1,720,989 research outputs found
Lyapunov vectors and assimilation in the unstable subspace: Theory and applications
No full textChaos, Data AssimilationBased on a limited number of noisy observations, estimation algorithms provide a complete description of the state of a system at current time. Estimation algorithms that go under the name of assimilation in the unstable subspace (AUS) exploit the nonlinear stability properties of the forecasting model in their formulation. Errors that grow due to sensitivity to initial conditions are efficiently removed by confining the analysis solution in the unstable and neutral subspace of the system, the subspace spanned by Lyapunov vectors with positive and zero exponents, while the observational noise does not disturb the system along the stable directions. The formulation of the AUS approach in the context of four-dimensional variational assimilation (4DVar-AUS) and the extended Kalman filter (EKF-AUS) and its application to chaotic models is reviewed. In both instances, the AUS algorithms are at least as efficient but simpler to implement and computationally less demanding than their original counterparts. As predicted by the theory when error dynamics is linear, the optimal subspace dimension for 4DVar-AUS is given by the number of positive and null Lyapunov exponents, while the EKF-AUS algorithm, using the same unstable and neutral subspace, recovers the solution of the full EKF algorithm, but dealing with error covariance matrices of a much smaller dimension and significantly reducing the computational burden. Examples of the application to a simplified model of the atmospheric circulation and to the optimal velocity model for traffic dynamics are given. This article is part of a special issue of Journal of Physics A: Mathematical and Theoretical devoted to 'Lyapunov analysis: from dynamical systems theory to applications'. © 2013 IOP Publishing Ltd
Towards a Flux-Partitioning Procedure Based on the Direct Use of High-Frequency Eddy-Covariance Data
No full textScanlon and Sahu (Water Resour Res 44(10):W10418, 2008) proposed an interesting method to estimate assimilation, respiration, evaporation and transpiration directly using high-frequency eddy-covariance measurements. In this note we critically revise this method and, in particular, using the Descartes' rule of sign, we show that one branch of solutions can be directly neglected reducing the analytical complexity of the procedure. We also discuss the stability of the results of the method with respect to the input parameters, especially to the water-use efficienc
Quantum measurement and entropy production
No full textWe study the time evolution of a quantum system without classical counterpart, undergoing a process of entropy increase due to the environment influence. We show that if the environment-induced decoherence is interpreted in terms of wave-function collapses, a symbolic sequence can be generated. We prove that the Kolmogorov-Sinai entropy of this sequence coincides with rate of von Neumann entropy increase. (C) 2001 Elsevier Science B.V. All rights reserved
A phenomenological estimate of the true scale of CoViD-19 from primary data
No full textEstimation of the prevalence of undocumented SARS-CoV-2 infections is critical for understanding the overall impact of CoViD-19, and for implementing effective public policy intervention strategies. We discuss a simple yet effective approach to estimate the true number of people infected by SARS-CoV-2, using raw epidemiological data reported by official health institutions in the largest EU countries and the USA
The water deficit and aridity indexes in the Capitanata plain calculated by statistical downscaling
No full textThis work describes the results obtained by the statistical downscaling
technique for the assessment of changes in precipitation (P),
potential evaporation (PE). In turn P and PE are used for computing
two indexes of water availability, namely the index of water deficit
(WDI) and the aridity index (AI). The analysis is carried out for the
Capitanata plain (South-East of Italy) and the A2 scenario of the IPCC
Assessment Report 4 (AR4). The large-scale temperature at the
1000hPa level and sea level pressure fields are used as predictors. The
local precipitation and potential evaporation time series are used as
predictands. The statistical downscaling technique used is based on
Canonical Correlation Analysis. A validation procedure of the model is
performed and the same technique is used for climatic projections of
P, PE and consequently WDI and AI. Climate analysis and projections at
this local space scale is an important issue not only for current water
management and planning, but also for improving the irrigation efficiency
considering future climate change scenarios
On the use of aggregated human mobility data to estimate the reproduction number
No full textThe reproduction number of an infectious disease, such as CoViD-19, can be described through a modified version of the susceptible-infected-recovered (SIR) model with time-dependent contact rate, where mobility data are used as proxy of average movement trends and interpersonal distances. We introduce a theoretical framework to explain and predict changes in the reproduction number of SARS-CoV-2 in terms of aggregated individual mobility and interpersonal proximity (alongside other epidemiological and environmental variables) during and after the lockdown period. We use an infection-age structured model described by a renewal equation. The model predicts the evolution of the reproduction number up to a week ahead of well-established estimates used in the literature. We show how lockdown policies, via reduction of proximity and mobility, reduce the impact of CoViD-19 and mitigate the risk of disease resurgence. We validate our theoretical framework using data from Google, Voxel51, Unacast, The CoViD-19 Mobility Data Network, and Analisi Distribuzione Aiuti
Fluoride crystals: 2 mu m Ho3+ laser emission and energy transfer mechanisms in Er3+
No full textA spectroscopic analysis of nonlinear energy transfer in BaY2F8 and LiYF4 doped with Er3+ is reported. We developed a new method to measure the upconversion parameter based on the sinusoidal modulation of the pump beam intensity and on the intensity spectrum analysis of the green fluorescence. We also present cw laser results in the 2 mum wavelength region of a Tm,Ho : LiYF4 crystal with 30% slope efficiency at room temperature. (C) 2001 Elsevier Science B.V. All rights reserved
Climate change assessment for Mediterranean agricultural areas by statistical downscaling
No full textMicroscopic treatment of upconversion in Nd3+-doped samples
No full textWe deal with the analysis of fluorescence decay of metastable manifolds of rare-earth ions in the presence of upconversion processes, with attention focused on Nd3+-doped crystals. In the literature this phenomenon is usually studied by means of rate equations or microscopic treatment. Here we show that only the second approach is correct in our experimental conditions, i.e., when the population dynamics is fast in comparison with the typical migration time tau(0), and tau(0) is considerably longer than the radiative lifetime. We studied the population dynamics after pulsed-laser excitation of some Nd3+-doped crystals, namely, Bay(2)F(8):Nd3+ at 3.75%, LiYF4:Nd3+ at 1.05%, and KLa(MoO4)(2):Nd3+ at 5.3%. We observed that the rate-equation formalism cannot reproduce the experimental data, therefore we used a microscopic treatment that gave much better results. From this analysis, after reaching the saturation regime, we were able to determine the donor-acceptor transfer constant Cd. for the samples under investigation in an unconventional way. (C) 2003 Optical Society of America
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