Time and space matter: how urban transitions create inequality

To analyze the response of cities to urban policies or transportation shocks, describing a succession of stationary states is not enough, and urban dynamics should be taken into account. To do so, the urban economics model NEDUM is proposed. This model reproduces the evolution of a monocentric city in continous time and captures the interaction between household moves, changes in at sizes, rent levels, and density of housing service supply. NEDUM allows, therefore, for a temporal and spatialized analysis of urban transitions. Applied to climate policies, this model suggests that the implementation of a transportation tax causes a larger welfare loss than can be inferred from traditional models. Moreover, such a tax increases signi cantly inequalities if its implementation is not anticipated enough. According to these results, therefore, smooth and early implementation paths of climate policies should be favored over delayed and aggressive action.City, Housing, Transportation

NAM-SCA: A Nonhydrostatic anelastic model with segmentally constant approximations

International audienceAn atmospheric convective system may be modeled as an ensemble of discrete plume elements. A representation of decomposited plumes provides the basis for mass-flux convective parameterization. A dry version of such a prototype model is constructed in a two-dimensional horizontally periodic domain. Each discrete plume element is approximated by a horizontally homogeneous segment such that the whole system is given by segmentally constant approximations (SCA) in the horizontal direction for each vertical level in a nonhydrostatic anelastic model (NAM). The distribution of constant segments is highly inhomogeneous in space and evolves with time in a highly adaptive manner. The basic modeling strategy from a physical point of view is to activate new segments vertically upward with time when a convective plume is growing and to deactivate segments when a plume event is over. The difference in physical values crossing segment interfaces is used as a criterion for numerically implementing this strategy. Whenever a large difference is found, the given interface is stretched vertically by subdividing an existing segment into two. In turn, when a segment interface difference is found below the threshold, the given interface is removed, thereby merging the two segments into one. This nonhydrostatic anelastic model with segmentally constant approximations (NAM-SCA) is tested on an idealized atmospheric convective boundary layer. It successfully simulates the evolution of convective plumes with a relatively limited number of segments (i.e., high compression) and with a much scarcer distribution of segments over nonplume environments (i.e., extremely inhomogeneous distribution of segments). Overall, this method compresses the size of the model up to 5 times compared to a standard NAM with homogeneous grid distribution without substantially sacrificing numerical accuracy. © 2010 American Meteorological Society

A formal analysis of the feedback concept in climate models. Part I: Exclusive and inclusive feedback analyses

International audienceClimate sensitivity and feedback are key concepts if the complex behavior of climate response to perturbation is to be interpreted in a simple way. They have also become an essential tool for comparing global circulation models and assessing the reason for the spread in their results. The authors introduce a formal basic model to analyze the practical methods used to infer climate feedbacks and sensitivity from GCMs. The tangent linear model is used first to critically review the standard methods of feedback analyses that have been used in the GCM community for 40 years now. This leads the authors to distinguish between exclusive feedback analyses as in the partial radiative perturbation approach and inclusive analyses as in the "feedback suppression" methods. This review explains the hypotheses needed to apply these methods with confidence. Attention is paid to the more recent regression technique applied to the abrupt 2×CO2 experiment. A numerical evaluation of it is given, related to the Lyapunov analysis of the dynamical feature of the regression. It is applied to the Planck response, determined in its most strict definition within the GCM. In this approach, the Planck feedback becomes a dynamical feedback among others and, as such, also has a fast response differing from its steady-state profile. © 2013 American Meteorological Society

Numerical Results for One Dimesionnal Configurations

0K or 500K with emissivities ffl = 1, 0:5 or 0:1. The gas is isothermal at ` = 1000K and is either pure carbon dioxide or pure water vapor at atmospheric pressure. In order to allow comparisons with published results, some simulations where held where only one spectral band is considered : the 3755cm \Gamma1 band for water vapor (extending from 2875cm \Gamma1 to 4250cm \Gamma1 ) and the 3715cm \Gamma1 band for carbon dioxide (extending from 3275cm \Gamma1 to 3875cm \Gamma1 ). Radiative band parameters are those published in Hartman et al. (1984), Soufiani et al. (1985), and Zhang et al. (1988). The discretization is that of Table 1. Surface radiation budgets at the walls and volumetric radi

An elicitation of the dynamic nature of water vapor feedback in climate change using a 1D model.

International audienceThe feedback concept has been used by several authors in the climatology field to describe model behavior and to assess the importance of different intervening mechanisms. Here, a simple global model of climate has been built to analyze the water vapor feedback, making use of elementary laws and parameterizations as determined by GCM results of CO2 doubling experiments. Beyond a static quantification of the water feedback, a more general formal definition of feedback gain based on the Tangent Linear System is introduced. This definition recovers the dynamical aspect of the system response to perturbation of Bode's original concept. Two conclusions are drawn from the model : (i) The water vapor effect is found to have a feedback gain of 36 % (1.6 factor), comparable with results from GCM analyzes, but with a very long characteristic time of over four years. (ii) The water vapor feedback is found negative for time scales below four years and positive for longer time scales. As a consequence, the water vapor feedback is fully active only in response to perturbations that last ten years at least. This suggests that the water vapor feedback could reduce the natural variability due to tropospheric temperature perturbations over short time scales while enhancing it over longer time scales

Analysis of Boundary-Layer Statistical Properties at Dome C, Antarctica

International audienceThe atmospheric boundary layer over the Antarctic Plateau is unique on account of its isolated location and extreme stability. Here we investigate the characteristics of the boundary layer using wind and temperature measurements from a 45-m high tower located at Dome C. First, spectral analysis reveals that both fields have a scaling behaviour from 30 min to 10 days (spectral slope β≈2 ). Wind and temperature time series also show a multifractal behaviour. Therefore, it is possible to fit the moment-scaling function to the universal multifractal model and obtain multifractal parameters for temperature ( α≈1.51,C1≈0.14 ) and wind speed ( α≈1.34,C1≈0.13 ). The same analysis is repeated separately in winter and summer at six different heights. The β parameter shows a strong stratification with height especially in summer, implying that properties of turbulence change surprisingly rapidly from the ground to the top of the tower

Feedback characteristics of nonlinear dynamical systems

International audienceWe propose a method to extend the concept of feedback gain to nonlinear models. The method is designed to dynamically characterise a feedback mechanism along the system natural trajectory. The numerical efficiency of the method is proved using the Lorenz (1963) classical model. Finally, a simple climate model of water vapour feedback shows how nonlinearity impacts feedback intensity along the seasonal cycle

Characterization of Atmospheric Ekman Spirals at Dome C, Antarctica

International audienceWe use wind speed and temperature measurements taken along a 45-m meteorological tower located at Dome C, Antarctica (75.06°S, 123.19)E) to highlight and characterize the Ekman spiral. Firstly, temperature records reveal that the atmospheric boundary layer at Dome C is stable during winter and summer nights (i.e., >85 % of the time). The wind vector, in both speed and direction, also shows a strong dependence with elevation. An Ekman model was then fitted to the measurements. Results show that the wind vector follows the Ekman spiral structure for more than 20 % of the year (2009). Most Ekman spirals have been detected during summer nights, that is, when the boundary layer is slightly stratified. During these episodes, the boundary-layer height ranged from 25 to 100 m, the eddy viscosity from 0.004 to 0.06 m2 s−1, and the Richardson number from zero to 1.6