DOCSLIB.ORG

Mixed Rossby–Gravity Wave–Wave Interactions

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Mixed Rossby–Gravity Wave–Wave Interactions JANUARY 2019 E D E N E T A L . 291 Mixed Rossby–Gravity Wave–Wave Interactions CARSTEN EDEN AND MANITA CHOUKSEY Institut fur€ Meereskunde, Universitat€ Hamburg, Hamburg, Germany DIRK OLBERS Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, and Center for Marine Environmental Sciences, University of Bremen, Bremen, Germany (Manuscript received 15 April 2018, in final form 18 October 2018) ABSTRACT Mixed triad wave–wave interactions between Rossby and gravity waves are analytically derived using the kinetic equation for models of different complexity. Two examples are considered: initially vanishing linear gravity wave energy in the presence of a fully developed Rossby wave field and the reversed case of initially vanishing linear Rossby wave energy in the presence of a realistic gravity wave field. The kinetic equation in both cases is numerically evaluated, for which energy is conserved within numerical precision. The results are validated by a corresponding ensemble of numerical model simulations supporting the validity of the weak-interaction assumption necessary to derive the kinetic equation. Since they are generated by nonresonant interactions only, the energy transfers toward the respective linear wave mode with vanishing energy are small in both cases. The total generation of energy of the linear gravity wave mode in the first case scales to leading order as the square of the Rossby number in agreement with independent estimates from laboratory experiments, although a part of the linear gravity wave mode is slaved to the Rossby wave mode without wavelike temporal behavior. 1. Introduction the interactions are known to transport energy pre- dominantly to larger scales (e.g., Rhines 1975; Salmon The dynamics of rotating stratified flows are often 1998). Internal gravity waves, on the other hand, evolve at described using linear Rossby and inertia–gravity waves much shorter temporal scales and also often have small (hereinafter called gravity waves), which have well- spatial scales down to a few meters, which renders them known properties. The interactions between these lin- hard to observe as well as to resolve in models. Olbers ear waves generated by the nonlinear terms in the (1976), Pomphrey et al. (1980), Müller et al. (1986),and equations of motions, however, are less well known but Henyey et al. (1986), for example, have discussed how can profoundly affect the evolution of the flow field. The wave–wave interactions of gravity waves in the ocean nonlinear wave–wave interactions within the gravity can lead to an energy transfer within the gravity wave and Rossby wave components along with the mixed in- spectrum to smaller scales through processes such as teractions between the two wave modes are the subject induced diffusion (e.g., Müller et al. 1986) and parametric of this paper. We discuss the energy transfers by weak subharmonic instability (e.g., Sutherland 2010). gravity, Rossby, and mixed wave–wave interactions in Rossby wave–wave interaction is often discussed and models of different complexity. understood as an energy flux from one wavenumber Rossby waves evolve at temporal scales much longer to its neighboring wavenumber, that is, local in wave- than the inertial period. Wave–wave interactions of number space, generated by the nonlinearities in the Rossby waves are typically characterized by motions model under investigation. Similar to the equivalent with spatial scales on the order of the deformation radius energy flux in the theory of isotropic turbulence by and larger, in both the atmosphere and the ocean, and Kolmogorov (1941), scaling arguments can then lead to an inference of the magnitude and wavenumber de- Corresponding author: Carsten Eden, carsten.eden@uni- pendency of the flux and the energy spectrum of Rossby hamburg.de waves (Salmon 1998). Such an energy flux can also be DOI: 10.1175/JPO-D-18-0074.1 Ó 2019 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses). Unauthenticated | Downloaded 09/26/21 12:46 PM UTC 292 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 49 diagnosed from observations or models (Frisch 1995) instability (Plougonven and Snyder 2007). Spontaneous and shows indeed a direction toward large scales (Scott generation of internal gravity waves from the balanced and Wang 2005). On the other hand, gravity wave–wave flow has been studied in laboratory (e.g., Williams et al. interactions are usually discussed in the framework 2005, 2008) and numerical studies (e.g., Molemaker of resonant triad interactions, where three waves of et al. 2005; Sugimoto and Plougonven 2016; Chouksey different wavenumbers and frequencies—satisfying a et al. 2018), where it was shown that this process can certain relation given below—exchange energy. Such transfer energy from the balanced flow to gravity waves. triad interactions can generate therefore an energy It was speculated that this process may represent a sig- transfer between rather different wavenumbers, that is, nificant energy sink for the balanced flow in the ocean nonlocal in wavenumber space. Although the nonlocal (e.g., Williams et al. 2008; Brüggemann and Eden 2015), triad interaction and the local energy flux framework which would modify our view of the energy cycle in the are in principle equivalent, the former presents more ocean and which would need consideration in parame- information since all waves can participate in the triad terizations for ocean models. Here we also discuss the interactions forming a complicated network. The net opposite scenario, the generation of Rossby waves or effect of these complex triad interactions can result in an balanced flow by gravity waves, which might have im- energy transfer in either upscale or downscale direction, portance for the energetics of the wave field. which is then diagnosed as the local energy flux in This study is structured into four sections. Section 2 wavenumber space. presents a theoretical layout of an evolution equation Understanding the triad of interactions between grav- for the wave energies in spectral space—the so-called ity waves, Rossby waves, and their mixed interactions kinetic equation or scattering integral—that is derived therefore can contribute to better understanding of and discussed in Fourier space for a quasigeostrophic the dynamics of atmosphere and ocean. Wave–wave in- layer model, for a single-layer reduced-gravity model, teractions have been discussed for the case of Rossby and for vertically resolved models using the weak- waves (e.g., Kenyon 1964; Longuet-Higgins et al. 1967; interaction assumption. The derivation is detailed Connaughton et al. 2015) and gravity waves (e.g., Olbers mainly for the reduced-gravity model but also holds for 1976; Nazarenko 2011) based on the weak-interaction the other models. From the general kinetic equation, assumption and the kinetic equation formulated for Rossby wave, gravity wave, and mixed Rossby–gravity geophysical applications first by Hasselmann (1962).The wave interactions are derived. Section 3 demonstrates wave–wave interaction between the two principal wave numerically the generation of linear gravity waves from modes in this framework, however, has attracted less in- balanced flow and vice versa and validates the results terest so far, an exception being Lelong and Riley (1991). with a numerical model. Section 4 presents a summary In a general derivation for models of different complex- and discussion of the results. ity, we here also allow for the resonant and nonresonant interactions of all wave modes present, that is, mixed triad 2. The kinetic equation or scattering integral interactions between (internal) gravity waves and Rossby waves, referred to as Rossby–gravity mixed interactions. We use as an algebraically simple model a reduced- Although usually separated by vastly different time gravity model for a single layer. All variables and also the scales, it is known that both wave modes can interact differential operators are scaled with a velocity scale U, with each other (e.g., Müller 1977; Lighthill 1978; Ford length scale L, and time scale 1/V,whereV denotes the et al. 2000). A general framework for the interaction magnitude of the constant Coriolis parameter. This yields between long and short waves has been set by Benney › 1 1 = 52 = (1977), investigating nonlinear differential equations tu f Hu h Rou Á u and that allow, in the linearized case, for long and short wave › h 1 c2= Á u 52Ro= Á hu, (1) solutions. The theory can be applied to the resonant t interaction of two short internal gravity waves with a where Ro 5 U/(VL) is the Rossby number, u denotes1 long Rossby wave. The resonance condition of such a the scaled layer velocity, and h is the layer thickness triad of waves is an approximate equality of the long perturbation scaled by g0/(VLU), with g0 being reduced wave phase velocity and the short wave group velocity. gravity. Furthermore, More recently, spontaneous emission or loss of balance (e.g., Vanneste 2013) has attracted considerable atten- tion and refers to the process of gravity wave generation 1 by a balanced flow spontaneously in the absence of any The vector Hu denotes anticlockwise rotation of the vector u by p/2; that is, u 5 (2y, u) for u 5 (u, y). external forcing, which often happens during baroclinic H Unauthenticated | Downloaded 09/26/21 12:46 PM UTC JANUARY 2019 E D E N E T A L . 293 qffiffiffiffiffiffiffi also yields Eq. (2) but with the three-dimensional defini- c 5 g0h/(VL) 5 Ro/Fr tion of potential vorticity given by is the scaled phase velocity of gravity waves without q 5 =2c 1 N2/f 2› c. rotation, h is the mean layer thickness, and zz qffiffiffiffiffiffiffi Fr 5 U/ g0h a. The quasigeostrophic layer model in Fourier space Before turning to the more complex models, the denotes the Froude number.
Load more

Recommended publications
  • Fourth-Order Nonlinear Evolution Equations for Surface Gravity Waves in the Presence of a Thin Thermocline
    J. Austral. Math. Soc. Ser. B 39(1997), 214-229 FOURTH-ORDER NONLINEAR EVOLUTION EQUATIONS FOR SURFACE GRAVITY WAVES IN THE PRESENCE OF A THIN THERMOCLINE SUDEBI BHATTACHARYYA1 and K. P. DAS1 (Received 14 August 1995; revised 21 December 1995) Abstract Two coupled nonlinear evolution equations correct to fourth order in wave steepness are derived for a three-dimensional wave packet in the presence of a thin thermocline. These two coupled equations are reduced to a single equation on the assumption that the space variation of the amplitudes takes place along a line making an arbitrary fixed angle with the direction of propagation of the wave. This single equation is used to study the stability of a uniform wave train. Expressions for maximum growth rate of instability and wave number at marginal stability are obtained. Some of the results are shown graphically. It is found that a thin thermocline has a stabilizing influence and the maximum growth rate of instability decreases with the increase of thermocline depth. 1. Introduction There exist a number of papers on nonlinear interaction between surface gravity waves and internal waves. Most of these are concerned with the mechanism of generation of internal waves through nonlinear interaction of surface gravity waves. Coherent three wave interactions of two surface waves and one internal wave have been investigated by Ball [1], Thorpe [22], Watson, West and Cohen [23] and others. Using the theoretical model of Hasselman [12] for incoherent three-wave interaction, Olber and Hertrich [18] have reported a mechanism of generation of internal waves by coupling with surface waves using a three-layer model of the ocean.
    [Show full text]
  • Internal Gravity Waves: from Instabilities to Turbulence Chantal Staquet, Joël Sommeria
    Internal gravity waves: from instabilities to turbulence Chantal Staquet, Joël Sommeria To cite this version: Chantal Staquet, Joël Sommeria. Internal gravity waves: from instabilities to turbulence. Annual Review of Fluid Mechanics, Annual Reviews, 2002, 34, pp.559-593. 10.1146/an- nurev.fluid.34.090601.130953. hal-00264617 HAL Id: hal-00264617 https://hal.archives-ouvertes.fr/hal-00264617 Submitted on 4 Feb 2020 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Distributed under a Creative Commons Attribution| 4.0 International License INTERNAL GRAVITY WAVES: From Instabilities to Turbulence C. Staquet and J. Sommeria Laboratoire des Ecoulements Geophysiques´ et Industriels, BP 53, 38041 Grenoble Cedex 9, France; e-mail: [email protected], [email protected] Key Words geophysical fluid dynamics, stratified fluids, wave interactions, wave breaking Abstract We review the mechanisms of steepening and breaking for internal gravity waves in a continuous density stratification. After discussing the instability of a plane wave of arbitrary amplitude in an infinite medium at rest, we consider the steep- ening effects of wave reflection on a sloping boundary and propagation in a shear flow. The final process of breaking into small-scale turbulence is then presented.
    [Show full text]
  • The Internal Gravity Wave Spectrum: a New Frontier in Global Ocean Modeling
    The internal gravity wave spectrum: A new frontier in global ocean modeling Brian K. Arbic Department of Earth and Environmental Sciences University of Michigan Supported by funding from: Office of Naval Research (ONR) National Aeronautics and Space Administration (NASA) National Science Foundation (NSF) Brian K. Arbic Internal wave spectrum in global ocean models Collaborators • Naval Research Laboratory Stennis Space Center: Joe Metzger, Jim Richman, Jay Shriver, Alan Wallcraft, Luis Zamudio • University of Southern Mississippi: Maarten Buijsman • University of Michigan: Joseph Ansong, Steve Bassette, Conrad Luecke, Anna Savage • McGill University: David Trossman • Bangor University: Patrick Timko • Norwegian Meteorological Institute: Malte M¨uller • University of Brest and The University of Texas at Austin: Rob Scott • NASA Goddard: Richard Ray • Florida State University: Eric Chassignet • Others including many members of the NSF-funded Climate Process Team led by Jennifer MacKinnon of Scripps Brian K. Arbic Internal wave spectrum in global ocean models Motivation • Breaking internal gravity waves drive most of the mixing in the subsurface ocean. • The internal gravity wave spectrum is just starting to be resolved in global ocean models. • Somewhat analogous to resolution of mesoscale eddies in basin- and global-scale models in 1990s and early 2000s. • Builds on global internal tide modeling, which began with 2004 Arbic et al. and Simmons et al. papers utilizing Hallberg Isopycnal Model (HIM) run with tidal foricng only and employing a horizontally uniform stratification. Brian K. Arbic Internal wave spectrum in global ocean models Motivation continued... • Here we utilize simulations of the HYbrid Coordinate Ocean Model (HYCOM) with both atmospheric and tidal forcing. • Near-inertial waves and tides are put into a model with a realistically varying background stratification.
    [Show full text]
  • Gravity Waves Are Just Waves As Pressure He Was Observing
    FLIGHTOPS Nearly impossible to predict and difficult to detect, this weather phenomenon presents Gravitya hidden risk, especially close to the ground. regory Bean, an experienced pi- National Weather Service observer re- around and land back at Burlington. lot and flight instructor, says he ferred to this as a “gravity wave.” Neither The wind had now swung around to wasn’t expecting any problems the controller nor Bean knew what the 270 degrees at 25 kt. At this point, he while preparing for his flight weather observer was talking about. recalled, his rate of descent became fromG Burlington, Vermont, to Platts- Deciding he could deal with the “alarming.” The VSI now pegged at the burgh, New York, U.S. His Piper Seneca wind, Bean was cleared for takeoff. The bottom of the scale. Despite the control checked out fine. The weather that April takeoff and initial climb were normal difficulty, he landed the aircraft without evening seemed benign. While waiting but he soon encountered turbulence “as further incident. Bean may not have for clearance, he was taken aback by a rough as I have ever encountered.” He known what a gravity wave was, but report of winds gusting to 35 kt from wanted to climb to 2,600 ft. With the he now knew what it could do to an 140 degrees. The air traffic control- vertical speed indicator (VSI) “pegged,” airplane.1 ler commented on rapid changes in he shot up to 3,600 ft. Finally control- Gravity waves are just waves as pressure he was observing. He said the ling the airplane, Bean opted to turn most people normally think of them.
    [Show full text]
  • Gravity Wave and Tidal Influences On
    Ann. Geophys., 26, 3235–3252, 2008 www.ann-geophys.net/26/3235/2008/ Annales © European Geosciences Union 2008 Geophysicae Gravity wave and tidal influences on equatorial spread F based on observations during the Spread F Experiment (SpreadFEx) D. C. Fritts1, S. L. Vadas1, D. M. Riggin1, M. A. Abdu2, I. S. Batista2, H. Takahashi2, A. Medeiros3, F. Kamalabadi4, H.-L. Liu5, B. G. Fejer6, and M. J. Taylor6 1NorthWest Research Associates, CoRA Division, Boulder, CO, USA 2Instituto Nacional de Pesquisas Espaciais (INPE), San Jose dos Campos, Brazil 3Universidade Federal de Campina Grande. Campina Grande. Paraiba. Brazil 4University of Illinois, Champaign, IL, USA 5National Center for Atmospheric research, Boulder, CO, USA 6Utah State University, Logan, UT, USA Received: 15 April 2008 – Revised: 7 August 2008 – Accepted: 7 August 2008 – Published: 21 October 2008 Abstract. The Spread F Experiment, or SpreadFEx, was per- Keywords. Ionosphere (Equatorial ionosphere; Ionosphere- formed from September to November 2005 to define the po- atmosphere interactions; Plasma convection) – Meteorology tential role of neutral atmosphere dynamics, primarily grav- and atmospheric dynamics (Middle atmosphere dynamics; ity waves propagating upward from the lower atmosphere, in Thermospheric dynamics; Waves and tides) seeding equatorial spread F (ESF) and plasma bubbles ex- tending to higher altitudes. A description of the SpreadFEx campaign motivations, goals, instrumentation, and structure, 1 Introduction and an overview of the results presented in this special issue, are provided by Fritts et al. (2008a). The various analyses of The primary goal of the Spread F Experiment (SpreadFEx) neutral atmosphere and ionosphere dynamics and structure was to test the theory that gravity waves (GWs) play a key described in this special issue provide enticing evidence of role in the seeding of equatorial spread F (ESF), Rayleigh- gravity waves arising from deep convection in plasma bub- Taylor instability (RTI), and plasma bubbles extending to ble seeding at the bottomside F layer.
    [Show full text]
  • Inertia-Gravity Wave Generation: a WKB Approach
    Inertia-gravity wave generation: a WKB approach Jonathan Maclean Aspden Doctor of Philosophy University of Edinburgh 2010 Declaration I declare that this thesis was composed by myself and that the work contained therein is my own, except where explicitly stated otherwise in the text. (Jonathan Maclean Aspden) iii iv Abstract The dynamics of the atmosphere and ocean are dominated by slowly evolving, large-scale motions. However, fast, small-scale motions in the form of inertia-gravity waves are ubiquitous. These waves are of great importance for the circulation of the atmosphere and oceans, mainly because of the momentum and energy they transport and because of the mixing they create upon breaking. So far the study of inertia-gravity waves has answered a number of questions about their propagation and dissipation, but many aspects of their generation remain poorly understood. The interactions that take place between the slow motion, termed balanced or vortical motion, and the fast inertia-gravity wave modes provide mechanisms for inertia-gravity wave generation. One of these is the instability of balanced flows to gravity-wave-like perturbations; another is the so-called spontaneous generation in which a slowly evolving solution has a small gravity-wave component intrinsically coupled to it. In this thesis, we derive and study a simple model of inertia-gravity wave generation which considers the evolution of a small-scale, small amplitude perturbation superimposed on a large-scale, possibly time-dependent flow. The assumed spatial-scale separation makes it possible to apply a WKB approach which models the perturbation to the flow as a wavepacket.
    [Show full text]
  • Chapter 16: Waves [Version 1216.1.K]
    Contents 16 Waves 1 16.1Overview...................................... 1 16.2 GravityWavesontheSurfaceofaFluid . ..... 2 16.2.1 DeepWaterWaves ............................ 5 16.2.2 ShallowWaterWaves........................... 5 16.2.3 Capillary Waves and Surface Tension . .... 8 16.2.4 Helioseismology . 12 16.3 Nonlinear Shallow-Water Waves and Solitons . ......... 14 16.3.1 Korteweg-deVries(KdV)Equation . ... 14 16.3.2 Physical Effects in the KdV Equation . ... 16 16.3.3 Single-SolitonSolution . ... 18 16.3.4 Two-SolitonSolution . 19 16.3.5 Solitons in Contemporary Physics . .... 20 16.4 RossbyWavesinaRotatingFluid. .... 22 16.5SoundWaves ................................... 25 16.5.1 WaveEnergy ............................... 27 16.5.2 SoundGeneration............................. 28 16.5.3 T2 Radiation Reaction, Runaway Solutions, and Matched Asymp- toticExpansions ............................. 31 0 Chapter 16 Waves Version 1216.1.K, 7 Sep 2012 Please send comments, suggestions, and errata via email to [email protected] or on paper to Kip Thorne, 350-17 Caltech, Pasadena CA 91125 Box 16.1 Reader’s Guide This chapter relies heavily on Chaps. 13 and 14. • Chap. 17 (compressible flows) relies to some extent on Secs. 16.2, 16.3 and 16.5 of • this chapter. The remaining chapters of this book do not rely significantly on this chapter. • 16.1 Overview In the preceding chapters, we have derived the basic equations of fluid dynamics and devel- oped a variety of techniques to describe stationary flows. We have also demonstrated how, even if there exists a rigorous, stationary solution of these equations for a time-steady flow, instabilities may develop and the amplitude of oscillatory disturbances will grow with time. These unstable modes of an unstable flow can usually be thought of as waves that interact strongly with the flow and extract energy from it.
    [Show full text]
  • Baroclinic Instability, Lecture 19
    19. Baroclinic Instability In two-dimensional barotropic flow, there is an exact relationship between mass 2 streamfunction ψ and the conserved quantity, vorticity (η)given by η = ∇ ψ.The evolution of the conserved variable η in turn depends only on the spatial distribution of η andonthe flow, whichisd erivable fromψ and thus, by inverting the elliptic relation, from η itself. This strongly constrains the flow evolution and allows one to think about the flow by following η around and inverting its distribution to get the flow. In three-dimensional flow, the vorticity is a vector and is not in general con­ served. The appropriate conserved variable is the potential vorticity, but this is not in general invertible to find the flow, unless other constraints are provided. One such constraint is geostrophy, and a simple starting point is the set of quasi-geostrophic equations which yield the conserved and invertible quantity qp, the pseudo-potential vorticity. The same dynamical processes that yield stable and unstable Rossby waves in two-dimensional flow are responsible for waves and instability in three-dimensional baroclinic flow, though unlike the barotropic 2-D case, the three-dimensional dy­ namics depends on at least an approximate balance between the mass and flow fields. 97 Figure 19.1 a. The Eady model Perhaps the simplest example of an instability arising from the interaction of Rossby waves in a baroclinic flow is provided by the Eady Model, named after the British mathematician Eric Eady, who published his results in 1949. The equilibrium flow in Eady’s idealization is illustrated in Figure 19.1.
    [Show full text]
  • On the Transport of Energy in Water Waves
    On The Transport Of Energy in Water Waves Marshall P. Tulin Professor Emeritus, UCSB [email protected] Abstract Theory is developed and utilized for the calculation of the separate transport of kinetic, gravity potential, and surface tension energies within sinusoidal surface waves in water of arbitrary depth. In Section 1 it is shown that each of these three types of energy constituting the wave travel at different speeds, and that the group velocity, cg, is the energy weighted average of these speeds, depth and time averaged in the case of the kinetic energy. It is shown that the time averaged kinetic energy travels at every depth horizontally either with (deep water), or faster than the wave itself, and that the propagation of a sinusoidal wave is made possible by the vertical transport of kinetic energy to the free surface, where it provides the oscillating balance in surface energy just necessary to allow the propagation of the wave. The propagation speed along the surface of the gravity potential energy is null, while the surface tension energy travels forward along the wave surface everywhere at twice the wave velocity, c. The flux of kinetic energy when viewed traveling with a wave provides a pattern of steady flux lines which originate and end on the free surface after making vertical excursions into the wave, even to the bottom, and these are calculated. The pictures produced in this way provide immediate insight into the basic mechanisms of wave motion. In Section 2 the modulated gravity wave is considered in deep water and the balance of terms involved in the propagation of the energy in the wave group is determined; it is shown again that vertical transport of kinetic energy to the surface is fundamental in allowing the propagation of the modulation, and in determining the well known speed of the modulation envelope, cg.
    [Show full text]
  • The Equilibrium Dynamics and Statistics of Gravity–Capillary Waves
    J. Fluid Mech. (2015), vol. 767, pp. 449–466. c Cambridge University Press 2015 449 doi:10.1017/jfm.2014.740 The equilibrium dynamics and statistics of gravity–capillary waves W. Kendall Melville1, † and Alexey V. Fedorov2 1Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92093-0213, USA 2Department of Geology and Geophysics, Yale University, 210 Whitney Avenue, PO Box 208109, New Haven, CT 06520, USA (Received 24 December 2013; revised 8 October 2014; accepted 19 December 2014; first published online 18 February 2015) Recent field observations and modelling of breaking surface gravity waves suggest that air-entraining breaking is not sufficiently dissipative of surface gravity waves to balance the dynamics of wind-wave growth and nonlinear interactions with dissipation for the shorter gravity waves of O.10/ cm wavelength. Theories of parasitic capillary waves that form at the crest and forward face of shorter steep gravity waves have shown that the dissipative effects of these waves may be one to two orders of magnitude greater than the viscous dissipation of the underlying gravity waves. Thus the parasitic capillaries may provide the required dissipation of the short wind-generated gravity waves. This has been the subject of speculation and conjecture in the literature. Using the nonlinear theory of Fedorov & Melville (J. Fluid Mech., vol. 354, 1998, pp. 1–42), we show that the dissipation due to the parasitic capillaries is sufficient to balance the wind input to the short gravity waves over some range of wave ages and wave slopes. The range of gravity wave lengths on which these parasitic capillary waves are dynamically significant approximately corresponds to the range of short gravity waves that Cox & Munk (J.
    [Show full text]
  • ATMS 310 Rossby Waves Properties of Waves in the Atmosphere Waves
    ATMS 310 Rossby Waves Properties of Waves in the Atmosphere Waves – Oscillations in field variables that propagate in space and time. There are several aspects of waves that we can use to characterize their nature: 1) Period – The amount of time it takes to complete one oscillation of the wave\ 2) Wavelength (λ) – Distance between two peaks of troughs 3) Amplitude – The distance between the peak and the trough of the wave 4) Phase – Where the wave is in a cycle of amplitude change For a 1-D wave moving in the x-direction, the phase is defined by: φ ),( = −υtkxtx − α (1) 2π where φ is the phase, k is the wave number = , υ = frequency of oscillation (s-1), and λ α = constant determined by the initial conditions. If the observer is moving at the phase υ speed of the wave ( c ≡ ), then the phase of the wave is constant. k For simplification purposes, we will only deal with linear sinusoidal wave motions. Dispersive vs. Non-dispersive Waves When describing the velocity of waves, a distinction must be made between the group velocity and the phase speed. The group velocity is the velocity at which the observable disturbance (energy of the wave) moves with time. The phase speed of the wave (as given above) is how fast the constant phase portion of the wave moves. A dispersive wave is one in which the pattern of the wave changes with time. In dispersive waves, the group velocity is usually different than the phase speed. A non- dispersive wave is one in which the patterns of the wave do not change with time as the wave propagates (“rigid” wave).
    [Show full text]
  • TTL Upwelling Driven by Equatorial Waves
    TTL Upwelling driven by Equatorial Waves L09804 LIUWhat drives the annual cycle in TTL upwelling? ET AL.: START OF WATER VAPOR AND CO TAPE RECORDERS L09804 Liu et al. (2007) Ortland, D. and M. J. Alexander, The residual mean circula8on in the TTL driven by tropical waves, JAS, (in review), 2013. Figure 1. (a) Seasonal variation of 10°N–10°S mean EOS MLS water vapor after dividing by the mean value at each level. (b) Seasonal variation of 10°N–10°S EOS MLS CO after dividing by the mean value at each level. represented in two principal ways. The first is by the area of [8]ConcentrationsofwatervaporandCOnearthe clouds with low infrared brightness temperatures [Gettelman tropical tropopause are available from retrievals of EOS et al.,2002;Massie et al.,2002;Liu et al.,2007].Thesecond MLS measurements [Livesey et al.,2005,2006].Monthly is by the area of radar echoes reaching the tropopause mean water vapor and CO mixing ratios at 146 hPa and [Alcala and Dessler,2002;Liu and Zipser, 2005]. In this 100 hPa in the 10°N–10°Sand10° longitude boxes are study, we examine the areas of clouds that have TRMM calculated from one full year (2005) of version 1.5 MLS Visible and Infrared Scanner (VIRS) 10.8 mmbrightness retrievals. In this work, all MLS data are processed with temperatures colder than 210 K, and the area of 20 dBZ requirements described by Livesey et al. [2005]. Tropopause echoes at 14 km measured by the TRMM Precipitation temperature is averaged from the 2.5° resolution NCEP Radar (PR).
    [Show full text]