| dc.description.abstract | With the advent of space travel in 20th century, and discoveries of thousands of exoplanets in the 21st, the field of atmospheric dynamics faces new challenges and opportunities. Of particular interest is qualitative understanding of how the astronomical, physical and chemical parameters of terrestrial planets influence their general circulation, whose nature can bridge the gap between a potentially habitable planet and a habitable one. Previous research used idealized GCMs to study the tropospheres of planets with smooth surface and an Earth-like set of bulk properties, one of which was varied at a time. Dependent on the rotation rate, meridional radiative forcing gradient, and the dampling timescale, they identified several qualitatively distinct tropospheric circulation regimes.
This work aims to be a step towards building similar understanding for layers above the troposphere. The research question is what happens to a stratosphere with a constant radiative forcing similar to a solstice on Earth, when the relative planetary rotation rate $\Omega^*=\Omega/\Omega_{E}$ is lowered. The results indicate a regime transition from a wave-driven circulation for the Earth-like $\Omega^* \sim 1$ to a non-linear regime at $\Omega^*\leq10^{-1}$, with a stronger, radiatively driven Hadley-like cell in the stratosphere. This overturning is an inherently seasonal effect developping because of the downwelling present over the winter pole, and likely enhanced to some extent because of the warmth of the ozone layer in lower latitudes. It is centered around 30 hPa in winter mid-latitudes, and extends all the way to mid-latitudes of the summer hemisphere. A somewhat surprising result is the emergence of superrotation in the stratosphere, separate from the better-known superrotation region near tropopause. This result is less understood, it is hypothesised to be a result of gravity-wave breaking in the upper summer stratosphere, with angular momentum potentially advected across the equator by the Hadley-like cell. More research is needed to properly understand this phenomenon.
This work also tentatively proposes a suitable parameter space for a more detailed future study. Compared to the troposphere, frictional Taylor number should be replaced with radiative Taylor number, and a further parameter shown to be related to Burger number might be considered. | |
