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排序方式: 共有58条查询结果,搜索用时 15 毫秒
1.
A. Greco G. Zimbardo P. Veltri A.L. Taktakishvili L.M. Zelenyi 《Astrophysics and Space Science》2001,277(1-2):35-38
We study the ion dynamics in a magnetic field reversal with a constant electric field and with a model of three dimensional
magnetic turbulence. By computing the mean square displacements in the plane of the current sheet we find superdiffusive and
superballistic transport regimes. Since velocity increases with the length of the free path, we have accelerated Lévyflights.
The possibility to generate power law velocity distribution functions is pointed out, as well as the long memory effects and
non local properties of ion transport.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
2.
Fabio Fontanot Pierluigi Monaco Stefano Borgani 《Monthly notices of the Royal Astronomical Society》2003,341(2):692-706
We apply the ztrace algorithm to the optical NOG and infrared PSC z galaxy catalogues to reconstruct the pattern of primordial fluctuations that have generated our local Universe. We check that the density fields traced by the two catalogues are well correlated, and consistent with a linear relation [either in δ or in log (1 +δ) ] with relative bias (of NOG with respect to PSC z ) b rel = 1.1 ± 0.1 . The relative bias relation is used to fill the optical zone of avoidance at | b | < 20° using the PSC z galaxy density field.
We perform extensive testing on simulated galaxy catalogues to optimize the reconstruction. The quality of the reconstruction is predicted to be good at large scales, up to a limiting wavenumber klim ≃ 0.4 h Mpc−1 beyond which all information is lost. We find that the improvement arising from the denser sampling of the optical catalogue is compensated by the uncertainties connected to the larger zone of avoidance.
The initial conditions reconstructed from the NOG catalogue are found (analogously to those from the PSC z ) to be consistent with a Gaussian paradigm. We use the reconstructions to produce sets of initial conditions ready to be used for constrained simulations of our local Universe. 相似文献
We perform extensive testing on simulated galaxy catalogues to optimize the reconstruction. The quality of the reconstruction is predicted to be good at large scales, up to a limiting wavenumber k
The initial conditions reconstructed from the NOG catalogue are found (analogously to those from the PSC z ) to be consistent with a Gaussian paradigm. We use the reconstructions to produce sets of initial conditions ready to be used for constrained simulations of our local Universe. 相似文献
3.
Federico Murrone Fabio Rossi Pierluigi Claps 《Stochastic Environmental Research and Risk Assessment (SERRA)》1997,11(6):483-510
A conceptual-stochastic approach to short time runoff data modelling is proposed, according to the aim of reproducing the
hydrological aspects of the streamflow process and of preserving as much as possible the dynamics of the process itself. This
latter task implies preservation of streamflow characteristics at higher scales of aggregation and, within a conceptual framework,
involves compatibility with models proposed for the runoff process at those scales. At a daily time scale the watershed response
to the effective rainfall is considered as deriving from the response of three linear reservoirs, respectively representing
contributions to streamflows of large deep aquifers, with over-year response lag, of aquifers which run dry by the end of
the dry season and of subsurface runoff. The surface runoff component is regarded as an uncorrelated point process. Considering
the occurrences of effective rainfall events as generated by an independent Poisson process, the output of the linear system
represents a conceptually-based multiple shot noise process. Model identification and parameter estimation are supported by
information related to the aggregated runoff process, in agreement to the conceptual framework proposed, and this allows parameter
parsimony, efficient estimation and effectiveness of the streamflow reproduction. Good performances emerged from the model
application and testing made with reference to some daily runoff series from Italian basins. 相似文献
4.
Statistical identification of orographic effects in the regional analysis of extreme rainfall 下载免费PDF全文
Regional models of extreme rainfall must address the spatial variability induced by orographic obstacles. However, the proper detection of orographic effects often depends on the availability of a well‐designed rain gauge network. The aim of this study is to investigate a new method for identifying and characterizing the effects of orography on the spatial structure of extreme rainfall at the regional scale, including where rainfall data are lacking or fail to describe rainfall features thoroughly. We analyse the annual maxima of daily rainfall data in the Campania region, an orographically complex region in Southern Italy, and introduce a statistical procedure to identify spatial outliers in a low order statistic (namely the mean). The locations of these outliers are then compared with a pattern of orographic objects that has been a priori identified through the application of an automatic geomorphological procedure. The results show a direct and clear link between a particular set of orographic objects and a local increase in the spatial variability of extreme rainfall. This analysis allowed us to objectively identify areas where orography produces enhanced variability in extreme rainfall. It has direct implications for rain gauge network design criteria and has led to promising developments in the regional analysis of extreme rainfall. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
5.
On the dependence of the saturated hydraulic conductivity upon the effective porosity through a power law model at different scales 下载免费PDF全文
We study the scale dependence of the saturated hydraulic conductivity Ks through the effective porosity ne by means of a newly developed power‐law model (PLM) which allows to use simultaneously measurements at different scales. The model is expressed as product between a single PLM (capturing the impact of the dominating scale) and a characteristic function κ? accounting for the correction because of the other scale(s). The simple (closed form) expression of the κ?‐function enables one to easily identify the scales which are relevant for Ks. The proposed model is then applied to a set of real data taken at the experimental site of Montalto Uffugo (Italy), and we show that in this case two (i.e. laboratory and field) scales appear to be the main ones. The implications toward an important application (solute transport) in Hydrology are finally discussed. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
6.
Filip Lefebre Xavier Fettweis Hubert Gallée Jean-Pascal Van Ypersele Philippe Marbaix Wouter Greuell Pierluigi Calanca 《Climate Dynamics》2005,25(1):99-116
A simulation of the 1991 summer has been performed over south Greenland with a coupled atmosphere–snow regional climate model
(RCM) forced by the ECMWF re-analysis. The simulation is evaluated with in-situ coastal and ice-sheet atmospheric and glaciological
observations. Modelled air temperature, specific humidity, wind speed and radiative fluxes are in good agreement with the
available observations, although uncertainties in the radiative transfer scheme need further investigation to improve the
model’s performance. In the sub-surface snow-ice model, surface albedo is calculated from the simulated snow grain shape and
size, snow depth, meltwater accumulation, cloudiness and ice albedo. The use of snow metamorphism processes allows a realistic
modelling of the temporal variations in the surface albedo during both melting periods and accumulation events. Concerning
the surface albedo, the main finding is that an accurate albedo simulation during the melting season strongly depends on a
proper initialization of the surface conditions which mainly result from winter accumulation processes. Furthermore, in a
sensitivity experiment with a constant 0.8 albedo over the whole ice sheet, the average amount of melt decreased by more than
60%, which highlights the importance of a correctly simulated surface albedo. The use of this coupled atmosphere–snow RCM
offers new perspectives in the study of the Greenland surface mass balance due to the represented feedback between the surface
climate and the surface albedo, which is the most sensitive parameter in energy-balance-based ablation calculations. 相似文献
7.
8.
R. Bruno V. Carbone Z. Vörös R. D’Amicis B. Bavassano M. B. Cattaneo A. Mura A. Milillo S. Orsini P. Veltri L. Sorriso-Valvo T. Zhang H. Biernat H. Rucker W. Baumjohann D. Jankovičová P. Kovács 《Earth, Moon, and Planets》2009,104(1-4):101-104
At the end of August 2007, Venus, Earth and Ulysses were aligned within a few degrees. This unusual event gives the opportunity to attempt a coordinated study on the radial evolution of solar wind turbulence and coronal transients like CMEs between 0.7 and 1.4 AU. Interplanetary magnetic field data and moments of proton velocity distribution function such as density, speed and temperature are required for this programme and will be provided by ACE at Earth, Venus Express at Venus and Ulysses at 1.4 AU. This project has been recently proposed as a Coordinated Investigation Programme (CIP35) for the International Heliophysical Year. 相似文献
9.
The present paper proposes a numerical model to determine horizontal and vertical components of the hydrodynamic forces on a slender submarine pipeline lying at the sea bed and exposed to non-linear waves plus a current. The new model is an extension of the Wake II type model, originally proposed for sinusoidal waves (Soedigdo et al., 1999) and for combined sinusoidal waves and currents (Sabag et al., 2000), to the case of periodic or random waves, even with a superimposed current. The Wake II type model takes into account the wake effects on the kinematic field and the time variation of drag and lift hydrodynamic coefficients. The proposed extension is based on an evolutional analysis carried out for each half period of the free stream horizontal velocity at the pipeline. An analytical expression of the wake velocity is developed starting from the Navier–Stokes and the boundary layer equations. The time variation of the drag and lift hydrodynamic coefficients is obtained using a Gaussian integration of the start-up function. A reduced scale laboratory investigation in a large wave flume has been conducted in order to calibrate the empirical parameters involved in the proposed model. Different wave and current conditions have been considered and measurements of free stream horizontal velocities and dynamic pressures on a bottom-mounted pipeline have been conducted. The comparison between experimental and numerical hydrodynamic forces shows the accuracy of the new model in evaluating the time variation of peaks and phase shifts of the horizontal and vertical wave and current induced forces. 相似文献
10.
Groundwater Rn concentration has been monitored at several sites in the Campi Flegrei Caldera since the time of the last episode of bradyseismic activity. The results concerning the period 1983–88 indicate that the Rn concentration is spatially variable due to the different chemico-physical characteristics of the waters or structural features. Rn concentration is generally influenced by the long-term changes of meteorological factors. Rainfalls appear to affect the Rn content of the most superficial aquifers. Several short-term spike-like Rn anomalies have been detected both during and after the period of the bradyseismic crisis. They do not show correlation with temporal changes of the pH and/or temperature of waters. A systematic correlation between Rn anomalies and episodic strong rainfall events is also absent. Furthermore, a clear relationship between Rn spikes and both uplift velocity changes and seismic activity has not been found.The overall evidence makes it very difficult to understand the temporal changes of groundwater Rn concentration that have occurred during and after the 1982–84 bradyseismic crisis. Nevertheless, a clear causal relationship between Rn concentration and the dynamics of the bradyseism is observed in some monitored sites. Its appearance may have been favoured by the low Rn concentration in the groundwaters during periods of quiescent activity. 相似文献