The atmosphere consists of molecules in motion, yet it is often hard to find any mention of the fact in meteorological texts. This absence is also true of substantial areas of physics and chemistry which have evolved to provide quantitative descriptions of the behaviour of atoms and molecules in the gas phase: in particular, non-equilibrium statistical mechanics and molecular dynamics have had less overlap with the theory and observation of turbulence than perhaps might have been expected. Meteorology of course has had fluid mechanics at front and centre for over a century and has had to face issues in turbulence for over half that time. The purpose of this book is to show that atmospheric turbulence is an emergent property arising from the anisotropic environment of populations of gas molecules, linking molecular dynamics with fluid mechanics through the generation of vorticity. The anisotropies arise from gravity, planetary rotation, the solar beam, and the nature of the topography, the sea and ice surfaces, and the vegetative cover. We shall see that analysis of high resolution data of adequate quality, as yet available largely from only a few aircraft, leads to the emergence of a correlation of the multifractal, turbulent scaling at the smaller scales with some characteristics of the larger scale meteorological flow, such as the intensity and depth of jet streams. Lest the reader should think that the formulation of events at the microscopic scale has little or nothing to do with the central concerns of modern meteorology, we note that climate is determined through the absorption and emission of photons by molecules in the atmosphere and at the surface. The nature and distribution of these molecules is determined by photochemical kinetics acting in the presence of turbulent transport and biogeochemical fluxes from the surface. Quantitative calculation of the rates of these processes must necessarily account for both the skewed probability distributions of molecular velocities maintained interactively by vorticity structures and the effect of the scale invariant turbulent structures, on such large volumes of chemical reaction as the stratospheric polar vortex.
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