Geophysical and Astrophysical Fluid Dynamics

Abstract:

This dataset provides horizontal flow fields (velocity, relative vorticity, divergence, and shear strain rate) from nine rotating turbulence experiments carried out at TurLab at the University of Turin between November 2016 and February 2017 by a team led by Oxford scientists in collaboration with others at the time from Università degli Studi di Torino, University of South Florida, Università del Piemonte Orientale and Sapienza Università di Roma. The experimental device was a rotating cylinder, 5 m in diameter filled with fresh water to a depth of 56 cm. The tank had a conical bottom, sloping upwards towards the centre from the outer edge over a radius of 2.25 m at an angle of 11.1◦. The flow was mechanically forced by a moving comb of vertical paddles partially immersed in the water, which moved backwards and forwards along a near-radial line over 100–150 s. The resulting flow was visualised using a laser sheet and microscopic particles, imaged using two cameras, and then processed into horizontal velocity fields and derived quantities using UVMAT/CIV (http://servforge.legi. grenoble-inp.fr/projects/soft-uvmat). The main quantity varied between the experiments included in this dataset is the rotation period of the tank. There were minor differences in the comb configuration at each rotation period. †

Abstract:

This dataset comprises time-resolved, two-component horizontal velocity fields from experiments in a rotating, differentially heated fluid annulus. The experiments were conducted at the GFDLab, University of Oxford.

Abstract:

Barotropic instability represents a class of instabilities, usually of parallel shear flows, for which gravity and buoyancy play a negligible role, at least in their energetics. It is not restricted to purely barotropic fluids (for which ρ = ρ(p), where ρ is density and p is pressure) but can also apply to flows which are stratified and exhibit vertical shear, often leading to instabilities with mixed barotropic and baroclinic characteristics. The primary attribute of barotropic instability is usually taken to be the dominance of energy exchanges in which the kinetic energy of a perturbation grows principally at the expense of the kinetic energy of the basic state. Here we present an introduction to the basic mechanisms involved and the factors that determine the necessary and/or sufficient conditions for instability. Several examples are presented and the occurrence and subsequent nonlinear evolution of the instability is illustrated with reference to both laboratory experiments and observations in the atmospheres and oceans of the Earth and other planets in the Solar System.

Abstract:

Recent analyses of wind measurements obtained from tracking cloud motions in spacecraft images of Jupiter and Saturn[1,2] indicate that nonlinear scale-to-scale transfers of kinetic energy act from small to large scales over a wide range of length scales, much as anticipated for 2D or geostrophic turbulence paradigms. At the smallest resolvable scales, however, there is evidence in observations of a forward (downscale) transfer, at least at low and middle latitudes on Jupiter, much like in the Earth’s atmosphere. Moreover, the upscale transfers at the largest spatial scales are evidently dominated by spectrally non-local, highly anisotropic eddy-zonal interactions associated with the generation of intense zonal jets and equatorial super-rotation by direct eddy-zonal flow exchanges. Most analyses to date have emphasised the global mean interactions for both planets, thereby focusing on the spatially homogeneous and isotropic components of the turbulence. Here we present some new analyses of spectral energy transfers on both Jupiter and Saturn that resolve variations in latitude[cf 3], thereby shedding new light on non-homogeneous aspects of jovian turbulent interactions. The results indicate significant variability and inhomogeneity between different locations, with a clear distinction between the tropics, the extratropical middle latitudes and the polar regions. We discuss these in light of other observations and models of gas giant circulation and related laboratory experimental analogues.[1] Antu˜nano, A., del Río-Gaztelurrutia, T., Sánchez-Lavega, A., & Hueso, R. (2015) Dynamics of Saturn’s polar regions. J. Geophys. Res.: Planets, 120 , 155–176. doi:10.1002/2014JE004709[2] Read, P. L., Antu˜nano, A., Cabanes, S., Colyer, G., del Río-Gaztelurrutia, T., Sánchez-Lavega, A. (2022). Energy exchanges in Saturn’s polar regions fromCassini observations: Eddy-zonal flow interactions. J. Geophys. Res., 127 , e2021JE006973. https://doi.org/10.1029/2021JE006973[3] Chemke, R., & Kaspi, Y. (2015). The latitudinal dependence of atmospheric jet scales and macroturbulent energy cascades. J. Atmos. Sci., 72 , 3891–3907. doi: 10.1175/JAS-D-15-0007.