Prof. Peter Read

Abstract:

A hexagonal structure has been observed at ∼76°N on Saturn since the 1980s (Godfrey, D.A. [1988]. Icarus 76, 335-356). Recent images by Cassini (Baines, K., Momary, T., Roos-Serote, M., Atreya, S., Brown, R., Buratti, B., Clark, R., Nicholson, P. [2007]. Geophys. Res. Abstr. 9, 02109; Baines, K., Momary, T., Fletcher, L., Kim, J., Showman, A., Atreya, S., Brown, R., Buratti, B., Clark, R., Nicholson, P. [2009]. Geophys. Res. Abstr. 11, 3375) have shown that the feature is still visible and largely unchanged. Its long lifespan and geometry has puzzled the planetary physics community for many years and its origin remains unclear. The measured rotation rate of the hexagon may be very close to that of the interior of the planet (Godfrey, D.A. [1990]. Science 247, 1206-1208; Caldwell, J., Hua, X., Turgeon, B., Westphal, J.A., Barnet, C.D. [1993]. Science 206, 326-329; Sánchez-Lavega, A., Lecacheux, J., Colas, F., Laques, P. [1993]. Science 260, 329-332), leading to earlier interpretations of the pattern as a stationary planetary wave, continuously forced by a nearby vortex (Allison, M., Godfrey, D.A., Beebe, R.F. [1990]. Science 247, 1061-1063). Here we present an alternative explanation, based on an analysis of both spacecraft observations of Saturn and observations from laboratory experiments where the instability of quasi-geostrophic barotropic (vertically uniform) jets and shear layers is studied. We also present results from a barotropic linear instability analysis of the saturnian zonal wind profile, which are consistent with the presence of the hexagon in the North Pole and absence of its counter-part in the South Pole. We propose that Saturn's long-lived polygonal structures correspond to wavemodes caused by the nonlinear equilibration of barotropically unstable zonal jets. © 2009 Elsevier Inc. All rights reserved.

Abstract:

We investigate three of the most common hypotheses underpinning parameterizations of baroclinic eddy fluxes in the context of the differentially heated rotating annulus experiment. The investigation is carried out over a region of parameter space which embraces the onset of baroclinic instability, the regular wave regime and the onset of irregular flows, the latter of which is arguably most relevant to oceanic conditions. Through diagnostics from a 2D axisymmetric and a 3D eddy-resolving numerical model, it was found that the transport of heat by baroclinic eddies is not strictly an adiabatic process but that diffusive 'ventilation' of the flow in the thermal boundary layers is significant during the nonlinear development of the flow. Total heat transport, however, is conserved overall. Depending on the stages of flow evolution and on the region in parameter space under consideration, either heat, quasi-geostrophic potential vorticity (QGPV) or relative vorticity (QGRV) may become a suitable variable on which to parameterize baroclinic eddy fluxes in a down-gradient manner. These results raise issues for eddy parameterization schemes that rely on these assumptions in ocean and atmosphere models. © 2009 Elsevier Ltd.

Abstract:

Long-term (2004-2009) on-orbit observations by Cassini Composite Infrared Spectrometer are analyzed to precisely measure Saturn's emitted power and its meridional distribution. Our evaluations suggest that the average global emitted power is 4.952 ± 0.035 W m-2 during the period of 2004-2009. The corresponding effective temperature is 96.67 ± 0.17 K. The emitted power is 16.6% higher in the Southern Hemisphere than in the Northern Hemisphere. From 2005 to 2009, the global mean emitted power and effective temperature decreased by ∼2% and ∼0.5%, respectively. Our study further reveals the interannual variability of emitted power and effective temperature between the epoch of Voyager (∼1 Saturn year ago) and the current epoch of Cassini, suggesting changes in the cloud opacity from year to year on Saturn. The seasonal and interannual variability of emitted power implies that the energy balance and internal heat are also varying. Copyright © 2010 by the American Geophysical Union.

Abstract:

The first Martian year and a half of observations by the Mars Climate Sounder aboard the Mars Reconnaissance Orbiter has revealed new details of the thermal structure and distributions of dust and water ice in the atmosphere. The Martian atmosphere is shown in the observations by the Mars Climate Sounder to vary seasonally between two modes: a symmetrical equinoctial structure with middle atmosphere polar warming and a solstitial structure with an intense middle atmosphere polar warming overlying a deep winter polar vortex. The dust distribution, in particular, is more complex than appreciated before the advent of these high (∼5 km) vertical resolution observations, which extend from near the surface to above 80 km and yield 13 dayside and 13 nightside pole-to-pole cross sections each day. Among the new features noted is a persistent maximum in dust mass mixing ratio at 15-25 km above the surface (at least on the nightside) during northern spring and summer. The water ice distribution is very sensitive to the diurnal and seasonal variation of temperature and is a good tracer of the vertically propagating tide. Copyright 2010 by the American Geophysical Union.

Abstract:

Synchronization is now well established as representing coherent behaviour between two or more otherwise autonomous nonlinear systems subject to some degree of coupling. Such behaviour has mainly been studied to date, however, in relatively low-dimensional discrete systems or networks. But the possibility of similar kinds of behaviour in continuous or extended spatiotemporal systems has many potential practical implications, especially in various areas of geophysics. We review here a range of cyclically varying phenomena within the Earth's climate system for which there may be some evidence or indication of the possibility of synchronized behaviour, albeit perhaps imperfect or highly intermittent. The exploitation of this approach is still at a relatively early stage within climate science and dynamics, in which the climate system is regarded as a hierarchy of many coupled sub-systems with complex nonlinear feedbacks and forcings. The possibility of synchronization between climate oscillations (global or local) and a predictable external forcing raises important questions of how models of such phenomena can be validated and verified, since the resulting response may be relatively insensitive to the details of the model being synchronized. The use of laboratory analogues may therefore have an important role to play in the study of natural systems that can only be observed and for which controlled experiments are impossible. We go on to demonstrate that synchronization can be observed in the laboratory, even in weakly coupled fluid dynamical systems that may serve as direct analogues of the behaviour of major components of the Earth's climate system. The potential implications and observability of these effects in the long-term climate variability of the Earth is further discussed. © 2010 Springer-Verlag Berlin Heidelberg.