AS1.7

EDI
Internal Gravity Waves

Internal gravity waves (IGWs) still pose major questions both to the atmospheric and ocean sciences, and to stellar physics. Important issues are IGW radiation from their various relevant sources, IGW reflection at boundaries, their propagation through and interaction with a larger-scale flow, wave-induced mean flow, wave-wave interactions in general, wave breaking and its implications for mixing, and the parameterization of these processes in models not explicitly resolving IGWs. The observational record, both on a global scale and with respect to local small-scale processes, is not yet sufficiently able to yield appropriate constraints. The session is intended to bring together experts from all fields of geophysical and astrophysical fluid dynamics working on related problems. Presentations on theoretical, modelling, experimental, and observational work with regard to all aspects of IGWs are most welcome, including those on major collaborative projects, such as DataWave.

Co-organized by NP7
Convener: Claudia StephanECSECS | Co-conveners: Ulrich Achatz, Alvaro de la Camara, Riwal Plougonven, Chantal Staquet
Presentations
| Wed, 25 May, 13:20–15:46 (CEST)
 
Room 0.31/32

Presentations: Wed, 25 May | Room 0.31/32

Chairpersons: Claudia Stephan, Katherine Grayson
13:20–13:30
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EGU22-10645
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ECS
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solicited
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Presentation form not yet defined
Neil Hindley, Lars Hoffmann, M. Joan Alexander, Cathryn Mitchell, Scott Osprey, Cora Randall, Corwin Wright, and Jia Yue

At around 04:14 UTC on the 15th January 2022, a major volcanic eruption began beneath the Tongan islands of Hunga Tonga and Hunga Ha’apai (175.4W, 20.5S). Located under only a shallow depth of water, the volcano rapidly launched a plume of super-heated ash and vapourised water upwards into the atmosphere. Over the next few hours, satellite observations reveal unprecedented large-scale concentric waves in the mid-stratosphere (near 40km altitude) radiating away from the eruption across the entire Pacific Ocean. In this presentation, we show brightness temperature perturbations in the 4.3 micron bands of the AIRS/Aqua, CrIS/Suomi-NPP and CrIS/JPSS-1 instruments that reveal three groups of atmospheric waves of special interest. First, an initial concentric wave is found travelling near the stratospheric speed of sound, likely to be an acoustic compression wave. There then follows a gap, which corresponds to phase speeds not permitted by theory, then a second group of waves likely to be gravity waves. These gravity waves are shown to be travelling near the maximum phase speed permitted, and there is a suggestion that some may travel the whole way around the globe in the tropics. Third, we observe small-scale gravity waves that pervade many thousands of kilometres across almost the entire Pacific Ocean, suggesting an extremely consistent heating source. All three of these wave observations are unprecedented in more than 20 years of stratospheric satellite observations, and this eruption may potentially have produced the first observations of an acoustic wave in the mid-stratosphere that can be measured from space. Now that we have space-borne instruments to observe it, this volcanic eruption provides a unique test of theoretical predictions of atmospheric wave phase speeds on some of the largest scales possible.

How to cite: Hindley, N., Hoffmann, L., Alexander, M. J., Mitchell, C., Osprey, S., Randall, C., Wright, C., and Yue, J.: The global reach of gravity waves at the stratospheric speed limit from the 2022 Hunga Tonga volcanic eruption, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10645, https://doi.org/10.5194/egusphere-egu22-10645, 2022.

Waves in the ocean
13:30–13:36
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EGU22-698
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Virtual presentation
Giovanni Dematteis, Kurt Polzin, and Yuri Lvov

The rate of diapycnal mixing, largely due to internal-wave breaking, is a key ingredient to understanding upwelling and horizontal circulation in the ocean. Here, we show a first-principles quantification of the downscale energy flux in the internal wavefield, that ultimately feeds the wave-breaking, shear-instability energy sink responsible for mixing. The approach is based on the wave kinetic equation that describes the inter-scale energy transfers via 3-wave nonlinear resonant interactions. Our results compare favorably with the phenomenological ‘Finescale Parameterization’ formula, by which deep ocean mixing is commonly implemented in the global models, and provide novel insights in the complex problem of oceanic energy transfers.

How to cite: Dematteis, G., Polzin, K., and Lvov, Y.: Energy flux quantification in the oceanic internal wavefield, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-698, https://doi.org/10.5194/egusphere-egu22-698, 2022.

13:36–13:42
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EGU22-2813
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Virtual presentation
Mariona Claret, M.-Pascale Lelong, Kraig B. Winters, and Yann Ourmières

Near-inertial waves (NIWs) are of major relevance to the global ocean circulation as they inject wind energy from the surface to the ocean interior and represent a primary source  of energy to the internal wave continuum. Eddies and fronts play a significant role in the downward penetration of NIW energy (from generation to propagation) and subsurface dissipation. Much of our understanding of NIW interactions with submeso- and mesoscale flows comes from limited observations as well as idealized theoretical and numerical processes, but these do not typically consider the presence of temporally evolving larger-scale flows. On the other hand, more realistic and time-evolving eddy fields from submesoscale-resolving Ocean General Circulation Models (OGCMs) forced with winds show truncated spectra at the subsurface due to the lack of vertical resolution -the subgrid vertical scale is 1-2 orders of magnitude larger than the scale at which dissipation occurs.  Since OGCMs are indeed very attractive tools to quantify global-regional impacts of small-scale phenomena, we propose to gain understanding of their biases in terms of wave-eddy interactions by using a novel approach.


This approach consists of nesting a non-hydrostatic Boussinesq model (Flow_Solve) into an OGCM configuration (NEMO-GLAZUR64) for the Gulf of Lion with O(1 km) horizontal and  O(30 m) vertical resolution. Preliminary analysis of NEMO-GLAZUR64 output reveals a highly energetic NIW field with intriguing distribution patterns relative to the eddies. We zoom into these patterns by following eddies with our nesting approach. The Boussinesq model provides a magnifying glass into dynamical processes that are either parameterized or fully unresolved in the OGCM. Wave energy budgets inferred from high-resolution process studies with Flow_Solve and NEMO-GLAZUR64 are then compared in order to better constrain model uncertainty in OGCMs due to NIW dynamics. 

How to cite: Claret, M., Lelong, M.-P., Winters, K. B., and Ourmières, Y.: Wave-eddy interactions in the Gulf of Lion: Bridging ocean general circulation models and process ocean simulations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2813, https://doi.org/10.5194/egusphere-egu22-2813, 2022.

13:42–13:48
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EGU22-6715
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ECS
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On-site presentation
Yangxin He and Kevin Lamb

We investigate the influence of a barotropic geostrophic current on
the internal wave (IW) generation over a shelf slope.
It is well known that most of the energy in the tide-topography
generated waves lies in waves with tidal frequency $\sigma_T$. 
Here we restrict our attention on the frequencies other than the dominant frequency $\sigma_T$. 
The current $V_g(x)$ is modeled as an idealized Gaussian function centered at
$x_0$ with width $x_r$ and maximum velocity $V_{max}$.
The bathymetry is modelled as a linear slope with smoothed corners.
Since the center of the current lies on the slope, there will always
be a region on the slope where the effective frequency $f_{eff}$ is
greater than the Coriolis parameter $f$ and another region where
$f_{eff} < f$. Parametric subharmonic instability (PSI) occurs where
waves with approximately half of the primary wave frequency, in this
case $\sigma_T/2$, are generated. In the presence of a large current,
PSI can occur where $f_{eff} < \sigma_T/2 < f$. This could not
happen without a current, i.e. $f_{eff} = f > \sigma_T/2$. Other interesting
interactions, including interharmonics and strong tidal harmonics, are also observed.

How to cite: He, Y. and Lamb, K.: Tide-topography interactions: the influence of an along-shelf current on the internal wave spectrum, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6715, https://doi.org/10.5194/egusphere-egu22-6715, 2022.

Wave-flow interactions
13:48–13:54
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EGU22-1077
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Presentation form not yet defined
On the shortwave instability of the stratified Kolmogorov flow
(withdrawn)
Michael Kurgansky
13:54–14:00
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EGU22-6834
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ECS
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Virtual presentation
Natalia Shmakova, Bruno Voisin, Joel Sommeria, and Jan-Bert Flor

An experimental study of the focused internal waves generated by a horizontally oscillating torus in a linearly stratified fluid is presented for a large range of Stokes numbers from 100 to 6000. For low Stokes number the waves are unimodal, i.e. in each propagation direction they diffuse to form a single wave beam, after their emission at the critical locations where the wave rays are tangential to the torus boundary. In that regime, the waves amplify in amplitude in a single focal zone. With increasing Stokes number the waves become bimodal, forming dual wave beams in each propagation direction and focusing in four zones of amplitude amplification.

Comparison of the experimental results at small oscillation amplitude with an original linear theory gives excellent agreement over the entire Stokes number range. As the oscillation amplitude increases the wave amplitude saturates in the focal zone. This saturation only appears at large oscillation amplitude for low Stokes number and is present already at moderate oscillation amplitude for high Stokes number.

Fourier analysis reveals triadic interactions of the fundamental wave with two subharmonic waves owing to focusing. This triadic resonance is visible only at large oscillation amplitude when viscous effects are high, i.e. for low Stokes number, but with increasing Stokes number it manifests itself at smaller oscillation amplitude. For high Stokes numbers, above 1800, and large oscillation amplitudes, greater than or equal to the minor radius of the torus, wave turbulence is observed.

The Stokes drift, calculated theoretically, appears as the key to understand the generation of vertical mean flow in the focal zone. At low and moderate Stokes numbers the mean flow is almost exactly opposed by the Stokes drift, while for higher Stokes numbers perturbations of this flow start to appear with time, possibly due to the generation of subharmonics.

How to cite: Shmakova, N., Voisin, B., Sommeria, J., and Flor, J.-B.: Effects of viscosity on internal wave focusing by an oscillating torus. , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6834, https://doi.org/10.5194/egusphere-egu22-6834, 2022.

14:00–14:06
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EGU22-13505
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Presentation form not yet defined
Manita Chouksey, Carsten Eden, and Dirk Olbers
  • The generation of internal gravity waves from an initially geostrophically balanced flow is diagnosed in non-hydrostatic numerical simulations of shear instabilities for varied dynamical regimes. A non-linear decomposition method up to third order in the Rossby number Ro is used as the diagnostic tool for a consistent separation of the balanced and unbalanced motions in the presence of their non-linear coupling. Wave emission is investigated in an Eady-like and a jet-like flow. For the jet-like case, geostrophic and ageostrophic unstable modes are used to initialize the flow in different simulations. Gravity wave emission is in general very weak over a range of values for Ro. At sufficiently high Ro, however, when the condition for symmetric instability is satisfied with negative values of local potential vorticity, significant wave emission is detected even at the lowest order. This is related to the occurrence of fast ageostrophic instability modes, generating a wide spectrum of waves. Thus, gravity waves are excited from the instability of the balanced mode to lowest order only if the condition of symmetric instability is satisfied and ageostrophic unstable modes obtain finite growth rates.

How to cite: Chouksey, M., Eden, C., and Olbers, D.: Gravity wave generation by shear instability of balanced flow, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13505, https://doi.org/10.5194/egusphere-egu22-13505, 2022.

14:06–14:12
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EGU22-3214
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ECS
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Virtual presentation
Michael Cox, Jacques Vanneste, and Hossein Kafiabad

The scattering of short inertia-gravity waves by large-scale geostrophic turbulence in the atmosphere and ocean can be described as a diffusion of wave action in wavenumber space. When the time dependence of the turbulent flow is neglected, waves conserve their frequency, which restricts the diffusion of energy to the constant-frequency cone. We relax the assumption of time independence and consider scattering by a flow that evolves slowly compared with the wave periods, consistent with a small Rossby number. The weak diffusion across the constant-frequency cone introduced by time dependence leads to a stationary energy spectrum that remains localised around the cone (specifically decaying as 1/σ5 with σ the angular deviation from the cone) corresponding to a small frequency broadening. We contrast our results with unbounded frequency broadening that arises for surface- or shallow-water waves.

How to cite: Cox, M., Vanneste, J., and Kafiabad, H.: Inertia-gravity wave diffusion by geostrophic turbulence: the impact of flow time dependence, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3214, https://doi.org/10.5194/egusphere-egu22-3214, 2022.

Modelling studies
14:12–14:18
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EGU22-6185
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On-site presentation
Sergiy Vasylkevych and Nedjeljka Žagar

Resolving inertia-gravity (IG, or gravity) waves poses a real challenge for the formulation of numerical schemes for numerical weather prediction (NWP) and climate models due to different time scales of Rossby wave dynamics and fast-propagating IG waves. With ever increasing emphasize placed on high-resolution simulations, the importance of the issue is growing due to the implications of Courant-Friedrich-Levy (CFL) stability criterium. It is especially prominent in the tropical atmosphere, where a significant part of variability is associated with divergence-dominated dynamics. Detangling gravity and Rossby wave dynamics in the tropics is a challenging problem due to a lack of sepaartion between the Rossby and gravity regime that is present in the extra-tropics.   

TIGAR (Transient Inertia Gravity and Rossby wave dynamics) targets this problem by employing the eigensolutions of the linearized primitive equations on the sphere as the basis functions for the numerical representation of dynamical variables. This leads to the description of dynamics in terms of physically identifiable structures, i.e. the Rossby and gravity waves, which are fully dynamically separated at the linearization level. The benefits of such approach can be reaped on analytical, modelling and computational sides. As a research tool, TIGAR allows to study wave-wave interections directly in the model, without the need of intermediate software for wave filtering. Simplified models aimed at particular dynamical regime can be obtained from a full model with a simple configuration change. For instance, retaining only the Rossby modes in the spectral expansion will result in the quasi-geostrophic model, while additionally keeping the Kelvin and mixed Rossby-gravity waves will reproduce essential features of tropical circulation. 

Numerically, high precision computation is achieved in TIGAR through the use of higher order exponential time-differencing schemes, which take advantage of the normal modes framework, leading to the major increase in computational efficiency and stability. The comparison with classical time-stepping schemes in the horizontal component of the model shows accuracy improvements of several orders of magnitude at the same computational cost. In our testing on multiscale flows, the stability gains associated with the enhanced representation of gravity wave dynamics raise CFL time-step bound for explicit schemes by a factor of 4-6. 

We present TIGAR solutions of some classical steady and time-dependent problems including barotropic and baroclinic instability tests.

How to cite: Vasylkevych, S. and Žagar, N.: TIGAR - a new global atmospheric model for the simulation of Transient Inertia-Gravity And Rossby wave dynamics, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6185, https://doi.org/10.5194/egusphere-egu22-6185, 2022.

14:18–14:24
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EGU22-10138
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ECS
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Virtual presentation
Christopher Kruse, M. Joan Alexander, Martina Bramberger, Padram Hassanzadeh, Ashesh Chattopadhyay, Brian Green, and Alison Grimsdell

Convection, both observed and modeled, generates gravity waves (GWs) that significantly impact large-scale circulations in the stratosphere and above. However, models that permit convection and resolve the GWs they generate cannot reproduce the timing, location, and intensity of the actual convective cells that generate the observed convective GWs. This issue prevents comparison of observed and modeled convective GWs and model validation/evaluation. 

Here, convective latent heating is predicted based on radar observations and provided to an idealized version of WRF, allowing WRF’s dynamics to generate convective updrafts/downdrafts and generated convective GWs both mechanically and diabatically. Two methods are used to predict convective latent heating: the composited lookup table of Bramberger et al. 2020 and neural networks (NNs) using the same, and additional, input variables. Offline performance of the NN-predicted latent heating can be improved over the previous method when more input variables are used. Preliminary comparisons of modeled and observed (via superpressure-balloon and satellite) convective GWs will be presented. 

How to cite: Kruse, C., Alexander, M. J., Bramberger, M., Hassanzadeh, P., Chattopadhyay, A., Green, B., and Grimsdell, A.: Simulating Convective GWs forced by Radar-Based, Neural-Network-Predicted Diabatic Heating, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10138, https://doi.org/10.5194/egusphere-egu22-10138, 2022.

14:24–14:30
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EGU22-6323
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ECS
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Virtual presentation
Emily Lear, Corwin Wright, Neil Hindley, and Inna Polichtchouk

Gravity waves impact the large scale circulation, and increasing our understanding of them is important to improve weather and climate models. This presentation focusses on atmospheric gravity waves in the stratosphere using data from the ECMWF ERA5 reanalysis, AIRS (Atmospheric Infrared Sounder) on NASA’s Aqua satellite and a high resolution run of the IFS operated at a km-scale spatial resolution. Data was examined during the first 2 weeks of November, as the high resolution model was initialized on the 1st of this month. Asia and surrounding regions are investigated, because preliminary studies of AIRS data suggested strong gravity wave activity in this region during this time period. Waves can also be seen in the ERA5 data at the same times and locations. The high resolution model also shows significant gravity wave activity in similar areas to where it is seen in the AIRS data, particularly over Russia. The 2D+1 S-Transform was used to find wave amplitudes, horizontal and vertical wavelengths and momentum flux for all three datasets. Weather models are advancing rapidly and km scales such as the experimental IFS run could become operational in next decade. At these grid scales, gravity waves must be resolved instead of parameterized so the models need to be tested to see if they do this correctly. This work provides information on how a cutting edge model resolves gravity waves compared to observations.

How to cite: Lear, E., Wright, C., Hindley, N., and Polichtchouk, I.: Comparing gravity waves in a kilometer scale run of the IFS to AIRS satellite observations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6323, https://doi.org/10.5194/egusphere-egu22-6323, 2022.

14:30–14:36
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EGU22-6694
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ECS
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Virtual presentation
Haruka Okui, Kaoru Sato, and Shingo Watanabe

Observations with high vertical resolution have shown that vertical wavenumber (m) power spectra of horizontal wind and temperature fluctuations have a universal shape with a steep slope that is roughly proportional to ~m–3. Several theoretical models explaining the universal spectra were proposed based on the assumption of gravity wave (GW) saturation. However, it has not yet been sufficiently confirmed that such characteristic spectra are fully composed of GWs. Thus, in the present study, we examine whether the m–3 spectra are due to GWs, using a GW-permitting general circulation model with a high top in the lower thermosphere. The model-simulated spectra have steep spectral slopes, which is consistent with observations. GWs are extracted as fluctuations having total horizontal wavenumbers of 21–639. From the comparison between spectra of the GWs and those of all simulated fluctuations, it is shown that GWs are dominant only at high ms, while disturbances other than the GWs largely contribute to the spectra at low ms even in the m–3 range. In addition, we examine vertical and geographical distributions of the characteristic wavenumbers, slopes, and amplitudes of GW spectra. The slopes of GW spectra are particularly steep near the eastward and westward jets in the middle atmosphere. It is theoretically shown that strong vertical shear below the jets is responsible for the formation of steep GW spectral slopes.

 

Reference:

Okui, H., Sato, K., and Watanabe, S., Contribution of gravity waves to the universal vertical wavenumber (m–3) spectra revealed by a gravity-wave permitting general circulation model, submitted to Journal of Geophysical Research Atmospheres.

How to cite: Okui, H., Sato, K., and Watanabe, S.: Contribution of gravity waves to the universal vertical wavenumber (m–3) spectra revealed by a gravity-wave permitting general circulation model, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6694, https://doi.org/10.5194/egusphere-egu22-6694, 2022.

14:36–14:42
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EGU22-3884
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ECS
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On-site presentation
Zuzana Procházková, Christopher Kruse, Aleš Kuchař, Petr Pišoft, and Petr Šácha

Terrestrial atmosphere supports propagation of various wave types. An important component of the dynamics especially in the middle atmosphere are the internal gravity waves (GWs) that are incessantly being generated from initial perturbations in a stably stratified atmosphere. Horizontal GW wavelengths range from a few to thousands of kilometres. Together with a wide range of temporal and vertical scales, this complicates their global observations and modeling, requiring high resolution model simulations. Subsequent analyses, nevertheless, contain a significant margin of uncertainty originating in the separation of GWs from the background flow, which is often performed by statistical means. In our work, we explore properties of a Gaussian high-pass filter method, using a deep WRF simulation with the horizontal resolution of 3 km in the region of the Drake Passage. Due to the revealed sensitivity of momentum flux and drag estimates to a filter cutoff parameter, we propose a new method that sets the value of the parameter on the basis of the horizontal spectra of horizontal kinetic energy.

How to cite: Procházková, Z., Kruse, C., Kuchař, A., Pišoft, P., and Šácha, P.: Detection of internal gravity waves by high-pass filtering, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3884, https://doi.org/10.5194/egusphere-egu22-3884, 2022.

14:42–14:48
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EGU22-4192
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ECS
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Virtual presentation
Christophe Bellisario, Pierre Simoneau, Alain Hauchecorne, Philippe Keckhut, Fabrice Chane-Ming, and Constantino Listowski

The infrared emission lines observed between 80 and 100 km known as nightglow allow the investigation of dynamic phenomena such as gravity waves. These perturbations act on local temperature and density. However, the observation of the local perturbations in the nightglow layer is mainly performed by spectrally broad cameras. Swenson and Gardner (1998) introduced the cancellation factor linking relative variations of intensity with relative variations of temperature. The cancellation factor is a function of the perturbation vertical wavelength estimated from simulation that do not include spectral variations. In this study, we intend to estimate the spectral variability of the cancellation factor, in particular within the range 0.9-1.7 µm corresponding to infrared InGaAs camera, used during measurement campaigns. We describe briefly the model that resolves the vibrational states of the nightglow main source (OH). Then vertically propagating gravity waves are applied on a 1D scheme and the cancellation factor is computed based on the impact on both temperature and intensity. Spectral variations of the cancellation factor are observed and compared along the variation of the vertical wavelength.

How to cite: Bellisario, C., Simoneau, P., Hauchecorne, A., Keckhut, P., Chane-Ming, F., and Listowski, C.: Spectral variations of the cancellation factor for temperature investigation in the mesospheric nightglow layer, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4192, https://doi.org/10.5194/egusphere-egu22-4192, 2022.

Coffee break
Chairpersons: Claudia Stephan, Katherine Grayson
15:10–15:16
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EGU22-8254
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ECS
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On-site presentation
Katherine Grayson, Stuart Dalziel, and Andrew Lawrie

With an aim of understanding the role of internal waves to oceanic mixing, various mechanisms have been cited as a possible explanation for how they transfer energy across the wavenumber and frequency spectra and eventually contribute to small-scale turbulence. Triadic Resonance Instability (TRI) has become increasingly recognised as potentially one of these mechanisms. This talk will summarise experimental work that examines the long-term temporal and spatial evolution of this instability in the more realistic scenario of a finite-width internal wave beam. Experiments have been conducted using a new generation of wave maker, featuring a flexible horizontal boundary driven by an array of independently controlled actuators. We present experimental results exploring the role the finite-width of a wave beam has on the evolution of TRI. Experimentally, we find that the approach to a saturated equilibrium state for the three triadic waves is not monotonic, rather their amplitudes continue to oscillate without reaching a steady equilibrium. A detailed study into the structure of the secondary waves shows that this behaviour is also witnessed in Fourier space. We show how a spectrum of these resonant frequencies in the flow ‘beat’ to cause interference patterns which manifest throughout the instability as slow amplitude modulations.

How to cite: Grayson, K., Dalziel, S., and Lawrie, A.: Experimental Investigation into the long-term spatial and temporal development of Triadic Resonance Instability in a finite-width internal wave beam , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8254, https://doi.org/10.5194/egusphere-egu22-8254, 2022.

Orographic waves
15:16–15:22
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EGU22-13076
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Virtual presentation
Markus Geldenhuys

The current orographic gravity wave drag parameterisation in the vicinity of low-level blocking is inadequate. Reducing the gravity wave amplitude (and thereby reducing the gravity wave drag) by assuming an effective mountain height dependent on the blocking depth is not realistic, yet this is implemented in most orographic gravity wave drag parameterisation schemes. The blocking layer acts as a sloped dynamic barrier that uplifts the air similarly to the mountain slope. Through a variety of mechanisms low-level blocking can induce more gravity waves or gravity waves with a higher momentum flux (compared to the current representation by parameterisation schemes). One possible solution is to modify the parameterisation scheme to not reduce the gravity wave momentum flux by the blocking depth. More realistic parameterisation schemes are likely to improve the models' performance.

How to cite: Geldenhuys, M.: On gravity wave parameterisation in vicinity of low-level blocking..., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13076, https://doi.org/10.5194/egusphere-egu22-13076, 2022.

15:22–15:28
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EGU22-6993
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ECS
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Virtual presentation
Sebastian Rhode, Peter Preusse, Manfred Ern, Lukas Krasauskas, Markus Geldenhuys, and Martin Riese

Observations and high resolution models suggest a high potential for gravity waves (GW) to propagate horizontally, which is usually not considered in current parameterizations of general circulation models (GCM). For a quantification of the oblique propagation of orographic GWs and their transport of momentum throughout the atmosphere, we present a mountain wave model (MWM) that describes the terrain induced GW sources, propagation and momentum flux. Being aware of horizontal wind gradients, the model also allows for GW refraction which leads to a turning of the wave vector.

The MWM we present here is a simplified model identifying orographic GW sources from topography data. It is similar to the one presented in Bacmeister et.al. (1994). First, the topography is smoothed using a Gaussian bandpass filter, which allows to consider the different scales of generated MWs separately. This smoothed topography is afterwards reduced to the inherent ridge structure (i.e. to the arêtes of mountains) by employing edge and line detection algorithms from computer vision. Using this underlying arête structure in combination with a fit of idealized Gaussian-shaped mountain ridges to the topography gives us a straightforward way of determining MW parameters for launching a ray, i.e. source location, orientation and size of the wave vector as well as the displacement amplitude. These parameters are then used to calculate the propagation in space and time in given atmospheric backgrounds (determined from smoothed ERA5 (European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis 5th Generation) data) with the ray tracer GROGRAT. The results can then be binned in terms of momentum flux and drag or used for a reconstruction of 3D temperature perturbations for a given time.

The MWM presented here has been validated against global satellite data as well as local measurements to a new quality compared to previous studies. The validation has been performed by applying an instrument-specific observational filter to the model data before considering global maps of momentum flux distributions and horizontal cross-sections of temperature perturbations. Comparisons of these to satellite data and limb measurement retrievals respectively will be shown in this presentation.

How to cite: Rhode, S., Preusse, P., Ern, M., Krasauskas, L., Geldenhuys, M., and Riese, M.: Quantification of oblique orographic gravity wave propagation deduced from a mountain wave model, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6993, https://doi.org/10.5194/egusphere-egu22-6993, 2022.

15:28–15:34
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EGU22-10667
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ECS
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Virtual presentation
Timothy Banyard, Corwin Wright, Neil Hindley, and Gemma Halloran

As the first Doppler wind lidar in space, ESA’s flagship Aeolus satellite provides us with a unique opportunity to study the propagation of gravity waves (GWs) from near the surface to the tropopause and UTLS. Existing space-based measurements of GWs in this altitude range are spatially limited and, where available, use temperature as a proxy for wind perturbations. Recent research confirms Aeolus’ ability to measure GWs, and so this and future spaceborne wind lidars have the potential to transform our understanding of these critically-important dynamical processes.

Here, we present results from a special campaign onboard Aeolus, involving a change to the satellite’s range-bin settings designed to allow better observations of orographic GWs over the Southern Andes during winter 2021. In line with recent research, we expect to see GW wind structures extending down to near the wave sources, enabling detailed measurements of vertical and horizontal wavelength, pseudo-momentum flux and wave intermittency. Such parameters will feed into the next generation of NWP and global circulation models, which will resolve waves at higher resolutions and employ more advanced parametrization schemes.

How to cite: Banyard, T., Wright, C., Hindley, N., and Halloran, G.: Atmospheric Gravity Wave Observations from a Special Aeolus Campaign over the Southern Andes during Winter 2021, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10667, https://doi.org/10.5194/egusphere-egu22-10667, 2022.

Waves in the tropics
15:34–15:40
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EGU22-8251
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ECS
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On-site presentation
Valentino Neduhal, Nedjeljka Žagar, and Žiga Zaplotnik
In contrast to the kinetic energy spectrum of the horizontal motions, the spectrum of kinetic energy of vertical motions (vertical kinetic energy spectrum) is poorly known because the vertical velocity is not an observed quantity of the global observing system. The vertical kinetic energy spectra can be simulated by non-hydrostatic models but are difficult to validate. Furthermore, contributions to the vertical kinetic energy spectrum from the Rossby and gravity waves have traditionally been treated separately using the quasi-geostrophic omega equations and the polarization relations for the stratified Boussinesq fluid, respectively. This approach is difficult to apply in the tropics, where the Rossby and gravity wave regimes are nonseparable and the frequency gap between the Rossby and gravity waves, present in the extra-tropics, is filled with the Kelvin and mixed Rossby-gravity waves.  

We apply a unified framework for the derivation of vertical velocities of the Rossby and inertia-gravity waves and associated kinetic energy spectra using the eigensolutions of the linearized primitive equations. It can be considered applicable to the hydrostatic atmosphere with horizontal scales up to around 10 km.  The derivation involves the analytical evaluation of divergence of the horizontal wind associated with the Rossby and inertia-gravity modes. The new framework is applied to the ECMWF analysis in August 2016 and August 2018. Latitude and altitude dependence of the horizontal wind divergence and vertical kinetic energy spectra within the tropics are discussed and compared with observations over the tropical Atlantic. In particular, we discuss the slope of the vertical kinetic energy spectra for the two dynamical regimes.

How to cite: Neduhal, V., Žagar, N., and Zaplotnik, Ž.: Zonal wavenumber spectra of the vertical velocity and horizontal wind divergence associated with the Rossby and non-Rossby waves in the tropics, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8251, https://doi.org/10.5194/egusphere-egu22-8251, 2022.

15:40–15:46
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EGU22-13062
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ECS
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Presentation form not yet defined
Martina Bramberger, M. Joan Alexander, Sean M. Davis, Aurelien Podglajen, Albert Hertzog, Lars Kalnajs, Terry Deshler, J. Douglas Goetz, and Sergey Khaykin

Atmospheric waves in the tropical tropopause layer are recognized as a significant influence on processes that impact global climate. For example, waves drive the quasi-biennial oscillation (QBO) in equatorial stratospheric winds and modulate occurrences of cirrus clouds. However, the QBO in the lower stratosphere and thin cirrus have continued to elude accurate simulation in state-of-the-art climate models and seasonal forecast systems. We use first-of-their-kind profile measurements deployed beneath a long-duration balloon to provide new insights into impacts of fine-scale waves on equatorial cirrus clouds and the QBO just above the tropopause. Analysis of these balloon-borne measurements reveals previously uncharacterized fine-vertical-scale waves (<1km) with large horizontal extent (>1000km) and multiday periods. These waves affect cirrus clouds and QBO winds in ways that could explain current climate model shortcomings in representing these stratospheric influences on climate. Accurately simulating these fine-vertical-scale processes thus has the potential to improve sub-seasonal to near-term climate prediction.

How to cite: Bramberger, M., Alexander, M. J., Davis, S. M., Podglajen, A., Hertzog, A., Kalnajs, L., Deshler, T., Goetz, J. D., and Khaykin, S.: First measurements of fine-vertical-scale wave impacts on the tropical lower stratosphere, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13062, https://doi.org/10.5194/egusphere-egu22-13062, 2022.