Geophysical and Astrophysical Fluid Dynamics

Abstract:

Polar vortices on Mars provide case-studies to aid understanding of geophysical vortex dynamics and may help to resolve long-standing issues regarding polar vortices on Earth. Due to the recent development of the first publicly available Martian reanalysis dataset (MACDA), for the first time we are able to characterise thoroughly the structure and evolution of the Martian polar vortices, and hence perform a systematic comparison with the polar vortices on Earth. The winter atmospheric circulations of the two planets are compared, with a specific focus on the structure and evolution of the polar vortices. The Martian residual meridional overturning circulation is found to be very similar to the stratospheric residual circulation on Earth during winter. While on Earth this residual circulation is very different from the Eulerian circulation, on Mars it is found to be very similar. Unlike on Earth, it is found that the Martian polar vortices are annular, and that the Northern Hemisphere vortex is far stronger than its southern counterpart. While winter hemisphere differences in vortex strength are also reported on Earth, the contrast is not as large. Distinctions between the two planets are also apparent in terms of the climatological vertical structure of the vortices, in that the Martian polar vortices are observed to decrease in size at higher altitudes, whereas on Earth the opposite is observed. Finally, it is found that the Martian vortices are less variable through the winter than on Earth, especially in terms of the vortex geometry. During one particular major regional dust storm on Mars (Martian year 26), an equatorward displacement of the vortex is observed, sharing some qualitative characteristics of sudden stratospheric warmings on Earth.

Abstract:

The dataset is documented in readme.pdf. The data files are in uncompressed .tar format. This dataset contains 11 1/2 hours of horizontal velocity measurements from four experiments using AOPP's 'small annulus' thermally-driven rotating annulus laboratory experiment. The experiments cover regular (2S, 3AV) and weakly chaotic (3SV) flow regimes. The apparatus consists of two concentric right circular cylinders with height 14.0cm and radii 2.5cm and 8.0cm, with a 17% glycerol / 83% water mixture (by volume) between them. The outer cylinder is heated and the inner cylinder cooled relative to the working fluid, with a temperature difference of approximately 4K, and the apparatus rotates about the co-incident axis of the two cylinders at rates between 0.75 and 3.1 rad/s. This setup mimics the main effects acting on a planetary atmosphere: gravity, rotation, and a heating gradient between low and high latitudes.

Abstract:

This chapter focuses on the "classical" thermally driven, rotating annulus system. It reviews the current state of understanding of the rich and diverse range of flow regimes that may be exhibited in thermal annulus experiments from the viewpoint of experimental observation, numerical simulation, and fundamental theory. This includes interpretation of various empirical experimental observations in relation to both linear and weakly nonlinear baroclinic instability theory. The chapter then examines how heat is transported within the baroclinic annulus across the full range of control parameters, associated with both the boundary layer circulation and baroclinically unstable eddies. It considers the overall role of annulus experiments in the laboratory in continuing to advance understanding of the global circulation of planetary atmospheres and oceans, reviewing the current state of research on delineating circulation regimes obtained in large-scale circulation models in direct comparison with the sequences of flow regimes and transitions in the laboratory.