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131.
Lukas Siebicke Martina Hunner Thomas Foken 《Theoretical and Applied Climatology》2012,109(1-2):109-131
Observations of vegetation–atmosphere exchange of carbon dioxide (CO2) by the eddy covariance (EC) technique are limited by difficult conditions such as nighttime and heterogeneous terrain. Thus, advective flux components are included into the net ecosystem exchange (NEE) budget. However, advection measurements are experimentally challenging and do not always help to solve the night flux problem of the EC technique. This study investigates alternative methods for the observation of horizontal advection, in particular horizontal concentration gradients, as well as different approaches to coordinate rotation and vertical advection. Continuous high-frequency measurements of the horizontal CO2 concentration field are employed and compared to the often used discontinuous sequential sampling. Significant differences were found in the case of 30-min mean concentration values between the conventional discontinuous sampling approach and the complete observation of the time series by continuous sampling. Estimates of vertical advection rely on accurate estimates of vertical wind velocity ( $\emph{w}$ ). Therefore, different approaches to the planar fit coordinate rotation have been investigated. Sector-wise rotation was able to eliminate directional dependencies of mean $\emph{w}$ . Furthermore, the effect of the data set length used for rotation (window length) was investigated and was found to have significant impact on estimates of vertical advection, with larger window lengths yielding about 50% larger vertical advection. A sequential planar fit with controlled window length is proposed to give reproducible results. The different approaches to the measurement and calculation of horizontal and vertical advection presented are applied to data obtained during the exchange processes in mountainous region experiment at the FLUXNET site Waldstein–Weidenbrunnen (DE-Bay). Estimates of NEE including advection are compared to NEE from turbulent and storage flux alone without advection. NEE including vertical advection with sector-wise planar fit rotation and controlled window length and including horizontal advection from continuous gradient measurements, which were comprehensively bias corrected by a new approach, did compare well with the expected night flux error, with meteorological drivers of the fluxes and with soil chamber measurements. Unrealistically large and noisy values of horizontal advection from the conventional discontinuous sampling approach, which lead to unrealistic values of NEE, could be eliminated by the alternative approaches presented. We therefore suggest the further testing of those approaches at other sites in order to improve the accuracy of advection measurements and, subsequently, estimates of NEE. 相似文献
132.
The lower limit on the drag coefficient under hurricane force winds is determined by the break-up of the air–sea interface
due to Kelvin–Helmholtz instability and formation of the two-phase transition layer consisting of sea spray and air bubbles.
As a consequence, a regime of marginal stability develops. In this regime, the air–sea drag coefficient is determined by the
turbulence characteristics of the two-phase transition layer. The upper limit on the drag coefficient is determined by the
Charnock-type wave resistance. Most of the observational estimates of the drag coefficient obtained in hurricane conditions
and in laboratory experiments appear to lie between the two extreme regimes: wave resistance and marginal stability. 相似文献
133.
Cyril Durand Didier Marquer Lukas Baumgartner Philippe Goncalves Philippe Boulvais Michel Rossy 《Contributions to Mineralogy and Petrology》2009,157(6):749-763
Chemical mass transfer was quantified in a metacarbonate xenolith enclosed within the granodiorite of the Quérigut massif
(Pyrenees, France). Mass balance calculations suggest a strong decrease of CaO, SrO and CO2 contents (up to −90%), correlated with a decrease of modal calcite content as the contact is approached. Most other chemical
elements behave immobile during metasomatism. They are therefore passively enriched. Only a small increase of SiO2, Al2O3 and Fe2O3 contents occurs in the immediate vicinity of the contact. Hence, in this study, skarn formation is characterized by the lack
of large chemical element influx from the granitoid protolith. A large decrease of the initial carbonate volume (up to −86%)
resulted from a combination of decarbonation reactions and loss of CaO and CO2. The resulting volume change has potentially important consequences for the interpretation of stable isotope profiles: the
isotope alteration could have occured over greater distances than those observed today. 相似文献
134.
A. W. Sandman M. J. Aschwanden M. L. DeRosa J. P. Wülser D. Alexander 《Solar physics》2009,259(1-2):1-11
The Solar Terrestrial Relations Observatory (STEREO) provides the first opportunity to triangulate the three-dimensional coordinates of active region loops simultaneously from two different vantage points in space. Three-dimensional coordinates of the coronal magnetic field have been calculated with theoretical magnetic field models for decades, but it is only with the recent availability of STEREO data that a rigorous, quantitative comparison between observed loop geometries and theoretical magnetic field models can be performed. Such a comparison provides a valuable opportunity to assess the validity of theoretical magnetic field models. Here we measure the misalignment angles between model magnetic fields and observed coronal loops in three active regions, as observed with the Extreme Ultraviolet Imager (EUVI) on STEREO on 30 April, 9 May, and 19 May 2007. We perform stereoscopic triangulation of some 100?–?200 EUVI loops in each active region and compute extrapolated magnetic field lines using magnetogram information from the Michelson Doppler Imager (MDI) on the Solar and Heliospheric Observatory (SOHO). We examine two different magnetic extrapolation methods: (1) a potential field and (2) a radially stretched potential field that conserves the magnetic divergence. We find considerable disagreement between each theoretical model and the observed loop geometries, with an average misalignment angle on the order of 20°?–?40°. We conclude that there is a need for either more suitable (coronal rather than photospheric) magnetic field measurements or more realistic field extrapolation models. 相似文献
135.
Several hundred radio bursts in the decimetric wavelength range (300–1000 MHz) have been compared with simultaneous soft and hard X-ray emission. Long lasting (type IV) radio events have been excluded. The association of decimetric emission with hard X-rays has been found to be surprisingly high (48%). The association rate increases with bandwidth, duration, number of structural elements, and maximum frequency. Type III-like bursts are observed up to the upper limit of the observed band. This demonstrates that the corona is transparent up to densities of about 1010 cm–3, contrary to previous assumptions. This can only be explained in an inhomogeneous corona with the radio source being located in a dense structure. The short decimetric bursts generally occur during the impulsive phase, i.e. simultaneously with hard X-rays. The times of maximum flux are well correlated (within 2 s). The HXR emission lasts 4 times longer then the radio emission in the average. This work finds a close relationship between decimetric and HXR emission with sufficient statistics offering additional information on the flare process. 相似文献
136.
M. J. Aschwanden A. O. Benz R. A. Schwartz R. P. Lin R. M. Pelling W. Stehling 《Solar physics》1990,130(1-2):39-55
We present observations of the solar flare on 1980 June 27, 16:14–16:33 UT, which was observed by a balloon-borne 300 cm2 phoswich hard X-ray detector and by the IKARUS radio spectrometer. This flare shows intense hard X-ray (HXR) emission and an extreme productivity of (at least 754) type III bursts at 200–400 MHz. A linear correlation was found between the type III burst rate and the HXR fluence, with a coefficient of 7.6 × 1027 photons keV–1 per type III burst at 20 keV. The occurrence of 10 type III bursts per second, and also the even higher rate of millisecond spikes, suggests a high degree of fragmentation in the acceleration region. This high quantization of injected beams, assuming the thick-target model, shows up in a linear relationship between hard X-ray fluence and the type III rate, but not as fine structures in the HXR time profile.The generation of a superhot isothermal HXR component in the decay phase of the flare coincides with the fade-out of type III production.Universities Space Research Associates.ST Systems Corporation. 相似文献
137.
Christoph Schnabel Valérie Olive Mariko Atarashi-Andoh Andrew Dougans Robert M. Ellam Stewart Freeman Colin Maden Martin Stocker Hans-Arno Synal Lukas Wacker Sheng Xu 《Applied Geochemistry》2007
This work constitutes the first survey of I isotope ratios for Scottish sea water including the first data for the west of Scotland. These data are of importance because of the proximity to the world’s second largest emission source of 129I to the sea, the Sellafield nuclear reprocessing plant, because of the increasing importance of the sea to land transfer of 129I and also as input data for dose estimates based on this pathway of 129I. 129I/127I ratios in SW Scotland reached 3 × 10−6 in 2004. No strong variation of I isotope ratios was found from 2003 to 2005 in Scottish sea waters. Iodine isotope ratios increased by about a factor of 6 from 1992 to 2003 in NE Scotland, in agreement with the increase of liquid 129I emissions from Sellafield over that time period. It is demonstrated that 129I/127I ratios agree better than 129I concentrations for samples from similar locations taken in very close temporal proximity, indicating that this ratio is more appropriate to interpret than the radionuclide concentration. 相似文献
138.
139.
G. A. Meehl R. Lukas G. N. Kiladis K. M. Weickmann A. J. Matthews M. Wheeler 《Climate Dynamics》2001,17(10):753-775
Interactions involving various time and space scales, both within the tropics and between the tropics and midlatitudes, are
ubiquitous in the climate system. We propose a conceptual framework for understanding such interactions whereby longer time
scales and larger space scales set the base state for processes on shorter time scales and smaller space scales, which in
turn have an influence back on the longer time scales and larger space scales in a continuum of process-related interactions.
Though not intended to be comprehensive, we do cite examples from the literature to provide evidence for the validity of this
framework. Decadal time scale base states of the coupled climate system set the context for the manifestation of interannual
time scales (El Nino/Southern Oscillation, ENSO and tropospheric biennial oscillation, TBO) which are influenced by and interact
with the annual cycle and seasonal time scales. Those base states in turn influence the large-scale coupled processes involved
with intraseasonal and submonthly time scales, tied to interactions within the tropics and extratropics, and tropical–midlatitude
teleconnections. All of these set the base state for processes on the synoptic and mesoscale and regional/local space scales.
Events at those relatively short time scales and small space scales may then affect the longer time scale and larger space
scale processes in turn, reaching back out to submonthly, intraseasonal, seasonal, annual, TBO, ENSO and decadal. Global coupled
models can capture some elements of the decadal, ENSO, TBO, annual and seasonal time scales with the associated global space
scales. However, coupled models are less successful at simulating phenomena at subseasonal and shorter time scales with hemispheric
and smaller space scales. In the context of the proposed conceptual framework, the synergistic interactions of the time and
space scales suggest that a high priority must be placed on improved simulations of all of the time and space scales in the
climate system. This is particularly important for the subseasonal time scales and hemispheric and smaller space scales, which
are not well simulated at present, to improve the prospects of successfully forecasting phenomena beyond the synoptic scales.
Received: 3 April 2000/ Accepted: 6 November 2000 相似文献
140.
Guilhem Amin Douillet Benjamin Bernard Mlanie Bouysson Quentin Chaffaut Donald Bruce Dingwell Lukas Gegg Inga Hoelscher Ulrich Kueppers Clia Mato Vanille Ariane Ritz Fritz Schlunegger Patrick Witting 《Sedimentology》2019,66(5):1531-1559
Pyroclastic currents are catastrophic flows of gas and particles triggered by explosive volcanic eruptions. For much of their dynamics, they behave as particulate density currents and share similarities with turbidity currents. Pyroclastic currents occasionally deposit dune bedforms with peculiar lamination patterns, from what is thought to represent the dilute low concentration and fluid‐turbulence supported end member of the pyroclastic currents. This article presents a high resolution dataset of sediment plates (lacquer peels) with several closely spaced lateral profiles representing sections through single pyroclastic bedforms from the August 2006 eruption of Tungurahua (Ecuador). Most of the sedimentary features contain backset bedding and preferential stoss‐face deposition. From the ripple scale (a few centimetres) to the largest dune bedform scale (several metres in length), similar patterns of erosive‐based backset beds are evidenced. Recurrent trains of sub‐vertical truncations on the stoss side of structures reshape and steepen the bedforms. In contrast, sporadic coarse‐grained lenses and lensoidal layers flatten bedforms by filling troughs. The coarsest (clasts up to 10 cm), least sorted and massive structures still exhibit lineation patterns that follow the general backset bedding trend. The stratal architecture exhibits strong lateral variations within tens of centimetres, with very local truncations both in flow‐perpendicular and flow‐parallel directions. This study infers that the sedimentary patterns of bedforms result from four formation mechanisms: (i) differential draping; (ii) slope‐influenced saltation; (iii) truncative bursts; and (iv) granular‐based events. Whereas most of the literature makes a straightforward link between backset bedding and Froude‐supercritical flows, this interpretation is reconsidered here. Indeed, features that would be diagnostic of subcritical dunes, antidunes and ‘chute and pools’ can be found on the same horizon and in a single bedform, only laterally separated by short distances (tens of centimetres). These data stress the influence of the pulsating and highly turbulent nature of the currents and the possible role of coherent flow structures such as Görtler vortices. Backset bedding is interpreted here as a consequence of a very high sedimentation environment of weak and waning currents that interact with the pre‐existing morphology. Quantification of near‐bed flow velocities is made via comparison with wind tunnel experiments. It is estimated that shear velocities of ca 0·30 m.s?1 (equivalent to pure wind velocity of 6 to 8 m.s?1 at 10 cm above the bed) could emplace the constructive bedsets, whereas the truncative phases would result from bursts with impacting wind velocities of at least 30 to 40 m.s?1. 相似文献