Is Gliese 581d habitable? Some constraints from radiative-convective climate modeling

The recently discovered exoplanet Gl581d is extremely close to the outer edge of its system's habitable zone, which has led to much speculation on its possible climate. We have performed a range of simulations to assess whether, given simple combinations of chemically stable greenhouse gases, the planet could sustain liquid water on its surface. For best estimates of the surface gravity, surface albedo and cloud coverage, we find that less than 10 bars of CO2 is sufficient to maintain a global mean temperature above the melting point of water. Furthermore, even with the most conservative choices of these parameters, we calculate temperatures above the water melting point for CO2 partial pressures greater than about 40 bar. However, we note that as Gl581d is probably in a tidally resonant orbit, further simulations in 3D are required to test whether such atmospheric conditions are stable against the collapse of CO2 on the surface.Comment: 9 pages, 11 figures. Accepted for publication in Astronomy & Astrophysic

Path-tracing Monte Carlo Library for 3D Radiative Transfer in Highly Resolved Cloudy Atmospheres

Interactions between clouds and radiation are at the root of many difficulties in numerically predicting future weather and climate and in retrieving the state of the atmosphere from remote sensing observations. The large range of issues related to these interactions, and in particular to three-dimensional interactions, motivated the development of accurate radiative tools able to compute all types of radiative metrics, from monochromatic, local and directional observables, to integrated energetic quantities. In the continuity of this community effort, we propose here an open-source library for general use in Monte Carlo algorithms. This library is devoted to the acceleration of path-tracing in complex data, typically high-resolution large-domain grounds and clouds. The main algorithmic advances embedded in the library are those related to the construction and traversal of hierarchical grids accelerating the tracing of paths through heterogeneous fields in null-collision (maximum cross-section) algorithms. We show that with these hierarchical grids, the computing time is only weakly sensitivive to the refinement of the volumetric data. The library is tested with a rendering algorithm that produces synthetic images of cloud radiances. Two other examples are given as illustrations, that are respectively used to analyse the transmission of solar radiation under a cloud together with its sensitivity to an optical parameter, and to assess a parametrization of 3D radiative effects of clouds.Comment: Submitted to JAMES, revised and submitted again (this is v2

Net-Exchange parameterization of infrared radiative transfers in Venus' atmosphere

International audienceThermal radiation within Venus atmosphere is analyzed in close details. Prominent features are identified, which are then used to design a parameterization (a highly simplified and yet accurate enough model) to be used in General Circulation Models. The analysis is based on a net exchange formulation, using a set of gaseous and cloud optical data chosen among available referenced data. The accuracy of the proposed parameterization methodology is controlled against Monte Carlo simulations, assuming that the optical data are exact. Then, the accuracy level corresponding to our present optical data choice is discussed by comparison with available observations, concentrating on the most unknown aspects of Venus thermal radiation, namely the deep atmosphere opacity and the cloud composition and structure

COMBINED CONDUCTIVE-RADIATIVE HEAT TRANSFER ANALYSIS IN COMPLEX GEOMETRY USING THE MONTE CARLO METHOD

Deterministic methods are commonly used to solve the heat balance equation in three-dimensional (3D) geometries. This article presents a preliminary study to the use of a stochastic method for the computation of the temperature in complex 3D geometries where the combined conductive and radiative heat transfers are coupled in the porous solid phase. The Monte Carlo algorithm and its results are validated by a comparison with the results obtained with a conventional finite-volume method

Radiative transfer and spectroscopic databases: A line-sampling Monte Carlo approach

Issu de : Eurotherm conference n° 105 - Computational thermal radiation in participating media V, Albi, FRANCE, 1-3 April 2015International audienceDealing with molecular-state transitions for radiative transfer purposes involves two successive steps that both reach the complexity level at which physicists start thinking about statistical approaches: (1) constructing line-shaped absorption spectra as the result of very numerous state-transitions, (2) integrating over optical-path domains. For the first time, we show here how these steps can be addressed simultaneously using the null-collision concept. This opens the door to the design of Monte Carlo codes directly estimating radiative transfer observables from spectroscopic databases. The intermediate step of producing accurate high-resolution absorption spectra is no longer required. A Monte Carlo algorithm is proposed and applied to six one-dimensional test cases. It allows the computation of spectrally integrated intensities (over 25 cm−1 bands or the full IR range) in a few seconds, regardless of the retained database and line model. But free parameters need to be selected and they impact the convergence. A first possible selection is provided in full detail. We observe that this selection is highly satisfactory for quite distinct atmospheric and combustion configurations, but a more systematic exploration is still in progress

Present and Last Glacial Maximum climates as states of maximum entropy production

The Earth, like other planets with a relatively thick atmosphere, is not locally in radiative equilibrium and the transport of energy by the geophysical fluids (atmosphere and ocean) plays a fundamental role in determining its climate. Using simple energy-balance models, it was suggested a few decades ago that the meridional energy fluxes might follow a thermodynamic Maximum Entropy Production (MEP) principle. In the present study, we assess the MEP hypothesis in the framework of a minimal climate model based solely on a robust radiative scheme and the MEP principle, with no extra assumptions. Specifically, we show that by choosing an adequate radiative exchange formulation, the Net Exchange Formulation, a rigorous derivation of all the physical parameters can be performed. The MEP principle is also extended to surface energy fluxes, in addition to meridional energy fluxes. The climate model presented here is extremely fast, needs very little empirical data and does not rely on ad hoc parameterizations. We investigate its range of validity by comparing its performances for pre-industrial climate and Last Glacial Maximum climate with corresponding simulations with the IPSL coupled atmosphere-ocean General Circulation Model IPSL_CM4, finding reasonable agreement. Beyond the practical interest of this result for climate modelling, it supports the idea that, to a certain extent, climate can be characterized with macroscale features with no need to compute the underlying microscale dynamics.Comment: Submitted to the Quarterly Journal of the Royal Meteorological Societ

Net exchange reformulation of radiative transfer in the CO2 15um band on Mars

International audienceThe Net Exchange Formulation (NEF) is an alternative to the usual radiative transfer formulation. It was proposed by two authors in 1967, but until now, this formulation has been used only in a very few cases for atmospheric studies. The aim of this paper is to present the NEF and its main advantages, and to illustrate them in the case of planet Mars. In the NEF, the radiative fluxes are no more considered. The basic variables are the net exchange rates between each pair of atmospheric layers i,j. NEF offers a meaningful matrix representation of radiative exchanges, allows to quantify the dominant contributions to the local heating rates and provides a general framework to develop approximations satisfying reciprocity of radiative transfer as well as first and second principle of thermodynamic. This may be very useful to develop fast radiative codes for GCMs. We present a radiative code developed along those lines for a GCM of Mars. We show that computing the most important optical exchange factors at each time step and the others exchange factors only a few times a day strongly reduces the CPU time without any significant precision lost. With this solution, the CPU time increases proportionally to the number N of the vertical layers and no more proportionally to its square N^2. We also investigate some specific points such as numerical instabilities that may appear in the high atmosphere and errors that may be introduced if inappropriate treatments are performed when reflection at the surface occurs

Dayside thermal structure of Venus' upper atmosphere characterized by a global model

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94843/1/jgre3057.pd