8,653 research outputs found
Modeling the Impact of Climate Change on Runoff and Annual Water Balance of an Arctic Headwater Basin
Climate change will be an important issue facing Arctic areas in the coming decades since climate models are projecting warmer and wetter conditions for many northern regions. From a hydrological perspective, critical issues include a shortened snow cover season, changes in winter snow cover properties, and changes in the timing and volume of snowmelt runoff. To assess the impacts of projected temperature and precipitation changes on the hydrology of a small Arctic headwater basin, the distributed hydrological model WATFLOOD was used in conjunction with selected Global Circulation Models (GCMs) and future climate scenarios. It was found that the hydrological model simulated basin runoff adequately either with input climate data collected in the study area or with input data from a long-term climate station located approximately 50 km south. WATFLOOD was then used to predict future runoff using GCM outputs for the 2040–69 and 2070–99 time periods. The results gave dates of first and peak runoff that were, on average, up to 25 days earlier than in current (1961–90) climate. In addition, future runoff and evaporation volumes increased by up to 48% as a result of projected increases in temperature and precipitation. Furthermore, a large number of simulated years showed midwinter melt periods, which will have major impacts on snowpack properties and, in turn, on human, animal, and plant life in this region.Au cours des décennies à venir, puisque les modèles climatiques projettent des conditions plus chaudes et plus humides pour de nombreuses régions nordiques, les régions arctiques feront face à l’important enjeu du changement climatique. Du point de vue hydrologique, les enjeux critiques se traduisent par une saison de couverture de neige plus courte, par des changements du point de vue des propriétés de la couverture de neige hivernale ainsi que par des changements par rapport au moment et au volume d’écoulement de la fonte des neiges. Nous avons utilisé le modèle hydrologique distribué WATFLOOD, certains modèles de circulation globale et des scénarios climatiques futurs pour évaluer les incidences des changements projetés en matière de températures et de précipitations sur l’hydrologie d’un petit bassin d’amont de l’Arctique. Le modèle hydrologique a permis de simuler, de manière adéquate, l’écoulement du bassin soit grâce à l’introduction des données climatiques recueillies dans la région visée par l’étude, soit grâce aux données à long terme provenant d’une station climatique située à une cinquantaine de kilomètres au sud. Ensuite, WATFLOOD a permis de prédire l’écoulement futur en recourant au débit des modèles de circulation globale pour les périodes allant de 2040 à 2069 et de 2070 à 2099. D’après les résultats obtenus, les dates du premier écoulement et de l’écoulement de pointe seraient devancées de jusqu’à 25 jours par rapport au climat actuel (période de 1961 à 1990). De plus, les volumes d’écoulement et d’évaporation futurs connaissaient une augmentation atteignant jusqu’à 48 % en raison des élévations prévues de températures et de précipitations. De plus, un grand nombre d’années simulées a permis de constater des périodes de fonte en plein milieu de l’hiver, ce qui aura une grande incidence sur les propriétés de la couverture de neige et, par conséquent, sur les êtres humains, les animaux et la vie végétale dans cette région
Scattered light images of spiral arms in marginally gravitationally unstable discs with an embedded planet
Scattered light images of transition discs in the near-infrared often show
non-axisymmetric structures in the form of wide-open spiral arms in addition to
their characteristic low-opacity inner gap region. We study self-gravitating
discs and investigate the influence of gravitational instability on the shape
and contrast of spiral arms induced by planet-disc interactions.
Two-dimensional non-isothermal hydrodynamical simulations including viscous
heating and a cooling prescription are combined with three-dimensional dust
continuum radiative transfer models for direct comparison to observations. We
find that the resulting contrast between the spirals and the surrounding disc
in scattered light is by far higher for pressure scale height variations, i.e.
thermal perturbations, than for pure surface density variations. Self-gravity
effects suppress any vortex modes and tend to reduce the opening angle of
planet-induced spirals, making them more tightly wound. If the disc is only
marginally gravitationally stable with a Toomre parameter around unity, an
embedded massive planet (planet-to-star mass ratio of ) can trigger
gravitational instability in the outer disc. The spirals created by this
instability and the density waves launched by the planet can overlap resulting
in large-scale, more open spiral arms in the outer disc. The contrast of these
spirals is well above the detection limit of current telescopes.Comment: Accepted for publication in MNRAS; 13 pages, 8 figure
Correlations of Rydberg excitations in an ultra-cold gas after an echo sequence
We show that Rydberg states in an ultra-cold gas can be excited with strongly
preferred nearest-neighbor distance if densities are well below saturation. The
scheme makes use of an echo sequence in which the first half of a laser pulse
excites Rydberg states while the second half returns atoms to the ground state,
as in the experiment of Raitzsch et al. [Phys. Rev. Lett. 100 (2008) 013002].
Near to the end of the echo sequence, almost any remaining Rydberg atom is
separated from its next-neighbor Rydberg atom by a distance slightly larger
than the instantaneous blockade radius half-way through the pulse. These
correlations lead to large deviations of the atom counting statistics from a
Poissonian distribution. Our results are based on the exact quantum evolution
of samples with small numbers of atoms. We finally demonstrate the utility of
the omega-expansion for the approximate description of correlation dynamics
through an echo sequence.Comment: 8 pages, 6 figure
Observability of Forming Planets and their Circumplanetary Disks I. -- Parameter Study for ALMA
We present mock observations of forming planets with ALMA. The possible
detections of circumplanetary disks (CPDs) were investigated around planets of
Saturn, 1, 3, 5, and 10 Jupiter-masses that are placed at 5.2 AU from their
star. The radiative, three dimensional hydrodynamic simulations were then
post-processed with RADMC3D and the ALMA Observation Simulator. We found that
even though the CPDs are too small to be resolved, they are hot due to the
accreting planet in the optically thick limit, therefore the best chance to
detect them with continuum observations in this case is at the shortest ALMA
wavelengths, such as Band 9 (440 microns). Similar fluxes were found in the
case of Saturn and Jupiter-mass planets, as for the 10
gas-giant, due to temperature weighted optical depth effects: when no deep gap
is carved, the planet region is blanketed by the optically thick circumstellar
disk leading to a less efficient cooling there. A test was made for a 52 AU
orbital separation, showed that optically thin CPDs are also detectable in band
7 but they need longer integration times (5hrs). Comparing the gap profiles
of the same simulation at various ALMA bands and the hydro simulation confirmed
that they change significantly, first because the gap is wider at longer
wavelengths due to decreasing optical depth; second, the beam convolution makes
the gap shallower and at least 25% narrower. Therefore, caution has to be made
when estimating planet masses based on ALMA continuum observations of gaps.Comment: Accepted for publication at MNRAS. Typos are corrected since previous
version. 11 pages, 5 tables, 4 figure
Electron Temperature Evolution in Expanding Ultracold Neutral Plasmas
We have used the free expansion of ultracold neutral plasmas as a
time-resolved probe of electron temperature. A combination of experimental
measurements of the ion expansion velocity and numerical simulations
characterize the crossover from an elastic-collision regime at low initial
Gamma_e, which is dominated by adiabatic cooling of the electrons, to the
regime of high Gamma_e in which inelastic processes drastically heat the
electrons. We identify the time scales and relative contributions of various
processes, and experimentally show the importance of radiative decay and
disorder-induced electron heating for the first time in ultracold neutral
plasmas
Experimental Realization of an Exact Solution to the Vlasov Equations for an Expanding Plasma
We study the expansion of ultracold neutral plasmas in the regime in which
inelastic collisions are negligible. The plasma expands due to the thermal
pressure of the electrons, and for an initial spherically symmetric Gaussian
density profle, the expansion is self-similar. Measurements of the plasma size
and ion kinetic energy using fluorescence imaging and spectroscopy show that
the expansion follows an analytic solution of the Vlasov equations for an
adiabatically expanding plasma.Comment: 4 pages, 4 figure
Revisit of non-linear Landau damping for electrostatic instability driven by blazar-induced pair beams
We revisit the effect of non-linear Landau (NL) damping on the electrostatic
instability of blazar-induced pair beams, using a realistic pair-beam
distribution. We employ a simplified 2D model in -space to study the
evolution of the electric-field spectrum and to calculate the relaxation time
of the beam. We demonstrate that the 2D model is an adequate representation of
the 3D physics. We find that non-linear Landau damping, once it operates
efficiently, transports essentially the entire wave energy to small wavenumbers
where wave driving is weak or absent. The relaxation time also strongly depends
on the IGM temperature, , and for eV, and
in the absence of any other damping mechanism, the relaxation time of the pair
beam is longer than the inverse Compton (IC) scattering time. The weak
late-time beam energy losses arise from the accumulation of wave energy at
small , that non-linearly drains the wave energy at the resonant
of the pair-beam instability. Any other dissipation process
operating at small would reduce that wave-energy drain and hence lead to
stronger pair-beam energy losses. As an example, collisions reduce the
relaxation time by an order of magnitude, although their rate is very small.
Other non-linear processes, such as the modulation instability, could provide
additional damping of the non-resonant waves and dramatically reduce the
relaxation time of the pair beam. An accurate description of the spectral
evolution of the electrostatic waves is crucial for calculating the relaxation
time of the pair beam
Relativistic models for quasi-elastic neutrino scattering
We present quasi-elastic neutrino-nucleus cross sections in the energy range
from 150 MeV up to 5 GeV for the target nuclei 12C and 56Fe. A relativistic
description of the nuclear dynamics and the neutrino-nucleus coupling is
adopted. For the treatment of final-state interactions (FSI) we rely on two
frameworks succesfully applied to exclusive electron-nucleus scattering: a
relativistic optical potential and a relativistic multiple-scattering Glauber
approximation. At lower energies, the optical-potential approach is considered
to be the optimum choice, whereas at high energies a Glauber approach is more
natural. Comparing the results of both calculations, it is found that the
Glauber approach yields valid results down to the remarkably small nucleon
kinetic energies of 200 MeV. We argue that the nuclear transparencies extracted
from A(e,e'p) measurements can be used to obtain realistic estimates of the
effect of FSI mechanisms on quasi-elastic neutrino-nucleus cross sections. We
present two independent relativistic plane-wave impulse approximation (RPWIA)
calculations of quasi-elastic neutrino-nucleus cross sections. They agree at
the percent level, showing the reliability of the numerical techniques adopted
and providing benchmark RPWIA results.Comment: revised version,28 pages, 7 figures, accepted in Phys.Rev.
Absorption Imaging and Spectroscopy of Ultracold Neutral Plasmas
Absorption imaging and spectroscopy can probe the dynamics of an ultracold
neutral plasma during the first few microseconds after its creation.
Quantitative analysis of the data, however, is complicated by the inhomogeneous
density distribution, expansion of the plasma, and possible lack of global
thermal equilibrium for the ions. In this article we describe methods for
addressing these issues. Using simple assumptions about the underlying
temperature distribution and ion motion, the Doppler-broadened absorption
spectrum obtained from plasma images can be related to the average temperature
in the plasma.Comment: 14 pages, 8 figure
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