10,039 research outputs found
Comment on \u201cCan assimilation of crowdsourced data in hydrological modelling improve flood prediction?\u201d by Mazzoleni et al. (2017)
Citizen science and crowdsourcing are gaining increasing attention among hydrologists. In a recent contribution, Mazzoleni et al. (2017) investigated the integration of crowdsourced data (CSD) into hydrological models to improve the accuracy of real-time flood forecasts. The authors used synthetic CSD (i.e. not actually measured), because real CSD were not available at the time of the study. In their work, which is a proof-of-concept study, Mazzoleni et al. (2017) showed that assimilation of CSD improves the overall model performance; the impact of irregular frequency of available CSD, and that of data uncertainty, were also deeply assessed. However, the use of synthetic CSD in conjunction with (semi-)distributed hydrological models deserves further discussion. As a result of equifinality, poor model identifiability, and deficiencies in model structure, internal states of (semi-)distributed models can hardly mimic the actual states of complex systems away from calibration points. Accordingly, the use of synthetic CSD that are drawn from model internal states under best-fit conditions can lead to overestimation of the effectiveness of CSD assimilation in improving flood prediction. Operational flood forecasting, which results in decisions of high societal value, requires robust knowledge of the model behaviour and an in-depth assessment of both model structure and forcing data. Additional guidelines are given that are useful for the a priori evaluation of CSD for real-time flood forecasting and, hopefully, for planning apt design strategies for both model calibration and collection of CSD
Modelling urban floods using a finite element staggered scheme with an anisotropic dual porosity model
In porosity models for urban flooding, artificial porosity is used as a statistical descriptor of the urban medium. Buildings are treated as subgrid-scale features and, even with the use of relatively coarse grids, their effects on the flow are accounted for. Porosity models are attractive for large-scale applications due to limited computational demand with respect to solving the classical Shallow Water Equations on high-resolution grids. In the last decade, effective schemes have been developed that allowed accounting for a wealth of sub-grid processes; unfortunately, they are known to suffer from over-sensitivity to mesh design in the case of anisotropic porosity fields, which are typical of urban layouts. In the present study, a dual porosity approach is implemented into a two-dimensional Finite Element numerical scheme that uses a staggered unstructured mesh. The presence of buildings is modelled using an isotropic porosity in the continuity equation, to account for the reduced water storage, and a tensor formulation for conveyance porosity in the momentum equations, to account for anisotropy and effective flow velocity. The element-by-element definition of porosities, and the use of a staggered grid in which triangular cells convey fluxes and continuity is balanced at grid nodes, allow avoiding undesired mesh-dependency. Tested against refined numerical solutions and data from a laboratory experiment, the model provided satisfactory results. Model limitations are discussed in view of applications to more complex, real urban layouts
Modelling urban floods using a finite element staggered scheme with porosity and anisotropic resistance
Artificial porosity models for urban flooding use porosity as a statistical descriptor for the presence of buildings, which are then treated as subgridscale features. Computational efficiency makes porosity models attractive for large-scale applications. These models are typically implemented in the framework of two-dimensional (2D) finite volume collocated schemes. The most effective schemes, falling under the category of Integral Porosity models, allow accounting for a wealth of sub-grid processes, but they are known to suffer from oversensitivity to mesh design in the case of anisotropic porosity fields. In the present exploratory study, a dual porosity approach is implemented into a staggered finite element numerical model. The free surface elevation is defined at grid nodes, where continuity equation is solved; fluxes are conveyed by triangular cells, which act as 2D-links between adjacent grid nodes. The presence of building is modelled using an isotropic porosity in the continuity equation to account for the reduced water storage, and an anisotropic conveyance porosity in the momentum equations to compute bottom shear stress. Both porosities are defined on an element-by-element basis, thus avoiding mesh-dependency. Although suffering a number of limitations, the model shows promising results
Horava-Lifshitz gravity with detailed balance
Horava-Lifshitz gravity with "detailed balance" but without the
projectability assumption is discussed. It is shown that detailed balance is
quite efficient in limiting the proliferation of couplings in Horava-Lifshitz
gravity, and that its implementation without the projectability assumption
leads to a theory with sensible dynamics. However, the (bare) cosmological
constant is restricted to be large and negative.Comment: Contribution to the proceedings of NEB 15 conference, Chania, 20-23
June 2012 (talk given by D.V.
On the difference-to-sum power ratio of speech and wind noise based on the Corcos model
The difference-to-sum power ratio was proposed and used to suppress wind
noise under specific acoustic conditions. In this contribution, a general
formulation of the difference-to-sum power ratio associated with a mixture of
speech and wind noise is proposed and analyzed. In particular, it is assumed
that the complex coherence of convective turbulence can be modelled by the
Corcos model. In contrast to the work in which the power ratio was first
presented, the employed Corcos model holds for every possible air stream
direction and takes into account the lateral coherence decay rate. The obtained
expression is subsequently validated with real data for a dual microphone
set-up. Finally, the difference-to- sum power ratio is exploited as a spatial
feature to indicate the frame-wise presence of wind noise, obtaining improved
detection performance when compared to an existing multi-channel wind noise
detection approach.Comment: 5 pages, 3 figures, IEEE-ICSEE Eilat-Israel conference (special
session
Asymmetric Dark Matter in the Sun and the Diphoton Excess at the LHC
It has been recently pointed out that a momentum-dependent coupling of the
asymmetric Dark Matter (ADM) with nucleons can explain the broad disagreement
between helioseismological observables and the predictions of standard solar
models. In this paper, we propose a minimal simplified ADM model consisting of
a scalar and a pseudoscalar mediator, in addition to a Dirac fermionic DM, for
generating such momentum-dependent interactions. Remarkably, the pseudoscalar
with mass around 750 GeV can simultaneously explain the solar anomaly and the
recent diphoton excess observed by both ATLAS and CMS experiments in the early
TeV LHC data. In this framework, the total width of the resonance
is naturally large, as suggested by the ATLAS experiment, since the resonance
mostly decays to the ADM pair. The model predicts the existence of a new light
scalar in the GeV range, interacting with quarks, and observable dijet, monojet
and signatures for the 750 GeV resonance at the LHC.Comment: 7 pages, 4 figures. Version to appear in PR
Group field theories for all loop quantum gravity
Group field theories represent a 2nd quantized reformulation of the loop
quantum gravity state space and a completion of the spin foam formalism. States
of the canonical theory, in the traditional continuum setting, have support on
graphs of arbitrary valence. On the other hand, group field theories have
usually been defined in a simplicial context, thus dealing with a restricted
set of graphs. In this paper, we generalize the combinatorics of group field
theories to cover all the loop quantum gravity state space. As an explicit
example, we describe the GFT formulation of the KKL spin foam model, as well as
a particular modified version. We show that the use of tensor model tools
allows for the most effective construction. In order to clarify the
mathematical basis of our construction and of the formalisms with which we
deal, we also give an exhaustive description of the combinatorial structures
entering spin foam models and group field theories, both at the level of the
boundary states and of the quantum amplitudes.Comment: version published in New Journal of Physic
Gravity with Auxiliary Fields
Modifications of General Relativity usually include extra dynamical degrees
of freedom, which to date remain undetected. Here we explore the possibility of
modifying Einstein's theory by adding solely nondynamical fields. With the
minimal requirement that the theory satisfies the weak equivalence principle
and admits a covariant Lagrangian formulation, we show that the field equations
generically have to include higher-order derivatives of the matter fields. This
has profound consequences for the viability of these theories. We develop a
parametrization based on a derivative expansion and show that - to next to
leading order - all theories are described by just two parameters. Our approach
can be used to put stringent, theory-independent constraints on such theories,
as we demonstrates using the Newtonian limit as an example.Comment: 5 pages, no figures; v2: clarifications and minor improvements,
matches published versio
QCD resummation for semi-inclusive hadron production processes
We investigate the resummation of large logarithmic perturbative corrections
to hadron production in electron-positron annihilation and semi-inclusive
deep-inelastic scattering. We find modest, but significant, enhancements of
hadron multiplicities in the kinematic regimes accessible in present
high-precision experiments. Our results are therefore relevant for the
determination of hadron fragmentation functions from data for these processes.Comment: 14 pages, 11 figure
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