Radiative transfer in planetary atmospheres with clouds

Kurzfassung

The work presented here is focused on the study of the radiative transfer in cloudy atmospheres of Earth-like extrasolar planets. Clouds can influence the different atmospheric molecular signatures appearing in the spectra through absorption and scattering events and may also influence the energy budget of a planet through the modification of the planetary albedo and atmospheric temperature profile. The main objective is a detailed analysis of the thermal infrared radiation of cloud covered Earth-like planets around F, G, K, and M stars by studying the spectral appearance useful for atmospheric remote sensing. This study is conducted in order to determine what atmospheric information may be exposed or hidden in the presence of clouds. To locate the altitude regions contributing the most at a specific wavelength, visualization of weighting functions was especially useful. For this assessment, an in-depth understanding of clear-sky spectra is essential. To distinguish physical and instrumental limits of atmospheric characterization, spectra of different resolutions are considered. Modeling spectra of cloud-covered planets required scattering contributions in the radiative transfer. Therefore, an already existing line-by-line radiative transfer model originally developed for the gaseous Earth atmosphere was extended with multiple scattering. Verification was done by comparing with spectra modeled by other radiative transfer codes for different atmospheric scenarios and spectral regions. The model was also validated by simulating near infrared Venus spectra and comparing them to satellite measurements, what even allowed to estimate the height of the main cloud deck. Analysis of spectra and weighting functions of clear sky exoplanets allowed the location of the radiation source regions, what already gives hints about the feasibility to identify molecules in cloudy atmospheres. Differences in the state of the planetary atmospheres due to host star and cloud cover could be seen in the spectra. A stratospheric temperature inversion has a big influence on the shape of the ozone band. Clouds modified the intensities and shapes of the molecular spectral bands, and some bands were even strongly masked, preventing the identification of the molecule. At lower spectral resolution, clouds were found to be essential to detect certain molecular bands that could not be identified in cloud-free spectra. Radiation in the carbon dioxide bands is emitted over a large altitude range and is less affected by clouds. For clear-sky and cloudy atmospheres, knowledge of the atmospheric temperatures, which can be estimated from the carbon dioxide bands and the atmospheric window regions, is essential for determining the composition, especially to avoid false negative interpretations of molecular signatures.