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It is a great pleasure to introduce this first volume of ‘Perspectives in geophysical and astrophysical fluids’ as a special feature of Proceedings of the Royal Society of London A.
The aim of this initiative is to bring together world-leading experts in the broad area of geophysical and astrophysical fluid mechanics to give their ‘perspective’ on a current field of research. The importance of the field of geophysical and astrophysical fluids (GAFD) for our understanding of the universe can hardly be overstated. For the Earth, it enables the description (inter alia) of the motion of the fluid in the atmosphere and oceans, the slow movement of the mantle, the interaction of convection and magnetic fields in the fluid outer core and the dynamics of avalanches, volcanoes and ice-shelves. Moving away from the Earth, the processes involved in planetary dynamics, star formation and stellar evolution and dynamics, turbulence in the inter-stellar medium and the solar wind and the extremely powerful (relativistic) jets emitted from black holes (among others) can only be understood using the techniques of GAFD.
These perspectives are not reviews in the sense that their primary aim is not to look backwards and summarize the state of the field—although of course some discourse on the current state of affairs is warranted. Rather the aim is to look forward, and to give the authors' personal opinion of how the field will (or should) develop and the lines of attack that will lead to significant breakthroughs in the future. As such, these Perspectives allow significantly more freedom than a regular journal article or review. The authors are encouraged to be speculative and opinionated, whether in proposing new theories (as some have done) or simply in identifying promising lines of approach (or dead-end lines of research). As such, the opinion of the authors is paramount, though of course the moderating influence of multiple referees has managed to keep the authors away from statements that are too outlandish. As expected, the authors have approached this task in a variety of different ways (some co-opting the aid of colleagues, others writing a more personal account) but a common theme in all of the perspectives in this volume is the imagination of future avenues of research that is the hallmark of great science.
The perspectives in this first volume span a wide variety of fields, from theoretical models of turbulence, to plasma reconnection in the farthest reaches of the Universe (via the Earth's core, oceans and atmospheres—and those of other planets). The distinguished authors who have contributed have all made a considerable effort to make their field accessible to the interested fluid dynamicist and physicist.
This volume comprises eight perspectives with a summary of the invited authors and their fields of exposition below.
Jon Aurnou is Professor of Planetary Physics at the University of California, Los Angeles. His research interests broadly include buoyancy- and mechanically-driven turbulent phenomena in geophysical and astrophysical fluid systems. His perspective elucidates the meaning of ‘magnetostrophic balance’ in planetary dynamos, in which Lorentz and Coriolis form the primary balance of forces, and speculates that an effective cut-off length scale exists only below which magnetostrophic convection physics can manifest in natural dynamo settings.
James C. McWilliams is the Louis Slichter Professor of Earth Sciences in the Department of Atmospheric and Oceanic Sciences at the University of California at Los Angeles. His expertise is geophysical fluid dynamics, especially related to oceanic currents. His perspective details the importance of submesoscale currents in the ocean and describes their generation, instabilities, life-cycles, their importance for the force balance, their transition to turbulence and their role in the transport and dispersion of materials.
Geoffrey Vallis is Professor of Applied Mathematics at the University of Exeter. His research interests include geophysical fluid dynamics, climate dynamics and turbulence. His perspective discusses the role of and need for a theoretical understanding of the fluid dynamics of the atmosphere and ocean in an age in which we can often simulate phenomena using large computers, without really understanding the phenomena in a meaningful way. In a wide-ranging article, he gives examples in both meteorology and oceanography, from jets and turbulence to paleoclimate, and shows that the increasing complexity of the problems we address actually increases the need for theoretical understanding alongside computer models, and that only with such understanding will the subject progress.
Gregory Falkovich is Professor of Physics at the Weizmann Institute of Science in Israel. His research interests include fluid mechanics and statistical physics. His perspective elucidates the recent attempts in analytic theory of interaction between coherent flow and turbulence in two dimensions and discusses what coherent flows may be generated by inverse turbulence cascades.
Tim Palmer is a Royal Society Research Professor of Climate Physics at the University of Oxford. His research interests evolve around the dynamics and predictability of weather and climate, though he also has side interests in the foundations of quantum theory, following a PhD in general relativity theory. He is concerned that we don't treat the problem of predicting climate with sufficient ambition and has long promoted the establishment of a small number of international Tier-0 supercomputing centres completely dedicated to climate prediction. He believes that human resources for such centres can in part be drawn from post-PhD talent in theoretical physics, and pure and applied mathematics. He discusses these ideas in his contributed paper.
Keith Moffatt is Emeritus Professor of Mathematical Physics at the University of Cambridge. He has worked since the 1960s on aspects of dynamo theory in geophysical and astrophysical contexts. His perspective covers the complementary processes of dynamo action and magnetic relaxation, and addresses in particular the vexed question of turbulent dynamo action at high magnetic Reynolds number and the associated saturation mechanism.
Ray Pierrehumbert is the Halley Professor of Physics at the University of Oxford, having formerly served on the faculties of the University of Chicago, Princeton and M.I.T. His chief current research interests are in the area of exoplanet climate dynamics, particularly for planets of the Super-Earth class and smaller. Part of the aim of this research is to generalize our understanding of the way atmospheres evolve through interchange with the deep (and often rocky) planetary material and through escape to space. He also maintains research interests in policy-relevant areas of climate physics related to global change problems. His perspective explores the completely novel territory of climate dynamics of atmospheres in which the condensible component can make up a substantial portion of atmospheric mass.
Ellen Zweibel is William L. Kraushaar Professor of Astronomy and Physics and Vilas Distinguished Achievement Professor at the University of Wisconsin-Madison. Her research interests cover plasma astrophysics from the Sun to clusters of galaxies. Her perspective integrates theoretical and experimental progress on magnetic reconnection and its application to astrophysical systems. Her co-author Masaaki Yamada is Distinguished Laboratory Research Fellow at Princeton Plasma Physics Laboratory and the Head of the Magnetic Reconnection Experiment research programme. He received the Maxwell Prize in 2015 and the 2002 Award for Excellence in Plasma Physics from the American Physical Society.