M3: Towards Consistent Subgrid Momentum Closures

Principal investigators: Dr. Almut Gassmann (Leibnitz Institute of Atmospheric Sciences), Dr. Stephan Juricke (Constructor University), Prof. Marcel Oliver (Constructor University/Catholic University of Eichstätt-Ingolstadt), Dr. Peter Korn (Max Planck Institute for Meteorology)

Computational models of atmosphere and ocean can only resolve a limited range of scales. Dynamical processes on scales smaller than the grid scale are parameterized or simply truncated.

When the grid scale lies within one of the classical turbulent inertial ranges, the use of viscous closures is well-understood, but in practice, simulations tend to be over-dissipative because of insufficient scale separation between numerical dissipation and the forcing scale. This problem is particularly severe when forcing occurs near the grid scale, for example through the process of baroclinic instability which converts available potential energy into kinetic energy in eddy-permitting or barely eddy-resolving simulations of the ocean. In general, momentum closures, as well as momentum advection, must aim at being minimally dissipative, which may require active reinjection of energy into the resolved kinetic energy range, a process termed in the context of oceanic momentum closures as kinetic energy backscatter.

This project will evaluate existing closure and advection schemes and develop new ones with a particular focus on (i) properly analysing their precise discrete behaviour, especially the energy budget near the resolution scale and the propagation of linear waves on unstructured meshes, (ii) developing closures that work on unstructured and variable grids (the B-grid in FESOM, the hexagonal and triangular C-grids in ICON-IAP and ICON-o), (iii) gaining physical understanding of numerically induced processes, and (iv) developing and assessing stochastic closures.

We will, first, continue the development of ocean kinetic energy backscatter which, as we demonstrated in the first project phase, is an effective means of improving energy consistency of eddy-permitting or barely eddy-resolving ocean simulations. Second, we will investigate the behaviour of models near the grid scale (i) top-down, by analysing the effective resolution of different choices of grids, discrete operators, and momentum closures empirically in the context of realistic model runs, and (ii) bottom-up, by building physical and mathematical understanding of the explicit as well as implicit choices of momentum closures. The combination of these methods and the application to the two different main models of the TRR181 – i.e., FESOM and ICON – will ensure a holistic approach to the development of efficient, energetically consistent and optimal discretizations of the momentum equations.

Next phase outlook:

➢ Spurious interfacial waves on unstructured meshes

➢ Further development and adjustment of kinetic energy backscatter schemes in global ocean models

➢ Investigation of effective resolution of general circulation models on various grids

The union of different grids (B) and the local inclusions of refined mesh areas (C) imply the appearance of a new type of waves, namely guided waves propagating at the interface, and local waves. These types of waves, sometimes called Rayleigh-Stoneley waves, considered to be "spurious" in the current context and should be muffled, since there are no such waves in the original uniform grid (A) reflecting the homogeneity of real models.

Research Stay in Pusan by Ekaterina Bagaeva (Sep 23)

Since the end of 2022, the topic of research stay slowly began to appear in our discussions with my supervisor.  We were considering various research groups studying eddies' modeling but overall decided to contact Prof. Dr. Christian Franzke, an expert in atmospheric stochastic modeling. As I found out later during the research stay he has a broad range of scientific interests, but first things first. We reached out to Prof. Franzke, who confirmed his interest in hosting a guest PhD student. And I haven’t told you yet about the destination. South Korea, Pusan National University, what an exotic place for the research stay! 

I began preparations. Timing was settled (right after the summer break), duration was defined (3 weeks) and the tickets to cross the entire Eurasia  were bought. Pleasant little things have remained: to have online meetings to discuss the working plan for these 3 weeks, to find the perfect Airbnb near the university, to plan the weekend trips…  

On September 1st, when all the students in my home country started the new academic year, I  moved to the East. Everything went very smoothly and on Monday we were already having the meeting with Prof. Franzke. On the same day, I was introduced to the working group of young scientists, and we had lunch together in the Korean mensa. I think after these three weeks, the concentration of kimchi in my stomach reached a critical amount.

I am very grateful to Prof. Franzke, who found time to meet and discuss intermediate results every second day and also was always available by phone, despite the high workload. We ended up with the idea that I’m currently implementing in FESOM2. And what is also great is that we are in contact after the research stay and keep meeting biweekly. During the last week, I had the opportunity to give a talk about my PhD work in front of an ocean modelling group. I believe it went well, because there were quite some questions.  

It's time to come up with the summary. During this short time South Korea gave me a chance to get to know it. The working environment, the urban design, the Korean weather with sunny and rainy days, delicious food and the most important for me - people there. As a nice bonus I did weekend trips to Seoul, to the DMZ between two Koreas, did several hikes to the temples and around Pusan.

First Steps in Stochastic Ocean Modelling

To find a good stochastic kinetic energy backscatter parameterization is my local aim so far.

Ekaterina Bagaeva, PhD M3

My name is Ekaterina and I’m a PhD Student of project M3 “Towards Consistent Subgrid Momentum Closures”. I work at Jacobs University in Bremen and at AWI in Bremerhaven as well. Due to Corona pandemic the personal contacts are quite limited, but I was lucky enough to meet already my colleagues in AWI and also regularly meet the colleagues in Jacobs University.

Before joining to TRR I’ve got a master degree in Mathematical Modelling following an Erasmus Program in Europe. My master thesis was related to stochastic modelling of the evolution of an epidemic. The stochastic area of my thesis brings me to the current work in TRR.

My global aim in the projects is to improve the representation of ocean variability that represents by ocean eddies. Some ocean eddies are already resolved for certain degrees of resolution, but still there are variations where explicit simulation is not possible. Some of these variabilities could be resolved by bringing back to the model kinetic energy, so called kinetic energy backscatter, obtained through the stochastic parametrization. And to find a good stochastic kinetic energy backscatter parameterization is my local aim so far.

To date, we are on the second stage of the project, so the first months of my work were devoted to acquaintance with the FESOM model, its configurations and code itself. The other significant part was related to understanding of papers which were published by members of M3 during the first stage of the project. Currently I start to implement the idea of identifying the process based on EOF (Empirical Orthogonal Functions) analysis of kinetics energy difference between realization of the model on the fine and coarse grids.

I see the evolution of my work in the application of smarter and more sophisticated approached of stochastic parametrization, preserving the model intuitively understandable and available for computation.