16×107K. In both cases the temperature at the lower boundary of the convection zone is about 9.8×105K. It is shown that this temperature corresponds approximately to the transition temperatureT
tr to the (almost) isothermal core of the white dwarf. This value is considerably lower than the values ofT
tr discussed in the literature until now.The outer convection zone consists of an upper completely non-degenerate part and a lower part with moderate degeneracy. In this lower part the degree of degeneracy is practically independent of depth. 相似文献
32.
Tilo Usbeck Peter Waldner Matthias Dobbertin Christian Ginzler Christian Hoffmann Flurin Sutter Charlotte Steinmeier Richard Volz Gustav Schneiter Martine Rebetez 《Theoretical and Applied Climatology》2012,108(3-4):451-462
This study compares the surface wind speed and forest damage data of two exceptionally severe winter storms, Vivian 1990 and Lothar 1999. The study area comprises the region that suffered damage in Switzerland. The wind speed data were derived from simulations of MeteoSwiss (Federal Office of Meteorology and Climatology), measurements during the storm periods and expert analyses of the data. The remotely sensed forest damage data were provided by the Federal Office for the Environment and the forest cover data by Swiss Federal Statistical Office. We compared data on the peak gust and maximum average wind speed, with data on the spatially related forest area and forest damage area, and found some clear differences in the correlations between the different wind data and forest damage. Our results point generally to the damage-causing role of near-surface gusts at maximum wind speeds during the storm. These tended to be spatially distributed on a fine scale. In only a few cases were the results statistically significant. However, these results could probably be improved with better wind data. For example, gust measurements spatially closer to forests or simulations of gusts at maximum wind speed could be produced with a spatially higher resolution. 相似文献
33.
Conventional bias correction of simulated climate time series for impact models is done separately for climate variables and hence leads to inconsistencies between them. However, agricultural models mostly use several variables, and meteorological consistency is essential. The present work points out meteorological inconsistency due to quantile mapping and describes a new method of consistent bias correction by an optimization approach. Time series of hourly precipitation and global radiation from the regional model REMO5.7 (Run UBA C20/A1B_1) were corrected with site observations from the German Meteorological Service. The results urge to check conventionally corrected series for consistency before using them for multidimensional models. Here, quantile mapping resulted in underestimation of diffuse radiation at hours with precipitation. This deficit was minimized by the developed procedure. 相似文献
34.
Jürgen Oberst Valéry Lainey Christophe Le Poncin-Lafitte Veronique Dehant Pascal Rosenblatt Stephan Ulamec Jens Biele J?rn Spurmann Ralph Kahle Volker Klein Ulrich Schreiber Anja Schlicht Nicolas Rambaux Philippe Laurent Beno?t Noyelles Bernard Foulon Alexander Zakharov Leonid Gurvits Denis Uchaev Scott Murchie Cheryl Reed Slava G. Turyshev Jesus Gil Mariella Graziano Konrad Willner Kai Wickhusen Andreas Pasewaldt Marita W?hlisch Harald Hoffmann 《Experimental Astronomy》2012,34(2):243-271
GETEMME (Gravity, Einstein??s Theory, and Exploration of the Martian Moons?? Environment), a mission which is being proposed in ESA??s Cosmic Vision program, shall be launched for Mars on a Soyuz Fregat in 2020. The spacecraft will initially rendezvous with Phobos and Deimos in order to carry out a comprehensive mapping and characterization of the two satellites and to deploy passive Laser retro-reflectors on their surfaces. In the second stage of the mission, the spacecraft will be transferred into a lower 1500-km Mars orbit, to carry out routine Laser range measurements to the reflectors on Phobos and Deimos. Also, asynchronous two-way Laser ranging measurements between the spacecraft and stations of the ILRS (International Laser Ranging Service) on Earth are foreseen. An onboard accelerometer will ensure a high accuracy for the spacecraft orbit determination. The inversion of all range and accelerometer data will allow us to determine or improve dramatically on a host of dynamic parameters of the Martian satellite system. From the complex motion and rotation of Phobos and Deimos we will obtain clues on internal structures and the origins of the satellites. Also, crucial data on the time-varying gravity field of Mars related to climate variation and internal structure will be obtained. Ranging measurements will also be essential to improve on several parameters in fundamental physics, such as the Post-Newtonian parameter ?? as well as time-rate changes of the gravitational constant and the Lense-Thirring effect. Measurements by GETEMME will firmly embed Mars and its satellites into the Solar System reference frame. 相似文献
35.
Emissions of Volatile Organic Compounds from Sunflower and Beech: Dependence on Temperature and Light Intensity 总被引:11,自引:0,他引:11
G. Schuh A. C. Heiden Th. Hoffmann J. Kahl P. Rockel J. Rudolph J. Wildt 《Journal of Atmospheric Chemistry》1997,27(3):291-318
Emissions of volatile organic compounds (VOCs) from sunflower (Helianthus annuus L. cv. giganteus) were measured in a continuously stirred tank reactor. The compounds predominantly emitted from sunflower were: isoprene, the monoterpenes -pinene, -pinene, sabinene, 3-carene and limonene, an oxygenated terpene, not positively identified so far and the sesquiterpene -caryophyllene. Emission rates ranged from 0.8 x 10–16 to 4.3 x 10 –15 mol cm–2 s–1 at a temperature of 25°C and at a light intensity of 820 µEm–2 s–1. A dependence of the emission rates on temperature as well as on light intensity was observed. The emission rates of -pinene, sabinene and thujene from beech (Fagus sylvatica L.) were also affected by temperature as well as by light intensity. Our results suggest that an emission algorithm for all compounds emitted from sunflower and beech has to consider temperature and light intensity simultaneously. The observations strongly indicate that the emissions of VOCs from sunflower and beech are in part closely coupled to the rate of biosynthesis and in part originate from diffusion out of pools. The emission rates can be described by an algorithm that combines the model given by Tingey and coworkers with the algorithm given by Guenther and coworkers after slight modification. 相似文献
36.
The application of satellite differential SAR interferometry-derived ground displacements in hydrogeology 总被引:7,自引:1,他引:6
The application of satellite differential synthetic aperture radar (SAR) interferometry, principally coherent (InSAR) and to a lesser extent, persistent-scatterer (PSI) techniques to hydrogeologic studies has improved capabilities to map, monitor, analyze, and simulate groundwater flow, aquifer-system compaction and land subsidence. A number of investigations over the previous decade show how the spatially detailed images of ground displacements measured with InSAR have advanced hydrogeologic understanding, especially when a time series of images is used in conjunction with histories of changes in water levels and management practices. Important advances include: (1) identifying structural or lithostratigraphic boundaries (e.g. faults or transitional facies) of groundwater flow and deformation; (2) defining the material and hydraulic heterogeneity of deforming aquifer-systems; (3) estimating system properties (e.g. storage coefficients and hydraulic conductivities); and (4) constraining numerical models of groundwater flow, aquifer-system compaction, and land subsidence. As a component of an integrated approach to hydrogeologic monitoring and characterization of unconsolidated alluvial groundwater basins differential SAR interferometry contributes unique information that can facilitate improved management of groundwater resources. Future satellite SAR missions specifically designed for differential interferometry will enhance these contributions. 相似文献
37.
GIS互操作的实现途径评述 总被引:19,自引:1,他引:18
文中主要探讨了开放式GIS的规程和关键技术,促进数据、系统和机构的互操作,总结了世界上著名公司最近GIS产品实现互操作的技术途径,给出了国际上有关GIS互操作、开放式GIS研究的最新进展以及近期发展方向。 相似文献
38.
The significance of organic matter origin for carbon oxidation via sulfate and iron reduction in the sediments of three acid mine lakes is analyzed. Carbon reactivity was estimated by fitting first‐order expressions to measured rates. Carbon oxidation rates via sulfate and ferric iron reduction ranged from 3.4 to 4.7 mmol m–2 d–1 and resembled those reported for freshwater lakes. The estimated reaction constants increased from about 10–3 a–1 at the interface to the former mine grounds to 0.05 to 0.2 a–1 at the current sediment‐water interface. Aquatic organic matter accounted for an estimated 45...75% of total carbon oxidation rates while it amounted only to about 5...14% of the total organic matter that had been deposited. The results of this study suggest that in highly acidic mine lakes the reactivity of the deposited organic matter can rapidly increase after flooding, enhancing carbon oxidation and internal neutralization rates in the sediments. 相似文献
39.
40.
The heavily eutrophicated Limfjorden (Denmark) provides a good illustration of the value of long-term monitoring, especially
if this is combined with an experimental, interdisciplinary research approach. Here, we first give a short overview of the
environmental status of Limfjorden, including the historical development of nutrient overloading and subsequent oxygen depletion
in near-bottom water, and how the annual landings of edible bottom-dwelling fish species (plaice, flounder, eel and others)
caught in Limfjorden have decreased from about 2,500 t in the early 1920s to only about 20 t in recent years where the fish
have been replaced by an increasing number of especially the moon jellyfish, Aurelia aurita, which mainly preys on zooplankton. Next, we evaluate the ecological consequences of the present high number of jellyfish,
based on data from recent years’ research on the abundance of jellyfish, their population dynamics and predation impact. In
Limfjorden, the benthic polyp stage of A. aurita ensures a large number of small ephyrae in the early spring and subsequently a large population of adult medusae that control
the zooplankton during summer and autumn. The holopelagic invasive ctenophore Mnemiopsis leidyi, which was observed in Limfjorden for the first time in 2007, is a second carnivore adding additional predation pressure
of the indigenous A. aurita so that copepods and other mesozooplankton organisms may be virtually absent, as observed in 2008 and 2009 where ciliates
made up a substantial part of the zooplankton biomass. Marine environmental management programmes should be aware of the increasing
importance of both indigenous and new invasive jellyfish species that may show mass occurrence in especially eutrophicated
and over-fished areas. 相似文献