148 research outputs found
Lithium depletion in solar-like stars: effect of overshooting based on realistic multi-dimensional simulations
We study lithium depletion in low-mass and solar-like stars as a function of
time, using a new diffusion coefficient describing extra-mixing taking place at
the bottom of a convective envelope. This new form is motivated by
multi-dimensional fully compressible, time implicit hydrodynamic simulations
performed with the MUSIC code. Intermittent convective mixing at the convective
boundary in a star can be modeled using extreme value theory, a statistical
analysis frequently used for finance, meteorology, and environmental science.
In this letter, we implement this statistical diffusion coefficient in a
one-dimensional stellar evolution code, using parameters calibrated from
multi-dimensional hydrodynamic simulations of a young low-mass star. We propose
a new scenario that can explain observations of the surface abundance of
lithium in the Sun and in clusters covering a wide range of ages, from
50 Myr to 4 Gyr. Because it relies on our physical model of convective
penetration, this scenario has a limited number of assumptions. It can explain
the observed trend between rotation and depletion, based on a single additional
assumption, namely that rotation affects the mixing efficiency at the
convective boundary. We suggest the existence of a threshold in stellar
rotation rate above which rotation strongly prevents the vertical penetration
of plumes and below which rotation has small effects. In addition to providing
a possible explanation for the long standing problem of lithium depletion in
pre-main sequence and main sequence stars, the strength of our scenario is that
its basic assumptions can be tested by future hydrodynamic simulations.Comment: 7 pages, 3 figures, Accepted for publication in ApJ Letter
Global precipitation response to changing forcings since 1870
Predicting and adapting to changes in the hydrological cycle is one of the major challenges for the 21st century. To better estimate how it will respond to future changes in climate forcings, it is crucial to understand how the hydrological cycle has evolved in the past and why. In our study, we use an atmospheric global climate model with prescribed sea surface temperatures (SSTs) to investigate how, in the period 1870–2005, changing climate forcings have affected the global land temperature and precipitation. We show that between 1870 and 2005, prescribed SSTs (encapsulating other forcings and internal variability) determine the decadal and interannual variabilities of the global land temperature and precipitation, mostly via their influence in the tropics (25° S–25° N). In addition, using simulations with prescribed SSTs and considering the atmospheric response alone, we find that between 1930 and 2005 increasing aerosol emissions have reduced the global land temperature and precipitation by up to 0.4 °C and 30 mm yr<sup>−1</sup>, respectively, and that between about 1950 and 2005 increasing greenhouse gas concentrations have increased them by up to 0.25 °C and 10 mm yr<sup>−1</sup>, respectively. Finally, we suggest that between about 1950 and 1970, increasing aerosol emissions had a larger impact on the hydrological cycle than increasing greenhouse gas concentrations
Assessment of parameters describing representativeness of air quality in-situ measurement sites
The atmospheric layer closest to the ground is strongly influenced by variable surface fluxes (emissions, surface deposition) and can therefore be very heterogeneous. In order to perform air quality measurements that are representative of a larger domain or a certain degree of pollution, observatories are placed away from population centres or within areas of specific population density. Sites are often categorised based on subjective criteria that are not uniformly applied by the atmospheric community within different administrative domains yielding an inconsistent global air quality picture. A novel approach for the assessment of parameters reflecting site representativeness is presented here, taking emissions, deposition and transport towards 34 sites covering Western and Central Europe into account. These parameters are directly inter-comparable among the sites and can be used to select sites that are, on average, more or less suitable for data assimilation and comparison with satellite and model data. Advection towards these sites was simulated by backward Lagrangian Particle Dispersion Modelling (LPDM) to determine the sites' average catchment areas for the year 2005 and advection times of 12, 24 and 48 h. Only variations caused by emissions and transport during these periods were considered assuming that these dominate the short-term variability of most but especially short lived trace gases. The derived parameters describing representativeness were compared between sites and a novel, uniform and observation-independent categorisation of the sites based on a clustering approach was established. Six groups of European background sites were identified ranging from <i>generally remote</i> to more polluted <i>agglomeration</i> sites. These six categories explained 50 to 80% of the inter-site variability of median mixing ratios and their standard deviation for NO<sub>2</sub> and O<sub>3</sub>, while differences between group means of the longer-lived trace gas CO were insignificant. The derived annual catchment areas strongly depended on the applied LPDM and input wind fields, the catchment settings and the year of analysis. Nevertheless, the parameters describing representativeness showed considerably less variability than the catchment geometry, supporting the applicability of the derived station categorisation
An ensemble-based approach to climate reconstructions
Data assimilation is a promising approach to obtain climate reconstructions that are both consistent with observations of the past and with our understanding of the physics of the climate system as represented in the climate model used. Here, we investigate the use of ensemble square root filtering (EnSRF) – a technique used in weather forecasting – for climate reconstructions. We constrain an ensemble of 29 simulations from an atmosphere-only general circulation model (GCM) with 37 pseudo-proxy temperature time series. Assimilating spatially sparse information with low temporal resolution (semi-annual) improves the representation of not only temperature, but also other surface properties, such as precipitation and even upper air features such as the intensity of the northern stratospheric polar vortex or the strength of the northern subtropical jet. Given the sparsity of the assimilated information and the limited size of the ensemble used, a localisation procedure is crucial to reduce "overcorrection" of climate variables far away from the assimilated information
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Changes in atmospheric shortwave absorption as important driver of dimming and brightening
The amount of solar (shortwave) radiation that reaches the Earth’s surface underwent substantial variations over recent decades. Since the 1950s, surface shortwave radiation gradually decreased at widespread locations. In Europe, this so-called surface dimming continued until the late 1980s, when surface brightening set in and surface shortwave radiation increased again. In China, the dimming levelled off in the 1980s, but did not turn into brightening until 2005. Changes in clouds and aerosol are the prime potential causes for the phenomenon, but the scientific community has not yet reached a consensus about the relative role of the different potential forcing agents. Here we bring together co-located long-term observational data from surface and space to study decadal changes of the shortwave energy balance in Europe and China from 1985 to 2015. Within this observation-based framework, we show that an increasing net shortwave radiation at the top of the atmosphere and a decreasing atmospheric shortwave absorption each contribute roughly half of the observed brightening trends in Europe. For China, we find that the continued dimming until 2005 and the subsequent brightening occurred despite opposing trends in the top-of-the-atmosphere net shortwave radiation. This shows that changes in atmospheric shortwave absorption are a major driver of European brightening and the dominant cause for the Chinese surface trends. Although the observed variations cannot be attributed unambiguously, we discuss potential causes for the observed changes
One-dimensional thermal pressure-driven expansion of a pair cloud into an electron-proton plasma
Recently a filamentation instability was observed when a laser-generated pair
cloud interacted with an ambient plasma. The magnetic field it drove was strong
enough to magnetize and accelerate the ambient electrons. It is of interest to
determine if and how pair cloud-driven instabilities can accelerate ions in the
laboratory or in astrophysical plasma. For this purpose, the expansion of a
localized pair cloud with the temperature 400 keV into a cooler ambient
electron-proton plasma is studied by means of one-dimensional particle-in-cell
(PIC) simulations. The cloud's expansion triggers the formation of electron
phase space holes that accelerate some protons to MeV energies. Forthcoming
lasers might provide the energy needed to create a cloud that can accelerate
protons.Comment: 5 pages 4 figures, accepted for publication in Physics of Plasma
Three-dimensional structure and stability of discontinuities between unmagnetized pair plasma and magnetized electron-proton plasma
We study with a 3D PIC simulation discontinuities between an
electron-positron pair plasma and magnetized electrons and protons. A pair
plasma is injected at one simulation boundary with a speed 0.6 along its
normal. It expands into an electron-proton plasma and a magnetic field that
points orthogonally to the injection direction. Diamagnetic currents expel the
magnetic field from within the pair plasma and pile it up in front of it. It
pushes electrons, which induces an electric field pulse ahead of the magnetic
one. This initial electromagnetic pulse (EMP) confines the pair plasma
magnetically and accelerates protons electrically. The fast flow of the
injected pair plasma across the protons behind the initial EMP triggers the
filamentation instability. Some electrons and positrons cross the injection
boundary and build up a second EMP. Electron-cyclotron drift instabilities
perturb the plasma ahead of both EMPs seeding a Rayleigh-Taylor-type
instability. Despite equally strong perturbations ahead of both EMPs, the
second EMP is much more stable than the initial one. We attribute the rapid
collapse of the initial EMP to the filamentation instability, which perturbed
the plasma behind it. The Rayleigh-Taylor-type instability transforms the
planar EMPs into transition layers, in which magnetic flux ropes and
electrostatic forces due to uneven numbers of electrons and positrons slow down
and compress the pair plasma and accelerate protons. In our simulation, the
expansion speed of the pair cloud decreased by about an order of magnitude and
its density increased by the same factor. Its small thickness implies that it
is capable of separating a relativistic pair outflow from an electron-proton
plasma, which is essential for collimating relativistic jets of pair plasma in
collisionless astrophysical plasma.Comment: 25 pages, 12 figures, provisionally accepted for publication by the
New Journal of Physic
Comparison of two- and three-dimensional compressible convection in a pre-main sequence star
Extending our recent studies of two-dimensional stellar convection to 3D, we
compare three-dimensional hydrodynamic simulations to identically set-up
two-dimensional simulations, for a realistic pre-main sequence star. We compare
statistical quantities related to convective flows including: average velocity,
vorticity, local enstrophy, and penetration depth beneath a convection zone.
These statistics are produced during stationary, steady-state compressible
convection in the star's convection zone. Our simulations with the MUSIC code
confirm the common result that two-dimensional simulations of stellar
convection have a higher magnitude of velocity on average than
three-dimensional simulations. Boundary conditions and the extent of the
spherical shell can affect the magnitude and variability of convective
velocities. The difference between 2D and 3D velocities is dependent on these
background points; in our simulations this can have an effect as large as the
difference resulting from the dimensionality of the simulation. Nevertheless,
radial velocities near the convective boundary are comparable in our 2D and 3D
simulations. The average local enstrophy of the flow is lower for
two-dimensional simulations than for three-dimensional simulations, indicating
a different shape and structuring of 3D stellar convection. We perform a
statistical analysis of the depth of convective penetration below the
convection zone, using the model proposed in our recent study (Pratt et al.
2017). Here we analyze the convective penetration in three dimensional
simulations, and compare the results to identically set-up 2D simulations. In
3D the penetration depth is as large as the penetration depth calculated from
2D simulations.Comment: 13 pages, 8 figure
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