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Maddalena Spada Gabriele Ghisellini Davide Lazzati † Annalisa Celotti 《Monthly notices of the Royal Astronomical Society》2001,325(4):1559-1570
The central engine causing the production of jets in radio sources may work intermittently, accelerating shells of plasma with different mass, energy and velocity. Faster but later shells can then catch up slower earlier ones. In the resulting collisions shocks develop, converting some of the ordered bulk kinetic energy into magnetic field and random energy of the electrons which then radiate. We propose that this internal shock scenario , which is the scenario generally thought to explain the observed gamma-ray burst radiation, can also work for radio sources in general, and for blazars in particular. We investigate in detail this idea, simulating the birth, propagation and collision of shells, calculating the spectrum produced in each collision, and summing the locally produced spectra from those regions of the jet which are simultaneously active in the observer's frame. We can thus construct snapshots of the overall spectral energy distribution, time-dependent spectra and light curves. This allows us to characterize the predicted variability at any frequency, study correlations between the emission at different frequencies, specify the contribution of each region of the jet to the total emission, and find correlations between flares at high energies and the birth of superluminal radio knots and/or radio flares. The model has been applied to reproduce qualitatively the observed properties of 3C 279. Global agreement in terms of both spectra and temporal evolution is found. In a forthcoming work, we will explore the constraints that this scenario sets on the initial conditions of the plasma injected in the jet and the shock dissipation for different classes of blazars. 相似文献
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Sensitivity of the Asian summer monsoon to the horizontal resolution: differences between AMIP-type and coupled model experiments 总被引：1，自引：0，他引：1
A set of experiments forced with observed SST has been performed with the Echam4 atmospheric GCM at three different horizontal resolutions (T30, T42 and T106). These experiments have been used to study the sensitivity of the simulated Asian summer monsoon (ASM) to the horizontal resolution. The ASM is reasonably well simulated by the Echam4 model at all resolutions. In particular, the low-level westerly flow, that is the dominant manifestation of the Asian summer monsoon, is well captured by the model, and the precipitation is reasonably simulated in intensity and space appearance. The main improvements due to an higher resolution model are associated to regional aspects of the precipitation, for example the Western Ghats precipitation is better reproduced. The interannual variability of precipitation and wind fields in the Asian monsoon region appears to be less affected by an increase in the horizontal resolution than the mean climatology is. A possible reason is that the former is mainly SST-forced. Besides, the availability of experiments at different horizontal resolution realized with the Echam4 model coupled to a global oceanic model allows the possibility to compare these simulations with the experiments previously described. This analysis showed that the coupled model is able to reproduce a realistic monsoon, as the basic dynamics of the phenomenon is captured. The increase of the horizontal resolution of the atmospheric component influences the simulated monsoon with the same characteristics of the forced experiments. Some basic features of the Asian summer monsoon, as the interannual variability and the connection with ENSO, are further investigated. 相似文献
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Claudio ZUCCA Annalisa CANU Raniero DELLA PERUTA 《国际泥沙研究》2006,21(2):132-144
1 INTRODUCTION Many studies on soil erosion by water have been carried out in the Mediterranean area, since erosion is considered as one of the most important land degradation processes in these environments (UNEP, 1994). Those studies mainly focused on r… 相似文献
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This paper considers the dynamical behaviour of a structural model with foundation uplift. The equations of motion of the system considered are derived for large displacements thus allowing for the eventual overturning of the system. The transition conditions between successive phases of motion, derived in terms of the specific Lagrangian co‐ordinates used in the formulation of the equations of motion, present innovative aspects which resolve some previously inexplicable behaviour in the structural response reported in the literature. The dynamical behaviour of the model is considered under impulsive and long‐duration ground motions. The minimum horizontal acceleration impulses for the uplift and the overturning of the system are evaluated in analytical form. The sensitivity of the model to uplifting and to overturning under impulsive excitations is established as a function of few significant structural parameters. Numerical applications have been performed changing either the structural parameters or the loading parameter, in order to analyse several dynamical behaviours and also to validate the analytical results. For earthquake ground motions the results, reported in the form of response spectra, show that linearized models generally underestimate, sometimes significantly, the structural response. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
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Transport properties in small-scale coastal flows: relative dispersion from VHF radar measurements in the Gulf of La Spezia 总被引：2，自引：2，他引：0
Angelique C. Haza Tamay M. Özgökmen Annalisa Griffa Anne Molcard Pierre-Marie Poulain Germana Peggion 《Ocean Dynamics》2010,60(4):861-882
Lagrangian transport characteristics in the Gulf of La Spezia, a 5 × 10-km area along the western coast of Italy, are investigated
using data collected from a very high frequency (VHF) radar system with 250 m and 30-min resolution and two clusters of Coastal
Dynamics Experiment surface drifters during 2 weeks in the summer of 2007. The surface drifters are found to follow the temporal
and spatial evolution of the finite-scale Lyapunov exponents (FSLEs) computed by the VHF radar, indicating the precision of
both the radar measurements and the diagnostic FSLE in mapping accurately the transport pathways. In light of this agreement,
an analysis of the relative dispersion is conducted. It is found that the average FSLE value varies within a narrow range
of 4 day-1 ￡ l ￡ 7 day-14 \;\mbox{day}^{-1} \leq \lambda \leq 7 \;\mbox{day}^{-1} and displays an exponential regime over the entire extent of the measurements. The dynamical implication is that relative
dispersion is controlled nonlocally, namely by slow, persistent, energetic mesoscale structures as opposed to the rapidly
evolving high-gradient small-scale turbulent features. The value of the exponent is about an order of magnitude larger than
those found in previous modeling studies and analysis of SCULP data in the Gulf of Mexico but somewhat smaller than that estimated
from CLIMODE drifters in the Gulf Stream region. Scaling of the FSLE using a metric of resolved gradients of the Eulerian
fields in the form of a positive Okubo–Weiss criterion is useful, but not as precise as in modeling studies. The horizontal
flow convergence is found to have a small yet tangible effect on relative dispersion. 相似文献
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Results from visual sightings of large floating debris are presented, taken in the Ligurian Sea, a sub-basin of the north-western Mediterranean Sea which belongs to the recently stated "Cetacean Sanctuary". Data have been collected during three oceanographic cruises, during the summer of 1997 and 2000. Results for the 1997 data suggest a debris density of the order of 15-25 objects km(-2), while for the 2000 data, a lower density of the order of 3-1.5 objects km(-2) is found. The difference between the two results appears statistically significant using simple tests. Possible reasons for the observed variability are discussed, including meteorological forcing, marine currents and debris input variability. 相似文献