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Reconstruction of constitutive parameters in isotropic linear elasticity from noisy full-field measurements
No full textInternational audienceWithin the framework of linear elasticity we assume the availability of internal full-field measurements of the continuum deformations of a non-homogeneous isotropic solid. The aim is the quantitative reconstruction of the associated moduli. A simple gradient system for the sought constitutive parameters is derived algebraically from the momentum equation, whose coefficients are expressed in terms of the measured displacement fields and their spatial derivatives. Direct integration of this system is discussed to finally demonstrate the inexpediency of such an approach when dealing with noisy data. Upon using polluted measurements, an alternative variational formulation is deployed to invert for the physical parameters. Analysis of this latter inversion procedure provides existence and uniqueness results while the reconstruction stability with respect to the measurements is investigated. As the inversion procedure requires differentiating the measurements twice, a numerical differentiation scheme based on an ad hoc regularization then allows an optimally stable reconstruction of the sought moduli. Numerical results are included to illustrate and assess the performance of the overall approach
Monetary and Fiscal Policy in a Liquidity Trap with Inflation Persistence
No full textThis paper relies on the new Keynesian model with inflation persistence to characterize the optimal monetary and fiscal policy in a liquidity trap. It shows that, with a Phillips curve that is both forward and backward looking, the monetary policy that is implemented during a liquidity trap episode can lift the economy out of depression. The central bank does not need to commit beyond the end of the crisis to get some traction on the level of economic activity. Regarding fiscal policy, inflation persistence justifies some front-loading of government expenditures to get ination started, which reduces the real interest rate. The magnitude of the optimal fiscal stimulus is decreasing in the degree of inflation persistence. Finally, if inflation persistence is due to adaptive expectations, rather than to price indexation, then monetary policy is ineffective while the optimal fiscal stimulus is large and heavily front-loaded
Weather Index Drought Insurance: An Ex Ante Evaluation for Millet Growers in Niger
No full textInternational audienceIn the Sudano-Sahelian region, which includes South Niger, the inter-annual variability of the rainy season is high and irrigation is limited. As a consequence, bad rainy seasons have a massive impact on crop yield and regularly result in food crises. Traditional insurance policies based on crop damage assessment are not available because of asymmetric information and high transaction costs compared to the value of production. We assess the risk mitigation capacity of an alternative form of insurance which has been implemented at a large scale in India since 2003: insurance based on a weather index. We compare the efficiency of various weather indices to increase the expected utility of a representative risk-averse farmer. We show the importance of using plot-level yield data rather than village averages, which bias results due to the presence of idiosyncratic shocks. We also illustrate the need for out-of-sample estimations in order to avoid overfitting. Even with the appropriate index and assuming substantial risk aversion, we find a limited gain of implementing insurance, which roughly corresponds to, or slightly exceeds, the cost observed in India for implementing such insurance policies. However, when we separately treat the plots with and without fertilisers separately, we see that the benefit of insurance is slightly higher in the former case. This suggests that insurance policies may slightly increase the use of risk-increasing inputs such as fertilisers and improved cultivars, and hence improve average yields, which remain very low in the region. © 2013 Springer Science+Business Media Dordrecht
The feasibility of retrieving vertical temperature profiles from satellite nadir UV observations: A sensitivity analysis and an inversion experiment with neural network algorithms
No full textInternational audienceAtmospheric temperature profiles are inferred from passive satellite instruments, using thermal infrared or microwave observations. Here we investigate on the feasibility of the retrieval of height resolved temperature information in the ultraviolet spectral region. The temperature dependence of the absorption cross sections of ozone in the Huggins band, in particular in the interval 320-325. nm, is exploited. We carried out a sensitivity analysis and demonstrated that a non-negligible information on the temperature profile can be extracted from this small band. Starting from these results, we developed a neural network inversion algorithm, trained and tested with simulated nadir EnviSat-SCIAMACHY ultraviolet observations. The algorithm is able to retrieve the temperature profile with root mean square errors and biases comparable to existing retrieval schemes that use thermal infrared or microwave observations. This demonstrates, for the first time, the feasibility of temperature profiles retrieval from space-borne instruments operating in the ultraviolet. © 2014 Elsevier Ltd
Turbulence compressible dans les plasmas spatiaux et astrophysiques : approche analytique et traitement des données du vent solaire
No full textMy thesis work is principally dedicated in understanding the role of compressibility in lowfrequency turbulence of space plasmas (the solar wind, magnetospheric plasma etc.) and astrophysical plasmas (interstellar molecular cloud, the core of a star etc.). Three new exact relations have been derived analytically in the framework of isothermal and polytropic hydrodynamic turbulence and also for an isothermal MHD plasma. By using these relations, various universal scalingproperties of compressible turbulence have been investigated. In addition, plausible phenomenologieshave been proposed in order to theoretically reproduce different power laws for energy power spectra which had been obtained in previous numerical simulations of compressible turbulence. A semi-qualitativedistinction between sub-sonic and supersonic regime of turbulence is hence concluded. In the second part, an analysis using THEMIS spacecraft data is also performed in a view to explainingthe effect of the compressibility in the turbulence of the fast solar wind. A remarkable smooth scalingin comparison with incompressible law is obtained for several intervals of fast solar wind. The corresponding turbulent energy flux is also found to be sufficient to explain the anomalous heating of fast solar wind.Ma thèse a pour but de comprendre le rôle de la compressibilité dans la turbulence aux basses fréquences dans les plasmas spatiaux (le vent solaire, les plasmas magnétosphériques etc.) et astrophysiques (nuage moléculaire interstellaire, le cœur d'une étoile etc.). Trois nouvelles relations exactes ont été déduites dans le cadre de la turbulence compressible dans un fluide isotherme et polytrope et dans un plasma MHD isotherme afin de comprendre différentes propriétés universelles de la turbulence compressible. De plausibles phénoménologies ont été proposées aussi en vue d'une compréhension de différentes lois de spectre obtenues grâce aux simulations numériques de la turbulence compressible. Une distinction qualitative entre la turbulence sous-sonique et supersonique est ainsi décrite.Une analyse utilisant des données d'observation des sondes spatiales THEMIS est également réalisée dans le but d'expliquer l'effet de la compressibilité dans la turbulence du vent solaire rapide. Une amélioration remarquable par rapport a "incompressible scaling" est observée avec la nouvelle "compressible scaling". Le flux d'énergie correspondant ainsi trouve est estime suffisant pour expliquer le chauffage anormal du vent solaire rapide
Unbiased black-box complexities of jump functions: how to cross large plateaus
No full textInternational audienceWe analyze the unbiased black-box complexity of jump functions with large jump sizes. Among other results, we show that when the jump size is (1/2 - epsilon)n, that is, only a small constant fraction of the fitness values is visible, then the unbiased black-box complexities for arities 3 and higher are of the same order as those for the simple OneMax function. Even for the extreme jump function, in which all but the two fitness values n/2 and n are blanked out, polynomial-time mutation-based (i.e., unary unbiased) black-box optimization algorithms exist. This is quite surprising given that for the extreme jump function almost the whole search space (all but a Theta(n-1/2) fraction) is a plateau of constant fitness.To prove these results, we introduce new tools for the analysis of unbiased black-box complexities, for example, selecting the new parent individual not by comparing the fitnesses of the competing search points, but also by taking into account the (empirical) expected fitnesses of their offspring
CRITICAL WETTING FOR A RANDOM LINE IN LONG-RANGE POTENTIAL
No full textWe consider a restricted Solid-on-Solid interface in Z_+, subject to apotential V(n) behaving at infinity like -w/n^2. Whenever there is a wetting transition as b_0=exp V(0) is varied, we prove the following results for the density of returns m(b_0) to the origin: If w1/8, there is no wetting transition
Effects of solar activity on noise in CALIOP profiles above the South Atlantic Anomaly
No full textInternational audienceWe show that nighttime dark noise measurements from the spaceborne lidar CALIOP contain valuable infor-mation about the evolution of upwelling high-energy ra-diation levels. Above the South Atlantic Anomaly (SAA), CALIOP dark noise levels fluctuate by ±6 % between 2006 and 2013, and follow the known anticorrelation of local par-ticle flux with the 11-year cycle of solar activity (with a 1-year lag). By analyzing the geographic distribution of noisy profiles, we are able to reproduce known findings about the SAA region. Over the considered period, it shifts westward by 0.3 • year −1 , and changes in size by 6 • meridionally and 2 • zonally, becoming larger with weaker solar activity. All results are in strong agreement with previous works. We pre-dict SAA noise levels will increase anew after 2014, and will affect future spaceborne lidar missions most near 2020
Direct radiative effect of the Russian wildfires and its impact on air temperature and atmospheric dynamics during August 2010
No full textInternational audienceIn this study, we investigate the shortwave aerosol direct radiative forcing (ADRF) and its feedback on air temperature and atmospheric dynamics during a major fire event that occurred in Russia during August 2010. The methodology is based on an offline coupling between the CHIMERE chemistry-transport and the Weather Research and Forecasting (WRF) models. First, simulations for the period 5–12 August 2010 have been evaluated by using AERONET (AErosol RObotic NETwork) and satellite measurements of the POLarization and Directionality of the Earth's Reflectance (POLDER) and the Cloud-Aerosol LIdar with Orthogonal Polarization (CALIOP) sensors. During this period, elevated POLDER aerosol optical thickness (AOT) is found over a large part of eastern Europe, with values above 2 (at 550 nm) in the aerosol plume. According to CALIOP observations, particles remain confined to the first five kilometres of the atmospheric layer. Comparisons with satellite measurements show the ability of CHIMERE to reproduce the regional and vertical distribution of aerosols during their transport from the source region. Over Moscow, AERONET measurements indicate an important increase of AOT (340 nm) from 0.7 on 5 August to 2–4 between 6 and 10 August when the aerosol plume was advected over the city. Particles are mainly observed in the fine size mode (radius in the range 0.2–0.4 μm) and are characterized by elevated single-scattering albedo (SSA) (0.95–0.96 between 440 and 1020 nm). Comparisons of simulations with AERONET measurements show that aerosol physical–optical properties (size distribution, AOT, SSA) have been well simulated over Moscow in terms of intensity and/or spectral dependence. Secondly, modelled aerosol optical properties have been used as input in the radiative transfer code of WRF to evaluate their direct radiative impact. Simulations indicate a significant reduction of solar radiation at the ground (up to 80–150 W m−2 in diurnal averages over a large part of eastern Europe due to the presence of the aerosol plume. This ADRF causes an important reduction of the near-surface air temperature between 0.2 and 2.6° on a regional scale. Moscow has been affected by the aerosol plume, especially between 6 and 10 August. During this period, aerosol causes a significant reduction of surface shortwave radiation (up to 70–84 W m−2 in diurnal averages) with a moderate part (20–30%) due to solar absorption within the aerosol layer. The resulting feedbacks lead to a cooling of the air up to 1.6° at the surface and 0.1° at an altitude of 1500–2000 m (in diurnal averages), that contribute to stabilize the atmospheric boundary layer (ABL). Indeed, a reduction of the ABL height of 13 to 65% has been simulated during daytime in presence of aerosols. This decrease is the result of a lower air entrainment as the vertical wind speed in the ABL is shown to be reduced by 5 to 80% (at midday) when the feedback of the ADRF is taken into account. However, the ADRF is shown to have a lower impact on the horizontal wind speed, suggesting that the dilution of particles would be mainly affected by the weakening of the ABL development and associated vertical entrainment. Indeed, CHIMERE simulations driven by the WRF meteorological fields including this ADRF feedback result in a large increase in the modelled near-surface PM10 concentrations (up to 99%). This is due to their lower vertical dilution in the ABL, which tend to reduce model biases with the ground PM10 values observed over Moscow during this specific period
Barotropic, baroclinic, and inertial instabilities of the easterly Gaussian jet on the equatorial β-plane in rotating shallow water model
No full textInternational audienceA detailed linear stability analysis of an easterly barotropic Gaussian jet centered at the equator is performed in the long-wave sector in the framework of one- and two-layer shallow-water models on the equatorial β-plane. It is shown that the dominant instability of the jet is due to phase-locking and resonance between Yanai waves, although the standard barotropic and baroclinic instabilities due to the resonance between Rossby waves are also present. In the one-layer case, this dominant instability has non-zero growth rate at zero wavenumber for high enough Rossby and low enough Burger numbers, thus reproducing the classical symmetric inertial instability. Yet its asymmetric counterpart has the highest growth rate. In the two-layer case, the dominant instability may be barotropic or baroclinic, the latter being stronger, with the maximum of the growth rate shifting towards smaller downstream wavenumbers as Rossby number increases at fixed Burger number, and given thickness and density ratios. At large enough Rossby numbers this instability has a non-zero growth rate limit at zero wavenumber, giving the baroclinic symmetric inertial instability. Again, the maximal growth rate is achieved at small but non-zero wavenumbers, corresponding to the asymmetric inertial instability. At high enough Rossby number and low enough Burger number not only the baroclinic, but also the barotropic symmetric instability appears, as well as higher meridional modes of the baroclinic symmetric instability. Still, all of them are dominated by their asymmetric counterparts. Direct numerical simulations of the saturation of the leading instabilities are performed, showing that the barotropic species of the instability saturates by forming a double vortex street subject to nonlinear oscillations, while the baroclinic, the most vigorous one, saturates by producing strong vertical shears and related dissipation and mixing. © 2014 AIP Publishing LL