Clouds in the atmospheres of extrasolar planets

IV. On the scattering greenhouse effect of CO₂ ice particles: Numerical radiative transfer studies

¹ Zentrum für Astronomie und Astrophysik, Technische Universität Berlin, Hardenbergstr. 36, 10623 Berlin, Germany
e-mail: kitzmann@astro.physik.tu-berlin.de
² Institut für Planetenforschung, Deutsches Zentrum für Luft- und Raumfahrt (DLR), Rutherfordstr. 2, 12489 Berlin, Germany

Received: 16 July 2012
Accepted: 17 June 2013

Abstract

Context. Owing to their wavelength-dependent absorption and scattering properties, clouds have a strong impact on the climate of planetary atmospheres. The potential greenhouse effect of CO₂ ice clouds in the atmospheres of terrestrial extrasolar planets is of particular interest because it might influence the position and thus the extension of the outer boundary of the classic habitable zone around main sequence stars. Such a greenhouse effect, however, is a complicated function of the CO₂ ice particles’ optical properties.

Aims. We study the radiative effects of CO₂ ice particles obtained by different numerical treatments to solve the radiative transfer equation. To determine the effectiveness of the scattering greenhouse effect caused by CO₂ ice clouds, the radiative transfer calculations are performed over the relevant wide range of particle sizes and optical depths, employing different numerical methods.

Methods. We used Mie theory to calculate the optical properties of particle polydispersion. The radiative transfer calculations were done with a high-order discrete ordinate method (DISORT). Two-stream radiative transfer methods were used for comparison with previous studies.

Results. The comparison between the results of a high-order discrete ordinate method and simpler two-stream approaches reveals large deviations in terms of a potential scattering efficiency of the greenhouse effect. The two-stream methods overestimate the transmitted and reflected radiation, thereby yielding a higher scattering greenhouse effect. For the particular case of a cool M-type dwarf, the CO₂ ice particles show no strong effective scattering greenhouse effect by using the high-order discrete ordinate method, whereas a positive net greenhouse effect was found for the two-stream radiative transfer schemes. As a result, previous studies of the effects of CO₂ ice clouds using two-stream approximations overrated the atmospheric warming caused by the scattering greenhouse effect. Consequently, the scattering greenhouse effect of CO₂ ice particles seems to be less effective than previously estimated. In general, higher order radiative transfer methods are needed to describe the effects of CO₂ ice clouds accurately as indicated by our numerical radiative transfer studies.