Geophysical and Astrophysical Fluid Dynamics

Abstract:

Scatter plots of Ertel potential vorticity, Q, versus Bernoulli streamfunction, B, on potential temperature surfaces, θ, are investigated for Mars using the global Mars Analysis Correction Data Assimilation (MACDA) reanalysis, which spans Mars Year (MY) 24.39 to 27.24. In midlatitudes, Mars exhibits monotonic, function-like Q(B) correlations on θ surfaces similar to those observed for Earth. We quantify this with linear regressions of Q versus B over the vertical range θ=400 to 900 K (∼30 to 60 km). In autumn, winter and spring, in both hemispheres, the non-dimensionalized correlation generally lies between zero and unity and gradually decreases with height, whereas in northern summer, it swings negative. These characteristics match Earth's lower mesosphere (θ= 2000 to 3000 K; z≈ 48 to 62 km) during the same seasons. The exception is southern summer, when the correlation on Mars nearly vanishes. In time series, the transition into and out of northern summer is sinuous and centred just after solar longitude Ls = 90°, whereas in southern summer it is abrupt and spans ΔLs≈120°, which is one third of a Mars year. A striking feature seen on Mars but not on Earth is a large range of Q over the narrow domain of B poleward of each winter polar jet, particularly in the north, which is consistent with the known annular structure of the Martian polar vortex. Froude number calculations suggest the existence of a planetary-scale hydraulic jump associated with the winter polar jet.

Abstract:

The atmospheric circulation in Venus is well known to exhibit strong super-rotation. However, the atmospheric mechanisms responsible for the formation of this super-rotation are still not fully understood. In this work, we developed a new Venus general circulation model to study the most likely mechanisms driving the atmosphere to the current observed circulation. Our model includes a new radiative transfer, convection and suitably adapted boundary layer schemes and a dynamical core that takes into account the dependence of the heat capacity at constant pressure with temperature.The new Venus model is able to simulate a super-rotation phenomenon in the cloud region quantitatively similar to the one observed. The mechanisms maintaining the strong winds in the cloud region were found in the model results to be a combination of zonal mean circulation, thermal tides and transient waves. In this process, the semi-diurnal tide excited in the upper clouds has a key contribution in transporting axial angular momentum mainly from the upper atmosphere towards the cloud region. The magnitude of the super-rotation in the cloud region is sensitive to various radiative parameters such as the amount of solar radiative energy absorbed by the surface, which controls the static stability near the surface. In this work, we also discuss the main difficulties in representing the flow below the cloud base in Venus atmospheric models.Our new radiative scheme is more suitable for 3D Venus climate models than those used in previous work due to its easy adaptability to different atmospheric conditions. This flexibility of the model was crucial to explore the uncertainties in the lower atmospheric conditions and may also be used in the future to explore, for example, dynamical-radiative-microphysical feedbacks.

Abstract:

In protoplanetary discs, planetary cores must be at least 0.1 M+ at 1 au for migration to be significant; this mass rises to 1 M+ at 5 au. Planet formation models indicate that these cores form on million year timescales. We report here a study of the evolution of 0.1 M+ and 1 M+ cores, migrating from about 2 and 5 au respectively, in million year old photoevaporating discs. In such a disc, a gap opens up at around 2 au after a few million years. The inner region subsequently accrete onto the star on a smaller timescale. We find that, typically, the smallest cores form systems of non{resonant planets beyond 0.5 au with masses up to about 1.5 M+. In low mass discs, the same cores may evolve in situ. More massive cores form systems of a few earth masses planets. They migrate within the inner edge of the disc gap only in the most massive discs. Delivery of material to the inner parts of the disc ceases with opening of the gap. Interestingly, when the heavy cores do not migrate significantly, the type of systems that are produced resembles our solar system. This study suggests that low mm ux transition discs may not form systems of planets on short orbits but may instead harbour earth mass planets in the habitable zone.