排序方式: 共有40条查询结果,搜索用时 31 毫秒
31.
We present simulations performed with a three dimensional global ocean general circulation model which show that simulated
salinities and amounts of convective mixing are very sensitive to vertical mixing of surface buoyancy fluxes. If, as usual,
surface buoyancy fluxes are placed entirely in the topmost model level, our model produces excessive convective mixing in
the Southern Ocean. This results in poor stimulated salinity in the Southern Ocean. In this simulation, we assume, as usual,
that both surface buoyancy forcing and vertical mixing are homogeneous within each grid cell. If, on the other hand, destabilizing
surface fluxes are instantaneously mixed into the subsurface ocean, the model produces much less convective mixing and much
more realistic salinities. The vertical mixing of surface buoyancy fluxes performed in this simulation is equivalent to assuming
that those fluxes affect only a small fraction of each grid cell, and cause vertical mixing only in that limited area. Our
interpretation of these results is that the usual assumption that both surface buoyancy forcing and vertical mixing are uniform
within each grid cell has a detrimental effect on model results; these results could be significantly improved by good parametrizations
which treat the horizontal inhomogeneity of surface buoyancy forcing and of vertical mixing.
Received: 25 February 1998 / Accepted: 9 September 1998 相似文献
32.
Black carbon aerosols absorb solar radiation and decrease planetary albedo, and thus can contribute to climate warming. In this paper, the dependence of equilibrium climate response on the altitude of black carbon is explored using an atmospheric general circulation model coupled to a mixed layer ocean model. The simulations model aerosol direct and semi-direct effects, but not indirect effects. Aerosol concentrations are prescribed and not interactive. It is shown that climate response of black carbon is highly dependent on the altitude of the aerosol. As the altitude of black carbon increases, surface temperatures decrease; black carbon near the surface causes surface warming, whereas black carbon near the tropopause and in the stratosphere causes surface cooling. This cooling occurs despite increasing planetary absorption of sunlight (i.e. decreasing planetary albedo). We find that the trend in surface air temperature response versus the altitude of black carbon is consistent with our calculations of radiative forcing after the troposphere, stratosphere, and land surface have undergone rapid adjustment, calculated as “regressed” radiative forcing. The variation in climate response from black carbon at different altitudes occurs largely from different fast climate responses; temperature dependent feedbacks are not statistically distinguishable. Impacts of black carbon at various altitudes on the hydrological cycle are also discussed; black carbon in the lowest atmospheric layer increases precipitation despite reductions in solar radiation reaching the surface, whereas black carbon at higher altitudes decreases precipitation. 相似文献
33.
Transpiration in Quercus suber trees under shallow water table conditions: the role of soil and groundwater
下载免费PDF全文
![点击此处可从《水文研究》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Clara A. Pinto Nadezhda Nadezhdina Jorge S. David Cathy Kurz‐Besson Maria C. Caldeira Manuel O. Henriques Fernando G. Monteiro João S. Pereira Teresa S. David 《水文研究》2014,28(25):6067-6079
Water is one of the major environmental factors limiting plant growth and survival in the Mediterranean region. Quercus suber L. woodlands occupy vast areas in the Iberian Peninsula, frequently under shallow water table conditions. The relative magnitude of soil and groundwater uptake to supply transpiration is not easy to evaluate under these circumstances. We recently developed a conceptual framework for the functioning of the root system in Q. suber that simulates well tree transpiration, based on two types of root behaviour: shallow connected and deep connected. Although this significantly improved knowledge on the functional traits of Mediterranean Q. suber, the approach has the limitation of requiring root sap flow data, which are seldom available. In this work, we present alternative methodologies to assess if trees are connected to groundwater and to estimate the soil and groundwater contributions to tree transpiration. We provide evidence on the tree unrestricted access to groundwater solely based on meteorological, stem sap flow and leaf water potential data. Using a soil mass balance approach, we estimated the yearly soil and groundwater contributions to tree transpiration: 69.7% and 30.3%, respectively. Groundwater uptake became dominant in the dry summer: 73.2% of tree transpiration. Results reproduce extremely well those derived from root modelling. Because of its simplicity both in formulation and data requirements, our approach is potentially liable to be adapted to other groundwater‐dependent Mediterranean oak sites, where interactions between land use and water resources may be relevant. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
34.
Summary In this paper, the authors address a predictability study of the NAO index by using the time series analysis method known
as “detrended fluctuation analysis” (DFA). This technique provides a quantitative measure of predictability by computing several
piecewise fits (either linear or higher degree polynomial ones) to the cumulative series of fluctuations associated to the
original series. The measurements, made on several examples of data available in the Internet, agree with other previous analyses
(Fernández et al, 2003) assessing that the NAO signal is a slightly red one, whose prediction needs a deeper understanding
of the underlying physics. 相似文献
35.
Ken Caldeira 《Climatic change》2007,85(3-4):267-269
36.
37.
Ihsane Bensaid Taj-Eddine Cherkaoui Fida Medina Bento Caldeira Elisa Buforn Anas Emran Youssef Hahou 《Journal of Seismology》2012,16(1):35-53
The Tafilalt region, located at the eastern end of the Anti-Atlas chain in Morocco, was shaken on 23 and 30 October 1992 by
two moderate earthquakes of magnitude mb ∼ 5 and intensity ∼ VI MSK64, which caused two deaths and great damage in the area between Erfoud and Rissani. The review
of data available on the seismic crisis allowed us to improve the knowledge on the macroseismic, instrumental and source parameters
of the earthquakes. The main results of the present study are: (1) location of the epicentres with the help of data from a
close portable network allowed us to propose new epicentral coordinates at 31.361° N, 4.182° W (23 October) and 31.286° N,
4.347° W (30 October); both events have focal depths of 2 km; (2) the shock of 30 October was followed by a series of 305
aftershocks, most of which were located west of Rissani; the 61 best-constrained events had focal depths of 5 to 19 km and
magnitudes 0.7 to 3; (3) the largest damage was located in an area between the two epicentres within the Tafilalt valley and
was probably amplified by site effects due to the proximity of the water table within the Quaternary sediments; (4) focal
mechanisms of the main events correspond to strike-slip faulting with fault planes oriented N–S (left lateral) and E–W (right
lateral); the only mechanism available for the aftershocks also corresponds to strike-slip faulting; (5) spectral analysis
shows that the scalar seismic moment (Mo) of the first event is slightly larger than the second; the corresponding values of Mw are 5.1 and 5.0, respectively; (6) the dimensions of the faults for a circular fault model are 7.7 ± 1.4 and 7.4 ± 1.2 km,
respectively; the average displacement is 4 cm for the first event and 3.7 cm for the second; the stress drop is 0.4 and 0.3 MPa,
respectively, in agreement with standard values; (7) the Coulomb Stress test performed for both earthquakes suggests a relationship
between both events only when the used location is at the limit of the horizontal uncertainty; (8) finally, the occurrence
of these shocks suggests that the Anti-Atlas is undergoing tectonic deformation in addition to thermal uplift as suggested
by recent publications. 相似文献
38.
Complex geophysical wake flows 总被引:1,自引:1,他引:0
Idealized studies of island wakes often use a cylinder-like island to generate the wake, whereas most realistic studies use
a close representation of the oceanic bathymetry immersed in a complex representation of the “ambient” geophysical flows.
Here, a system of multiple islands was placed into numerical and experimental channels, in order to focus on the complexity
of the archipelago wake, including (a) the influence of small neighboring islands and (b) the role of the island-shelf. The
numerical geostrophic and stratified channel was built using a three-dimensional primitive equation model, considering a realistic
representation of the Madeira archipelago bathymetry, with prescribed initial and boundary conditions. Results from the simulations
show that the neighboring islands alter the near-field wake. Small eddies generated by the neighboring islands lead to destabilization
of the shear layers of the larger island. Laboratory experiments carried out in the Coriolis rotating tank corroborated this
near-field disruptive mechanism. The neighboring island perturbation effect was present whatever the direction of the incoming
flow, but under different regimes. North–south wakes produced geostrophic eddies (≥ R
d), whereas west–east wakes produced (exclusively) ageostrophic submesoscale eddies (< < R
d) which traveled offshore with wave-like motion. The archipelago shelf contributed to the asymmetric vertical migration of
oceanic vorticity. Cyclonic vorticity dominated the surface dynamics, whereas anticyclonic circulation prevailed at the bottom
part of the linearly stratified upper layer. This study identifies several likely wake scenarios induced by the Madeira archipelago,
and may serve as guide for future multiscale numerical studies and in situ campaigns. 相似文献
39.
Directivity effects are a characteristic of seismic source finiteness and are a consequence of the rupture spread in preferential
directions. These effects are manifested through seismic spectral deviations as a function of the observation location. The
directivity by Doppler effect method permits estimation of the directions and rupture velocities, beginning from the duration
of common pulses, which are identified in waveforms or relative source time functions. The general model of directivity that
supports the method presented here is a Doppler analysis based on a kinematic source model of rupture (Haskell, Bull Seismol
Soc Am 54:1811–1841, 1964) and a structural medium with spherical symmetry. To evaluate its performance, we subjected the method to a series of tests
with synthetic data obtained from ten typical seismic ruptures. The experimental conditions studied correspond with scenarios
of simple and complex, unilaterally and bilaterally extended ruptures with different mechanisms and datasets with different
levels of azimuthal coverage. The obtained results generally agree with the expected values. We also present four real case
studies, applying the method to the following earthquakes: Arequipa, Peru (M
w = 8.4, June 23, 2001); Denali, AK, USA (M
w = 7.8; November 3, 2002); Zemmouri–Boumerdes, Algeria (M
w = 6.8, May 21, 2003); and Sumatra, Indonesia (M
w = 9.3, December 26, 2004). The results obtained from the dataset of the four earthquakes agreed, in general, with the values
presented by other authors using different methods and data. 相似文献
40.
Claudia L. Caldeira Virginia S.T. Ciminelli Kwadwo Osseo-Asare 《Geochimica et cosmochimica acta》2010,74(6):1777-1789
The mechanism of pyrite oxidation in carbonate-containing alkaline solutions at 80 °C was investigated with the help of rate experiments, thermodynamic modeling and diffuse reflectance infrared spectroscopy (DRIFTS). Pyrite oxidation rate increased with pH and was enhanced by addition of bicarbonate/carbonate ions. The carbonate effect was found to be limited to moderately alkaline conditions (pH 8-11). Metastable Eh-pH diagrams, at 25 °C, indicate that soluble iron-carbonate complexes (FeHCO3−, FeCO30, Fe(CO3)(OH)− and FeCO32−) may coexist with pyrite in the pH range of 6-12.5. Above pH 11 and 13, the Fe(II) and Fe(III) hydroxocomplexes, respectively, become stable, even in the presence of carbonate/bicarbonate ions. Surface-bound carbonate complexes on iron were also identified with DRIFTS as products of pyrite oxidation in addition to iron oxyhydroxides and soluble sulfate species. The conditions under which thermodynamic and DRIFTS analyses indicate the presence of carbonate compounds also correspond to those in which the fastest rate of pyrite oxidation in carbonate solutions was observed. Following the Singer-Stumm model for pyrite oxidation in acidic solutions, it is assumed that Fe(III) is the preferred pyrite oxidant under alkaline conditions. We propose that carbonate ions facilitate the electron transfer from soluble iron(II)-carbonate to O2, increase the iron solubility, and provide buffered, favorable alkaline conditions at the reaction front, which in turn favors the overall kinetics of pyrite oxidation. Therefore, the electron transfer from sulfur atoms to O2 is facilitated by the formation of the cycle of Fe(II)-pyrite/Fe(III)-carbonate redox couple at the pyrite surface. 相似文献