Dynamical circulation regimes in planetary (and exo-planetary) atmospheres

Thesis

In this thesis, we study the effect of diurnally- and seasonally-varying forcing on the global circulation of planetary atmospheres explored within a large parameter space. This work focusses on studying the spacial and spectral energy budgets across a large range of planetary parameters as well as the momentum transfer as a response to diurnal and seasonal effects.

We simulate planetary atmospheres using PUMA-GT, a simple GCM co-developed for this work, that is forced by a semi-grey two-band radiative-convective scheme, dissipated by Rayleigh friction and allows for temporally varying insolation. Our parameter regime includes the variation of the planetary rotation rate, frictional timescale in the boundary layer, the thermal inertia of the surface and the atmosphere, as well as the short-wave optical thickness.

We calculate the energy transfer in Martian atmosphere to have a reference case of an atmosphere that is subject to very strong seasonal and diurnal variation. For this we present the first Lorenz energy budget calculated from reanalysis data of a non-Earth planet. A comparison between Martian and Earth atmosphere reveals a fundamentally different behaviour of the barotropic conversion term in the global mean. A significant impact of the thermal tide can be discerned in the generation of eddy kinetic energy, especially during global dust storms.

Our study of seasonal variation reaffirms previous work that the equatorial super-rotating jet in the slow-rotating regime is arrested for strong seasonal variation. We find a novel explanation as to why the Titan atmosphere is able to maintain super-rotation despite strong surface seasonality; for non-zero short-wave absorption in the atmosphere the mechanism that hinders equatorial super-rotation is weakened.

Diurnally-varying forcing can significantly enhance the equatorial super-rotation in cases with non-zero short-wave absorption. In our simulations this enhancement is maintained by a convergence of vertical momentum flux at the equator. Efforts to identify the atmospheric waves involved in this enhancement point towards thermally-excited gravity waves.

Authors: Tabataba-Vakili, F

• DOI: 10.5287/ora-jxrbqx77k
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: seasonal forcing; super-rotation; planetary atmospheres; macroturbulence; diurnal forcing
• Subjects: radiative transfer; geophysical fluid dynamics