scholarly journals Surface manifestation of stochastically excited internal gravity waves

2021 ◽  
Author(s):  
Daniel Lecoanet ◽  
Matteo Cantiello ◽  
Evan H Anders ◽  
Eliot Quataert ◽  
Louis-Alexandre Couston ◽  
...  
Keyword(s):  
Transfer Function ◽  
Gravity Waves ◽  
Massive Stars ◽  
Theoretical Calculations ◽  
Low Frequency ◽  
Wave Theory ◽  
Internal Gravity Waves ◽  
Finite Width ◽  
Linear Wave ◽  
Surface Manifestation

Abstract Recent photometric observations of massive stars show ubiquitous low-frequency ‘red-noise’ variability, which has been interpreted as internal gravity waves (IGWs). Simulations of IGWs generated by convection show smooth surface wave spectra, qualitatively matching the observed red-noise. Theoretical calculations using linear wave theory by Shiode et al (2013) and Lecoanet et al (2019) predict IGWs should manifest at the surface as regularly-spaced peaks associated with standing g-modes. In light of the apparent discrepancy between these theories and simulations/observations, we test the theories with simplified 2D numerical simulations of wave generation by convection. The simulations agree with the transfer function calculations presented in Lecoanet et al (2019), demonstrating that the transfer function correctly models linear wave propagation. However, there are differences between our simulations and the g-mode amplitude predictions of Shiode et al (2013). This is because that work did not take into account the finite width of the g-mode peaks; after correcting for this finite width, we again find good agreement between theory and simulations. This paper verifies the theoretical approach of Lecoanet et al (2019) and strengthens their conclusion that internal gravity waves generated by core convection do not have a surface manifestation consistent with observed low-frequency variability of massive stars.

scholarly journals Photometric detection of internal gravity waves in upper main-sequence stars

2020 ◽  
Vol 640 ◽  
pp. A36 ◽  
Author(s):  
D. M. Bowman ◽  
S. Burssens ◽  
S. Simón-Díaz ◽  
P. V. F. Edelmann ◽  
T. M. Rogers ◽  
...  
Keyword(s):  
Gravity Waves ◽  
Massive Stars ◽  
Pressure Fluctuations ◽  
Tangential Velocity ◽  
Chemical Species ◽  
Internal Gravity Waves ◽  
Spectral Lines ◽  
Photometric Variability ◽  
Near Surface ◽  
Photometric Detection

Context. Massive stars are predicted to excite internal gravity waves (IGWs) by turbulent core convection and from turbulent pressure fluctuations in their near-surface layers. These IGWs are extremely efficient at transporting angular momentum and chemical species within stellar interiors, but they remain largely unconstrained observationally. Aims. We aim to characterise the photometric detection of IGWs across a large number of O and early-B stars in the Hertzsprung–Russell diagram, and explain the ubiquitous detection of stochastic variability in the photospheres of massive stars. Methods. We combined high-precision time-series photometry from the NASA Transiting Exoplanet Survey Satellite with high-resolution ground-based spectroscopy of 70 stars with spectral types O and B to probe the relationship between the photometric signatures of IGWs and parameters such as spectroscopic mass, luminosity, and macroturbulence. Results. A relationship is found between the location of a star in the spectroscopic Hertzsprung–Russell diagram and the amplitudes and frequencies of stochastic photometric variability in the light curves of massive stars. Furthermore, the properties of the stochastic variability are statistically correlated with macroturbulent velocity broadening in the spectral lines of massive stars. Conclusions. The common ensemble morphology for the stochastic low-frequency variability detected in space photometry and its relationship to macroturbulence is strong evidence for IGWs in massive stars, since these types of waves are unique in providing the dominant tangential velocity field required to explain the observed spectroscopy.

Seasonal modulation of trapped gravity waves and their imprints on trade wind clouds

2020 ◽  
Author(s):  
Claudia Stephan
Keyword(s):  
Gravity Waves ◽  
Large Scale ◽  
Climate Models ◽  
Global Climate ◽  
Wave Theory ◽  
Internal Gravity Waves ◽  
Global Climate Models ◽  
Trade Wind ◽  
Trapped Waves ◽  
Convective Boundary

<p>Idealized simulations have shown decades ago that shallow clouds generate internal gravity waves, which under certain atmospheric background conditions become trapped inside the troposphere and influence the development of clouds. These feedbacks, which occur at horizontal scales of up to several tens of km are neither resolved, nor parameterized in traditional global climate models (GCMs), while the newest generation of GCMs is starting to resolve them. The interactions between the convective boundary layer and trapped waves have almost exclusively been studied in highly idealized frameworks and it remains unclear to what degree this coupling affects the organization of clouds and convection in the real atmosphere. Here, the coupling between clouds and trapped waves is examined in storm-resolving simulations that span the entirety of the tropical Atlantic and are initialized and forced by meteorological analyses. The coupling between clouds and trapped waves is sufficiently strong to be detected in these simulations of full complexity.  Stronger upper-tropospheric westerly winds are associated with a stronger cloud-wave coupling. In the simulations this results in a highly-organized scattered cloud field with cloud spacings of about 19 km, matching the dominant trapped wavelength. Based on the large-scale atmospheric state wave theory can reliably predict the regions and times where cloud-wave feedbacks become relevant to convective organization. Theory, the simulations and satellite imagery imply a seasonal cycle in the trapping of gravity waves. </p>

scholarly journals Meteorological effects in the lower ionosphere as based on VLF/LF signal observations

2014 ◽  
Vol 2 (4) ◽  
pp. 2789-2812 ◽  
Author(s):  
A. Rozhnoi ◽  
M. Solovieva ◽  
B. Levin ◽  
M. Hayakawa ◽  
V. Fedun
Keyword(s):  
Gravity Waves ◽  
Atmospheric Pressure ◽  
Low Frequency ◽  
Lower Ionosphere ◽  
Signal Amplitude ◽  
Internal Gravity Waves ◽  
Atmospheric Parameters ◽  
Low Latitudes ◽  
Far Eastern ◽  
Meteorological Effects

Abstract. Very low and low frequency (VLF/LF) data recorded in the Far Eastern stations Petropavlovsk-Kamchatsky (158.92° E, 53.15° N), Yuzhno-Sakhalinsk (142.75° E, 46.95° N) and Yuzhno-Kurilsk (145.861° E, 44.03° N) are investigated to study the meteorological effects in the lower ionosphere. The results demonstrate the sensitivity of the VLF/LF signals to the variations of atmospheric pressure, humidity, wind velocity and temperature, and the VLF/LF record at the station of Yuzhno-Kurilsk is found to be most sensitive to those variations of atmospheric parameters. The region under consideration is characterized by high winter cyclonic activity in midlatitudes and strong summer and autumn typhoon activity in low latitudes. VLF/LF signal variations during 8 tropical cyclones (TCs) with different intensity are considered. Negative nighttime anomalies in the signal amplitude that are most probably caused by TC activity are found for 6 events. Those anomalies are observed during 1–2 days when TCs move inside the sensitivity zones of the subionospheric paths. Perturbations of the VLF signal observed during 2 TCs can be caused by both the TC influence and seismic activity, but no correlation between TC intensity and magnitude of the signal anomalies is found. Spectral analysis of the typhoon-induced disturbed signals revealed the fluctuations with time periods in the range of 7–16 and 15–55 min that corresponds to the range of internal gravity waves periods.

A Note on the Second-Order Contribution to Extreme Waves Generated During Hurricanes

2019 ◽  
Vol 141 (4) ◽  
Author(s):  
Mark L. McAllister ◽  
Thomas A. A. Adcock ◽  
Paul H. Taylor ◽  
Ton S. van den Bremer
Keyword(s):  
Wind Waves ◽  
Low Frequency ◽  
Nonlinear Behavior ◽  
Wave Theory ◽  
Linear Range ◽  
Second Order ◽  
Linear Wave ◽  
Extreme Waves ◽  
Order Difference ◽  
Hurricane Wind

High wind speeds generated during hurricanes result in the formation of extreme waves. Extreme waves by nature are steep meaning that linear wave theory alone is insufficient in understanding and predicting their occurrence. The complex, highly transient nature of the direction of wind and hence of waves generated during hurricanes affects this nonlinear behavior. Herein, we examine how this directionality can affect the second-order nonlinearity of extreme waves generated during hurricanes. This is achieved through both deterministic calculations and experiments based on the observations of Young (2006, “Directional Spectra of Hurricane Wind Waves,” J. Geophys. Res. Oceans, 111(C8), epub). Our calculations show that interactions between the tail and peak of the spectrum can become significant when they travel in different directions, resulting in second-order difference components that exist in the linear range of frequencies. These calculations are generally supported by experimental observations, but we note the difficulty of generating and focusing the high-frequency tail of the spectrum experimentally. Bound second-order difference components or subharmonics typically exist as low frequency infra-gravity waves. Components that exist in the linear range of frequencies may be missed by conventional methods of processing field data where low-pass filtering is used and hence overlooked. In this note, we show that in idealized directional spreading conditions representative of a hurricane, failing to account for second-order difference components may lead to underestimation of extreme wave height.

Generation of surf beat by non-linear wave interactions

1971 ◽  
Vol 49 (1) ◽  
pp. 1-20 ◽  
Author(s):  
Brent Gallagher
Keyword(s):  
Gravity Waves ◽  
Numerical Evaluation ◽  
Low Frequency ◽  
Linear Wave ◽  
Transfer Energy ◽  
Wave Interactions ◽  
Frequency Spectra ◽  
Non Linear ◽  
Linear Interactions ◽  
Low Frequency Waves

Non-linear interactions among wind-generated gravity waves transfer energy to low frequency waves in a coastal zone. A transfer function is derived for a straight coastline of constant bottom slope. This model is applied to three actual cases, and numerical evaluation of the energy transfer produces low frequency spectra which are compared with observations.

Visualization and measurement of internal waves by ‘synthetic schlieren’. Part 1. Vertically oscillating cylinder

1999 ◽  
Vol 390 ◽  
pp. 93-126 ◽  
Author(s):  
BRUCE R. SUTHERLAND ◽  
STUART B. DALZIEL ◽  
GRAHAM O. HUGHES ◽  
P. F. LINDEN
Keyword(s):  
Density Gradient ◽  
Gravity Waves ◽  
Wave Attenuation ◽  
Time Derivative ◽  
Low Frequency ◽  
High Sensitivity ◽  
Internal Gravity Waves ◽  
Quantitative Agreement ◽  
Gradient Field ◽  
Displacement Distribution

We present measurements of the density and velocity fields produced when an oscillating circular cylinder excites internal gravity waves in a stratified fluid. These measurements are obtained using a novel, non-intrusive optical technique suitable for determining the density fluctuation field in temporally evolving flows which are nominally two-dimensional. Although using the same basic principles as conventional methods, the technique uses digital image processing in lieu of large and expensive parabolic mirrors, thus allowing more flexibility and providing high sensitivity: perturbations of the order of 1% of the ambient density gradient may be detected. From the density gradient field and its time derivative it is possible to construct the perturbation fields of density and horizontal and vertical velocity. Thus, in principle, momentum and energy fluxes can be determined.In this paper we examine the structure and amplitude of internal gravity waves generated by a cylinder oscillating vertically at different frequencies and amplitudes, paying particular attention to the role of viscosity in determining the evolution of the waves. In qualitative agreement with theory, it is found that wave motions characterized by a bimodal displacement distribution close to the source are attenuated by viscosity and eventually undergo a transition to a unimodal displacement distribution further from the source. Close quantitative agreement is found when comparing our results with the theoretical ones of Hurley & Keady (1997). This demonstrates that the new experimental technique is capable of making accurate measurements and also lends support to analytic theories. However, theory predicts that the wave beams are narrower than observed, and the amplitude is significantly under-predicted for low-frequency waves. The discrepancy occurs in part because the theory neglects the presence of the viscous boundary layers surrounding the cylinder, and because it does not take into account the effects of wave attenuation resulting from nonlinear wave–wave interactions between the upward and downward propagating waves near the source.

scholarly journals The Art of Identifying Equatorial Waves

2021 ◽  
Author(s):  
Peter Knippertz ◽  
Juliana Dias ◽  
Andreas H. Fink ◽  
Maria Gehne ◽  
George Kiladis ◽  
...  
Keyword(s):  
Gravity Waves ◽  
Deep Convection ◽  
Water System ◽  
Wave Theory ◽  
Lead Times ◽  
Linear Wave ◽  
Equatorial Waves ◽  
Easterly Waves ◽  
Advantages And Disadvantages ◽  
Shallow Water System

<div> <div> <div> <div> <p>Equatorial waves are synoptic- to planetary-scale propagating disturbances at low latitudes with frequencies from a few days to several weeks. Here this term includes Kelvin waves, equatorial Rossby waves, mixed-Rossby gravity waves, and inertio-gravity waves, which are closely related to linear wave theory, but also tropical disturbances, African easterly waves, and the intraseasonal Madden-Julian Oscillation. These waves can couple with deep convection, leading to a substantial modulation of rainfall. Recent work has shown that equatorial waves are amongst the dynamical features internal to the troposphere with the longest intrinsic predictability and that some models forecast them with an exploitable level of skill at lead times of up to a few weeks.</p> <p>A number of methods have been developed to identify and objectively isolate equatorial waves, both in (usually satellite) observations and in model fields. Most of these rely on (or at least refer to) the adiabatic, frictionless linearized primitive equations or shallow water system on the tropical beta plane. Common ingredients to these methods are longitude-time filtering (Fourier or wavelet) and/or projections onto predefined empirical or theoretical dynamical patterns. This paper aims to give an overview of the different methods to isolate the waves and their structures, to discuss underlying assumptions, to provide a systematic comparison, and to reveal advantages and disadvantages of each method. This way this study helps to optimally choose an approach suited to a given problem at hand and to avoid misuse and misinterpretation of the results.</p> </div> </div> </div> </div>

scholarly journals Photometric detection of internal gravity waves in upper main-sequence stars

2019 ◽  
Vol 621 ◽  
pp. A135 ◽  
Author(s):  
D. M. Bowman ◽  
C. Aerts ◽  
C. Johnston ◽  
M. G. Pedersen ◽  
T. M. Rogers ◽  
...  
Keyword(s):  
Numerical Simulations ◽  
Gravity Waves ◽  
Main Sequence ◽  
Low Frequency ◽  
Internal Gravity Waves ◽  
Main Sequence Stars ◽  
Photometric Detection ◽  
F Stars ◽  
Low Frequency Power ◽  
Frequency Power

Context. Main sequence stars with a convective core are predicted to stochastically excite internal gravity waves (IGWs), which effectively transport angular momentum throughout the stellar interior and explain the observed near-uniform interior rotation rates of intermediate-mass stars. However, there are few detections of IGWs, and fewer still made using photometry, with more detections needed to constrain numerical simulations. Aims. We aim to formalise the detection and characterisation of IGWs in photometric observations of stars born with convective cores (M ≳ 1.5 M⊙) and parameterise the low-frequency power excess caused by IGWs. Methods. Using the most recent CoRoT light curves for a sample of O, B, A and F stars, we parameterised the morphology of the flux contribution of IGWs in Fourier space using an MCMC numerical scheme within a Bayesian framework. We compared this to predictions from IGW numerical simulations and investigated how the observed morphology changes as a function of stellar parameters. Results. We demonstrate that a common morphology for the low-frequency power excess is observed in early-type stars observed by CoRoT. Our study shows that a background frequency-dependent source of astrophysical signal is common, which we interpret as IGWs. We provide constraints on the amplitudes of IGWs and the shape of their detected frequency spectrum across a range of mass, which is the first ensemble study of stochastic variability in such a diverse sample of stars. Conclusions. The evidence of a low-frequency power excess across a wide mass range supports the interpretation of IGWs in photometry of O, B, A and F stars. We also discuss the prospects of observing hundreds of massive stars with the Transiting Exoplanet Survey Satellite (TESS) in the near future.

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