The Experts below are selected from a list of 6915 Experts worldwide ranked by ideXlab platform
Jacques Vanneste - One of the best experts on this subject based on the ideXlab platform.
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exponential smallness of inertia gravity wave generation at small Rossby Number
Journal of the Atmospheric Sciences, 2008Co-Authors: Jacques VannesteAbstract:Abstract This paper discusses some of the mechanisms whereby fast inertia–gravity waves can be generated spontaneously by slow, balanced atmospheric and oceanic flows. In the small Rossby Number regime relevant to midlatitude dynamics, high-accuracy balanced models, which filter out inertia–gravity waves completely, can in principle describe the evolution of suitably initialized flows up to terms that are exponentially small in the Rossby Number e, that is, of the form exp(−α/e) for some α > 0. This suggests that the mechanisms of inertia–gravity wave generation, which are not captured by these balanced models, are also exponentially weak. This has been confirmed by explicit analytical results obtained for a few highly simplified models. These results are reviewed, and some of the exponential-asymptotic techniques that have been used in their derivation are presented. Two types of mechanisms are examined: spontaneous-generation mechanisms, which generate exponentially small waves from perfectly balanced i...
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inertia gravity wave generation by balanced motion revisiting the lorenz krishnamurthy model
Journal of the Atmospheric Sciences, 2004Co-Authors: Jacques VannesteAbstract:Abstract The spontaneous generation of inertia–gravity waves by balanced motion at low Rossby Number is examined using Lorenz's five-component model. The mostly numerical analysis by Lorenz and Krishnamurthy of a particular (homoclinic) balanced solution is complemented here by an asymptotic analysis. An exponential–asymptotic technique provides an estimate for the amplitude of the fast inertia–gravity oscillations that are generated spontaneously, through what is shown to be a Stokes phenomenon. This estimate is given by 2πκϵ−2 exp[−π/(2ϵ)], where ϵ ≪ 1 is proportional to the Rossby Number and the prefactor κ is determined from recurrence relations. The nonlinear dependence of κ on the O(1) rotational Froude Number indicates that the feedback of the inertia–gravity waves on the balanced motion directly affects their amplitude. Numerical experiments confirm the analytic results. Optimally truncated slaving relations are used to separate the exponentially small inertia–gravity oscillations from the (much l...
Jonathan L. Mitchell - One of the best experts on this subject based on the ideXlab platform.
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wave mean flow interactions and the maintenance of superrotation in a terrestrial atmosphere
Journal of the Atmospheric Sciences, 2016Co-Authors: Joao Rafael Dias Pinto, Jonathan L. MitchellAbstract:AbstractThe interplay between mean meridional circulation and transient eddies through wave–mean flow interaction processes defines the general behavior of any planetary atmospheric circulation. Under a higher-Rossby-Number regime, equatorward momentum transports provided by large-scale disturbances generate a strong zonal flow at the equatorial region. At intermediate Rossby Numbers, equatorial Kelvin waves play a leading role in maintaining a superrotating jet over the equator. However, at high Rossby Numbers, the Kelvin wave only provides equatorward momentum fluxes during spinup, and the wave–mean flow process that maintains this strongly superrotating state has yet to be identified. This study presents a comprehensive analysis of the tridimensional structure and life cycle of atmospheric waves and their interaction with the mean flow, which maintains the strong, long-lived superrotating state in a higher-Rossby-Number-regime atmosphere. The results show that the mean zonal superrotating circulation i...
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Spontaneous Superrotation and the Role of Kelvin Waves in an Idealized Dry GCM
Journal of the Atmospheric Sciences, 2014Co-Authors: Samuel F. Potter, Geoffrey K. Vallis, Jonathan L. MitchellAbstract:The nondimensional parameter space of an idealized dry primitive equation model is explored to find superrotating climate states. The model has no convective parameterization and is forced using a simple thermal relaxation to a prescribed radiative equilibrium temperature. It is demonstrated that, of four nondimensional parameters that determine the model’s state, only the thermal Rossby Number has a significant effect on superrotation. The mode that drives the transition to superrotation in an intermediate-thermalRossby-Number atmosphere is shown to behave like a Kelvin wave in the tropics.
Spencer A Hill - One of the best experts on this subject based on the ideXlab platform.
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constraints from invariant subtropical vertical velocities on the scalings of hadley cell strength and downdraft width with rotation rate
Journal of the Atmospheric Sciences, 2021Co-Authors: Spencer A HillAbstract:Weak-temperature-gradient influences from the tropics and quasigeostrophic influences from the extratropics plausibly constrain the subtropical-mean static stability in terrestrial atmospheres. Because mean descent acting on this static stability is a leading-order term in the thermodynamic balance, a state-invariant static stability would impose constraints on the Hadley cells, which this paper explores in simulations of varying planetary rotation rate. If downdraft-averaged effective heating (the sum of diabatic heating and eddy heat flux convergence) too is invariant, so must be vertical velocity -- an "omega governor." In that case, the Hadley circulation overturning strength and downdraft width must scale identically -- the cell can strengthen only by widening or weaken only by narrowing. Simulations in two idealized, dry GCMs with a wide range of planetary rotation rates exhibit nearly unchanging downdraft-averaged static stability, effective heating, and vertical velocity, as well as nearly identical scalings of the Hadley cell downdraft width and strength. In one, eddy stresses set this scaling directly (the Rossby Number remains small); in the other, eddy stress and bulk Rossby Number changes compensate to yield the same, ({\sim}\Omega^{-1/3}) scaling. The consistency of this power law for cell width and strength variations may indicate a common driver, and we speculate that Ekman pumping could be the mechanism responsible for this behavior. Extending to moist atmospheres, in an idealized aquaplanet GCM the subtropical static stability is also insensitive to rotation rate but the effective heating and vertical velocity are not.
Geoffrey M Vasil - One of the best experts on this subject based on the ideXlab platform.
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heat transport in low Rossby Number rayleigh benard convection
Physical Review Letters, 2012Co-Authors: Keith Julien, A Rubio, Edgar Knobloch, Geoffrey M VasilAbstract:We demonstrate, via simulations of asymptotically reduced equations describing rotationally constrained Rayleigh-B\'enard convection, that the efficiency of turbulent motion in the fluid bulk limits overall heat transport and determines the scaling of the nondimensional Nusselt Number $\mathrm{Nu}$ with the Rayleigh Number $\mathrm{Ra}$, the Ekman Number $E$, and the Prandtl Number $\ensuremath{\sigma}$. For $E\ensuremath{\ll}1$ inviscid scaling theory predicts and simulations confirm the large $\mathrm{Ra}$ scaling law $\mathrm{Nu}\ensuremath{-}1\ensuremath{\approx}{C}_{1}{\ensuremath{\sigma}}^{\ensuremath{-}1/2}\mathrm{R}{\mathrm{a}}^{3/2}{E}^{2}$, where ${C}_{1}$ is a constant, estimated as ${C}_{1}\ensuremath{\approx}0.04\ifmmode\pm\else\textpm\fi{}0.0025$. In contrast, the corresponding result for nonrotating convection, $\mathrm{Nu}\ensuremath{-}1\ensuremath{\approx}{C}_{2}\mathrm{R}{\mathrm{a}}^{\ensuremath{\alpha}}$, is determined by the efficiency of the thermal boundary layers (laminar: $0.28\ensuremath{\lesssim}\ensuremath{\alpha}\ensuremath{\lesssim}0.31$, turbulent: $\ensuremath{\alpha}\ensuremath{\sim}0.38$). The $3/2$ scaling law breaks down at Rayleigh Numbers at which the thermal boundary layer loses rotational constraint, i.e., when the local Rossby Number $\ensuremath{\approx}1$. The breakdown takes place while the bulk Rossby Number is still small and results in a gradual transition to the nonrotating scaling law. For low Ekman Numbers the location of this transition is independent of the mechanical boundary conditions.
P A Davidson - One of the best experts on this subject based on the ideXlab platform.
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A physical conjecture for the dipolar-multipolar dynamo transition
'Cambridge University Press (CUP)', 2019Co-Authors: Br Mcdermott, P A DavidsonAbstract:In numerical simulations of planetary dynamos there is an abrupt transition in the dynamics of both the velocity and magnetic fields at a 'local' Rossby Number of 0.1. For smaller Rossby Numbers there are helical columnar structures aligned with the rotation axis, which efficiently maintain a dipolar field. However, when the thermal forcing is increased, these columns break down and the field becomes multi-polar. Similarly, in rotating turbulence experiments and simulations there is a sharp transition at a Rossby Number of ∼ 0.4. Again, helical axial columnar structures are found for lower Rossby Numbers, and there is strong evidence that these columns are created by inertial waves, at least on short time scales. We perform direct numerical simulations of the flow induced by a layer of buoyant anomalies subject to strong rotation, inspired by the equatorially biased heat flux in convective planetary dynamos. We assess the role of inertial waves in generating columnar structures. At high rotation rates (or weak forcing) we find columnar flow structures that segregate helicity either side of the buoyant layer, whose axial length scale increases linearly, as predicted by the theory of low-frequency inertial waves. As the rotation rate is weakened and the magnitude of the buoyant perturbations is increased, we identify a portion of the flow which is more strongly three-dimensional. We show that the flow in this region is turbulent, and has a Rossby Number above a critical value Rocrit ∼ 0.4, consistent with previous findings in rotating turbulence. We suggest that the discrepancy between the transition value found here (and in rotating turbulence experiments), and that seen in the numerical dynamos (Rocrit ∼ 0.1), is a result of a different choice of the length scale used to define the local Ro. We show that when a proxy for the flow length scale perpendicular to the rotation axis is used in this definition, the numerical dynamo transition lies at Rocrit ∼ 0.5. Based on this we hypothesise that inertial waves, continually launched by buoyant anomalies, sustain the columnar structures in dynamo simulations, and that the transition documented in these simulations is due to the inability of inertial waves to propagate for Ro > Rocrit
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on the formation of cyclones and anticyclones in a rotating fluid
Physics of Fluids, 2008Co-Authors: Binod Sreenivasan, P A DavidsonAbstract:It is commonly observed that the columnar vortices that dominate the large scales in homogeneous, rapidly rotating turbulence are predominantly cyclonic. This has prompted us to ask how this asymmetry arises. To provide a partial answer to this we look at the process of columnar vortex formation in a rotating fluid and, in particular, we examine how a localized region of swirl (an eddy) can convert itself into a columnar structure by inertial wave propagation. We show that, when the Rossby Number (Ro) is small, the vortices evolve into columnar eddies through the radiation of linear inertial waves. When the Rossby Number is large, on the other hand, no such column is formed. Rather, the eddy bursts radially outward under the action of the centrifugal force. There is no asymmetry between cyclonic and anticyclonic eddies for these two regimes. However, cyclones and anticyclones behave differently in the intermediate regime of Ro∼1. Here we find that the transition from columnar vortex formation to radial bu...
