Lars Umlauf

Leibniz Institute for Baltic Sea Research

Author Statistics

Papers

Citation

H-Index

i-10 index

Research Trends

Author Order

Document Type

Co-Authors

Hans Burchard

Leibniz Institute for Baltic Sea Research

Peter Holtermann

Leibniz Institute for Baltic Sea Research

Lars Arneborg

Swedish Meteorological and Hydrological Institute

Knut Klingbeil

Leibniz Institute for Baltic Sea Research

Volker Mohrholz

Leibniz Institute for Baltic Sea Research

Karsten Bolding

Aarhus University

Christian Stranne

Stockholm University

Julia Muchowski

Stockholm University

Christoph Humborg

Stockholm University

Mira Schmitt

Leibniz Institute for Baltic Sea Research

Cooperative Institutions

Leibniz Institute for Baltic Sea Research

Woods Hole Oceanographic Institution

Universität Hamburg

Scripps Institution of Oceanography

Helmholtz-Zentrum Hereon

University of Rostock

University of California, San Diego

Stockholm University

GEOMAR Helmholtz Centre for Ocean Research Kiel

Centre National de la Recherche Scientifique

Author Statistics

Papers

Citation

H-Index

i-10 index

Research Field

Energetics of deep-water mixing in a stratified basin without tides

EGU General Assembly Conference Abstracts (2010)

Peter Holtermann Lars Umlauf

Energetics

Source

Cite

Citations (0)

Potential and Limitations of a Commercial Broadband Echo Sounder for Remote Observations of Turbulent Mixing

Journal of Atmospheric and Oceanic Technology (2022)

Julia Muchowski Lars Umlauf Lars Arneborg Peter Holtermann Elizabeth Weidner

Abstract Stratified oceanic turbulence is strongly intermittent in time and space, and therefore generally underresolved by currently available in situ observational approaches. A promising tool to at least partly overcome this constraint are broadband acoustic observations of turbulent microstructure that have the potential to provide mixing parameters at orders of magnitude higher resolution compared to conventional approaches. Here, we discuss the applicability, limitations, and measurement uncertainties of this approach for some prototypical turbulent flows (stratified shear layers, turbulent flow across a sill), based on a comparison of broadband acoustic observations and data from a free-falling turbulence microstructure profiler. We find that broadband acoustics are able to provide a quantitative description of turbulence energy dissipation in stratified shear layers (correlation coefficient r = 0.84) if the stratification parameters required by the method are carefully preprocessed. Essential components of our suggested preprocessing algorithm are 1) a vertical low-pass filtering of temperature and salinity profiles at a scale slightly larger than the Ozmidov length scale of turbulence and 2) an automated elimination of weakly stratified layers according to a gradient threshold criterion. We also show that in weakly stratified conditions, the acoustic approach may yield acceptable results if representative averaged vertical temperature and salinity gradients rather than local gradients are used. Our findings provide a step toward routine turbulence measurements in the upper ocean from moving vessels by combining broadband acoustics with in situ CTD profiles.

Stratification (seeds)

10.1175/jtech-d-21-0169.1

Cite

Citations (11)

Stratification and mixing on sloping boundaries

Geophysical Research Letters (2008)

Andreas Lorke Lars Umlauf Volker Mohrholz

In lakes as well as in the ocean vertical transport is, to a large extent, controlled by localized mixing along the boundaries of the respective basin. Direct measurements of turbulent mixing and density stratification in the bottom‐boundary layer of a large and deep lake reveal the importance of cross‐slope advection near sloping boundaries for local and basin‐scale vertical transport. The periodic, internal wave driven, occurrence of up‐ and downslope directed currents results in periodic heating and cooling of the boundary layer. Cooling of the boundary layer during periods of up‐slope flow is associated with unstable stratification and turbulent diffusivities exceed those observed during periods of down‐slope flow by almost one order of magnitude. The periodic replacement of water in this layer with water from the interior of the basin maintains vertical density gradients and thus enhances the overall efficiency of the boundary mixing.

Stratification (seeds)

Stratified flows

Vertical mixing

10.1029/2008gl034607

Cite

Citations (59)

The vertical structure and entrainment of subglacial melt water plumes

Hans Burchard Karsten Bolding Adrian Jenkins Martin Lösch Markus Reinert

<p>Basal melting of marine-terminating glaciers, through its impact on the forces that control the flow of the glaciers, is one of the major factors determining sea level rise in a world of global warming. Detailed quantitative understanding of dynamic and thermodynamic processes in melt-water plumes underneath the ice-ocean interface is essential for calculating the subglacial melt rate. The aim of this study is therefore to develop a numerical model of high spatial and process resolution to consistently reproduce the transports of heat and salt from the ambient water across the plume into the glacial ice. Based on boundary layer relations for momentum and tracers, stationary analytical solutions for the vertical structure of subglacial non-rotational plumes are derived, including entrainment at the plume base. These solutions are used to develop and test convergent numerical formulations for the momentum and tracer fluxes across the ice-ocean interface. After implementation of these formulations into a water-column model coupled to a second-moment turbulence closure model, simulations of a transient rotational subglacial plume are performed. The simulated entrainment rate of ambient water entering the plume at its base is compared to existing entrainment parameterizations based on bulk properties of the plume. A sensitivity study with variations of interfacial slope, interfacial roughness and ambient water temperature reveals substantial performance differences between these bulk formulations. An existing entrainment parameterization based on the Froude number and the Ekman number proves to have the highest predictive skill. Recalibration to subglacial plumes using a variable drag coefficient further improves its performance.</p>

Entrainment (biomusicology)

10.5194/egusphere-egu22-1158

Cite

Citations (1)

Mixing rates in the deep Baltic Sea revealed by a tracer release experiment

Toste Tanhua Gregor Rehder P. Holterman J.J. Wainek Lars Umlauf

TRACER

Baltic sea

Source

Cite

Citations (0)

Estuarine circulation versus tidal pumping: Sediment transport in a well‐mixed tidal inlet

Journal of Geophysical Research Oceans (2016)

Johannes Becherer Götz Flöser Lars Umlauf Hans Burchard

Abstract High‐resolution water column observations have been carried out in the Wadden Sea to understand suspended particulate matter (SPM) transport in well‐mixed tidal channels. These observations include more than 4000 consecutive CTD, microstructure shear and turbidity profiles from a free‐falling microstructure probe, as well as velocity data from an ADCP and SPM samples for calibration. A horizontal density gradient was established by a landward temperature gradient built up during an extraordinarily warm and calm spring season. Tidal averaging along σ ‐layers (relative depth) provides the first direct observations of along‐channel estuarine circulation in the Wadden Sea, with net inflow near the bottom and outflow near the surface. Increased westerly (up‐estuary) winds during the second part of the campaign weakened and eventually even reversed estuarine circulation and yielded a net barotropic eastward transport. SPM concentrations showed a strong quarter‐diurnal signal with maxima near full flood and full ebb and were generally lower during the calm period and increased during the windy period, mainly due to wave‐related resuspension over nearby intertidal flats. The sediment flux analysis was based on a decomposition of the vertically integrated SPM flux into a barotropic advective component, an estuarine circulation component and a tidal pumping component. As a result, tidal pumping (due to ebb‐dominance weakly seaward) dominated the SPM flux during calm conditions, whereas barotropic advection dominated the strong landward SPM flux during the windy period. Along‐channel estuarine circulation is found to be of minor importance for the net SPM transport in such well‐mixed systems.

Estuarine water circulation

Barotropic fluid

Discharge

10.1002/2016jc011640

Cite

Citations (53)

High‐Resolution Simulations of Submesoscale Processes in the Baltic Sea: The Role of Storm Events

Journal of Geophysical Research Oceans (2021)

Evridiki Chrysagi Lars Umlauf Peter Holtermann Knut Klingbeil Hans Burchard

Abstract Recently discovered, ocean submesoscales have attracted considerable attention due to their ability to change the upper ocean stratification, affect vertical transport, and induce a downscale cascade of energy toward dissipation. In this paper, we highlight the effect of submesoscale fronts and filaments on surface layer properties and dynamics during storm events, extending previous idealized simulations toward real‐ocean applications. Here, we use the Baltic Sea as a natural laboratory to study the rich submesoscale activity of this system with the help of realistic high‐resolution numerical simulations. These simulations reveal a wealth of cold submesoscale filaments with sharp lateral buoyancy gradients, strong surface convergence, and high vertical velocities. Highly heterogeneous Mixed Layer Depth (MLD) modulations are associated with these features, maintaining locally reduced MLDs even during storm events due to vigorous submesoscale restratification. The interaction of near‐surface turbulence and submesoscale restratification results in strong and highly efficient mixing inside the submesoscale fronts. Focusing on a typical filament, it is shown that frontogenesis is associated with the convergent cross‐front circulation arising from a turbulent thermal wind balance, consistent with previous studies. We show that the sharp fronts generated by this process may become symmetrically unstable, and show enhanced (modeled) energy dissipation rates that are in good quantitative agreement with existing theoretical estimates.

Frontogenesis

Stratification (seeds)

Mixed layer

Energy cascade

10.1029/2020jc016411

Cite

Citations (36)

Comment on egusphere-2023-920

Julia Muchowski Martin Jakobsson Lars Umlauf Lars Arneborg Bo G. Gustafsson

Abstract. Turbulent diapycnal mixing is important for the estuarine circulation between basins of the Baltic Sea as well as for its local ecosystems, in particular with regard to eutrophication and anoxic conditions. While the interior of the basins is overall relatively calm, stratified flow over steep bathymetric features is known as a source for strong turbulent mixing. Yet, current in situ observations often cannot capture dynamic and intermittent turbulent mixing related to overflow over rough bathymetry. We present observational oceanographic data together with openly accessible high-resolution bathymetry from a prototypical sill and an adjacent deep channel in the sparsely-sampled Southern Quark located in the Åland Sea, connecting the Northern Baltic Proper with the Bothnian Sea. Our data include high resolution broadband acoustic observations of turbulent mixing, in situ microstructure profiler measurements, and current velocities from Acoustic Doppler Current Profilers and were acquired during two one-week cruises in February–March 2019 and 202. A temporally reversing non-tidal stratified flow over the steep bathymetric sill created a dynamic and extremely energetic environment. Saltier, warmer, and less oxygenated deep water south of the sill was partly blocked, the reversing flow was at times hydraulically controlled with hydraulic jumps occurring on both sides of the sill, and sub-mesoscale processes in the surface layer leading to high spatial variability at small scales. Mixing and vertical salt flux rates were increased by 3–4 orders of magnitude in the entire water column in the vicinity of the sill compared to reference stations not directly influenced by the overflow. We suggest based on acoustic observations and in situ measurements that underlying mechanisms for the highly increased mixing across the halocline are a combination of shear and topographic lee waves which are breaking at the halocline interface. We anticipate that the resulting deep- and surface-water modification in the Southern Quark directly impacts exchange processes between the Bothnian Sea and the Northern Baltic Proper and that the observed mixing is likely important for oxygen and nutrient conditions in the Bothnian Sea. Our results contribute to the knowledge on turbulent mixing processes in the Åland Sea and can help to improve mixing parametrizations in numerical models of the area.

Sill

Stratification (seeds)

Internal tide

10.5194/egusphere-2023-920-rc1

Cite

Citations (0)

Vertical mixing in the Baltic Sea and consequences for eutrophication – A review

Progress In Oceanography (2009)

J. H. Reissmann Hans Burchard Rainer Feistel Eberhard Hagen Hans Ulrich Lass

Halocline

Entrainment (biomusicology)

Eddy