465 research outputs found
Opportunities for Increasing Resilience and Sustainability of Urban Social–Ecological Systems: Insights from the URBES and the Cities and Biodiversity Outlook Projects
Urban futures that are more resilient and sustainable require an integrated social–ecological system approach to urban policymaking, planning, management, and governance. In this article, we introduce the Urban Biodiversity and Ecosystem Services (URBES) and the Cities and Biodiversity Outlook (CBO) Projects as new social–ecological contributions to research and practice on emerging urban resilience and ecosystem services. We provide an overview of the projects and present global urbanization trends and their effects on ecosystems and biodiversity, as a context for new knowledge generated in the URBES case-study cities, including Berlin, New York, Rotterdam, Barcelona, and Stockholm. The cities represent contrasting urbanization trends and examples of emerging science–policy linkages for improving urban landscapes for human health and well-being. In addition, we highlight 10 key messages of the global CBO assessment as a knowledge platform for urban leaders to incorporate state-of-the-art science on URBES into decision-making for sustainable and resilient urban development
A finite volume method for two-moment cosmic-ray hydrodynamics on a moving mesh
We present a new numerical algorithm to solve the recently derived equations
of two-moment cosmic ray hydrodynamics (CRHD). The algorithm is implemented as
a module in the moving mesh Arepo code. Therein, the anisotropic transport of
cosmic rays (CRs) along magnetic field lines is discretised using a
path-conservative finite volume method on the unstructured time-dependent
Voronoi mesh of Arepo. The interaction of CRs and gyroresonant Alfv\'en waves
is described by short-timescale source terms in the CRHD equations. We employ a
custom-made semi-implicit adaptive time stepping source term integrator to
accurately integrate this interaction on the small light-crossing time of the
anisotropic transport step. Both the transport and the source term integration
step are separated from the evolution of the magneto-hydrodynamical equations
using an operator split approach. The new algorithm is tested with a variety of
test problems, including shock tubes, a perpendicular magnetised discontinuity,
the hydrodynamic response to a CR overpressure, CR acceleration of a warm
cloud, and a CR blast wave, which demonstrate that the coupling between CR and
magneto-hydrodynamics is robust and accurate. We demonstrate the numerical
convergence of the presented scheme using new linear and non-linear analytic
solutions.Comment: 24 pages, 15 figures, submitted to MNRAS, comments are welcome
Coupling multi-fluid dynamics equipped with Landau closures to the particle-in-cell method
The particle-in-cell (PIC) method is successfully used to study magnetized
plasmas. However, this requires large computational costs and limits
simulations to short physical run-times and often to setups in less than three
spatial dimensions. Traditionally, this is circumvented either via hybrid-PIC
methods (adopting massless electrons) or via magneto-hydrodynamic-PIC methods
(modelling the background plasma as a single charge-neutral
magneto-hydrodynamical fluid). Because both methods preclude modelling
important plasma-kinetic effects, we introduce a new fluid-PIC code that
couples a fully explicit and charge-conservative multi-fluid solver to the PIC
code SHARP through a current-coupling scheme and solve the full set of
Maxwell's equations. This avoids simplifications typically adopted for Ohm's
Law and enables us to fully resolve the electron temporal and spatial scales
while retaining the versatility of initializing any number of ion, electron, or
neutral species with arbitrary velocity distributions. The fluid solver
includes closures emulating Landau damping so that we can account for this
important kinetic process in our fluid species. Our fluid-PIC code is
second-order accurate in space and time. The code is successfully validated
against several test problems, including the stability and accuracy of shocks
and the dispersion relation and damping rates of waves in unmagnetized and
magnetized plasmas. It also matches growth rates and saturation levels of the
gyro-scale and intermediate-scale instabilities driven by drifting charged
particles in magnetized thermal background plasmas in comparison to linear
theory and PIC simulations. This new fluid-SHARP code is specially designed for
studying high-energy cosmic rays interacting with thermal plasmas over
macroscopic timescales.Comment: 35 pages, 11 figures, submitted to JPP. Comments are welcom
Accurate determination of elastic parameters for multi-component membranes
Heterogeneities in the cell membrane due to coexisting lipid phases have been
conjectured to play a major functional role in cell signaling and membrane
trafficking. Thereby the material properties of multiphase systems, such as the
line tension and the bending moduli, are crucially involved in the kinetics and
the asymptotic behavior of phase separation. In this Letter we present a
combined analytical and experimental approach to determine the properties of
phase-separated vesicle systems. First we develop an analytical model for the
vesicle shape of weakly budded biphasic vesicles. Subsequently experimental
data on vesicle shape and membrane fluctuations are taken and compared to the
model. The combined approach allows for a reproducible and reliable
determination of the physical parameters of complex vesicle systems. The
parameters obtained set limits for the size and stability of nanodomains in the
plasma membrane of living cells.Comment: (*) authors contributed equally, 6 pages, 3 figures, 1 table; added
insets to figure
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