W1: Gravity Wave Parameterization for the Atmosphere

Principal investigators: Prof. Ulrich Achatz (Goethe University Frankfurt), Prof. Carsten Eden (Universität Hamburg), Prof. Franz-Josef Lübken (Leibniz Institute of Atmospheric Physics)

Present-day approaches to parameterize mesoscale motions in atmospheric models are oversimplified and over-tuned and hence not sufficiently reliable. Neither gravity-wave (GW) transience nor GW horizontal propagation is taken into account, although there is mounting evidence that they play a significant role. We will resolve these issues by the extension of the Lagrangian spectral GW model (MS-GWaM) in the atmosphere model ICON-a by lateral propagation and by the implementation of the related IDEMIX-a closure into the same model. 
While MS-GWaM is based on the fully resolved spectral GW energy equation, the related IDEMIX approach is simpler, but for the same reason also more efficient. Moreover, progress in GW-resolving modelling, using a novel dynamic macro-turbulence parameterization based on the Smagorinsky model (DSM), and lidar measurements in subproject T1 suggested merging these activities into W1. Highly resolved model and lidar data will be used for the validation of the parameterization approaches.

The following core themes will be in focus:

  • Implementation of lateral propagation into MS-GWaM in ICON-a. Validation against lidar measurements and GW resolving simulations.
  • Implementation of IDEMIX-a into ICON-a, including also lateral propagation. Validation against MS-GWaM, lidar data, and data from GW-resolving simulations.
  • Extension of the DSM parameterization as a unified anisotropic closure scheme for atmospheric flows from the troposphere above the boundary layer to the upper mesosphere.
  • Lidar observations covering the complete range of temporal and spatial scales relevant for GWs and the transition to turbulence, for the validation of the parameterizations.

The recently proposed parameterization module "Internal wave Dissipation Energy and MIXing" (IDEMIX) describes the generation, propagation, interaction, and dissipation of the internal gravity wave field and can be used in ocean general circulation models to account for vertical mixing (and friction) in the interior of the ocean. It is based on the radiative transfer equation of a weakly interacting internal wave field, for which spectrally integrated energy compartments are used as prognostic model variables. IDEMIX is central to the concept of an energetically consistent ocean model, since it enables to link all sources and sinks of internal wave energy and furthermore all parameterized forms of energy in an ocean model without spurious sources and sinks of energy.

Gravity waves are an important part of the energy cycle of the atmosphere and exchange momentum and energy with the mean flow due to wave breaking and wave refraction. Wave breaking and the resulting mean-flow effects need special parameterization in global climate models as they usually resolve at most a small part of the full spectrum of gravity waves. In W1 we apply the IDEMIX concept to develop corresponding gravity wave schemes for atmospheric circulation models. We propose to base a new, energetically consistent gravity wave parameterization on the radiative transfer equation for a field of waves. This method is fundamentally different from conventional schemes which describe the superposition of monochromatic waves launched at a particular level and which make the strong assumption of a stationary mean flow. As for the ocean, the wave field is represented by the wave-energy density in physical and wavenumber space. This new concept goes far beyond conventional gravity wave schemes which are based on the single column approximation. The radiative transfer equation has – to our knowledge – never been considered in the atmospheric community as a framework for sub-grid-scale parameterization. The proposed parameterization will, for the first time, 1) include all relevant sources continuously in space and time and 2) accommodate all gravity wave sources (orography, fronts, and convection) in a single parameterization framework. Moreover, the new scheme is formulated in a precisely energy preserving fashion.

The IDEMIX concept was shown to be successful for ocean applications but instead of focussing on the mixing effect by breaking waves as for the oceanic case, the focus in the atmospheric application is on the wave-mean flow interaction, i.e. the gravity wave drag and the energy deposition. We will extend the concept of energetically consistent closures to atmospheric gravity wave closures. The project will contribute to a transfer of knowledge from the oceanic community to the atmospheric community and vice versa.

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.

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.

Georg Sebastian Voelker, Postdoc in W1, S2

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.