Based on a previous study for temperature, a new method for the calculation of non-stationary return levels for extreme rainfall is described and applied to Extremadura, a region of southwestern Spain, using the peaks-over-threshold approach. Both all-days and rainy-days-only datasets were considered and the 20-year return levels expected in 2020 were estimated taking different trends into account: first, for all days, considering a time-dependent threshold and the trend in the scale parameter of the generalized Pareto distribution; and second, for rainy days only, considering how the mean, variance, and number of rainy days evolve. Generally, the changes in mean, variance and number of rainy days can explain the observed trends in extremes, and their extrapolation gives more robust estimations. The results point to a decrease of future return levels in 2020 for spring and winter, but an increase for autumn. 相似文献
Upwelling coastal systems can be used to understand how dissolved oxygen and biological productivity control the accumulation of redox-sensitive metals in marine sediments. The aluminium (Al), cadmium (Cd), iron (Fe), nickel (Ni), molybdenum (Mo), vanadium (V), total organic carbon (TOC), total nitrogen (TN) and total sulfur (TS) contents in surficial sediment collected from different water depths (30, 70, and 120?m) in three northern Chilean bays influenced by coastal upwelling and oxygen minimum zones (OMZs) were measured. Principal component analysis (PCA), cluster analysis, and Spearman?s rank correlation were used to identify the mechanisms responsible for the redox-sensitive metal accumulation. The content of redox-sensitive metals and organic components in sediment increased with increasing water column depth, whereas lithogenic metals decreased. In the Mejillones del Sur and Caldera bays, the enrichment factors of the redox-sensitive metals showed enrichment for all metals with depth. The Cd and V enrichments are mainly the product of biogenic flow to the seabed, and the Mo and Ni enrichments are due to preservation under low subsurface oxygen conditions. Sulfate reduction is not an important mechanism in the accumulation of redox-sensitive metals in the sediment of the three bays. The PCA showed that the behaviors of the redox-sensitive metals and organic components reflect differences in the effects of the OMZ in sediment along the coast of northern Chile, with a more intense OMZ in Mejillones del Sur bay and weaker OMZs in Caldera and Inglesa bays. However, the high degree of enrichment in redox-sensitive metals in Caldera Bay can be attributed to the intense activity of the mining industry near the bay, a situation that produces geochemical behavior similar to that observed in Mejillones del Sur Bay. 相似文献
Both semi-empirical methods and CFD simulations use real climate datasets as a basis for determining the building facade exposure to wind-driven rain and simultaneous wind pressure. The time resolution of these datasets and the number of variables considered (commonly rainfall intensity, wind speed and wind direction) determine the required calculation effort and the accuracy of the result. Omitting the wind direction, a former article (Part I of this research) has analysed the effect of this time resolution on two scalar exposure indices obtained by semi-empirical methods: driving rain index (aDRI) and driving-rain wind pressure (DRWP). However, the wind direction during precipitation events also causes significant exposure variations between possible facade orientations. Thus, it is also necessary to clarify the influence of the time resolution of the dataset, on the accuracy of the directional semi-empirical calculation of aDRI and DRWP. To meet this challenge, the article examines 10-min climate records collected between 2001 and 2016 at 6 Spanish locations, uses them to obtain hourly, daily, monthly and annual datasets, and analyses the accuracy of the directional exposure indices associated with each time resolution. The results show that a daily dataset would allow identifying the most exposed orientation with an error less than 45°. However, even the hourly datasets cause errors close to 10% in the exposure values identified on each facade orientation. Finally, adjustment relationships that allow estimating the maximum value of directional exposure from simple scalar indices are obtained.
We study the relationships between the seafloor structures and the axial magma chamber geometry in the 9°N overlapping spreading center (OSC) area on the fast spreading East Pacific Rise (EPR). Our observations are based on a new high resolution bathymetric map of the 9°N OSC area derived from picks of the seafloor arrival on 3D seismic data, and on previously published data that constrain the presence and distribution of melt below the 9°N OSC. Differences in the orientation of structures between the seafloor and the magma chamber indicate a sharp change in principal stress directions with depth, suggesting that the brittle crust above the melt sill is decoupled from the melt sill itself and the ductile crust underlying it. The stress-field within the brittle upper crust results from a local interaction of the two overlapping spreading centers, whereas the stress-field in the crust below the melt sill corresponds to the regional stress-field imposed by plate separation. Given this mechanical structure of the crust, the melt sill shape and location appear to be controlled by the following factors: the location of the deep melt source below the melt sill, the ambient stress-field at the depth of the melt sill, and the stress-field in the brittle upper crust above the melt sill, which thermally shapes the roof of the melt sill through repeated eruptions. 相似文献
The SCHA.DI.00 directional model for the geomagnetic field in Europe for the last 2000 years (Pavón–Carrasco
et al., 2008) has been updated by modelling the palaeointensity. This model, SCHA.DI.00, was developed from available Bayesian
European Palaeosecular Variation Curves using the regional Spherical Cap Harmonic Analysis technique. The comparison of the
palaeosecular variation curves, given by the regional model, with available archaeomagnetic data not used in its development
showed an improvement with respect to the fit obtained by global archaeomagnetic models. In this paper advantage is taken
of recently published palaeointensity databases to develop a complete (direction and intensity) regional archaeomagnetic model
for the last 2000 years valid for the European region: the SCHA.DI.00–F model. Not only does this complete model provide an
improvement for example for archaeomagnetic data studies, but it is also shown that this new regional model can be used to
study the recently proposed link between the centennial secular variation of the geomagnetic field and climate change. The
pattern of the archaeosecular variation of the field intensity obtained by SCHA.DI.00–F seems to verify the hypothesis presented
by Gallet et al. (2005) about a possible (causal) connection between changes in the geomagnetic field intensity and in climate parameters,
opening the door for more discussions on this challenging subject. 相似文献
A compact, time-explicit, approximate solution of the highly non-linear relative motion in curvilinear coordinates is provided under the assumption of circular orbit for the chief spacecraft. The rather compact, three-dimensional solution is obtained by algebraic manipulation of the individual Keplerian motions in curvilinear, rather than Cartesian coordinates, and provides analytical expressions for the secular, constant and periodic terms of each coordinate as a function of the initial relative motion conditions or relative orbital elements. Numerical test cases are conducted to show that the approximate solution can be effectively employed to extend the classical linear Clohessy–Wiltshire solution to include non-linear relative motion without significant loss of accuracy up to a limit of 0.4–0.45 in eccentricity and 40–45\(^\circ \) in relative inclination for the follower. A very simple, quadratic extension of the classical Clohessy–Wiltshire solution in curvilinear coordinates is also presented. 相似文献
The demand for sensitive biological tools to assess the environmental quality of coastal waters at broad spatial scales is increasing. Many of the tools used are based on the taxonomic composition of biotic assemblages. They usually require a valuable taxonomic expertise while are unique reflecting the overall ecosystem integrity. Here, we evaluate the potential indicator value of several features of the epiphytic community (overall assemblage composition, species richness, and proportion of the main taxonomic groups) growing on the seagrass Posidonia oceanica leaves. We do so by empirically examining their changes along a disturbance gradient where multiple human activities have interactive and cumulative impacts, sampling at different spatial scales and at two different depths (5 and 15 m). Our results show that the specific composition of the epiphytic assemblages (i.e. species composition) closely reflects, in the deep meadows, the combined effects of different anthropogenic stressors along the gradient, showing an integrative and non-specific response. Similarly, an increase in the proportion of hydrozoans, and a decrease in the proportion of rhodophytes and chlorophytes are observed in deep meadows along the gradient. In shallow meadows, grazing and biotic features of the seagrass seem the main forcing factors determining species composition, and therefore masking the response of epiphytes to the deterioration gradient. After address the effect of natural sources of variability (water depth, within- and between-meadow heterogeneity), changes in epiphyte assemblages and in the proportion of hydrozoans, rhodophytes and chlorophytes in relatively deep meadows seem promising monitoring tools for detecting coastal environmental deterioration. 相似文献