1,721,105 research outputs found
Beyond the linear fluctuation-dissipation theorem: the role of causality
No full textIn this paper we tackle the traditional problem of relating the fluctuations of a system to its response to external forcings and extend the classical theory in order to be able to encompass also nonlinear processes. With this goal, we try to build on Kubo's linear response theory and the response theory recently developed by Ruelle for nonequilibrium systems equipped with an invariant Sinai-Ruelle-Bowen (SRB) measure. Our derivation also sheds light on the link between causality and the possibility of relating fluctuations and response, both at the linear and nonlinear level. We first show, in a rather general setting, how the formalism of Ruelle's response theory can be used to derive in a novel way a generalization of the Kramers-Kronig relations. We then provide a formal extension at each order of nonlinearity of the fluctuation-dissipation theorem for general systems endowed with a smooth invariant measure. Finally, we focus on the physically relevant case of systems weakly perturbed from equilibrium, for which we present explicit fluctuation-dissipation relations linking the susceptibility describing the nth order response of the system with suitably defined correlations taken in the equilibrium ensemble
Entropy production and coarse graining of the climate fields in a general circulation model
No full textWe extend the analysis of the thermodynamics of the climate system by investigating the role played by processes taking place at various spatial and temporal scales through a procedure of coarse graining. We show that the coarser is the graining of the climatic fields, the lower is the resulting estimate of the material entropy production. In other terms, all the spatial and temporal scales of variability of the thermodynamic fields provide a positive contribution to the material entropy production. This may be interpreted also as that, at all scales, the temperature fields and the heating fields resulting from the convergence of turbulent fluxes have a negative correlation, while the opposite holds between the temperature fields and the radiative heating fields. Moreover, we obtain that the latter correlations are stronger, which confirms that radiation acts as primary driver for the climatic processes, while the material fluxes dampen the resulting fluctuations through dissipative processes. We also show, using specific coarse-graining procedures, how one can separate the various contributions to the material entropy production coming from the dissipation of kinetic energy, the vertical sensible and latent heat fluxes, and the large scale horizontal fluxes, without resorting to the full three-dimensional time dependent fields. We find that most of the entropy production is associated to irreversible exchanges occurring along the vertical direction, and that neglecting the horizontal and time variability of the fields has a relatively small impact on the estimate of the material entropy production. The approach presented here seems promising for testing climate models, for assessing the impact of changing their parametrizations and their resolution, as well as for investigating the atmosphere of exoplanets, because it allows for evaluating the error in the estimate of their thermodynamical properties due to the lack of high-resolution data. The findings on the impact of coarse graining on the thermodynamic fields on the estimate of the material entropy production deserve to be explored in a more general context, because they provide a way for understanding the relationship between forced fluctuations and dissipative processes in continuum systems. © 2014 Springer-Verlag Berlin Heidelberg
Supplementary Material for "Typicality of the 2021 Western North America Summer Heatwave" by V. Lucarini, V. M. Galfi, G. Messori, and J. Riboldi
No full textSupplementary material for the article Typicality of the 2021 Western North America Summer Heatwave" by V. Lucarini, V. M. Galfi, G. Messori, and J. Riboldi, </p
Hydrological cycle over South and Southeast Asian river basins as simulated by PCMDI/CMIP3 experiments
No full textWe investigate how the climate models contributing to the PCMDI/CMIP3 dataset describe the hydrological cycle over four major South and Southeast Asian river basins (Indus, Ganges, Brahmaputra and Mekong) for the 20th, 21st (13 models) and 22nd (10 models) centuries. For the 20th century, some models do not seem to conserve water at the river basin scale up to a good degree of approximation. The simulated precipitation minus evaporation (P-E), total runoff (R) and precipitation (P) quantities are neither consistent with the observations nor among the models themselves. Most of the models underestimate P-E for all four river basins, which is mainly associated with the underestimation of precipitation. This is in agreement with the recent results on the biases of the representation of monsoonal dynamics by GCMs. Overall, a modest inter-model agreement is found only for the evaporation and inter-annual variability of P-E. For the 21st and 22nd centuries, models agree on the negative (positive) changes of P-E for the Indus basin (Ganges, Brahmaputra and Mekong basins). Most of the models foresee an increase in the inter-annual variability of P-E for the Ganges and Mekong basins, thus suggesting an increase in large low-frequency drywet events. Instead, no considerable future change in the inter-annual variability of P-E is found for the Indus and Brahmaputra basins © Author(s) 2013
Influence of the Tumor Microenvironment on NK Cell Function in Solid Tumors
Full text linkNatural killer (NK) cells are a population of innate lymphoid cells playing a pivotal role in host immune responses against infection and tumor growth. These cells have a powerful cytotoxic activity orchestrated by an intricate network of inhibitory and activating signals. The importance of NK cells in controlling tumor growth and in mediating a robust anti-metastatic effect has been demonstrated in different experimental mouse cancer models. Consistently, high density of tumor-infiltrating NK cells has been linked with a good prognosis in multiple human solid tumors. However, there are also tumors that appear to be refractory to NK cell-mediated killing for the presence of an immunosuppressive microenvironment affecting NK cell function. Immunotherapeutic strategies aimed at restoring and increasing the cytotoxic activity of NK cells in solid tumors, including the adoptive transfer of NK and CAR-NK cells, are currently employed in preclinical and clinical studies. In this review, we outline recent advances supporting the direct role of NK cells in controlling expansion of solid tumors and their prognostic value in human cancers. We summarize the mechanisms adopted by cancer cells and the tumor microenvironment to affect NK cell function, and finally we evaluate current strategies to augment the antitumor function of NK cells for the treatment of solid tumors
Statistics of the seasonal cycle of the 1951-2000 surface temperature records in Italy and in the Mediterranean area
No full textWe present an analysis of the seasonal cycle of the last 50 years of records of surface temperature in Italy, as described by observations of maximum and minimum daily temperature, and of the surface and upper air temperature of the whole Mediterranean area, as described by the 1951-2000 NCEP reanalysis.
We compute the best estimate of the seasonal cycle of the variables considered by adopting the cyclograms’ technique. In the case of the Italian surface temperature, we observe that in general the minimum temperature cycle lags behind the maximum
temperature cycle, and that the cycles of the Southern Italy temperatures records lag behind the corresponding cycles referring to Northern Italy. In the case of the NCEP reanalysis data for the whole Mediterranean area, we observe that at surface the phase and amplitude of the seasonal cycle are strongly characterized by the signature of the underlying surface, while in the upper air large-scale features related to ocean-continent contrast come into play. All seasonal cycles lag considerably behind the solarcycle. In all cases considered, the amplitude and phase of the seasonal cycles do not show any statistically significant trend in the time interval considered. This works supports the idea that climate change studies are much more reliable when upper air data are taken into account
Statistics of the seasonal cycle of the 1951-2000 surface temperature records in Italy
No full text|We present an analysis of seasonal cycle of the last 50 years of records of surface temperature in Italy. We consider two data sets which synthesize the surface temperature fields of Northern and Southern Italy. Such data sets consist of
records of daily maximum and minimum temperature. We compute the best estimate of the seasonal cycle of the variables considered by adopting the cyclograms'
technique. We observe that in general the minimum temperature cycle lags behind the maximum temperature cycle, and that the cycles of the Southern Italy temper-
atures records lag behind the corresponding cycles referring to Northern Italy. All seasonal cycles lag considerably behind the solar cycle. The amplitude and phase of the seasonal cycles do not show any statistically signi ̄cant trend in the time interval considered
Linear and fractional response for the SRB measure of smooth hyperbolic attractors and discontinuous observables
No full textWe consider a smooth one-parameter family t → ( ft : M → M) of diffeomorphisms with compact transitive Axiom A attractors λt denoting by dpt the SRB measure of fttλt. Our first result is that for any function θ in the Sobolev space Hrp(M), with 1π-rfpagπ ∞ and 0 π r π 1/p, the map tx→ ∫ θ dpt is ?-Hölder continuous for all r. This applies to(x) = h(x) θ (g(x) ? a) (for all >1) for h and g smooth and θ the Heaviside function, if a is not a critical value of g. Our second result says that for any such function -(x) = h(x) θ (g(x) ? a) so that in addition the intersection of {x|g(x) = a} with the support of h is foliated by admissible stable leaves of ft, the map t d-t is differentiable. (We provide distributional linear response and fluctuation-dissipation formulas for the derivative.) Obtaining linear response or fractional response for such observables θ is motivated by extreme-value theory
Bistability of the climate around the habitable zone: A thermodynamic investigation
No full textThe goal of this paper is to explore the potential multistability of the climate for a planet around the habitable zone. We apply our methodology to the Earth system, but our investigation has more general relevance. A thorough investigation of the thermodynamics of the climate system is performed for very diverse conditions of energy input and infrared atmosphere opacity. Using PlaSim, an Earth-like general circulation model, the solar constant S* is modulated between 1160 and 1510Wm-2 and the CO2 concentration, [CO2], between 90 and 2880ppm. It is observed that in such a parameter range the climate is bistable, i.e. there are two coexisting attractors, one characterised by warm, moist climates (W) and one by completely frozen sea surface (Snowball Earth, SB). The tipping points of both the transitions (W→SB and SB →W) are located along straight lines in the (S*,log[CO2]) space. The dynamical and thermodynamical properties - energy fluxes, Lorenz energy cycle, Carnot efficiency, material entropy production - of the W and SB states are very different: W states are dominated by the hydrological cycle and latent heat is prominent in the material entropy production; the SB states are eminently dry climates where heat transport is realised through sensible heat fluxes and entropy mostly generated by dissipation of kinetic energy. We also show that the Carnot efficiency regularly increases towards each transition between W and SB, with a large discontinuous decrease at the point of each transition. Finally, we propose well-defined empirical functions allowing for expressing the global non-equilibrium thermodynamical properties of the system in terms of either the mean surface temperature or the mean planetary emission temperature. While the specific results presented in this paper depend on some characteristics of the Earth system (e.g. rotation rate, position of the continents), this paves the way for the possibility of proposing efficient parameterisations of complex non-equilibrium properties and of practically deducing fundamental properties of a planetary system from a relatively simple observable. As a preliminary result, we obtain that when reducing the rotation rate of the planet by a factor of two, the multistability properties, the quantitative estimators of the thermodynamics of the system, and the approximate parameterisations in terms of the surface of emission temperature are only weakly affected. © 2013
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