Three-dimensional simulations of the ozone layer and atmospheric dynamics of earth-like habitable planets
Creators: name: Proedrou, Elisavet
Publisher: name: Universität Bern
Year: 2016
Subjects: 620 Engineering

description: This thesis investigates how the atmospheric circulation and ozone distribution of a planet with the size, the mass, the continental distribution and topography, the oceans, and the atmospheric composition and circulation of the present day Earth (Earth-like planet) is altered by local and global radiative forcing changes using three-dimensional simulations. These simulations are generated using the coupled 3D chemistry-climate model CESM1(WACCM), which incorporates the entire atmosphere up to an altitude of 140 km, as well as parametrizations for the full atmospheric chemistry, photochemistry, cloud microphysics and small-scale gravity wave flux. These features allow for a realistic simulation of the composition and dynamics of the Earth's atmosphere. The investigation is composed of three studies. In the first study, the effects in the lower tropospheric dynamics generated by a local radiative forcing change on the present day Earth are investigated. The forcing change is implemented by changing the local soil colour and therefore the local albedo. In order to isolate the generated perturbation from the background waves, a small-scale perturbation analysis is performed for the first 5 days of the simulation. The soil colour change generates an upwards propagating convective perturbation, which induces a radially propagating circular wave at an altitude of 2 km. This wave has a mean wave velocity of (v) = 200 ± 50 m/s, a mean horizontal wavelength (λ) = 3000 ± 500 km and a mean wave period (p) = 4 ± 1 h. In addition to this wave, a secondary wave is also generated over the tropical Amazon convection zone when the primary wave collides with it. The secondary wave has a mean wave velocity (v) = 220± 40 m/s, a mean horizontal wavelength (λ) = 2600 ± 600 km and a mean wave period (p) = 3 ± 1 h. The second study expands the scope of the first study by investigating how a global radiative forcing change affects the atmospheric circulation and ozone distribution of an Earth-like planet orbiting a Sun-like star. In this study, the forcing change is implemented by tidally locking the planet. The simulations reveal that, when the full photochemistry and atmospheric dynamics are included, the planet's middle atmosphere adjusts to the new conditions within a relatively short time (roughly 80 days from the start of the simulation) and its atmospheric circulation and ozone distribution are altered. The Brewer-Dobson circulation is replaced by a day side upwelling and a night side downwelling. The total ozone content of the tidally locked planet is reduced by 19.3% compared to the Earth due to radiation and transport changes. Specifically, the total ozone content mean is reduced by 23.21% on the day side and by 15.52% on the night side. The middle stratospheric ozone accumulates on the day side of the planet resulting in a day-night variation of 40%. In comparison, the Earth's daynight variation is only 2%. The lower stratospheric ozone is mainly influenced by the altered circulation and is characterised by enhanced night side zone and depleted day side regions. The planet's mesospheric ozone is similar to the Earth's mesospheric ozone distribution, with decreased ozone on the day side and enhanced ozone on the night side. For a distant observer, the planet's total ozone content will vary up to 23% during its revolution around its parent star. Finally, the third study is an extension of the second study. It investigates, in more detail, the 3D atmospheric circulation of a tidally locked planet. An intercomparison with the fast rotating Earth is performed and the effects of the sea surface temperature (SST) on the middle atmosphere of the tidally locked planet are simulated and analysed. For this study, two extreme SSTs are used: a present day Earth SST and a tidally locked aquaplanet SST. The simulation shows that the SST has a limited influence on the middle atmosphere. The warmer present day Earth SST generated, on average, a lower tropospheric heating of 3.7 K, an upper tropospheric cooling of 4 K, a lower stratospheric heating of 3.8 K, a lower mesospheric cooling of 1.13 K and an upper mesospheric heating of 4.3 K. The lower stratospheric heating is possibly generated by the increased infrared radiation flux from the warmer present day Earth SST surface, as the lower stratospheric ozone will absorb the increased infrared radiation at 9.6 μm. The SST change has no significant influence on the primary ozone layer, while the warmer SST leads to a strong increase of the secondary ozone layer. The tropospheric and stratospheric results are in agreement with past studies of the influence of SST variability on the Earth's troposphere and stratosphere. The lower mesospheric cooling is consistent with increased mesospheric wave-breaking due to the warmer present day Earth SST. Both simulations are characterised by an upwelling on the day side and downwelling on the night side, while the stratospheric and mesospheric circulation is only weakly influenced by the underlying SST. Generally, the reduced Coriolis force of the tidally locked planet leads to enhanced meridional mixing and consequently to a relatively isothermal temperature distribution of the middle atmosphere. The occurrence of large-scale vortices and variable jet streams depends, to some extent, on the SST distribution.