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Distinguished Lecture

Distinguished Lecture - PS

Peter READ
University of Oxford

Mon- 29 Jul, 12:00-12:30pm
Nicoll 3, Level 3

"The Turbulent Dynamics of Jupiter’s and Saturn’s Weather Layers: Order Out of Chaos?"

The cloudy atmospheres of Jupiter and Saturn have long fascinated observers and theoreticians because they exhibit a wealth of phenomena where long-lived, coherent features, such as large-scale vortices and zonally banded jets, coexist with intense chaotic motion on smaller scales. The persistent pattern of zonal jets accounts for around 90% of the kinetic energy of the circulation near Jupiter’s cloud tops and forms a nearly unchanging flow structure on timescales ≤ 100 yrs, yet seems to be maintained by the action of a turbulent spectrum of waves and eddies. Since the early work of Gareth Williams in the 1970s, theoretical and numerical models have suggested the importance of rapid rotation, spheroidal planetary curvature and vertical stratification in directing the transfer of kinetic energy into zonal jets. But the origins of the energetic eddies and their vertical structure and depth of penetration have remained poorly constrained and understood, despite a wealth of observations from missions such as Voyager, Galileo, Cassini and Juno.

In recent work, we have re-examined the observed interactions between eddies and zonal flows on Jupiter, using for the first time a full spectral decomposition of cloud-tracked winds from Cassini’s closest approach in 2000. These clearly demonstrate the anisotropic nature of the inverse cascade of kinetic energy from fairly small-scale eddies towards larger scales and, in particular, into zonal flows. At the smallest resolved scales, however, with horizontal wavelengths ≤ 3000 km, the sense of kinetic energy transfer is evidently direct, i.e. towards even smaller scales. This kind of bi-directional “dual cascade” has only recently been identified as a fundamental paradigm for rotating, stratified flows, and further suggests a kinetic energy source for eddies on Jupiter at scales around 3000 km. Given this is close to estimates of the Rossby radius of deformation on Jupiter, such an energy source is most likely to be from the potential energy of the stably-stratified background thermal field of the weather layer itself, released through a form of baroclinic instability. Such a process, however, is likely to differ significantly in form from baroclinic cyclones found on Earth and other terrestrial planets, for which interactions with the underlying surface play an important role.

This interpretation has also received support from other recent work in our group using a numerical general circulation model (GCM) of Jupiter’s weather layer (spanning pressures from 20 bars to 10 mb). This model is able to reproduce many qualitative features of the observed cloud-level circulation on Jupiter and Saturn, including their extra-tropical zonal jets and prograde equatorial jet, without invoking a deep convective circulation. A spectral analysis of energy transfers within these model simulations demonstrates a similar pattern of kinetic energy cascades to those observed, including a dual cascade with an energy source at scales around 3000 km and eddy-zonal flow interactions that peak close to the tropopause. The energy source in our model is due to conversion from available potential energy by baroclinic instabilities at zonal wavelengths around 2000-5000 km, centred around the tropopause.

In this talk I will discuss both observations and models of turbulent cascades in the weather layers of Jupiter and Saturn and their implications for our understanding of the atmospheric circulations of these planets and the associated transport of momentum and constituents. One feature of the observed cloud level circulation that is not well represented in most gas giant GCMs is the spontaneous development of large-scale, long-lived oval vortices such as the Great Red Spot, Oval BA or the recently discovered polar vortices. The reasons for this are not well understood, but possible implications will be discussed, together with desirable directions for future research.

Peter Read is currently Professor of Atmospheric, Oceanic and Planetary Physics in the Department of Physics at the University of Oxford, where he has worked since 1991. He graduated in Physics at the University of Birmingham (UK) in 1975 and obtained a Ph.D. in Radioastronomy at the University of Cambridge in 1980. After completing his Ph.D., he joined the research staff of the UK Meteorological Office, first in remote sensing of stratospheric ozone and later in experimental and theoretical geophysical fluid dynamics before moving to become Head of the Met. Office unit at Oxford University in 1988. His research interests cover a range of problems in fundamental fluid dynamics and planetary meteorology and climate. Fundamental studies have included both laboratory and theoretical investigations of finite-dimensional nonlinear dynamics and chaos in rotating fluids (both stratified and homogeneous), geostrophic turbulence and inertia-gravity wave generation in balanced flows. His group has also developed a range of numerical circulation and climate models for the atmospheres of Mars, Jupiter, Saturn and Venus. He has published more than 150 refereed scientific papers and review articles, and a major research monograph on the Martian atmosphere and climate. He has received a number of awards, including the Adrian Gill Prize of the Royal Meteorological Society and the Lewis Fry Richardson Medal of the European Geosciences Union.