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In many respects 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. Also 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. Besides, this year we welcome abstracts reporting results on the SouthTRAC campaign in the Southern Hemisphere, as well as any other major collaborative projects such as MS-GWaves.

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Co-organized by AS1/NP7
Convener: Alvaro de la CamaraECSECS | Co-conveners: Ulrich Achatz, Chantal Staquet, Claudia StephanECSECS
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| Fri, 08 May, 14:00–15:45 (CEST)

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Chat time: Friday, 8 May 2020, 14:00–15:45

D2392 |
EGU2020-6118
| solicited
Christopher Kruse, Joan Alexander, Lars Hoffmann, Inna Polichtchouk, Annelize van Niekerk, Riwal Plougonven, Corwin Wrioght, Julio Bacmeister, Manfred Ern, Kaoru Sato, Ryosuke Shibuya, Catrin Meyer, Olaf Stein, Laura Holt, Petr Šácha, and Sonja Gisinger

Orographic gravity wave (OGW) drag is one of the fundamental physics parametrizations employed in every global numerical model across timescales from weather to climate. These parameterizations have significant influences, both direct and indirect, on the atmosphere’s general circulation from the troposphere at least through the mesosphere. Despite their significant influence, observational constraints on these parameterizations are still largely lacking.

Presented here is a team project jointly supported by SPARC and the International Space Science Institute with the overall objective of providing new quantitative constraints for OGW drag parameterizations. Specific objectives are to evaluate methods that quantify vertical fluxes of horizontal momentum (MF) from satellite observations via an observing system simulation experiment (OSSE), a validation of WRF, UKMO, ECMWF, and ICON models against satellite and balloon observations, and an inter-comparison of OGW properties (e.g. MF and drag) within these models. Evaluation of satellite-based estimates of MF and model validation/inter-comparison will help to better quantify actual MF in the stratosphere, providing the best stratospheric MF and drag estimates for parameterizations to reproduce to date.

Two unique aspects of the project are that all models involved are deep, extending up to 1 Pa. The motivations for doing so was to include entire instrument weighting functions for AIRS observations, allowing direct, quantitative comparison between AIRS (and other satellite-borne) observations and the models. The second is the effort to perform an OSSE within the simulations, allowing comparison between MF from satellite-based methods within the models to the true MF in the models.

Preliminary results show that higher-res models (dx = 3 km) compare well and produce significantly more MF than lower-res global models, but the higher-res models still underrepresent OGW amplitudes. Mesospheric tides in analyses used to force the models significantly modulate resolved GWs and their drags.

D2393 |
EGU2020-4347
Haruka Okui and Kaoru Sato

Using long-term high-resolution operational radiosonde observation data from nine stations in the subtropics and mid-latitudes of Japan, this study performed statistical analysis of the dynamical characteristics of gravity waves (GWs). Wave generation by shear instability in summer was a particular focus because orographic GWs cannot propagate deep into the middle atmosphere through their critical layer in the lower stratosphere. The kinetic energy of summer stratospheric GWs is markedly large south of 37°N. Hodograph analysis revealed that GWs propagating eastward relative to the ground are dominant in summer. The percentage of GWs propagating energy upward (downward) is large above (below) the height at which the mean occurrence frequency of shear instability is high. The time series of the kinetic energy of stratospheric GWs exhibited statistically significant positive correlation with the occurrence frequency of shear instability slightly below the tropopause. These findings strongly suggest the possibility of excitation of summer stratospheric GWs by shear instability above the jet. The shear instability condition is satisfied more frequently in the region 30°–35°N. This is probably related to two characteristics of the background fields slightly below the tropopause: larger vertical shear of zonal winds at higher latitudes and lower static stability at lower latitudes.

D2394 |
EGU2020-6474
Michael Reeder and Adam Morgan

During rapid extratropical cyclogenesis in the Australian region, distinctive striated, triangular-shaped clouds commonly form on the poleward side of the jet exit near the axis of inflection between an upstream trough and downstream ridge. These clouds are called striated deltas and the striations are shown here to be caused by radiating inertia-gravity waves. An analysis of 28 striated delta clouds shows that the striated deltas have a mean length of 1034 km and width of 537 km, and a striation wavelength of 74 km. Large parcel accelerations, nonlinear flow imbalance and active convection within the striated delta are features of the composite upper-tropospheric environment. Patterns of Q-vector illustrates the unique shape of striated delta clouds to be coincident with a delta-shaped forcing of adiabatic ascent in the poleward jet exit. One of the extratropical cyclones analysed and its associated striated delta is simulated with WRF-ARW both with and without diabatic heating. Both simulations produce pronounced gravity wave packets along the surface cold front, along the downstream upper jet axis and a delta-shaped packet in the lower stratosphere above the jet exit. Ray tracing identifies the source region of the waves in the stratosphere to be the upper jet. Vertically-radiating gravity waves originating in the vicinity of the jet during rapid extratropical cyclogenesis, propagate both upwards and downwards, imprinting striations onto the cloud in the jet exit and setting the spacing between the convective bands.

D2395 |
EGU2020-9876
Manfred Ern, Peter Preusse, and Martin Riese

Sources of atmospheric gravity waves are mostly located in the troposphere and the lower stratosphere. Gravity waves propagate away from their sources, re-distribute energy and momentum in the atmosphere, and exert drag on the atmospheric background flow where they dissipate. Therefore they are important drivers of the atmospheric circulation. In climate models, their effect on the background circulation is usually parameterized because of their relatively short horizontal and vertical wavelengths that are of the order of 10-1000km and 1-100km, respectively. Gravity wave parametrizations are very simplified. For example, they often neglect the fact that gravity wave source processes and gravity wave propagation conditions can vary on short temporal and spatial scales. Therefore the global distribution of gravity wave activity is very intermittent, which has also important consequences where gravity waves dissipate and exert drag on the background flow, and which should be accounted for in parametrizations.
For guiding models, global observations of the gravity wave distribution and its intermittency are needed. We derive gravity wave potential energies and absolute momentum fluxes from observations of the High Resolution Dynamics Limb Sounder (HIRDLS) and the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) satellite instruments. As a measure of intermittency, we calculate global distributions of Gini coefficients. We find that our results are qualitatively in good agreement with previous findings from satellite, and similar in magnitude to intermittency obtained from previous superpressure balloon campaigns. In the stratosphere, strongest intermittency is found over orographic gravity wave sources, followed by gravity wave activity in the polar night jets. Intermittency in the tropical stratosphere is weakest. However, in the tropical upper mesosphere intermittency is increased, which is likely caused by the modulation of the gravity wave distribution by tides.

D2396 |
EGU2020-19821
Neil Hindley, Corwin Wright, Lars Hoffmann, Tracy Moffat-Griffin, M. Joan Alexander, and Nicholas Mitchell

Atmospheric gravity waves are a fundamental component of the Earth’s dynamical system. These mesoscale waves play a key role in the coupling of different atmospheric layers, acting as crucial drivers of the middle atmospheric circulation through the transport and deposition of energy and momentum. As Global Circulation Models (GCMs) achieve ever higher resolution in the stratosphere, there is a need to ensure that the simulated gravity waves resolved in these models are well constrained by observations, which in turn ensures that modelled circulations are realistic and not over-dependent on tuning with parameterisations. However, obtaining 3-D gravity-wave measurements in the real atmosphere is notoriously difficult. Global 3-D observations of wave properties in the stratosphere are required in order to accurately estimate gravity wave fluxes that can be compared to models. Here we analyse a unique long-term satellite dataset of specialised high-resolution 3-D temperature measurements from NASA’s AIRS/Aqua instrument from 2002-2020. By analysing these data with a 3-D Stockwell transform (3DST) using high-performance computing, we can reveal global distributions of gravity-wave amplitudes, wavelengths, intermittency and directional momentum fluxes in the stratosphere across two decades - the largest such 3-D study of stratospheric gravity waves performed yet. This long-term dataset reveals solar-cycle variability of gravity-wave amplitudes in the tropics, significant reductions in gravity-wave fluxes during southern Sudden Stratospheric Warmings (SSWs) and the persistent oblique propagation of wintertime gravity waves into the southern polar vortex around 60S each year, a phenomenon that is not observed in the northern hemisphere. With these new observations we can begin to better constrain simulated gravity waves and their impacts in GCMs, ultimately leading to better forecasts of weather and climate.

D2397 |
EGU2020-12918
Markus Rapp, Bernd Kaifler, Andreas Dörnbrack, Sonja Gisinger, Tyler Mixa, Robert Reichert, Natalie Kaifler, Andreas Giez, Peter Preusse, Markus Geldenhuis, Lukas Krasauskas, Wolfgang Woiwode, Felix Friedl-Vallon, Alejandro de la Torre, Jose Luis Hormaechea, Martin Riese, Björn-Martin Sinnhuber, Peter Hoor, and Andreas Engel

The region around Southern Argentina and the Antarctic peninsula is known as the world’s strongest hotspot of stratospheric gravity wave activity. In this region, large tropospheric winds are perturbed by the orography of the Andes and the Antarctic peninsula resulting in the excitation of mountain waves which might propagate all the way up into the upper mesosphere when the polar night jet is intact. In addition, satellite observations also show large stratospheric wave activity in the region of the Drake passage, i.e., in between the Andes and the Antarctic peninsula, and along the corresponding latitudinal circle of 60°S. The origin of these waves is currently not entirely understood. Several hypotheses are currently being investigated, like for example the idea that the mountain waves that were originally excited over the Andes and the Antarctic peninsula propagate horizontally to 60°S and along the latitudinal circle. In order to investigate this and other hypotheses the German research aircraft HALO was deployed to Rio Grande, Tierra del Fuego, at the Southern Tip of Argentina in September and November 2019 in the frame of the SOUTHTRAC (Southern hemisphere Transport, Dynamics, and Chemistry) research mission. A total of 6 dedicated research flights with a typical length of 7000km were conducted to obtain gravity wave observations with the newly developed ALIMA (ALIMA=Airborne LIdar for Middle Atmosphere research)-instrument and the GLORIA (GLORIA=Gimballed Limb Observer for Radiance Imaging of the Atmosphere) limb sounder. While ALIMA measures temperatures and temperature perturbations in the altitude range from 20-90 km, GLORIA observations allow to characterize wave perturbations in temperatures and trace gas concentrations below flight level (<~14 km). This paper gives an overview of the mission objectives, the prevailing atmospheric conditions during the HALO deployment, and highlights some outstanding initial results of the gravity wave observations.

D2398 |
EGU2020-19489
Tyler Mixa, Andreas Dörnbrack, Bernd Kaifler, and Markus Rapp

We present numerical simulations of a deep orographic gravity wave (GW) event observed by the ALIMA airborne lidar on 11-12 September 2019 over Southern Argentina. The measurements are taken from the 2019 SOUTHTRAC Campaign, employing a comprehensive suite of remote sensing and in-situ instruments onboard the HALO research aircraft to study the stratospheric GW hotspot over Tierra del Fuego and the Antarctic Peninsula. Wind conditions on 11-12 September exhibit local and large-scale directional shear from the ground to the polar night jet, creating a complex propagation environment supporting multiple orientations of GW propagation and strong potential for local GW breaking and secondary GW generation. Using high resolution numerical models, we simulate the 3D evolution of the orographic GW field to analyze the remote sensing and in-situ measurements from the event.

D2399 |
EGU2020-10176
Bruno Ribstein, Christophe Millet, Francois Lott, and Alvaro de la Camara

A multiwave non-orographic gravity wave (GW) scheme is adapted to represent waves of small intrinsic phase speed and sources located at all altitudes in the troposphere and middle atmosphere. Using reanalysis data, these changes impose larger amplitude saturated waves everywhere in the middle atmosphere, which permits to produce more realistic GW vertical spectra than when the phase speeds are large and the sources are in the troposphere only. The same scheme, tested online in the Laboratoire de Météorologie Dynamique Zoom(LMDz) general circulation model, performs at least as well as the operational non-orographic GW scheme.  Some modest benefits are seen, for instance, in the equatorial tilt with altitude of the winter jets in the middle atmosphere. Although the scheme includes the effects of inertial waves, which are more and more often detected in the mesosphere, the configuration that gives a reasonable climatology in LMDz hinders the vertical propagation of these parameterized waves and do not generally reach mesospheric altitudes.

D2400 |
EGU2020-5248
Young-Ha Kim, Gergely Bölöni, Sebastian Borchert, Hye-Yeong Chun, and Ulrich Achatz

The intermittency of gravity waves (GWs) is investigated using Multi-Scale Gravity Wave Model (MS-GWaM) implemented in the upper-atmosphere extension of ICON model. The intermittency of GWs is originated from that of wave sources but altered during propagation of the waves. Conventional GW parametrization (GWP), which diagnoses vertical profiles of GW properties under the steady-state assumption, can take into account the source intermittency if the GWP employs flow-dependent sources, while it cannot present the change of intermittency by transient evolutions of GWs. MS-GWaM is a prognostic model that explicitly solves the evolution of positions of waves (as well as their wavenumbers and amplitudes) in time and thus capable of describing the intermittency change. In order to include the source intermittency and variability, we couple the convective source, as diagnosed by subgrid-scale cumulus parametrization in ICON, to MS-GWaM, based on an analytic formulation of GW response to this source. In addition to this, a spatio-temporally uniform, persistent source is prescribed in the extratropics to take into account other non-orographic sources. Orographic sources are currently not used. The GW intermittency is measured by the Gini index, and is found to be quite high in the tropics, compared to that in the extratropics. In both regions, the index has similar values to those obtained from superpressure balloon observations reported in previous studies. A control experiment is performed using GWP based on the steady-state assumption, but coupled to the same wave sources, to assess the effects of transient modelling using MS-GWaM on the simulated intermittency. From comparison to the control experiment, the intermittency is found to increase largely for GWs from the uniform source but to decrease for convective GWs by the transient modelling.

D2401 |
EGU2020-8371
Georg Sebastian Voelker, Triantaphyllos Akylas, and Ulrich Achatz

Internal gravity waves are a well known mechanism of energy transport in stratified fluids such as the atmosphere and the ocean. Their abundance and importance for various geophysical processes like ocean mixing and momentum deposition in atmospheric jets are widely accepted. While resonant wave-wave interactions of monochromatic disturbances have received intensive study, little work has been done on triad interactions between wave trains that are modulated by a variable mean flow.

Using the method of multiple scale asymptotics we consider a weakly non-linear Boussinesq WKBJ theory for interacting gravity wave trains propagating through a finite amplitude background flow. Consequently the wave trains are allowed to spectrally pass through resonance conditions and exchange energy when sufficiently close to resonance. We find a global optimal threshold for the deviation from resonance and derive a corresponding parametrization for the triad interaction applicable to ray tracing schemes.

We test the theory with idealized simulations in which two wave trains generate a third by passing through resonance in a sinusoidal background shear flow with varying vertical scales. Comparing WKBJ simulations with wave resolving large eddy simulations we find qualitative and quantitative agreement. Furthermore we assess the impact of the strength of the modulation as well as the effect of the wave amplitudes on the energy exchange between the interacting wave triad.

D2402 |
EGU2020-18419
| solicited
Géraldine Davis, Thierry Dauxois, Sylvain Joubaud, Timothée Jamin, Nicolas Mordant, and Clément Savaro

Stratified fluids may develop simultaneously turbulence and internal wave turbulence, the latter describing a set of a large number of dispersive and weakly nonlinear interacting waves. The description and understanding of this regime for internal gravity waves (IGW) is really an open subject, in particular due to their very unusual dispersion relation. In this presentation, I will show experimental signatures of a large set of weakly interacting IGW obtained in a 2D trapezoidal tank.

Due to the peculiar linear reflexion law of IGW on inclined slopes, this setup - for given excitation frequencies - focuses all the input energy on a closed loop called attractor. If the forcing is large enough, this attractor destabilizes and the system eventually achieves a nonlinear cascade in frequencies and wavevectors via triadic resonant interactions, which results at large forcing amplitudes in a k^-3 spatial energy spectrum. I will also show some results obtained in a much larger set-up -the Coriolis facility in Grenoble- with signature of 3D internal wave turbulence.

D2403 |
EGU2020-9374
Costanza Rodda and Uwe Harlander

Inertia-gravity waves (IGWs) are known to play an essential role in the terrestrial atmospheric dynamics as they can lead to energy and momentum flux when they propagate upwards. An open question is to which extent nearly linear IGWs contribute to the total energy and to flattening of the energy spectrum observed at the mesoscale.
In this work, we present an experimental investigation of the energy distribution between the large-scale balanced flow and the small-scale imbalanced flow. Weakly nonlinear IGWs emitted from baroclinic jets are observed in the differentially heated rotating annulus experiment. Similar to the atmospheric spectra, the experimental kinetic energy spectra reveal the typical subdivision into two distinct regimes with slopes k-3 for the large scales and k-5/3 for smaller scales. By separating the spectra into a vortex and wave part, it emerges that at the largest scales in the mesoscale range the gravity waves observed in the experiment cause a flattening of the spectra and provide most of the energy. At smaller scales, our data analysis suggests a transition towards a turbulent regime with a forward energy cascade up to where dissipation by diffusive processes occurs.

D2404 |
EGU2020-4290
Xiaojiang Zhang, Weimin Zhang, Huizan Wang, and Ren Zhang

High temporal resolution mooring observations reported here revealed that there exist energy cascades from internal solitary wave (ISW) to turbulent mixing via smaller, high-frequency internal waves near the maximum local buoyancy frequency (near N-waves), which are transient, inhomogeneous in space. These near N-waves, riding on the parent ISW, emerged at the trough and gradually extended to the rear face of ISW with their ampltiudes becoming larger and larger. Most of the enlargement occurred in the primary stratifed layer, where the displacements between the density surfaces are largest. The near N-waves riding on a typical ISW held approximately 5 percent of the energy of ISW during its passage. Simulations of the KdV-Burgers equation confirmed the emergence of the near N-waves due to the energy cascade, similar as in the observation. The above results point out a new route of energy cascade from ISWs to turbulence in the ocean, which would be helpful on deepening the understanding of the mechanism of wave-induced mixing and energy cascade in internal waves.

D2405 |
EGU2020-8503
Yvan Dossmann and Callum Shakespeare

Internal tides / lee waves coupling : dynamics and impact on the ocean energy budget

Yvan Dossmann,

LEMTA, Université de Lorraine, CNRS, Nancy, France.

Callum Shakespeare,

Climate and Fluid Physics, The Australian National University, Canberra, Australia

 

Usual parameterizations of mixing in global models quantify independently the contribution of internal tides -generated by barotropic flows- and lee waves -generated by quasi-steady flows- relying on a linear approach based on the theory of Bell [1]. However the combined effects of the tidal and quasi-steady flows causes a linear coupling between internal tides and lee waves that has been overlooked in internal wave mixing parameterizations over the last decades [2]. This coupling induces major changes in the internal wave dynamics that has dramatic global impacts on :

  • the energy fluxes to lee waves that is cancelled by 20 % on a global scale and up to 90 % in key areas of the Meridional Overturning Circulation as the Drake passage.

  • the generation of Doppler-shifted internal tides beyond the critical latitudes.

  • the existence of a net wave stress above abyssal hills comparable to the local wind stress.

An accurate description of the cascade from generation to mixing is a necessary step to define relevant parameterizations at the ocean scale and significantly reduce the large uncertainties due to partially represented processes.

The experimental campaign LATMIX led at ANU Canberra in 2019 has confirmed the dynamical effects of this linear coupling on internal wave propagation, energy fluxes and mixing based on high resolution density measurements with the light attenuation technique (LAT). Strong nonlinear processes such as the formation of horizontal vortices have been measured in the bottom boundary layer. The generation of these vortices is only observed when the steady and tidal forcings are combined, while different strong nonlinear structures are present in the case of a pure steady flow [3]. Mixing induced by nonlinear processes overcomes internal wave induced mixing in most relevant parameter regimes for the ocean. These results provide insights to better understand and represent (non-)linear internal wave processes and their impact on mixing at regional and global scales. I will present the main results of this experimental campaign and discuss their implications for the representation of internal wave induced mixing at regional and global scales.

References

[1] Bell, T. H. : Topographically generated internal waves in open ocean », Journal of Geophysical Research, vol. 80, p. 320–327, 1975.

[2] Shakespeare, C. : Interdependence of internal tide and lee wave generation at abyssal hills: global calculations », in Press, Journal of Physical Oceanography, 2020.

[3] Dossmann, Y.; G. Rosevear, M.; Griffiths, R. W.; McC. Hogg, A.; Hughes, G. O. and Copeland, M.: Experiments with mixing in stratified flow over a topographic ridge, Journal of Geophysical Research: Oceans 121 : 6961-6977, 2016.

 

D2406 |
EGU2020-8789
Irina Strelnikova, Gerd Baumgarten, and Franz-Josef Lübken

In order to understand the generation, propagation and climatology of gravity waves (GWs)observations with high temporal and vertical resolution are required. The observation of gravity waves is important to understand the vertical coupling in the atmosphere.

Recent developments in lidar technology give us new possibilities to study GWs experimentally on a more or less regular basis and resolve spatial sales of 150
meters in the vertical and temporal scales of about 10 minutes. In particular, the capability to operate the lidar during daytime allows for long duration GW observations. The Doppler Rayleigh Iodine Spectrometer (DoRIS) in additionto the established hydrostatic temperature measurement technique yields simultaneous and common volume measurements of winds.

At the ALOMAR observatory in northern Norway (69°N, 16°E)
the gravity wave potential energy density (GWPED) in the stratosphere is shown to have a large seasonal variation with a maximum in winter and a minimum in summer.

In this work we use the phase relation between both zonal and meridional wind components and temperature. We study gravity waves sorted for up- and downward propagating waves under summer and winter conditions to investigate different wave propagation and generation scenarios. We discuss the winter/summer difference not only in terms of total GWPED, but in terms of wave characteristics obtained from our extended analysis technique. We demonstrate, for example, that amount of downward propagating waves is larger in winter than in summer. Also, other wave characteristics like phase speed and mean intrinsic period
will be discussed.

D2407 |
EGU2020-3253
Sarthak Srivastava and Amal Chandran

Ionospheric Total Electron Content (TEC) data from ground-based Global Positioning System (GPS) receiver networks have been used previously to detect Travelling Ionospheric Disturbances (TIDs). The TIDs have been shown to arise through coupling of lower atmosphere with the Ionosphere with Gravity Waves as the coupling mechanism. Gravity Waves generated by earthquakes, tsunamis, volcanoes, topography, convection and even solar eclipses have been detected using GPS TEC data. In this study, we identify Gravity Wave signatures in GPS TEC data derived from the Sumatran GPS Array (SuGAr) network. SuGAr is a network of 49 ground-based GPS stations along the convergent plate boundary between Indo-Australian and Asian tectonic plates in western Sumatra, Indonesia. Since initiation in 2002, data from SuGAr has primarily been used to study earthquakes and plate-tectonics in south-east Asia. Due to its location along the seismically-active region, SuGAr can provide valuable data for studying co-seismic Gravity Waves triggered by terrestrial-atmosphere coupling. Frequent occurrence of deep convective clouds in tropical region implies that SuGAr data also provides a unique opportunity to study atmospheric waves generated by convection. 

We have identified Gravity Waves across a wide spectrum corresponding to seismic and tropical convection events in Sumatran region. Upon identifying the wave signatures, we characterized the wave parameters and identified the wave sources through suitable ray tracing calculations. In this paper we show acoustic-gravity waves generated by the 2012 Sumatra great earthquake sequence consisting of 2 largest strike slip earthquakes ever recorded. Spectral analysis indicates the presence of fundamental resonant frequencies for solid Earth-atmosphere coupling. Using a geometric ray tracing method, we also trace the waves very close to the reported epicentres of the double earthquake sequence. We also discuss inertia-gravity waves generated due to convection in South-East Asia using SuGAr TEC data for 2018. Indication of deep convective clouds is confirmed through satellite-based cloud top brightness temperature data.  Ray tracing is performed to further trace the observed waves to the convective system location.

D2408 |
EGU2020-13569
Andreas Dörnbrack, Tyler Mixa, Bernd Kaifler, and Markus Rapp

At the end of the austral winter 2019, a sudden stratospheric warming led to an early breakdown of the polar vortex. The meteorological conditions during this event are documented and analysed by means of operational analyses of the Intgrated Forecast System (IFS) of the ECMWF and ERA5 data. Especially, we focus on the decline of stratospheric wave activity over the southern tip of South America. For this region, ground-based and airborne measurements are employed to compare selected diagnostics with fields from the ECMWF's numerical weather prediction model IFS. Furthmore, the meteorological conditions for one selected research flight during the SOUTHTRAC campaign are presented. This part serves as background information for a case study presented by Tyler Mixa.

D2409 |
EGU2020-13308
Fede Conte, Jorge Chau, Brian Harding, Ralph Latteck, and Jacobo Salvador

During September of 2019, a state-of-the-art multistatic meteor radar system called SIMONe (Spread Spectrum Interferometric Multistatic meteor radar Observing Network) was installed in southern Patagonia, Argentina. Its main goal being the study of mesospheric waves in one of the (theoretically predicted) most dynamically active regions of the world. SIMONe Patagonia consists of 5 linearly polarized Yagi antennas in a pentagon configuration on transmission, and 5 dual-polarization single Yagi antennas on reception, situated between 30 and 270 km from the transmitters, which locate at 49.6° S. Combining measurements from the 5 links allows for more accurate estimations of mean winds and horizontal momentum flux for altitudes between 75 and 105 km. Furthermore, given the significantly higher amount of meteor detections, one can determine wind fields within the limits of the illuminated volume every 1 hour and 1 km in time and height, respectively. Preliminary results indicate a dominant semidiurnal oscillation in both the zonal and meridional wind components, as well as an enhanced and sustained (in time) gravity wave activity, especially above 90 km of altitude. In addition, momentum flux analysis reveals that the gravity wave activity is stronger than in other parts of the Southern Hemisphere, confirming that Patagonia is indeed a very active region at mesospheric heights.  

D2410 |
EGU2020-13736
Lukas Krasauskas, Markus Geldenhuys, Peter Preusse, jörn Ungermann, Michael Höpfner, Felix Friedl-Vallon, Bernd Kaifler, Markus Rapp, and Riese Martin

The Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) is an aircraft-
based Fourier transform spectrometer with a 2D detector array jointly developed by Forschungszentrum
Jülich and Karlsruhe Institute of Technology. Air temperature and volume mixing ratios of various
trace gases are retrieved from the measured IR spectra. GLORIA's viewing direction can be panned between 45 and 135
w. r. t. the flight direction. Combining this capability with flight paths that encircle the observed
atmospheric region, multiple measurements of the same air mass can be performed, allowing for 3D
tomography of the atmosphere with a vertical resolution down to 250 m and horizontal resolution of
around 25 km.
GLORIA flew on the German HALO research aircraft during the SouthTRAC measurement cam-
paign held in southern Argentina in September-November 2019. One of the main goals of the campaign
was gravity wave study using GLORIA, as well as an upward looking ALIMA lidar instrument devel-
oped by Deutsches Zentrum für Luft- und Raumfahrt (DLR), and in situ instruments. During one of
the research flights, a large amplitude mountain wave was observed over the Andes. The air volume
near the mountains was encircled twice, providing a unique opportunity to study the time evolution
of an orographic gravity wave with a help of 3D time dependent temperature retrieval. We present
the initial analysis of this dataset, showing complex temperature structure with several overlapping
gravity wave families at altitudes of 9 to 14 km. GLORIA data is complemented by the ALIMA lidar
temperature retrieval at altitudes between about 20 and 60 km, providing insight into further upward
propagation and breaking of the observed mountain wave. We also compare our results with ECMWF model data.

D2411 |
EGU2020-20345
Bernd Kaifler, Andreas Dörnbrack, Tyler Mixa, Markus Rapp, Natalie Kaifler, Sonja Gisinger, and Robert Reichert

We present observations by airborne lidar which were obtained over the southern Andes during the SOUTHTRAC campaign in September 2019. Operated onboard the German research aircraft HALO, the Airborne LIdar for Middle Atmosphere research (ALIMA) acquired high resolution temperature profiles in the altitude range 20-80 km. The data show signatures of mountain waves located at the mountain ridges, but often these signatures also extend several hundred kilometer downstream. While during the first days of the campaign mountain waves were able to penetrate into the mesosphere, observations obtained in the following weeks indicate a downward shift of the breaking zone from the mesosphere to the stratosphere which is consistent with the early breakdown of the polar vortex. Our data also indicate evidence for generation of secondary gravity waves within the breaking zone of the mountain waves.

D2412 |
EGU2020-18470
Markus Geldenhuys, Isabell Krisch, Peter Preusse, Joern Ungermann, Inna Polichtchouk, Lukas Krasouskas, Felix Friedl-Vallon, and Martin Riese

Apart from orography, no specific sources of parametrized gravity waves are considered in most global circulation models. This is an inadequate generalization in the long run. In 2002, Charron and Manzini stated that no global gravity wave source climatology exist, and in the meantime, little has been done to address this observationally. A single observational case over Greenland is used to illustrate how difficult it is to disentangle the source of a gravity wave. The observation was made during the POLar STRAtosphere in a Changing Climate (POLSTRACC) campaign on 10 March 2016. The campaign was based in Kiruna, Sweden and investigated polar air during the polar vortex breakdown in spring. The gravity wave was observed between 10 and 15 km, with a horizontal wavelength of around 300 km and a vertical wavelength of around 2 km. Several plausible source mechanisms were present at the time of observation, a breaking Rossby wave, jet exit region, cold front, strong wind shear, and topography. POLSTRACC observations, ECMWF high resolution analysis and ERA 5 reanalysis were used to find the gravity wave source. Observations were obtained by the Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) instrument based on the High Altitude Long Range (HALO) German research aircraft. GLORIA is an infrared spectrometer that measures many trace gasses in the mid-infrared frequencies. Radiance is changed by inverse-modelling to temperatures. The temperature structure of the gravity wave was tomographically reconstructed. The 3D observations correlate well with the model data. ERA 5 data and the Gravity-wave Regional Or Global Ray Tracer (GROGRAT) was used to determine the exact location of where the gravity wave was emitted. Evidence exists that the gravity wave could have been released by the geostrophic imbalance in the jet. In contrast, dedicated ECMWF model runs suggest that the origin of the gravity wave is in fact orography. However, the Scorer parameter suggests no gravity wave can propagate from the surface upwards. Two ECMWF model runs were used to prove this, one normal operational model run, and the second, with a ‘flat’ orography. This work indicates that care needs to be exercised to diagnose the source of gravity waves, especially without a full informational analysis. Our study illustrates that a better parametrization scheme of gravity wave sources should be included in models for a more realistic representation.

D2413 |
EGU2020-12990
Cornelia Strube, Manfred Ern, Peter Preusse, and Martin Riese

Gravity waves are important drivers of dynamic processes in the middle atmosphere, but not the only process that could lead to small-scale perturbations. To analyse atmospheric data for gravity wave signals, gravity wave perturbations have to be separated from atmospheric variability caused by other dynamic processes. Common methods to separate small-scale gravity wave signals from a large-scale background comprise filtering methods in either the horizontal or vertical wavelength domain. Recently, studies showed that vertical wavelengths filtering can mistake other wave-like perturbations, such as inertial instability effects, for gravity wave perturbations.

We use artificial inertial instability perturbations, global model data and satellite observations to assess different spectral background removal approaches on their ability to separate gravity waves and inertial instabilities. Therefore, we investigate a horizontal background removal, applying a zonal wavenumber filter with additional smoothing of the spectral components in meridional and vertical direction, a sophisticated filter based on 2D time-longitude spectral analysis (see Ern et al., 2011) and a vertical wavelength Butterworth filter.

We analyse the results for critical thresholds of the vertical wavelength and zonal wavenumber, respectively. Vertical filtering has to remove a part of the gravity wave spectrum in order to eliminate inertial instability remnants from the perturbations. Horizontal filtering, however, separates the data at scales far beyond the expected gravity wave spectrum for the case we investigated. Furthermore, we show that it is possible to effectively separate inertial instabilities perturbations from gravity waves perturbations for infrared limb-sounding satellite profiles using a cutoff zonal wavenumber of 6.

D2414 |
EGU2020-1418
Kaoru Sato and Soichiro Hirano

The climatology of residual mean circulation – a main component of the Brewer–Dobson circulation – and the potential contribution of gravity waves (GWs) are examined for the annual mean state and each season in the whole stratosphere based on the transformed-Eulerian mean zonal momentum equation using four modern reanalysis datasets. Resolved and unresolved waves in the datasets are respectively designated as Rossby waves and GWs, although resolved waves may contain some GWs. First, the potential contribution of Rossby waves (RWs) to residual mean circulation is estimated from Eliassen–Palm flux divergence. The rest of residual mean circulation, from which the potential RW contribution and zonal mean zonal wind tendency are subtracted, is examined as the potential GW contribution, assuming that the assimilation process assures sufficient accuracy of the three components used for this estimation. The GWs contribute to drive not only the summer hemispheric part of the winter deep branch and low-latitude part of shallow branches, as indicated by previous studies, but they also cause a higher-latitude extension of the deep circulation in all seasons except for summer. This GW contribution is essential to determine the location of the turn-around latitude. The autumn circulation is stronger and wider than that of spring in the equinoctial seasons, regardless of almost symmetric RW and GW contributions around the Equator. This asymmetry is attributable to the existence of the spring-to-autumn pole circulation, corresponding to the angular momentum transport associated with seasonal variation due to the radiative process. The potential GW contribution is larger in September to November than in March-to-May in both hemispheres. The upward mass flux is maximized in the boreal winter in the lower stratosphere, while it exhibits semi-annual variation in the upper stratosphere. The boreal winter maximum in the lower stratosphere is attributable to stronger RW activity in both hemispheres than in the austral winter. Plausible deficiencies of current GW parameterizations are discussed by comparing the potential GW contribution and the parameterized GW forcing.

D2415 |
EGU2020-20423
Andrea Schneidereit, Hauke Schmidt, and Claudia Stephan

Several current general atmospheric circulation models provide sufficiently high resolutions to resolve important parts of the internal gravity wave spectrum allowing for numerical experiments without GW drag parameterizations. GWs start to be well resolved from horizontal wavelengths of about 7 times the horizontal grid spacing. How much does the resolved wave spectrum and its forcing on the mean circulation depend on the vertical resolution?

−1,The middle atmosphere summer hemisphere provides a suitable background to investigate this question. The mean stratospheric and mesospheric circulation is characterised by prevailing easterlies which prevent planetary wave propagation upwards and represents a mean state driven by IGWs. The sensitivity of the forcing by IGWs is analysed on the basis of the Eliassen-Palm (EP) flux divergence, which describes the forcing on the circulation by resolved eddies.
Model simulations are performed using the upper atmosphere version of the ICON (ICOsahedral Nonhydrostatic) general circulation model, UA-ICON (Borchert et al. 2019, GMD). The simulations start in October and run for an extended austral summer season until March with a horizontal grid spacing of roughly 20 km. The top of the model atmosphere is located at 150 km. Three different model configurations are used with 90, 180, and 360 vertical model layers. The mean vertical grid spacing ranges from roughly 1300 m (90 layers) to 320 m (360 layers) at stratospheric levels, and from roughly 2300 m to 500 m at mesospheric levels. Gravity wave drag parameterizations (orographic and non-orographic) are turned off. The resolved forcing on the mean state due to the EP flux divergence is decomposed into contributions of different scales with respect to horizontal wave numbers. For contributions of IGWs wave numbers above 20 are considered.

The stratospheric and mesospheric easterlies appear stronger in the lower resolution from October to the end of the austral summer season. Westerlies occur above the mesopause. This strong vertical gradient in the zonal mean zonal wind amplifies in the lower resolution. At the beginning of the simulation period, differences between the mean states are weak, of the order of 5 ms−1 , and strengthen during the summer season. The forcing due to internal GWs appears stronger in the lower resolution at higher altitudes and amplifies in the region of the strong vertical gradient of the zonal mean zonal wind. Furthermore, wave spectra are discussed. In accordance with previous studies, an increased vertical resolution results in a reduction of the IGW forcing close to strong zonal mean zonal wind gradients in the upper mesosphere/lower thermosphere.

D2416 |
EGU2020-7375
Pierre Léard, Daniel Lecoanet, and Michael Le Bars

In the Earth’s stratosphere, equatorial zonal winds reverse from easterlies to westerlies with a period of roughly 28 months. This phenomenon, known as Quasi-Biennial Oscillation (QBO), is driven by internal gravity waves (IGWs) propagating in the stratosphere and interacting with the ambient large-scale flow. Those waves are generated by the turbulent motions of the troposphere. In 1977, an idealised model describing the generation of a reversing large-scale flow by two counter-propagating monochromatic internal gravity waves was developed by Plumb [1]. In 1978, the famous Plumb & McEwan’s experiment [2] validated this model using oscillating membranes to force a standing wave pattern at the boundary of a linearly stratified salty-water layer in a cylindrical shell container.

Recently, the effects of the wave dissipation and wave energy were studied by Renaud et al. [3] using the Plumb model in order to explain the QBO disruption observed in 2016. It was found that as the Reynolds number increases, bifurcations from periodic to non-periodic regimes are seen for the large-scale flow oscillations.

Here, we present the results obtained from an extended version of the Plumb’s model, taking into account the stochastic generation of IGWs in Nature. Our new model includes a wide spectrum of waves as forcing for the large-scale flow. A gaussian distribution of energy is used in order to compare monochromatic forcing results (characterised by a gaussian energy spectrum with a small standard deviation) with multi-wave forcing results (large standard deviation). Unexpectedly, we find that in a large parameter domain, gathering the energy of the forcing into one frequency results in non-periodic oscillations for the QBO while spreading the same amount of energy among many frequencies results in periodic oscillations. We also investigate more realistic distribution of energy for the forcing including classical convective spectra, with or without rotation. We find that different forcings result in very similar reversals. This result is quite relevant for Global Circulation Models (GCMs) where internal gravity waves are parameterised in order to drive a realistic QBO. However, our study suggests that driving a QBO with realistic characteristics (amplitude, period) does not involve that the input forcing (i.e. the wave spectrum characteristics) is realistic as well.

References:

[1] R. A. Plumb, « The interaction of two internal waves with the mean flow: Implications for the theory of the quasi-biennial oscillation », Journal of the Atmospheric Sciences, 1977.

[2] R. A. Plumb and A. D. McEwan, « The instability of a forced standing wave in a viscous stratified fluid: a laboratory analogue of the quasi biennial oscillation », Journal of the Atmospheric Sciences, 1978.

[3] A. Renaud, L.-P. Nadeau, and A. Venaille, « Periodicity Disruption of a Model Quasibiennial Oscillation of Equatorial Winds », Phys. Rev. Lett., vol. 122, no 21, p. 214504, 2019.

D2417 |
EGU2020-4997
Zuzana Procházková, Petr Šácha, Aleš Kuchař, Petr Pišoft, and Christopher Kruse

Internal gravity waves (GWs) and their interaction with the atmospheric circulation present a complex problem for global climate models (GCMs) due to a variety of spatial and temporal scales involved. GWs and their effects in GCMs are parameterized by employing various simplifications and restrictions
(propagation, spectrum). Also, our incomplete knowledge of the GW properties in the real atmosphere complicates the situation. Global (satellite) observations of the GW activity are spatiotemporally sparse, making the quantification of the GW interaction with the circulation hardly possible. Recently, atmospheric models capable of resolving most of the GW spectrum have been emerging due to the increasing performance of computing systems. It is increasingly acknowledged that a combination of various types of observations with dedicated high-resolution, GW resolving, simulations has a potential to provide the most precise information about GWs. This combination will allow us to better understand the uncertainty of satellite observations of GW activity, which in turn will be used to develop new GW parameterizations or in development of GW resolving models.
In this study, we will analyze sensitivity of GW momentum flux and its divergence on background separation (and other GW detection) methods and approximations (Boussinesq, anelastic) used in the formulas. We analyze data from high-resolution model simulations produced for an observing system simulation experiment of the ISSI team "New Quantitative Constraints on Orographic GW Stress and Drag" (to be introduced in an invited presentation by Ch. Kruse).

D2418 |
EGU2020-20262
Matthäus Mai and Erich Becker

Gravity waves play an important role in the momentum and heat budgets of the middle atmosphere. Global circulation models used for long-term simulations need to parameterize the transport of wave momentum and energy from the lower to the middle atmosphere and the associated wave-mean flow interaction. This gravity wave-mean flow interaction is usually due to dynamical instability triggered by wave refraction and amplitude growth, giving rise to wave dissipation. In addition, gravity waves can interact with the mean flow through the passage of finite wave packets without dissipation. Conventional gravity wave parameterizations cannot describe this effect; nor can they account for wave sources being continuous in space and time, for the finite duration of vertical propagation, or wave acceleration induced by a temporally varying mean flow.
All these effects are accommodated when the wave field is described by the wave-energy density in wave number and physical space, and its evolution is computed by the radiative transfer equation for the wave field. A corresponding parameterization called IDEMIX has successfully been applied in ocean models. Here we present a corresponding parameterization for atmosphere models in single-column approximation. The new scheme is validated in off-line simulations. Results show that the evolution of wave packets forced in the troposphere and propagating upward into stratospheric and mesospheric jets is simulated consistently with theoretical expectations. This includes wave reflection and critical layers. Furthermore, an explicit diffusion scheme was added to account for wave dissipation due to dynamical instability.

D2419 |
EGU2020-3157
Gergely Bölöni, Young-Ha Kim, Sebastian Borchert, and Ulrich Achatz

The aim of the presented work is to improve the parameterization of subgrid-scale gravity wave (GW) effects on the resolved flow in atmospheric models in a large altitude range from the upper troposphere to the mesopause (~85km). State of the art GW parameterization schemes are using the steady-state approximation for the wave field and therefore assume an instantaneous GW propagation neglecting direct interactions between the GW field and the resolved flow within the whole altitude range mentioned above. As such, these schemes rely on dissipative processes - GW breaking and critical layer filtering - as the only mechanism to accelerate/decelerate the resolved flow. In contrast to this, by dropping the steady-state assumption a contribution to the mean-flow forcing emerges in the form of direct GW-mean-flow interactions. Several idealized studies show that, besides dissipative effects, direct GW-mean-flow interactions contribute to GW dynamics in an important extent (Bölöni et al., 2016, J. Atmos. Sci.}, 73, 4833-4852, Wilhelm et al., 2018, J. Atmos. Sci., 75, 2257-2280, Wei et al., 2019, J. Atmos. Sci., 76, 2715-2738). This motivates the implementation of a transient GW model (MS-GWaM: Multi Scale Gravity Wave Model) to UA-ICON, the upper atmosphere version of ICON (Borchert et al., 2019, Geosci. Model Dev., 12, 3541-3569) which does not rely on the steady-state assumption and thus includes direct GW-mean-flow interactions. As a reference and a representative of currently available GW parameterization schemes a steady-state version of MS-GWaM (ST-MS-GWaM) has been implemented to UA-ICON as well, which shares the treatment of all possible components (wave sources and wave saturation scheme) with the transient MS-GWaM scheme and differs from it "only" in the treatment of propagation, i.e. excluding direct GW-mean-flow interactions and thus transience. Both MS-GWaM and ST-MS-GWaM reproduce the observed wind and temperature climatology (e.g. URAP data: Swinbank, R. and D. A. Ortland, 2003, J. Geophys. Res., 108, D19, 4615) reasonably well, but the transient propagation makes a robust difference in the circulation in perpetual runs. The transient propagation in MS-GWaM substantially contributes to an increase of the GW intermittency in the whole altitude range, giving a better comparison with super-pressure balloon observations (e.g. Hertzog et al., 2012, J. Atmos. Sci., 69, 3433-3448), whereas the lack of transience prevents any occurrence of higher GW momentum flux values than the launch magnitude itself. This is explained by the fact that the direct GW-mean-flow interactions involve a highly transient evolution of the wave action and the vertical group velocity, which often leads to increased pseudo-momentum fluxes as compared to the launch magnitude.

D2420 |
EGU2020-10957
Fanni Dora Kelemen, Jan Weinkaemmerer, Gergely Bölöni, and Ulrich Achatz

Orography induced gravity waves are investigated in a Multi Scale Gravity Wave Model (MS-GWaM) over idealized topography. MS-GWaM is a prognostic gravity-wave model, which parametrizes both the propagation and dissipation of subgrid-scale GWs. It is a Lagrangian ray-tracer model, which applies WKB-theory and calculates the propagation of ray volumes in spectral space. Its novelty is that not only the dissipative effect, but also the non-dissipative effects due to direct wave-mean flow interaction are captured. In our conceptual studies we investigate mountain wave generation, which is induced via a time-dependent large-scale wind encountering a prescribed topography. The framework used in our experiments is the PincFloit model, which integrates the pseudo-incompressible equations. We use it both in low resolution with MS-GWaM and in high resolution LES mode as a wave resolving reference. In the reference LES simulations the idealized topography, a mountain chain, is represented with an immersed boundary method. In the MS-GWaM experiments there is no resolved topography, but its effect is modelled as a lower boundary condition. The lower boundary condition is represented by initializing ray volumes with wave number and wave action density depending on the mountain characteristics and the large scale wind speed, based on the assumption that the flow follows the terrain. In the wave resolving reference experiments the flow does not strictly follow the terrain, but other instabilities (rotor formation, boundary layer separation) arise around the mountains. These processes decrease the available momentum transported by GWs, which was initially not accounted for in MS-GWaM. Thus an overestimation of wind deceleration was found in the MS-GWaM parametrization compared to the wave resolving simulation. To correct for this overestimation, an effective mountain height is introduced into Ms-GWaM, which is calculated by a scaling function between mountain height and flow properties using the Froude number.

D2421 |
EGU2020-2903
Fabienne Schmid, Elena Gagarina, Rupert Klein, and Ulrich Achatz

Inertia–gravity waves (IGWs) emitted from jets and fronts are ubiquitous in the atmosphere and have a significant impact on atmospheric processes (Plougonven and Zhang, 2014). Since the mechanism responsible for the spontaneous emission of IGWs during the evolution of an initially balanced flow remain poorly understood, their representation in numerical weather prediction models is challenging (de la Cámara and Lott, 2015). Better understanding of this IGW source mechanism based on idealized numerical simulations is crucial to improve the accuracy of the forecasts. In this study, idealized baroclinic-wave life cycle experiments on the f-plane are performed to investigate spontaneous emission, using a finite-volume solver for the pseudo-incompressible equations (Rieper et al., 2013). In particular, the implementation of a semi-implicit time stepping scheme, along the lines of Smolarkiewicz and Margolin (1997) and Benaccio and Klein (2019), but adjusted to our staggered grid, permits longer simulation runs with much larger domains. A novelty is the implementation of a simple Newtonian heating function based on Held and Suarez (1994), which is used for forcing a baroclinically unstable temperature profile and allows the background state to vary in time (O’Neill and Klein, 2014). The results of the model with semi-implicit time stepping scheme will be documented and compared to an explicit Runge-Kutta scheme. The analysis may serve as a basis for the development and validation of a parameterization scheme for GWs emitted from jets and fronts.

References:

Benaccio, T., and R. Klein, 2019: A semi-implicit compressible model for atmospheric flows with seamless access to soundproof and hydrostatic dynamics. Mon. Wea. Rev., 147, 4221-4240.
de la Cámara, A., and F. Lott, 2015: A parameterization of gravity waves emitted by fronts and jets. Geophys. Res. Lett., 42, 2071-2078.
Held, I.M., and M.J. Suarez, 1994: A Proposal for the Intercomparison of the Dynamical Cores of Atmospheric General Circulation Models. Bull. Amer. Meteor. Soc., 75, 1825-1830.
O’Neill, W.P., and R. Klein, 2014: A moist pseudo-incompressible model. Atmos. Res., 142, 133-141. Plougonven R., and F. Zhang, 2014: Internal gravity waves from atmospheric jets and fronts. Rev. Geophys., 52, 33-76.
Rieper, F., Hickel, S., and U. Achatz, 2013: A conservative integration of the pseudo-incompressible equations with implicit turbulence parameterization. Mon. Wea. Rev., 141, 861-886. Smolarkiewicz, P.K., and L.G. Margolin, 1997: On forward-in-time differencing for fluids: an Eulerian/semi-Langrangian nonhydrostatic model for stratified flows. Atmosphere-Ocean, 35, 127- 152.

D2422 |
EGU2020-1584
Katherine Grayson, Stuart Dalziel, and Andrew Lawrie
D2423 |
EGU2020-10630
Thomas Eriksen, Carsten Eden, and Dirk Olbers

A key component in setting the large scale ocean circulation is the process of diapycnal mixing, since this can drive the meridional overturning circulation. Diapycnal mixing in the interior ocean is predominantly associated with the breaking of internal waves. Traditionally, diapycnal mixing has been represented in ocean models by a diapycnal diffusivity either constant or exponentially decreasing with depth. This approach, however, does not take into account the actual physics behind the breaking of internal waves. The energetically consistent internal wave model IDEMIX (Internal wave Dissipation, Energetics and MIXing), on the other hand, computes diffusivities directly on the basis of internal wave energetics. One such type of internal waves are lee waves. These are generated and subsequently dissipated when geostrophic currents interact with bottom topography and are therefore believed to be a source of energy for deep ocean mixing. In this study IDEMIX is coupled to a 1/12th degree regional model of the Atlantic. The lee wave energy flux is calculated and used as a bottom flux at each time step effectively allowing lee waves to propagate, interact with mean flow and waves, and subsequently dissipate. This setup enables not only an estimate of the lee wave energy flux but also a direct investigation of the influence of lee waves on dissipation, stratification and horizontal and overturning circulation.

D2424 |
EGU2020-13808
Mozhgan AmirAmjadi, Ali R. Mohebalhojeh, and Mohammad Mirzaei

One of the remaining issues in the parameterization of inertia-gravity waves is the estimation of wave characteristics such as wavenumber and intrinsic frequency. In this survey, we explore a new way to estimate the wave characteristics at the launch level. To this end, we retrieve the wavenumber using the Riesz Transform which is the generalized form of the Hilbert Transform applicable in the multi-dimensional analysis. For this purpose, the high-resolution horizontal divergence field has been employed since it filters the background flow and thus provides a reasonable representation of the inertia-gravity wave signal. This is followed by the application of machine learning to reconstruct the retrieved wavenumber using the coarse-grained resolvable variables including the horizontal wind speed, the large scale vertical velocity, the cross-stream ageostrophic wind speed, the frontogenesis function and the latent heat released during condensation as explanatory variables at the launch level.
We have employed the ERA5 dataset in this survey, having observed that the dataset can directly resolve the inertia-gravity waves at its full resolution. In order to avoid mountain waves and focus on non-orographic inertia-gravity waves, two areas far from the significant obstacles over the midlatitude of the Atlantic Ocean and Northern Pacific Ocean are considered from December 2018 to February 2019. The results show a reasonable correlation between the reconstructed wavenumber using low-resolution explanatory variables and the retrieved one using the Riesz Transform so that this method can be utilized to estimate the inertia-gravity wave number at the launch level.

D2425 |
EGU2020-12268
Kurt Polzin, Zhankun Wang, Binbin Wang, and Angel Ruiz Angulo

Moored data from the northern Deepwater Gulf of Mexico in the vicinity of DeepWater Horizon are presented.  Subinertial flows of O(0.1-0.2 m/s) are in the sense of Kelvin wave propagation and support a downwelling Ekman layer with reduced near boundary stratification.  The moored data document cross-slope and vertical buoyancy fluxes dominated by a frequency band that includes diurnal and inertial frequencies and extend to about an order of magnitude larger than inertial.  We refer to this frequency band as internal swash and the region of reduced stratification at the bottom boundary exhibiting these fluxes as the internal swash zone.  Vertical fluxes of cross-slope momentum associated with internal swash band frequencies are large, of similar order of magnitude as the drag associated with the viscous no-flow bottom boundary condition on the cross-slope subcentral current.  Typical mixing efficiencies of (Γ ~ 0.2) are found in association with elevated mixing O(100 times background) one-to-two hundred meters above the bottom. This enhanced turbulence appears in conjunction with near-inertial frequency motions that may be dynamically coupled to the mean flow.