Reports

Research Stay in Hokkaido by Nicolas Dettling (Nov 23)

My name is Nicolas Dettling, I am a PhD student in subproject T3 „Energy Transfers in Gravity Currents“. So far I have worked on applying and improving eddy parameterizations in the presence of gravity currents, such as the ones exporting dense water from the Antarctic continental shelves. Now, it is time to apply what we have found in idealised models to regional models of the Antarctic marginal seas.

For this purpose, I visited Prof. Yoshihiro Nakayama and his research group at the Institute for Low Temperature Science at Hokkaido University, Japan in November. Over the last years, Prof. Nakayama and his group have set up a number of regional ocean model simulations targeting key questions concerning the dynamics of the ocean around the Antarctic continental shelf and slope. Over the course of three weeks, I was introduced to a model of the Cape Darnley region, where dense water flows down the continental slope into the abyssal ocean, providing a nice test case for my previous parameterization work. I learned how to set up, run and interpret the model at different resolutions and we discussed the steps towards applying an eddy parameterization in the model. I am very grateful for the support and the fruitful discussions during my time at the institute.

The Hokkaido University campus is centred around a park and every morning I would walk along the Ginkgo Avenue enjoying the autumn colours on my way to work. Fortunately, there was also time to explore the City of Sapporo and the beautiful nature of Hokkaido, where the first snow of the season had already arrived. Luckily, the next Hokkaido Soup Curry or a bunch of Gyozas (fried dumplings) were never far away to recover after working or travelling.

I genuinely enjoyed to work in Japan and to engage in this cultural exchange. I would like to thank Prof. Nakayama and his group for their hospitality and I am very much looking forward to working together again in the future. Finally I want to thank the TRR 181 for funding the research visit.

Energy fluxes at the air-sea interface using high-resolution CFD simulations

Our goal is to resolve the small-scale processes that dominate the energy exchange as well as to identify the individual mechanisms as a function of the wind wave conditions.
Malte Loft, PhD T4

Hello everyone, my name is Malte Loft and I work on the ”T4 Surface Wave-Driven Energy Fluxes at the Air-Sea Interface” subproject as a PhD student at the Hamburg University of Technology (TUHH).

I studied dual mechanical engineering at the Hamburg University of Applied Sciences and specialised in fluid mechanics at the University of Rostock as part of a Master’s degree. In September 2021, I started my PhD to investigate the energy fluxes at the air-sea interface using high-resolution CFD simulations (WP2).

Our goal is to resolve the small-scale processes that dominate the energy exchange as well as to identify the individual mechanisms as a function of the wind wave conditions, e.g. the wave age or wave slope of the current sea state. Due to mostly very high Reynolds numbers, it is hardly possible to perform Direct Numerical Simulations (DNS). Therefore, a hybrid turbulence model (Detatched Eddy Simulation, DES) is used for our simulations. First, a numerical wind-wave tank is developed to reproduce relatively simple laboratory conditions and to validate the numerical model with experimental results (WP1). In the animation shown, a non-linear surface wave can be seen propagating from left to right, involving strong wind forcing. Air separation events and highly turbulent structures are clearly visible. Due to our fully coupled model, we are able to extract the pressure fields and surface stresses at any point in space and can also include the influence of surface tension effects in our investigations. Furthermore, we produce large amounts of data during our simulations in order to determine phase-averaged quantities using triple decomposition. In other words, fields of pressure or velocity that correlate with the respective sea state, detached from turbulent fluctuations. With all this data, we hope to gain deep insights into the physical processes that determine the mechanical energy flow at the air-sea interface.

In the future, we will extend the application of our model to more complex scenarios, e.g. to highly non-linear sea states of the Baltic Sea, including further phenomena such as wave breaking. Another goal is to formulate the findings into improved parameterisations, in particular to improve the boundary conditions of current ocean models (WP3).

Here you can see a short video.

Research Stay in Miami by Janina Tenhaus (Aug 22)

Last year I was asked if I would like to participate in a wind-wave project at the Alfred C. Glassell, Jr. SUSTAIN Laboratory in Miami, USA, for three weeks. After listening to the song "Miami" by Will Smith several times, I felt well prepared and started organizing the trip, especially the funding by the TRR. The wind-wave tank is top-notch, and I was very excited when everything was approved. After my arrival, I met the scientists from Columbia University, U.S. Naval Research Laboratory, University of New Hampshire, and of course University of Miami. From now on, we spent almost every day in the dark lab with no daylight – thanks to the Particle Image Velocimetry measurements. Outside it was summer and mosquito season, so we did not complain much. We survived working on weekends with strong Cuban coffee (do you really want the real one and no sleep for a week?). But the experimental work did not only take place in the lab; discovering the great dive sites of Miami was also part of my tight schedule. Shortly before my return flight, we cooled the tank, whereupon it began to leak as all the silicone seals contracted. This reminded me of the rainy weather in Hamburg, and I knew it was time to come home. I am very grateful for this experience and would recommend everyone not miss the opportunity to do a research stay.

To resolve or not to resolve?

I’m investigating effects of grid resolution on the modification of overflow and ocean energetics.
Deniz Aydin, PhD T3

Bathymetry of the study region with different resolutions.

I am Deniz and I work on the T3 ‘Energy transfers in gravity plumes’ project as a PhD candidate at AWI. In particularly we are interested in the Denmark Strait Overflow (DSO) which is between Greenland and Iceland. This location is special because the DSO carries most of the dense and cold Arctic water entering the North Atlantic. Thus contributing to the deep southward flowing part of the Atlantic meridional overturning circulation.

As soon as the dense water on the sill starts descending, it undergoes a significant amount of mixing and entrainment of ambient water. By 200km downstream of the sill, volume and tracer properties of the overflow water are substantially modified due to combination of different processes. In our subproject we try to understand the interactions of all these different processes at different scales using observational and numerical modeling analysis.

It’s difficult to properly represent overflows in a global ocean model with the coarse resolution climate models generally have. For my part in this subproject, I use a general circulation model (MITgcm) in a regional setup with a 1year of simulation period. I’m investigating effects of grid resolution on the modification of overflow and ocean energetics. For this purpose I use 6 different horizontal resolutions ranging from eddy resolving (1km) to coarse resolution (36km). At the moment, I am analyzing the results from higher resolution simulations. Soon coarser resolutions will come into the picture and analysis of eddy parameterization schemes along with them. My research will contribute to a better understanding of consequences of lacking smaller scale processes and better representation of them in coarser models.

Energy transfers in gravity plumes

The next step will be connecting this mesoscale activity with high frequency variability and mixing parameters in the plume.
Stylianos Kritsotalakis, PhD T3

Schematic illustrating the observed mesoscale activity ~120km downstream of the Denmark Strait Sill. The position of the moorings on the Greenland slope is marked with black dots. The direction of the mean flow is indicated with a solid black arrow.

Hello everyone, my name is Stylianos Kritsotalakis and I am a PhD student in the subproject “Energy transfers in gravity plumes” at AWI/MARUM. The aim of the project is to understand the pathways and processes by which kinetic energy is transferred from the mesoscale eddy field to submesoscales and dissipative turbulent scales. Using observational and numerical modeling efforts the project focuses in tackling the above problem within the Denmark Strait Overflow plume.

I am working , primarily, with mooring data aquired ~120km downstream of the Denmark Strait in late summer 2018. I have identified the mesoscale field associated with the plume which consists of eddy pairs with opposing sense of rotation (Fig.1) and at the moment I am comparing these findings with the existing literature. The next step will be connecting this mesoscale activity with high frequency variability and mixing parameters in the plume.

Atmospheric gravity waves from LIDAR observations

My mission is to develop data analysis from our observations and apply it to the output from the Kühlungsborn Mechanistic Circulation Model (KMCM).
Marwa Almowafy, PhD T1

My name is Marwa Almowafy, I am a PhD student in the subproject “T1: Mesoscale energy cascades in the lower and middle atmosphere”.

I am working on temperature perturbations in the upper stratosphere and mesosphere, between 30 and 80 km, caused by atmospheric gravity waves. These waves are mainly generated in the troposphere due to several processes, for example convection and flow of air over mountains. The waves are propagating upward carrying momentum and energy. Eventually this momentum and energy is deposited at higher altitudes. With the help of observations, we address the cycle of gravity propagation and dissipation which is important for understanding their role of modifying the background atmosphere.

At the Leibniz Institute for atmospheric Physics (IAP) we have a variety of observation techniques and facilities such as balloons, sounding rockets, radars and Lidars. In the frame work of my PhD, I am focusing on data from Lidar observations. Our Rayleigh/Mie/Raman (RMR) Lidar is used to study temperatures and winds in the middle atmosphere. This Lidar has the unique capability to operate even under full daylight. IAP is operating several Lidars, one of them being located in Kühlungsborn, Germany, and another one in Andenes, Northern Norway. This allows for studying the impact of latitudinal difference and upper atmospheric dynamics regarding gravity waves. We are comparing the seasonal variability of temperature fluctuations from both locations to available reanalysis and satellite retrievals. A step further will be to approve the results with our highly resolving models at IAP.

My mission as a part of TRR181 is to develop data analysis from our observations and also apply it to the output from the Kühlungsborn Mechanistic Circulation Model (KMCM). Furthermore, I plan to construct time series of gravity wave spectra from temperature and wind data to study the behavior of power spectral indices and compare them to expectations from theory.

Decoding the Energy Spectrum Using ICON-IAP

It is unrealistic to expect the numerical models to exactly simulate the real atmosphere for all observed penomena since the atmospheric flows are turbulent in nature.
Kesava Ramachandran, PhD T1

Hi, my name is Kesava Ramachandran from subproject T1. My work deals with the implementation of Dynamical Smagorinsky Model (DSM) to understand the effects of stratified turbulence due to gravity-wave breaking in the MLT region using high-resolution non-hydrostatic ICON-IAP model. In this context, the investigation of energy cycle by analyzing the spectral budgets of kinetic energy and potential energy will be carried out.

Numerical models are widely used for investigations of atmospheric conditions and behaviour. It is unrealistic to expect the numerical models to exactly simulate the real atmosphere for all observed phenomena since the atmospheric flows are turbulent in nature. The set of mathematical equations that describe such flows are nonlinear and it is impossible to solve them exactly. At least till now, no one has solved the complete set of equations. This leads to use of different modelling techniques where we resolve the wide range of time and length scales. Such atmospheric models normally consist of a dynamical core and physical parametrization.

ICON-IAP is one such atmospheric model with a novel discretization for strict representation of the conservation laws by the dynamical core. An issue not normally considered in the circulation models is the inherent diffusion due to the numerical formulation of the dynamical core. This inherent diffusion cannot be interpreted as physical dissipation. ICON-IAP discretizes the Poisson-brackets of the Hamiltonian system and guarantees consistent reversible energy pathways. As a reference for comparing, we have the observation data from Nastrom & Gage, where a -3 slope in the synoptic scale and -5/3 slope in mesoscale scale is noted for horizontal wind and temperature.

It is important to have an elaborate understanding of the different processes that contribute to the energy cycle and the interaction between different dynamical regimes since it will give us an idea on the scales at which the transport occurs. With respect to this, the governing equations are transformed so that the processes that do not contribute are made invisible. Using the transformed equation we can disentangle the contribution of the horizontal and vertical flux terms. We can also compare the spectral budgets of kinetic and a v a i l a b l e p o t e n t i a l energy and the individual fluxes between the transformed and the untransformed equation.

Analysing the kinetic and available potential energy spectrum will result in understanding the scales of the primary gravity waves transport of momentum from lower to middle atmosphere and a reasoning as to whether the concept of Stratified Macroturbulence applies when averaging about individual wave packet and to the energy cascade induced by the gravity wave breakdown in the mesosphere.

Meso-scale energy cascades in the lower and middle atmosphere

My task is to extend the recently developed parameterization for friction/diffusion for atmospheric flows to the middle atmosphere.
Serhat Can, PhD in T1

Hi, I am Serhat from subproject T1. As a PhD candidate, my task is to extend the recently developed parameterization for friction/diffusion for atmospheric flows to the middle atmosphere, including full accounting of the spectral budget for kinetic and available potential energy. Complex flows cover a wide range of spatial and temporal scales and it becomes practically illogical to expect existing computational technology to simulate a realistic atmosphere for all observed phenomena. Thus, the emergence of accounting for the effects of unresolved scales is inevitable, resulting in what is known as the turbulence closure problem.

Closure is handled via the so-called Dynamic Smagorinsky Model (DSM), in the Kühlungsborn Mechanistic general Circulation Model (KMCM). This scheme eliminates ad hoc tuning for the parameterization and allows a space-time dependent mixing length, fully determined by the resolved flow.

Observational data from Nastrom & Gage point to transition from synoptic -3 slope to -5/3 in mesoscales for horizontal motion and temperature, providing a solid reference information for comparison. Atmosphere being strongly effected by gravity, anisotropic formulation is needed for DSM and the arguments of Stratified Macro Turbulence (SMT) comes into play for the aid, yielding an additional constraint on the dependence of vertical form of DSM on its horizontal part.

On top of all these intertwined descriptions of turbulence, scale invariance sets the tone and dictates equations to keep their forms unchanged for inertial regimes, including parameterizations. A dynamically determined mixing length complies with this requirement and definition of parameterization is completed. It should be emphasized that sub-grid scale motion is considered as a modelling of friction from a thermodynamic point of view. In this manner, only forward energy cascade with no backscatter must result on average from the spectral analyses of the circulation model.

Reasoning for a unidirectional energy cascade stems from the Lorenz Energy Cycle, where the conversions between kinetic, available and unavailable potential energy drives the climate. To appropriately represent this cycle detailed description of entropy production, i.e. friction due to motion is crucial. DSM appears as a comprehensive method to address above-mentioned demands in general circulation modelling. As a result, friction/diffusion in atmosphere represented in the framework of turbulence modelling creates an exciting meeting of seemingly distant fields.

Investigating the Denmark Strait Overflow plume

Using the results of the observational and modeling components, we will investigate the role entrainment plays in the evolution of the plume.
Ryan North, Postdoc in T3

In October 2016 I joined the TRR 181 as a postdoc at the Universität Hamburg in the T3 subproject: Energy transfers in gravity plumes. Our subproject aims to improve our ability to parameterize the energetics and mixing within gravity plumes by investigating the Denmark Strait Overflow plume. This plume was chosen as an ideal study case because of its relevance to the global ocean circulation, and the long history of observational data in the Strait. My role within the subproject mainly involves working with this historical data and the collection of new data. The data will be used both on its own and for collaborative modelling work. Using the results of the observational and modeling components, we will investigate the role entrainment plays in the evolution of the plume. In particular, we are interested in investigating the hypothesis that enhanced entrainment occurs where the plume interacts with mesoscale eddies or topography. The modeling component will help to put the results in perspective across a range of scales, from the turbulent scale up to the mesoscale.

Prior to joining the Institute of Oceanography I followed a winding career path. Beginning at Canada’s Queen’s University, my career has taken me through structural and coastal engineering, lake, river and coastal hydrodynamic modeling, climate related hypoxia in lakes, and submesoscale eddies in the coastal ocean (at HZG in nearby Geesthacht). With this new position I have finally managed to move beyond inland waters and the coastal shelf break to reach properly deep water!

I am looking forwarding to meeting more members of TRR 181, and to opportunities to work together in the near future. Currently, I am onboard the FS Meteor helping out fellow TRR project members investigate filaments forming within the Benguela upwelling system off the coast of Namibia.