Reports
Research Stay in Lyon by Mohamed Mossad (Oct 23)
Exploring Atmospheric Dynamics
From the 1st of October and until the 14th, I had the opportunity to embark on a short research stay in Lyon, France, funded by the project TRR181 at École Centrale de Lyon. This period was not just a chance to collaborate and learn but also a stepping stone in my understanding of atmospheric dynamics, particularly regarding gravity wave (GW) spectra.
My time in Lyon was spent working alongside Raffaele Marino (scientist at CNRS, France) and the team at the Laboratory Mechanical Des Fluides Et D'acoustique (LMFA). The environment at LMFA was not only academically stimulating but also warmly welcoming, fostering both professional growth and personal connections.
One of the most enlightening aspects of this visit was the shift in my perspective on the processes which contribute to the canonical GW spectra. Discussions about turbulence and the scaling of gravity wave spectra opened my eyes to the broader physics underlying these phenomena. It was a transition from focusing merely on the slope of gravity wave spectra to understanding the vast, open-ended field of their scaling.
A highlight of my stay was exploring the relationship between the Froude number and statistics (kurtosis) of velocity and temperature fields. Although time constraints didn’t allow for its application on lidar data, the concepts presented were inspiring and thought-provoking.
Our work concentrated on validation of spectra from direct numerical simulations (DNS) against lidar data, scrutinizing how different wind regimes affect GW spectra. This involved a detailed comparison of integrated kinetic energy/scalar spectra with lidar data.
Leaving Lyon, I am armed with an array of studies and topics to delve into, especially regarding the comparison with DNS regimes. These studies are pivotal in enhancing our interpretation of GW data from lidar measurements. I am optimistic about the continuation of this collaboration in the future and the potential for significant findings.
Beyond the academic realm, Lyon itself proved to be a delightful experience. The city's transportation system was notably efficient which made commuting a breeze. The streets of Lyon are filled with friendly faces, many of them young students, also the 2023 Rugby World Cup was taking place there which added a lively and diverse vibe to the city. École Centrale de Lyon, nestled in this vibrant environment, struck me as an exceptional place for study and research, providing many chances to do sports as well.
I extend my deepest gratitude to the entire team at LMFA for their hospitality and support. Special thanks go to Rafaello Foldes and Fabio Feraco (IAP) for their invaluable help in answering my questions and assistance with data provision.
My research stay in Lyon was not only productive but also immensely rewarding. It has broadened my understanding and has surely impacted my approach to atmospheric science. I am grateful for this experience and hopeful that my contributions, though a fraction, have added value to our collective research endeavors.
Research Stay in San Diego by Pablo Sebastia Saez (May 23)
California Dreamin’
Nods, smiles… How’re the waves? This is the perfect conversation starter for an oceanographer spending the summer in California. Hella chillin, bro!
But let’s get to business… Just a little, after I’ll tell you about the fun part. During the summer of 2023 I spent almost two months at Scripps Institution of Oceanography in San Diego, California. The decision came to life after a brainstorming session with my supervisor on who (a topic-wise related and experienced researcher) and where (an oceanographic institute that could open new doors) could I visit, that could provide further, new, groundbreaking, specialized, state-of-the-art insights about internal gravity waves and their interaction with the background environment. And the winner was … (drum roll, please) Prof. Dr. William R. Young! After developing and proposing a feasable working structure for the research stay and several meetings…, after applying for the visa, finding accomodation, working myself through a whole load of paper work and packing my bags…, I was finally ready to catch this wave, ride it out, and let the californian vibe wash over me.
I went to the US with a clear and clean research topic, but as in surfing, sometimes it’s better to let the wave tune you into the right direction. During long eye-opening discussions with Prof. Young we stumble upon an unresolved problem within the direction of the scope of my dissertation, so we decided to reformulate the research question of my upcoming work. Within the two months at Scripps I had the chance to get to know other PhDs, PostDocs and professors from various research disciplines. I took advantage and attended seminars offered by both visiting as well as in-house researchers. I joined social events organized by the institute. And I got the great opportunity to present my work, discuss my interests and raise awareness of the research done within TRR181. But most importantly, I came back to Hamburg with far more new and exciting research questions to be answered!
What I liked best?
I arrived in San Diego just on time for ‘May Gray and June Gloom’ with cloudy and cool weather, but we slowly drifted into the summer season with plenty Sunkissed mornings and sunset wave rides. These together with endless nature getaways, road trips around Southern California and the legendary TGF were the highlights of this amazing journey! There remains naught else for me to express but my gratitude to the TRR181.
Upper-ocean energy spectrum, flux & dissipation
The use of a new technology combined with new sampling algorithms potentially offers unprecedented insights into deep ocean mixing and internal wave climate.
Hi! My name is Ilmar and I work as a PhD student at the MARUM/University of Bremen. I am supervised by Dr. Maren Walter (MARUM/University of Bremen) and Dr. Alexa Griesel (Universität Hamburg) and am part of the TRR subprojects L3
entitled “Meso- to Submesoscale Turbulence in the Ocean” and W5 „Internal Wave Energy Dissipation and Wavenumber Spectra: Adaptive Sampling in the Ocean Interior “.
Before I joined TRR, I lived in Kiel, where I got a bachelor degree in Physics of Earth and master degree in Climate Physics: Meteorology and Physical Oceanography at Christian Albrechts University Kiel & GEOMAR. I started my work as a part of TRR in September 2022.
In the first phase of L3, we assessed turbulence regimes with a focus on the Benguela upwelling region. Using a new scaling method and with adequate subsampling of the deployed surface drifters, we estimated a consistent energy transfer rate and identified an inverse cascade for scales 30-500 km close to the upwelling current. Now Our aim in the second phase is to extend the Lagrangian analyses and apply the structure-function diagnostic (in addition to the classical Lagrangian dispersion estimates) in an area offshore from the Benguela region that is characterized by high internal tide and eddy activity but without a deep baroclinic current. In a concerted effort (targeted measurements with gliders, ship ADCP, drifters) we will quantify horizontal wavenumber spectra for the upper ocean in the Walvis Ridge Region in close collaboration with W5 and W2. The subsampling methods developed from the analyses in the Benguela and Walvis Ridge regions, together with high-resolution modelling, will be used to extrapolate to the global ocean using the global drifter program.
The W5 Project is concerned with the shape of the internal wave energy spectrum, where our aim is to simultaneously observe the oceanic energy spectrum below the submesoscale range and the spatial distribution of energy dissipation, using adaptive/reactive sampling to guide the observations. For this purpose, we will deploy a new hybrid pelagic glider (developed by Prof. Ralf Bachmayer) using an innovative approach of combining advanced numerical model informed sampling techniques in real-time to observe internal wave spectra and turbulence in the ocean interior. As a key sensor, a pressure rated microstructure probe will be integrated into the pelagic glider system; this use of a new technology combined with new sampling algorithms potentially offers unprecedented insights into deep ocean mixing and internal wave climate. Obtained observational data will be contextualized by idealized and regional numerical modelling studies carried out in L3 and L2 and the results of this project will complement the observations, that will be jointly used to construct the upper- and pelagic oceanic energy spectrum within L3, and the observations towards obtaining a local energy budget.
A Memory of Pre-Pandemic Times and a Glimpse at the hopefully soon-to-be Future: My Visit at MIT and the AGU Fall Meeting 2021
With two successful talks, one on my research at the CRC181 and one on my science policy activities, I am more than happy with the received exposure and appreciation of our work.
Having been in the home office for a long time during the last two years I am sure everyone wonders: Remember how things were before the virus hit? And how things will be afterwards? I was asking myself the very same questions while having a travel grant available I had won mid 2020 from the DFG research unit MS-GWaves which was still sitting in the accounts waiting to be used. My visit had been planned for a long time but had also been delayed by the pandemic. So when the US started opening up to foreign visitors in late summer 2021 I decided to try to move forward with the plan we had been setting aside for so long. And despite the restrictions and insecurities linked to long distance travel I should very soon be rewarded. On November 8 I boarded an airplane to Cambridge, Massachusetts to visit the long research partner of out group, Prof. Triantaphyllos Akylas at the Massachusetts Institute of Technology.
Our former and ongoing research project with T. R. Akylas is concerned with the background-modulated wave-wave interaction of internal gravity waves. In a previous manuscript we had been able to show that wave modulation by a sheared mean flow can significantly inhibit the energy exchange through a near-resonant triadic interaction. However, the assumptions of Boussinesq dynamics and a constant stratification limited the applicability of the findings to the atmospheric context. We thus took on the task to extend the theory to semi-incompressible dynamics with both a variable stratification and sheared mean winds. Having derived the theory beforehand we used the 5 weeks together at MIT to explore the combined effects of the modulation by the wind and the stratification on the wave interaction. Interestingly the two modulation mechanisms can counteract each other opening up the possibility of strong interactions in regions with both changing stratification and strong shear. As the tropopause region typically exhibits these features it is of particular interest to be studied. A manuscript is now in preparation and planned to be submitted later this year.
Having already traveled to the US another possibility opened for me: the in-person attendance of the fall meeting of the American Geophysical Union in New Orleans. Traveling to conferences has always been one of my favorite parts of being a scientist. I am particular fond of getting to know places and people, exchanging ideas about our research, networking among peers and like-minded people and making friends throughout the world. The idea of attending a conference on site for the first time in two years was therefore especially tempting for me. Even though it came with the huge insecurity of sharing the venue with another 10,000 people during a pandemic the stringent health policies helped keeping the participants safe and the number of infections low.
With two successful talks, one on my research at the CRC181 and one on my science policy activities, I am more than happy with the received exposure and appreciation of our work. Fostering existing connections and forging new ones additionally rendered the conference experience as a very positive one. But maybe most importantly, I also realized what I had been missing out in the past months. Even though video conferences can account for the majority of the scientific collaboration it will not be able to replace the experience of and the human relationships associated to a person to person contact. Partnerships are build on these relationships and I am hoping that there will be a time soon where we can find a way to get back together. Personally I feel motivated to move forward and make progress in ways that I had not expected when I boarded that airplane on November 8. I would therefore like to particularly thank the CRC181, the research group MS-GWaves, the WilhelmHeraeus Visiting Professorship program and not at last Prof. Ulrich Achatz and Prof. Triantaphyllos Akylas for enabling this collaboration and the conference participation for me.
Internal Gravity Wave Interaction, Propagation, and Breaking in the Atmosphere
It has becomes clear that, even though the vertical propagation of gravity waves may be dominant for many atmospheric phenomena, the horizontal propagation cannot be neglected.
My name is Georg Sebastian and I am currently a postdoc in the TRRENERGYTRANSFERS, working in the project W1: “Gravity Wave Parameterization for the Atmosphere” under the supervision of Prof. Ulrich Achatz at the Goethe University Frankfurt am Main.
I had started out by studying the generation of internal gravity waves below the ocean surface by wind during my PhD under the supervision of Maren Walter. Recently, I moved to working on internal gravity waves in the atmosphere. Thus having studied both physical oceanography and atmospheric dynamics I am keen to investigate internal gravity waves in both environments considering various aspects of their dynamics.
The W1 Project is concerned with the lateral propagation of internal gravity waves in the atmosphere. In particular we employ the ray-tracing algorithm MS-GWaM to model sub-grid scale gravity waves including their transient propagation characteristics. That is, we model the lifetime of gravity waves including their interaction with the background – such as Doppler shifting or wave modulation by the mean flow.
In contrast, current state of the art parameterizations of gravity waves in atmospheric models assume that gravity waves propagate in the vertical instantaneously neglecting their finite vertical group velocity. Moreover these parameterizations do not allow for a horizontal propagation at all. However, it has becomes clear that, even though the vertical propagation of gravity waves may be dominant for many atmospheric phenomena, the horizontal propagation cannot be neglected. With the implementation of these effects in the numerical weather forecasting code ICON-NWP we hope to support that finding and gain new insight into the net effects and importance of the horizontal propagation.
Propagation is, however, only a part of a gravity wave’s life story. On its journey it can undergo a myriad of processes. Of special interest in various contexts are for instance the triad resonant interaction (TRI) and finally its breaking mechanisms.
Recently, we successfully described resonantly interacting gravity wave packets using a ray tracing algorithm utilizing weakly non-linear WBKJ theory in a Boussinesq environment including a slowly varying background flow. While interacting, the triad members are simultaneously modulated by a horizontal jet, leading to a reduction in energy exchange as the waves spectrally pass through the exact resonance conditions.
Moreover we are working on identifying instability mechanisms for strongly non-linear gravity waves in the vicinity of a mean-flow jet. Our theoretical study shows that in contrast to the upper jet edge the lower jet edge can sustain a novel type of modulational instability. New numerical evidence also shows that breaking mechanisms at the jet edges have distinct structures and might be associated to modulational instabilities or TRI (see Fig. 1 a and b).
This ongoing work is-among others–conducted in collaboration with Gergely Bölöni (DWD), Triantaphyllos Akylas (MIT, MA, USA), Mark Schlutow (FU Berlin, GER), and Ulrich Achatz (Goethe Universität Frankfurt).
Implementation of Lee Waves in IDEMIX
I’m investigating what and how big of a role lee waves play in transferring energy between large scale geostrophic motions and scmall scale turbulent mixing.
The purpose of my project is to investigate what and how big of a role lee waves play in transferring energy between large scale geostrophic motions and small scale turbulent mixing. Lee waves are formed when geostrophic motions interact with bottom topography. They radiate away from the topography and eventually break. When they break, the kinetic energy that they contain is used for dissipation, which, ultimately, raises potential energy. The issue of their role in the general circulation has been raised due to observed increased mixing rates near the ocean bottom in the Drake Passage and the Scotia Sea.
Previous estimates of the energy transfer from geostrophic motions into lee waves are around 1/3 of the energy input into gravity waves from winds. However, they are very few and differ roughly by a factor of 4. Furthermore, this energy transfer estimate has so far only been diagnosed and not used as in integral part of an ocean model. The contribution of lee waves in driving the large scale motions themselves – the overturning circulation, for example – are therefore largely unknown. The proper way of including lee waves in an energetically consistent ocean model would thus be to diagnose the energy c o n t a i n e d in lee waves e v e r y w h e r e in the ocean, let this energy travel and eventually be used for dissipation – in my case using an internal wave model – and then subtract it from its source.
This is exactly what is done in my model. The objective of my study is therefore to extend the IDEMIX model with an inclusion of lee wave energetics. This means that the energy being transferred into lee waves will be able to affect the rest of the ocean through diapycnal diffusivity – similarly to other types of gravity waves.
So far in my study, I have diagnosed the global energy transfer into lee waves to around 0.3TW. This is in accordance with previous estimates. The implementation of lee wave energetics into IDEMIX is underway. The lee wave energy flux is split into four directional compartments (N, S, E, W) will enter the gravity wave field as a bottom boundary flux, and the wave energy will thus be able to travel in the same manner as energy from other gravity waves. This is a fundamentally different way of treating lee waves compared to previous studies.
The next step is to study the differences in diapycnal diffusivity in model runs with and without lee waves. To what degree lee waves are able to account for the observed increased mixing rates in the deep Southern Ocean is till an open question, which I would like to answer. After this, I would like to address the question of what role lee waves play in setting the overturning circulation.
Investigating internal wave energy fluxes
In my current work, I also look into the impact of mesoscale motion on the energy flux in this dataset.
My name is Jonas and I am a PhD Student in the subproject W2 “Low mode waves” in the working group Oceanography at the University Bremen. In this project I calculate low mode internal wave energy fluxes from mooring measurements and compare the results with measurements from satellite altimetry and a 1/10° ocean model (STORMTIDE2). Energy flux is an important quantity for these models because its divergence identifies sources and sinks.
Internal gravity waves occur all over the stratified ocean and can be grouped in different categories varying on their generation mechanism. I focus mainly on internal tides in the semidiurnal frequency M2 generated by the barotropic tides over rough topography. Internal tides are a response of the astronomical gravitational forces of the ocean via oscillations in the sea surface elevation with horizontal tidal currents through the entire water column. These waves in the stratified ocean take the form of standing vertical oscillations of horizontal currents, called modes. The “zeroth” (barotropic) mode of horizontal velocity corresponds to horizontal ocean currents that are uniform from top to bottom. The first depth dependent (baroclinic) mode is characterized by flow in one direction at the top and in the opposite direct at the bottom. Higher modes have a more complicated vertical structure and their phase speed decreases with increasing mode number. The vertical structure of a mode can be calculated by the stratification, and velocity profiles can be fitted onto a linear combination of these modes. Low mode motions contain appreciable energy but quickly propagate away laterally. To study these low mode internal waves, we deployed a mooring inside a tidal beam in the eastern North Atlantic, south of the Azores, where a seamount chain stands out as a generation site for internal tides. In our study region the energy flux correlates reasonably well in direction, coherent – uncoherent portioning and mode ratio between mooring and model time series and satellite data. With regard to the total energy flux, the model and satellite observations underestimate the flux compared to the in situ data.
In my current work, I also look into the impact of mesoscale motion on the energy flux in this dataset. A surface eddy was crossing the mooring, and in the process dampening the energy flux in the first two modes by about one third, while a passing subsurface eddy dampened the energy mainly in the second mode. These observations support the idea that eddy interactions transfer energy from low modes into higher modes that can lead to increased dissipation. An open question is how much of the energy converted from lower to higher modes result in local dissipation, which is a crucial information in creating energy consistent ocean-climate models.
Parameterising Gravity Wave Effects in the Atmosphere
Without the gravity wave parameterisations the models do not simulate these large scale reversals so the inclusion of gravity wave parameterisations are essential to Earth system models for future climate change predictions.
I am a postdoctoral researcher working on adapting the IDEMIX internal gravity wave model for the atmosphere in subproject W1 together with Carsten Eden, Dirk Olbers, Matthäus Mai and Erich Becker.
Internal gravity waves in the atmosphere have a profound effect on the large-scale circulation in the atmosphere and contribute significantly to the mesoscale wind and temperature variance. Since their scales are too small to be represented in general circulation models, their effects must be included via sub-grid scale parameterisations. Most climate and numerical weather prediction models now include some form of gravity wave parameterisation in order to accurately simulate large-scale middle-atmospheric phenomena such as the Brewer-Dobson circulation, the seasonal zonal-mean wind reversal and the quasi-biennial oscillation. Without the gravity wave parameterisations the models do not simulate these large scale reversals so the inclusion of gravity wave parameterisations are essential to Earth system models for future climate change predictions.
However current gravity wave parameterisations have a major drawback in that they use much too simplified physics to determine the feedback of the waves on the large scale flow. They are steady state and use separate parameterisations for terrain-generated and convectively-generated gravity waves. The main goal of this project is to include more sophisticated physics in atmospheric gravity wave parameterisations by use of IDEMIX which simulates the evolution in time of the wave field, its energy exchange interaction with the background flow (shown in Figure 1) and can accommodate multiple wave sources at once, allowing for wave-wave interactions.
IDEMIX can easily incorporate the effect of critical layers which occur frequently in the atmosphere since the gravity waves have similar phase speeds to the background flow, typically tens of metres per second. A dynamically-determined dissipation is calculated when the waves break, to keep the wave field within convective stability limits. The wave energetics from IDEMIX are used to calculate the gravity wave drag which feeds back onto the largescale circulation to either accelerate or decelerate it. Compared to traditional parameterisations which tend to provide the drag around the mesopause, IDEMIX exhibits a better vertical distribution of gravity wave drag throughout the middle atmosphere which is more fitting to observations. The basic IDEMIX for the atmosphere is now being implemented into the ICON-A component of the ICON Earth System Model.
How the background mean flow effects internal gravity waves
From my work, hopefully general rules may be seen that can be included in parameterisations for internal gravity waves.
I am investigating the effect background mean flow has on the propagation of internal gravity waves. From this hopefully general rules may be seen that can be included in parameterisations for internal gravity waves. For this ray tracing is used to follow the positions and properties of wave packets that interact with an idealised current.
The test wave packets are populated randomly over a range of physical positions and also phase space, which allows exploration of the importance to various properties to how the test wave packets interact with the background current. The key property that is being tracked is the energy of the packets and from this the transfer of energy to and from the current can be seen.
Ray tracing simply propagates the position and wave numbers of the wave packets over a series of time steps given that background properties of background flow velocity, the local buoyancy frequency. The energy of the wave packets can be followed due to the conservation of Action. The results means that individual wave packets can be followed to different end conditions namely critical layer absorption, wave capture or refraction away from the current flow. The net energy transfer from the waves to the background flow (or from) can be seen by the end energy of the waves that enter critical layers or are captured by the current.
By varying the properties of the background current the effects of various shears in the current can be seen which will lead to more information about the key properties of both internal wave and background flow that lead to wave captures and critical layer absorption. In addition the background flow can be changed into configuration to simulate eddies, using the same processes.
Gravity Wave Parameterisation for the Atmosphere
The focus of W1 will be on the wave-mean flow interactions, i.e. the energy deposition and the gravity wave drag.
Gravity waves are an important part of the energy cycle of the atmosphere. They exchange momentum and energy with the mean flow due to wave breaking and wave refraction. In the subproject W1, we propose a new, energetically consistent gravity wave parameterisation, based on the radiative transfer equation for a field of waves.
Conventional parameterisations for gravity waves base on the assumption of stationary mean flow and a superposition of monochromatic waves launched at a certain level. The new scheme describes the wave field continuously in physical and wave number space. The IDEMIX concept (previously used for mixing by internal waves in the ocean) enables to simplify the radiative transfer equation with a few assumptions (propagation into a particular azimuth direction, single column approximation). The focus of W1 will be on the wave-mean flow interactions, i.e. the energy deposition and the gravity wave drag. This drag contributes strongly to drive the residual circulation in the middle atmosphere and to induce the quasi-biennial oscillation in the tropical stratosphere.
My part of the project is the derivation of realistic force functions of gravity waves (spatially and temporally continuous). These functions will contain gravity waves caused by flow over rough terrain, and convection and frontal activity. Another task will be the derivation of a vertical diffusion coefficient, which considers dynamical instability criterion. At last, this parameterisation has to be implemented in KMCM (Kühlungsborn Mechanistic general Circulation Model) and compared with the conventional schemes routinely used in KMCM and ICON.