Abstracts and Presentations/Posters

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COMOD 2018

Oral Presentations

Brüning, Thorger, Lorkowski, Ina, Schwichtenberg, Fabian, Schöngaßner, Thomas, Ruhnau, Marcel:
Innovations in BSH’s operational model system – technical status, new products and outlook

In recent years, there has been a growing demand for information about water quality in relation to the Marine Strategy Framework Directive, and about the impact of the use of the new scrubber technology in shipping industry. To provide valuable information on these topics, BSH (Federal Maritime and Hydrographic Agency) has developed new model components in various projects, which can be coupled interactively to the operational circulation model HBM (HIROMB-BOOS-MODEL) via a generic interface.

 An ecosystem component based on the ERGOM model (https://ergom.net) has been developed within the projects “DeMarine2” and “MeRamo. It is running in pre-operational mode for some time and already provides a daily forecast of nutrients and oxygen. In particular the predicted bottom oxygen is used in the planning procedure of measurement cruises and also in the implementation of the Marine Strategy Framework Directive. In the near future, the distribution of nutrients from the river Elbe is planned as another (pre-)operational forecast product. It will be generated with the help of the so-called “nutrient tagging” – a method to track nutrients or tracers in general from particular sources (such as rivers, the atmosphere or sediments).

Parallel to the ecosystem component, an interactively coupled version of an Eulerian propagation model has been developed within the project “Scrubber Water Survey” (SWS). In this model component various types of possible sources (area sources, moving sources, line and point sources) have been defined. Within the SWS-project, it is used to calculate scenarios for the use of scrubber technology in shipping. However, in operational mode this component can also be used to calculate and predict the spread of water-soluble substances from temporary sources (like river floods, ship accidents, etc.)

This contribution describes the current status of BSH’s operational model system. It presents the new forecast products and gives an outlook on the potential further improvement of existing model components and on the development of new model components.


Cahill, B., Fischer, J., Gräwe, U., Burchard, H., Wilkin, J., Warner, J., Ganju, N.:
The impact of water constituents on radiative heat transfer in the open ocean and shelf seas

We are using a coupled ocean-atmosphere circulation model (COAWST) incorporating a bio-optical module with multiple phytoplankton groups in tandem with an atmosphere-ocean radiative transfer model to explore the contribution of optically active water constituents to energy fluxes in the upper ocean and across the air-sea interface. Specifically, we are investigating:

How heterogeneity in optically active water constituents in shelf seas affects the characteristics of sub-mesoscale vertical turbulent mixing and advective fluxes, through feedbacks with upper ocean heating rates and water density?

What are the consequences for the supply of surface nutrients and the transport and transformation of phytoplankton biomass?

What is the seasonal modulation of the flux of thermal energy across the ocean atmosphere interface as a result of heating rates induced by optically active water constituents?

To what extent is variability in CDOM attenuation reflected by environmental conditions and phytoplankton community structure?

The coupled ocean-atmosphere-bio-optical circulation model is being applied to selected shelf sea regions characterized by different freshwater and nutrient regimes, and complex bio-optical and hydrodynamic processes (Western Baltic Sea, Southern North Sea and Northwest Atlantic). The bio-optical module, which explicitly follows the spectrally-resolved vertical light stream, accounts for optically active water constituents’ contribution to the divergence of the heat flux within the full hydrodynamic solution. This means that heating rates due to the highly variable concentrations of optically active water constituents can be estimated and their impact on ocean biophysical processes evaluated. We will evaluate these modelled heating rates against more rigourous co-located heating rate calculations performed usingthe atmosphere-ocean radiative transfer model, MOMO. The coupled 3D model solution will thus be optimized for regional applications. This will lead to improved net surface solar radiation forcing fields, an accurate underwater vertical description of the light field and a resolution of the radiative flux divergence between physics and biology in the ocean. In so doing, we expect to make a major contribution toward overcoming some of the obstacles inherent in studying optically complex shelf and coastal waters. The outcome will be a rigorous assessment of the impact of optically active water constituents on upper ocean heat budgets and will establish a framework for regional two-way coupled ocean-atmosphere investigations with important consequences for weather forecasting and climate change.

This project is funded by the German Research Foundation (DFG Grant No. CA 1347/2-1), 2018 – 2021.


Dissanayake, P. and H. Hofmann:
3D Modelling of nearshore morphodynamics in a topographically challenging environment

Keywords: ELCOM, Delft3D, temperature stratification, Sediment transport, Lake Constance.

 Though the models of sediment transport and morphodynamics are reaching a mature stage, applications in different environments still show limitations and complications, which are of interest for the developers as well as for the coastal and ocean modelling community. Two 3D hydrodynamic models, ELCOM (Estuary, Lake and Coastal Ocean Model: [2]) and Delft3D ([3]), were initially employed to simulate barotropic- and baroclinic-hydrodynamics in Lake Constance [1]. The models used a measured vertical temperature profile to set up the initial stratification and main river discharges, and were forced by a heterogeneous wind field from COSMO (Consortium for Small Scale Modelling, [4]). Heat exchange at the free surface was estimated using the meteorological data from German Weather Service in Konstanz. Model predicted temperatures and currents were compared with the field data (thermistors, ADCP and drifters).

Simulating nearshore morphodynamics was challenging due to steep bed gradient (~1:10) in the lake compared to the coastal environments (e.g. North Sea coast ~ 1:100). Therefore, different model setups were tested to transform offshore hydrodynamics into a high resolution nearshore model grid. Forcing with a measured storm event (Hs,max ~ 0.5 m), the morphodynamic evolution of a bay area of the lake, Kressbronn Bay [5], was finally simulated.

Predicted stratification of both models reasonably agreed with the measured temperatures. Currents at deep layers always had a better agreement with the data than currents at surface layers in both models. Results further indicated that the wave propagation into the bay is independent from the direction of wind approach. The eastern shore area experienced about 0.1 m erosion and accumulation within the simulated storm period. This study concludes, both ELCOM and Delft3D have almost similar model skill in generating baroclinic dynamics in Lake Constance. In contrast to the coastal/ocean environments, the model-nesting is not an option to transform offshore forcing into nearshore areas of the lake to simulate morphodynamics. This study finally established that a numerical model can be used to investigate bed evolution of lakes, which is quite novel for the morphodynamic modelling.  


[1] Dissanayake, P., Hofmann, H. and Peeters, F. (submitted), Comparison of results from two 3D hydrodynamic models with field data: Internal seiches and horizontal currents, Journal of Geophysical Research – Ocean.

[2] Hodges, B, Dallimore, C. (2006), Estuary, Lake and Coastal Ocean Model (ELCOM). v2.2 Science Manual, Centre for Water Research, University of Western Australia.

[3] Lesser, G.R, Roelvink, J.A, Van Kester, J.A.T.M, Stelling, G.S. (2004), Development and validation of a three-dimensional morphological model. Coastal Engineering 51, 883–915.

[4] Schättler, U. (2009), A description of the nonhydrostatic regional COSMO-Model Part V. Preprocessing: Initial and boundary data for the COSMO-Model, Available at http://www.cosmo-model.org

[5] Dissanayake, P. and Hofmann, H. (submitted), Modellierung des Sedimenttransports eines Ufergebiets vom Bodensee: Kressbronne Bucht, Hydrologie und Wasserbewirtschaftung.


Dreier, N. and Fröhle, P.:
Operational wave forecast in the German Bight as part of a sensor- and risk based early warning system

Today’s available flood- respectively storm surge warning systems for the German North Sea coast consist exclusively of water level forecasts. Other hydrodynamic loads caused by wind waves and local currents as well as the resistance of the flood protection structure itself (e.g. coastal dikes, flood protection walls etc.) are not taken into account in the early-warning system. Within the framework the German joint research project “EarlyDike” (2015-2018), founded by the federal ministry of education and research (BMBF), an operational wave now- and forecast system is set up that consists of available field measurements and data from numerical simulation of waves in the German Bight using a combined hybrid model approach with a phase integrated spectral wave forecast model (SWAN) and an empirical wave run-up approach (EurOtop) for the now- and forecast of e.g. average wave run-up heights at a sea dike on the North Frisian Island of Pellworm in the German Bight. The operational system is demonstrated exemplarily for the forecast of a severe storm event on the 5-6th December 2013 (German name "XAVER"). The qualitiy of the forecast system is assessed seperately for the local nearshore wave conditions as well as the average wave run-up heights on the basis of available field measurements that have been carried out by the local authority responsible for coastal and flood protection in the federal state of Schleswig-Holstein (LKN-SH).


F. Lemarié, L. Debreu, F. Auclair, N. Ducousso, F. Dumas, R. Benshila:
Recent Developments around the CROCO Initiative for Complex Regional to Coastal Modeling

CROCO (Coastal and Regional Ocean Community model [1]) is a new structured-grid oceanic modeling system built upon the dynamical kernel of the AGRIF version of ROMS (see [2] for a description of the various ROMS kernels). An objective behind CROCO is to aggregate recent achievements of the french regional/coastal modeling community including the pseudo-compressible non-hydrostatic (NH) capability of SNH [3] (and an incompressible alternative from [4]), the MUd and Sand TrAnsport modelliNG (MUSTANG) sediment dynamics module of Mars3D, as well as a newly developed multi-resolution capability based on a multigrid approach, etc. The implementation of both the multiresolution and pseudo-compressible NH features have been facilitated by a revision of the way the dissipation necessary to stabilize the time-integration of the baroclinic/barotropic mode-splitting is imposed [5].

The talk will thus focus on three original features which have been implemented in the CROCO model during the last few months and which are of interest for the coastal ocean modeling community:

1- A much reduced numerical dissipation to stabilize the baroclinic/barotropic mode-splitting

2- A 3-mode time-splitting approach relaxing the Boussinesq and hydrostatic assumptions

3- A truly multiresolution strategy as an alternative to unstructured strategy

The benefits of each of these numerical developments will be illustrated by idealized and realistic simulations. Finally, some thoughts on the computational efficiency of oceanic models and how it relates to numerical methods as well as future challenges and perspectives will be given. 


[1]  http://www.croco-ocean.org
[2]  Shchepetkin A. and J.C. McWilliams Correction and commentary for ocean forecasting in terrain-following coordinates: formulation and skill assessment of the Regional Ocean Modelling System by Haidvogel et al.. J. Comp. Phys. (2009)
[3]  Auclair F., L. Bordois, Y. Dossmann, T. Duhaut, A. Paci, C. Ulses, and C. Nguyen. A non-hydrostatic non-Boussinesq algorithm for free-surface ocean modelling. Ocean Modell., in revision (2018)
[4]  Roulet G., M. Molemaker, N. Ducousso, and T. Dubos. Compact symmetric Poisson equa- tion discretization for non-hydrostatic sigma coordinates ocean model, Ocean Modell. (2017)
[5]  Demange, J., L. Debreu, F. Lemarié, P. Marchesiello and E. Blayo Stability analysis of split-explicit oceanic models. J. Comp. Phys., submitted, (2018)


Gräwe, U., Holtermann, P., Klingbeil, K.:
Shelf Sea Modelling – where are we and where to go?

The North Sea and the Baltic Sea can be regarded as prototypes for shelf seas. Although both regions are tightly coupled, they do show unique features, which pose challenges to the modelling community.
Whereas the North Sea is dominated by tidal mixing, the mixing in the nearly tideless Baltic Sea is controlled by boundary mixing. However, both show strong temperature stratification during summer, which is limiting the exchange across the thermocline. Additionally, the stratification leads to a shrinking of the baroclinic Rossby radius, which limits the ability of numerical models to resolve this important length scale. Even if they are able to do so, the models might not be capable of representing the energy fluxes between the mesoscale and submesoscale, or the transition from the 2D geostrophic turbulence to the ageostrophic regime. Here, the horizontal mixing parameterisation, which is either not grid sensitive or based on isotropic 3D turbulence, is a limiting factor.  Moreover, giving a high enough spatial resolution, the internal wave field and its impact on the mixing is only partially resolved. This makes is hard to apply internal wave mixing parameterisations, like in global ocean models.
Since shelf sea models are always regional models, they rely on boundary conditions from global ocean models or a nesting hierarchy. Both approaches might miss variations on different time and length scales, limiting their applicability. The same holds for the atmospheric forcing. Although more state of the art forcing datasets are available, the effect of land-sea transition might also limit their usage for local modelling studies.
In summary, we will highlight some achievements, challenges and potential future pathways in our efforts to improve shelf sea modelling.

Kuznetsov, I., Androsov, A., Fofonova, V., Danilov, S., Rakowsky, N., Harig, S., Wiltshire, K. H.:
FESOM-C: coastal dynamics on mixed unstructured meshes

Numerical modelling of coastal ocean provides a basis for answering a wide range of questions related to coastal dynamics, engineering and ecology. We describe the coastal branch of FESOM (FESOM-C), which complements the large-scale FESOM and shares with it many aspects of numerics, in particular, its finite-volume cell-vertex discretization. The major differences in the dynamical core are in the implementation of time stepping, the explicit use of terrain-following vertical coordinate and the use of mixed meshes composed of triangles and quads. The first two distinctions were critical to code FESOM-C separately from the large-scale model. The ability to work with mixed meshes improves numerical efficiency, for quadrilateral cells involve less edges than triangular cells. They do not support spurious inertial modes of triangular cell-vertex discretization and need less dissipation. The support for mixed meshes allows one to use triangular mesh patches to join quadrilateral meshes of differing resolution. The description of numerical part is complemented with the description of simulations showing the performance of the model.The newly developed FESOM-C model applied to the South-East North Sea region. Model could reproduce both barotropic and baroclinic dynamics of the coastal and estuary regions reasonably well. An Elbe summer flood 2013 year event was well captured by the model.


Lehmann, A., Höflich, K.:
An Earth System Model with reduced complexity of the entire Baltic Sea

We present an Earth System model of the entire Baltic Sea including Kattegat and Skagerrak. The model system is based on an ocean general circulation model (Kiel Baltic Sea model, BSIOM) which is coupled with a dynamic-thermodynamic sea ice model, an oxygen consumption model (OXYCON) and a simple surface wave model. The model system is forced by ERA-Interim reanalysis data (1979-2017) which includes SLP, 2-m air temperature, 2-m dew pointtemperature, cloudiness and total precipitation. Wind speed and direction at 10 m height are calculated from geostrophic winds with respect to different degrees of roughness of the open sea and off the coast. River runoff is prescribed from a monthly mean runoff data set. At the western boundary, a simplified North Sea basin is connected to the model domain to provide characteristic North Sea water masses in terms of temperature and salinity profiles. Surface salinity in the North Sea Basin is relaxed to observed seasonal salinity data. The model is further forced by low frequency sea level variations in the North Sea/Skagerrak calculated from the Baltic Sea Index (BSI). The surface wave model is based on an empirical growth formula.

The model has mainly been utilizedfor studies on climate variability of temperature, salinity, oxygen as well as the 3-d current system. Further applications are on spatial-temporal dynamics of cod nursery areas and drift studies of cod eggs and larvae. The model has also been used to determine the wave climate, Stokes drift and total sea level changesof the entire Baltic Sea. We will present the Earth System model with reduced complexity of the entire Baltic Sea and show results from different applications.

Mathis, M., Elizalde, A., Mikolajewicz, U.:
The future regime of Atlantic nutrient supply to the Northwest European Shelf

Earth system models consistently project future decreases in subpolar North Atlantic mixed layer depths (MLD) and surface nutrient concentrations as a response to anthropogenic global warming. Accordingly, it is expected that Atlantic nutrient import to the Northwest European Shelf (NWES) decreases. To study the evolution of net primary production on the shelf we downscaled global MPI-ESM climate projections of emission scenarios RCP4.5 and RCP8.5 with a high-resolution regionally coupled ocean-atmosphere climate system model. Our simulations suggest that at the end of the 21st century the shoaling of eastern North Atlantic MLD causes a regime shift in the dynamics of Atlantic nutrient supply to the shelf. Upper ocean nutrient concentrations in the North Atlantic drop substantially, while on the shelf the nutrient decline is weaker due to deep ocean-shelf exchange at the continental margin, inducing a cross-shelf break nutrient front. As a consequence, primary production on the shelf becomes strongly modulated by multidecadal variations of Atlantic sub-mixed layer nutrient concentrations. Moreover, for scenario RCP8.5 shallow Atlantic MLDs reach the shelf edge at about 150-200 m depth, giving rise to enhanced interannual variability due to NAO-induced variations of vertical mixing at the shelf break.


Staneva, J., Schrum C., Behrens A., Grayek S., Ho-Hagemann, H., Breyvik O., Bidlot J., Morgensen K.:
North Sea-Baltic Sea regional models: coupling of ocean and atmosphere a through a dynamic wave interface

The coupling of models is a commonly used approach when addressing the complex interactions between different components of earth system. In climate and forecasting research and activities, advanced models are needed and there is an urge towards the use of coupled modelling. This study presents the developments and implementation of a high-resolution, coupled model system for the North Sea and the Baltic Sea, as a part of the Geestacht COAstal model SysTem GCOAST. We focus on the nonlinear feedback between strong tidal currents and wind-waves, which can no longer be ignored, in particular in the coastal zone where its role seems to be dominant. Ocean waves influence the circulation through number of processes: (1) The Stokes-Coriolis forcing; (2) Sea state dependent momentum flux; and (3) Sea state dependent energy flux. The proposed wave-atmosphere coupling parameterizations account for the feedback between of the upper ocean on the atmosphere by accounting for the effects the sea surface roughness. Sensitivity experiments are performed to estimate the individual and collective role of different coupling components. The performance of the coupled modelling system is illustrated for the cases of several extreme storm events. The model comparisons with data from new satellite altimeter and in-situ observations showed that the use of the coupled models reduces the errors, especially under severe storm events. For example, the inclusion of wave coupling leads to decreases strong winds through wave dependent surface roughness or changes sea surface temperature, the mixing and ocean circulation; leading to better agreement with  in-situ  and satellite measurements, especially in the coastal areas. The wave-induced forcing in the circulation model leads to surge simulations closer to observations during extremes. All this justifies the further developments and implementation of the wave model component in coupled model systems for both operational and climate research and development activities.

Wenyan Zhang, Richard Hofmeister, Kai Wirtz:
Integrating biotic-abiotic interactions at the sediment-water interface in coastal ocean models – what can we do?

The sediment-water interface represents one of the most dynamic interfaces in the earth system. This interface serves as habitat for benthic life ranging from microphytobenthos to worms, mussels, up to macroalgea, sea grass, or flatfish. The partitioning of material being buried as sedimentary record or recycled in water is dynamically modulated. Benthic organisms exert a significant influence on the material exchange at the sediment-water interface because of their presence both above and beneath the seafloor. The interaction between biotic and abiotic factors at the interface is two-way. On one hand, benthic organisms modify the benthic boundary layer flow through introducing additional physical roughness (burrow entrances and mounds, or simply by the body shape). They also affect the erodibility of the surface sediment layer through stabilizing (secrete binding) or destabilizing (diffusive mixing) behaviors. At the same time, bioturbation and bioirrigation determine exchange of nutrients and organic matter within the sediment. On the other hand, benthic boundary layer flow and organic matter flux control the abundance, positioning and behaviors of benthic organisms.

To understand the physical and biogeochemical dynamics in coastal seas, it is necessary to implement the dynamic interaction of biota and abiota in numerical models. In this study we present 1) a brief overview of current research status w.r.t. implementation of biotic-abiotic interactions at the sediment-water interface in coastal ocean modeling, and 2) introduce our recent progress toward this end.

Winter, C.:
Estuarine and Coastal Ocean Modelling

This introductory talk presents examples of recent model studies on estuarine and coastal ocean settings. With a regional focus on the German North Sea coast it is discussed which societal and scientific aims are followed, and which model assumptions are being applied in order to allow the application of models.  Examples for transport and morphodynamic modelling studies of different coastal elements introduce the effect of different hydrodynamic drivers and the role of diferent sediment fractions in nearshore transport patterns. The feasibility gap between highest spatiotemporal resolution on the one hand and the demand for large scale and long term predictions on the other hand is described.

Wirtz, K., Lemmen, C.:
Biological controls of primary production in the coastal ocean revealed by a new model and data system

One of the most striking patterns at the land--ocean interface is the persistent steep increase of chlorophyll-a (CHL) from continental shelves towards the coast, a phenomenon that is classically thought to reflect physical features. Here we show that the steep cross-shore CHL gradient has biological origins related to phytoplankton mortality, which are neglected in state-of-the-art biogeochemical models. Our ecosystem model as part of the coupled Modular System for Shelves and Coasts(MOSSCO) displays unprecedented skill in reproducing daily, seasonal, and inter-annual (2000--2014) dynamics and meso-scale patterns in macronutrients, zooplankton biomass, and CHL. These patterns were observed by remote sensors, Scanfish cruises, and a dense array of Dutch and German monitoring stations throughout the southern North Sea (SNS). Our reproduction of the nearshore CHL accumulation critically depends on assuming pathogen dynamics and a lateral gradient in carnivorous grazing. This gradient in carnivory reflects higher near-coast abundance of juvenile fish and mussels and effectively constrains near-shore zooplankton. The resulting mortality gradient for phytoplankton counteracts the strong rise in turbidity and light limitation in shallow waters of the SNS. Indeed, calculated primary production is maximal at intermediate water depths. Further simulations demonstrate a significant impact of mussels on primary production in the SNS and also, for the first time, quantify ecosystem effects of offshore wind-farms, which are mostly located within or adjacent to the zone of largest primary production.
Our new insights into coastal ecosystem functioning and sensitivity to natural and anthropogenic drivers rely on a research strategy, where model expertise such as on hydrodynamics (here GETM), sedimentology, and ecology, is distributed among different institutional partners. As a consequence, each partner is enabled to better advance individual components.

Poster Presentations

Bieser, J., Daewel, U., Schrum, C.:
Modeling mercury transport, transformation, and bio-accumulation in the coastal ocean

Mercury is a global pollutant that has been identified as one of the top ten chemicals of major public health concern. Thus, in 2017 the UN Minamata Convention on Mercury came into force with the aim to safeguard our food and ecosystems. Mercury is primarily emitted by anthropogenic activities into the atmosphere where it is transported on a hemispheric scale. Via atmospheric deposition and riverine inflow it eventually reaches the ocean. There it is transformed into highly toxic methylmercury that accumulates along the food chain. Moreover, biological parameters have a direct impact on the marine mercury chemistry. Phytoplankton activity leads to reduction and evasion of mercury from the ocean, while remineralization and anoxia are the major drivers for mercury methylation. Moreover, the abundance of biological particles influences phytodegradation and the vertical distribution of mercury.

The large sources, high primary production, and the abundance of fish in the coastal ocean make it a focal point of the global mercury cycle. Here we present results from the first coupled atmosphere-ocean-ecosystem 3d-hydrodynamical mercury model that is capable to model transport, transformation, and bio-accumulation of mercury. We applied the model to a regional domain in the North- and Baltic Sea and analyse the temporal and spatial variability of mercury methylation and bio-accumulation.

Burchard, H.:
Numerical issues of the Total Exchange Flow (TEF) analysis framework for quantifying estuarine circulation

Since more than a century, estuarine exchange flow is quantified by means of the Knudsen relations which provide bulk quantitites such as inflow and outflow volume fluxes and salinities. These relations are closely linked to estuarine mixing. The recently developed Total Exchange Flow (TEF) concept which leads to profiles of volume and salt fluxes as function of salinity allows for consistent calculation of the Knudsen relations. There are however numerical issues, since the method does not converge for an increasing number of salinity classes. In the present study, this problem is investigated, first by means of an analytical scenario for estuarine exchange flow and then by analysing results from a numerical model for the Western Baltic Sea. As a result, the number of discrete velocity and salinity values per salinity class must not be too small. Convergence towards an analytical solution is only obtained, when the number of salinity classes as well as the number of discrete values per salinity class increase.


Daewel, U., Schrum, C.:
On the impact of implementing higher trophic levels into NPZD modelling approaches: An application the North Sea and Baltic Sea ecosystem

The majorities of marine ecosystem models target only parts of the trophic food chain. This implicates difficulties to consistently simulate the major controls of marine ecosystems and to distinguish between ‘bottom-up’, ‘top-down’ or ‘wasp-waist’ controlled ecosystems. While one solution to this deficit is to couple specific models for the different trophic levels into an interlinked model approach, another approach would be enhancing the existing ecosystem models consistently by defining functional groups for higher trophic level similarly to those for lower trophic levels.  The latter would solve the requirements for closing the lower trophic food chain and hence could be used to address questions on spatial and temporal variations of ‘top-down’ impacts on lower trophic level dynamics, while it further allows estimates of fish production potential. Here we present such an NPZD-Fish modelling approach that bases on the fully coupled biological-physical ecosystem model ECOSMO II. The model represents both fish and macrobenthos as functional groups that are linked to the lower trophic levels via predator-prey relationships.

To understand the role of fish and macrobenthos in this model, especially for long term variations in the ecosystem, we will perform a 68 year long (1948-2015) hindcast simulations for the coupled North Sea and Baltic Sea ecosystem and analyse and discuss the relevance of the implemented higher trophic levels for long term changes in lower trophic level productivity and nutrient availability in the system. The long-term model integration together with sensitivity studies on critical model parameters and assumptions, e.g. the model closure through fisheries and apex predators, entails conclusions on the development needs for consistent functional type “End-to-End” modelling approaches.

Eggert, A., Friedland, R., Graiff, A., Karsten, U., Wahl, M., Bartsch, I.:
Modelling nearshore coastal ecosystems requires implementation of benthic macroalgae

Benthic macroalgae supply the majority of biomass to nearshore ecosystems of rocky and stony coasts. Macroalgae communities provide three-dimensional habitats and many ecological niches, contributing essentially to structural heterogeneity, food and shelter for associated marine flora and fauna. From a biogeochemical perspective, macroalgae are the dominant primary producers in the coastal zone and export about 43% of their production as particulate and dissolved organic carbon, both through local burial in coastal sediments and offshore transport over wide geographical scales. Thus, macroalgae impact benthic-pelagic coupling of nutrients, can increase water clarity but also buffer against coastal erosion through sediment trapping and damping of wave energy. This (incomplete) list of ecosystem services shows that macroalgae communities need to be implemented in nearshore ecosystem models - what has become possible! We have vast knowledge about the physiology and ecology of the key species, detailed information about the spatial distribution of macroalgae and a first boxmodel simulating the seasonal growth of a Baltic Fucus vesiculosus community.


Fatemeh Chegini, Peter Holtermann, Markus Kreus, Onur Kerimoglu, Christian Winter, Ulf Gräwe, Knut Klingbeil, Carsten Lemmen, Frank Kösters, Kai Wirtz, Hans Burchard:
Stratification and destratification processes in a high resolution (300 m) model of the German Bight

A high resolution ocean model of the German Bight is setup to study the dynamics of stratification and destratification processes. Observations in summer 2016 of vertical density structure in two nearby stations south of Helgoland, demonstrate different dynamics, with one station being vertically mixed and another showing periods of stratification. To investigate the processes leading to this different behavior, a model based on the General Estuarine Transport Model (GETM), with a horizontal resolution of 300 m will be used. The initial and boundary conditions are derived from a larger scale 600 m resolution model of the Southern North Sea. In this presentation, details of the high resolution model setup are discussed, including the model performance in comparison to observations. The model ability to predict the stratification and destratification dynamics in addition to preliminary analysis of related processes is presented.

The model is developed as part of the Modular System for Shelfs and Coasts (MOSSCO) and can be easily extended by sediment and biogeochemistry model components. In this framework, the effect of mesoscale physical features on observed variability in biology and sediment transport will be studied for the estuarine-offshore transition zone, which is in particular subject to extreme storm and flood events such as the 2013 Elbe flood.

Goodliff, M., Lorkowski, I., Bruening, T., Schwichtenberg, F., Nerger, L.:
Temperature Assimilation onto a Pre-Operational Coastal Ocean-Biogeochemical Model: Assessment of Weakly and Strongly Coupled Data Assimilation

The effect of the assimilation of satellite sea surface temperature onto the forecast quality of the coastal ocean-biogeochemical model HBM-ERGOM in the North- and Baltic Seas is studied. The HBM-ERGOM model is currently used pre-operationally, without data assimilation, by the Germany Federal Maritime and Hydrographic Agency (BSH). The model is configured with nested grids with a resolution of 5km in the North and Baltic Seas and a resolution of 900m in the German coastal waters. The biogeochemical model ERGOM contains three phytoplankton groups (Cyanobacteria, Flagellates, Diatoms), two zooplankton size groups, four nutrient groups (nitrate, ammonium, phosphate and silicate), two detritus groups (N-Detritus and Si-Detritus) and oxygen to simulate the biogeochemical cycling in the coastal seas.

To improve the predictions of the HBM-ERGOM model, data assimilation was added by coupling the model to the parallel data assimilation framework (PDAF, http://pdaf.awi.de). The ensemble-based error-subspace transform Kalman filter (ESTKF) is applied for the data assimilation.

As a first step to improve the biogeochemical forecasts the impact of assimilating satellite sea surface temperature data is assessed. Two cases are considered. First, the impact of weakly coupled data assimilation. In this case, the assimilation of temperature only directly influences the physical model variables in the correction step of the assimilation while the biogeochemical fields react dynamically to the changed physical model state during the subsequent ensemble forecasts using the coupled model. The second case is the strongly-coupled data assimilation in which next to the physical model fields also the biogeochemical fields are directly updated in the analysis step through the multivariate covariances estimated by the joined physical-biogeochemical ensemble of model states. Here, it is assessed whether these covariances are sufficiently well estimated to result in an improvement of the biogeochemical fields.

Hinrichs, C., Flagg, C.N., Wilson, R.E., Flood, R.:
Hydrodynamic modelling of multi-inlet-system with FVCOM, Small inlet – big effect!

Great South Bay is the central part of a shallow multi-inlet-lagoon system along the south shore of Long Island. In 2012, Superstorm Sandy caused a breach in the barrier island leading to a new inlet. A high-resolution coastal ocean model is applied to quantify the changes in circulation, flushing time and exposure time in the interior of Great South Bay. Simulations show that even though the new inlet produced only a very small change in tidal range in the lagoon, it did produce a marked change in the seasonal residual transport patterns characterized by a through-flow in central bay which is linked to Stokes transport within the new inlet. This reveals a potentially important mechanism by which the new inlet interacts with the other inlets. Changes in flushing time of central Great South Bay associated with the modified transport patterns are evaluated using an Eulerian passive tracer technique. Results show that the new inlet produced a significant decrease in the flushing time (approximately 35% reduction under summer wind conditions and 20% reduction under winter wind conditions).

Hofmeister, R., Zhang, W.:
Coastal ocean vs. Sea floor in coastal biogeochemical models

Biogeochemical modelling in shallow coastal seas requires to address the interplay of processes in the sea floor with the processes in the water column. This interplay gets more important with increasing horizontal resolution in the model applications, which resolves larger variability of water depths at the coasts as well as potentially larger areas of shallow water depths. Models of the coastal ocean are extended traditionally with some treatment of processes at or in the sea floor. However, the computational load, the model complexity, and the budget of nutrients is easily higher for the sea floor compared to the water body in shallow water. The results of a biogeochemical model in a coastal environment are presented for different complexities of the resolution and coupling, where coastal ocean and sea floor are resolved. The differences between the configurations show, that the benthic-pelagic coupling is a key feature of coastal biogeochemical models, which requires refined model architecture as well as detailed observations for further calibration. 

Jacob, B, Stanev, E.:
Interactions between wind and tidally induced currents in coastal and shelf basins

The impact of atmospheric variability on ocean circulation in tidal and non-tidal basins is adressed using a  unstructured-grid numerical model (SCHISM). The model resolves the dynamics in the coastal area, as well as in the straits connecting the North Sea and Baltic Sea. The model response to atmospheric forcing in different frequency intervals is quantified. The results demonstrate that the effects of the two mechanical drivers, tides and wind, are not additive, yet non-linear interactions play an important role. There is a tendency for tidally and wind-driven circulations to be coupled, in particular in the coastal areas and straits. High-frequency atmospheric variability tends to amplify the mean circulation and modify the exchange between the North and the Baltic Sea. The ocean response to different frequency ranges in the wind forcing is area-selective depending on specific local dynamics. The work done by wind on the oceanic circulation depends strongly upon whether the regional circulation is tidally or predominantly wind-driven. It has been demonstrated that the atmospheric variability affects the spring-neap variability very strongly.

Klingbeil, K., Lemarié, F., Debreu, L., Burchard, H.:
The numerics of hydrostatic structured-grid coastal ocean models: state of the art and future perspectives

The state of the art of the numerics of hydrostatic structured-grid coastal ocean models is reviewed here. First, some fundamental differences in the hydrodynamics of the coastal ocean, such as the large surface elevation variation compared to the mean water depth, are contrasted against large scale ocean dynamics. Then the hydrodynamic equations as they are used in coastal ocean models as well as in large scale ocean models are presented, including parameterisations for turbulent transports. As steps towards discretisation, coordinate transformations and spatial discretisations based on a finite-volume approach are discussed with focus on the specific requirements for coastal ocean models. As in large scale ocean models, splitting of internal and external modes is essential also for coastal ocean models, but specific care is needed when drying & flooding of intertidal flats is included. As one obvious characteristic of coastal ocean models, open boundaries occur and need to be treated in a way that correct model forcing from outside is transmitted to the model domain without reflecting waves from the inside. Here, also new developments in two-way nesting are presented. Single processes such as internal inertia-gravity waves, advection and turbulence closure models are discussed with focus on the coastal scales. Some overview on existing hydrostatic structured-grid coastal ocean models is given, including their extensions towards non-hydrostatic models. Finally, an outlook on future perspectives is made.


Lang, A., Mikolajewicz, U.:
Long term variability of extreme storm surges in the German Bight

Inundation due to storm floods bears a high damage potential for low-lying coastal environments such as the German Bight, where tide gauge observations show a marked variability on multiple timescales. While the mechanisms for shorter term variability at specific locations have been studied quite extensively, the variability on decadal and longer scales and their associated large scale climatic drivers has received little attention.
As observational records are not sufficiently long to derive a statistical relation between the aforementioned, we use here the regionally coupled atmosphere-ocean model REMO-MPIOM to simulate climate variations of the last millennium. Aside from conventional coupled Global Climate Models this model configuration includes tides and has a high enough resolution to realistically represent shelf sea processes and storm surges, while it still maintains a global domain and thus allows for the continuous propagation of climate signals.
The resulting 1000 years of model data reveal that extreme storm surges exhibit variability on timescales from years to centuries. The respective modes of variability are different to those of mean sea level, suggesting distinct large-scale forcing mechanisms: Climate conditions associated with high winter mean sea level comprise a SLP pattern resembling the positive phase of the NAO at multidecadal timescales and North Sea sea surface temperature variations on longer timescales up to centuries. Climate conditions favoring extreme storm surge activity are less pronounced and a direct link to the NAO is not evident, as a high variability over the Eastern North Atlantic masks clearer signals.

Lange, Xaver, Burchard, Hans:
Wind-induced preconditioning of river plumes in non-tidal estuaries

In the absence of tides wind straining becomes a key mechanism in estuarine and river plume dynamics. Previous studies have focused on along-shore winds when studying plume formation in terms of upwelling/downwelling events but little is known about the influence of cross-shore wind straining on the generation of a buoyant plume when preconditioned by wind-driven estuarine circulation. Here the response of a non-tidal river plume to along-estuary wind forcing is studied using the example of the Warnow river, the second largest German river entering the Baltic Sea. In order to estimate the influence of wind stress on thickness, hydrographic properties, mixing and volume transport of the plume we used a well-validated, realistic and highly-resolved 3D coastal ocean model (down to 20m) in combination with Eulerian and Lagrangian tracers. Main consequences of up-estuary wind conditions are the amplification of the near-surface seaward volume flux at the mouth of the estuary and an increase of plume area in the near-field. In contrast, down-estuary winds are even able to prohibit the generation of a plume by inverting the direction of density-driven estuarine circulation, when strong enough. The latter is found if the local Wedderburn number (ratio of non- dimensional wind stress to non-dimensional horizontal density gradient) is larger than 0.37.

Lorenz, M., Burchard, H.:
Numerical study of the overturning circulation of the Persian Gulf

In this study a model simulation for the Persian Gulf using the coastal ocean model GETM has been used to investigate the seasonal overturning circulation of the Persian Gulf. The Persian Gulf can be described by an inverse estuarine circulation with an outflow of saline thus dense water at the bottom and an inflow of less dense water at the surface. The model has been forced by ERA-Interim at the ocean-atmosphere interface and by HYCOM at the open boundary. The results below are derived from one simulation year and compare well to previous studies.

The seasonal cycle of the overturning circulation of the Persian Gulf is well reproduced. In spring when the heat flux is positive the inflowing Indian Ocean Surface Water (IOSW) reaches farther west due to a shallow thermocline restricting inflows to the upper 10-20 m. Because of persistent northwesterlies the IOSW forms a cyclonic circulation in the central and northern Gulf. In summer this circulation becomes strongest since the stratification reaches its maximum. In fall, when the heat flux decreases and evaporation increases, vertical mixing creates a deep mixed surface layer which weakens the spreading of IOSW. In addition, the cyclonic eddies dissolve into smaller eddies which dissipate in late fall/early winter. In winter almost the whole Persian Gulf is vertically homogeneously mixed and no significant surface circulation is found.

Besides the circulation the formation of the dense Persian Gulf Water (PGW) has been investigated. The densest water in the Persian Gulf can be found in winter around Bahrain, but does only contribute little to the PGW due to mixing. The PGW is mainly created in the northern and southern shallows during winter months. The dense water of the northern region sinks into the deep channel of the Gulf and moves to the Strait of Hormuz. The saline water of the southern shallows is too warm in summer and stratifies over the denser water in the channel and becomes part of a near surface recirculation. In winter the southern shallows are responsible for the high salinities of the PGW.

The transports through the Strait of Hormuz have been analysed using Total Exchange Flow theory (MacCready, 2011). The exchange flow shows a seasonal cycle which closely follows the seasonal cycle of the circulation. The annual mean quantities for the exchange flow are: Q in,year = 0.20 ± 0.02 Sv, Q out,year = −0.19 ± 0.02 Sv, s in,year = 37.02 g/kg and s out,year = 38.86 g/kg. The highest transport rates are found in summer and the lowest in fall.


Massmann, S.:
Steps towards testing of Lagrangian drift models in the North and Baltic Sea

Lagrangian models for oil, object and tracer drift forecasts in the ocean deliver important information for maritime emergency management and search & rescue activities. Their performance depends on the quality of the used hydrodynamic and atmospheric models and their representation of the processes in the ocean-atmosphere interface. The scope of this study is to establish a benchmark for the drift model Seatrack Web based on drifter observations. In May, June and July 2015 several drifters were deployed in the German Bight. To generate an ensemble of drift forecasts the drifter observations are interpolated to model times and 25h drift forecasts are started every 3 hours from these pseudo observations. Then forecast distance respectively velocity errors with respect to the original observations and typical error norms like the root mean square error and the scatter index are computed. For the forcing fields used operationally at BSH at that time – BSHcmod NOKU and COSMO-EU -  the following three main results are deduced: 1.) Within the first 24h with a probability of more than 80% the predicted location is less than 5 nm away from the observed one. 2.) The drift velocities are slightly underestimated. 3.) There is a tendency to find the object to the left of the line between start and simulated position. For future changes in the drift model or forcing fields we recommend to test the setup with the proposed benchmark. In order to give a more general performance for North and Baltic Sea it is indispensable to collect more validated drift observations in other regions and for different met-ocean conditions.

Pleskachevsky, A, Wiehle, S., Jacobsen, S, Schwarz, E., Daedelow, H., Krause, D.:
Supporting maritime forecast with Sentinel-1 satellite Near Real Time services: observations of wind, sea sate and coastal processes

Innovative algorithms for Sentinel-1 (S1) satellites allow daily observations of meteo-marine parameters, tracking of storm propagation, study of local sea state variability and coastal processes. In order to investigate geophysical processes, the sea state and wind fields estimated simultaneously from S1 scenes acquired twice daily are combined with numerical forecast model results and in-situ measurements. The focus of the investigations are the evolution and propagation of storm peaks/centres, storm front movements, and arrival of swell.

An example of efficient storm tracking in the Black Sea in April 2017 over three days is analysed. The HZG forecast spectral wave model running for the Black Sea reproduces the storm peak propagation near to the S1 observations. In detail, the storm peak observed by S1 is shifted ~80km towards the south in comparison to the model simulations. During this storm, the ship “Geroi Arsenala” of river-sea class licensed for inland waterways with access to the coastal seas was capsized about 40 kilometres to the south of the Kerch Strait in the open Black Sea, according to the associated press. The cargo ship was carrying grain from Russia to Turkey. Obviously, the course was taken too far from the coast to shorten the way across the sea with unexpected high sea state. Only one of the 12 people aboard was rescued. This is a tragic incident that proves the importance of Maritime Safety and Security. With the Sentinel satellites and the processing framework demonstrated in this work, we have appropriate tools to raise Maritime Situation Awareness (MSA) to unprecedented levels, which helps avoiding such accidents.

The rapid development of satellite techniques, information extraction algorithms and ground infrastructures during the last years enabled a series of oceanographic applications with near-real-time (NRT) capabilities. Several minutes after acquisition, the produced data with geo-coded information on wind speed and wave height can be transferred to the weather services for validation of the forecasting models. The different kind of data like coastline, wave height, surface wind speed, ice coverage, oil spills etc. can be processed in parallel for the same image and combined with other information (e.g. model results, ship traffic) for supporting MSA. The algorithms currently developed for this purpose are integrated into a prototype processor for Sentinel-1 imagery. The DLR Ground Station Neustrelitz applies this prototype as part of a near real-time demonstrator MSA service. The presented scientific service involves daily provision of surface wind and sea state parameters estimated fully automatically from S1 Wide Swath Mode (IW) Synthetic Aperture Radar (SAR) images of North and Baltic Sea. S1 IW covers area-strips of thousand kilometres of earth and ocean surface with a resolution of ~10m by sequences of multiple individual IW images with an approximate size of 200km×250km. The data are free to use and provide unprecedented observation possibilities of ocean processes and natural phenomena worldwide at a high repetition rate. Due to the independence of sunlight and cloud coverage, SAR data are an indispensable source of 2D information of the ocean surface for open sea and for coastal applications.

A series of storm cases for the North Sea and the Baltic Sea depicts the importance of remote sensing information for forecast modelling. Especially in cases of winds with strong gust cells, the corresponding clusters of increased waves can be observed. This additional information can be also provided parallel to forecast results for MSA users.

Ricker, M., Staneva, S., Krüger, O., Behrens, A., Carasco, R., Breivik, Ø., Bidlot, J., Schrum, C., Stanev, E. V.:
Particle transport model sensitivity on wave-induced processes in the coupled model system

Different effects of wind waves on the hydrodynamics in the North Sea and Baltic Sea are investigated using a coupled wave (WAM) and circulation (NEMO) model system as part of the Geesthacht Coupled cOAstal model SysTem GCOAST. The terms accounting for the wave-current interaction are: the Stokes-Coriolis force, the sea-state dependent momentum and energy flux. The role of the different wave parametrizations are investigated using a particle-drift model. Those particles can be considered as simple representations of either litter, oil fractions, or fish larvae. In the ocean circulation models the momentum flux from the atmosphere, which is related to the wind speed, is passed directly to the ocean and this is controlled by the drag coefficient. However, in the real ocean, the waves play also the role of a reservoir for momentum and energy because different amounts of the momentum flux from the atmosphere are taken up by the waves. In the coupled model system, the momentum transferred into the ocean model is estimated as the fraction of the total flux that goes directly to the currents plus the momentum lost from wave dissipation. Additionally, we demonstrate that the wave-induced Stokes-Coriolis force leads to a deflection of the current. During the extreme events the Stokes velocity is comparable in magnitude to the current velocity. So the resulting wave-induced drift is crucial for the transport of particles in the upper ocean. The performed sensitivity analyses demonstrate that the model skill depends on the chosen processes.
The using of a coupled model system reveals that the newly introduced wave effects are important for the drift-model performance, especially during extremes. Those effects cannot be neglected by litter, search and rescue, oil-spill, transport of biological material, or larva drift modelling.


Schakau, V., Lettmann, K. A., Wolff, J.-O., Feudel, U.:
Hydrodynamic drivers of Vibrio spp. dynamics and associated health risks in the German Bight

In recentyears, massive mortalities of oysters are frequently reported and have been associated with bacteria of the genus Vibrio. Some of these bacteria are also potentially pathogenic for humans and can causeharmful infections either by direct contact with seawater or by consumption of contaminated shellfish. Sincepopulation dynamics of these bacteria are greatly influenced by environmental factors, such as temperature and salinity, it is anticipated that under global warming conditions, the risk of the occurrence of (human-) pathogenic bacteriain summer seasons will further increase.

Here, we introduce a model based on ordinary differential equations to study the population dynamics of Vibrio spp. in the water column.Coupling this biological model to a hydrodynamic model of the Southern North Sea, based on the Regional Ocean Modeling System (ROMS),we investigate the temporal and spatial distribution of pathogens along the German North Sea coast. Moreover, we identify hot spots of growth and pathways of transport and finally predict the impact of environmental scenarios on distribution patterns and Vibrio spp. concentrations.

In a first study, this modeling system has been applied to a summer season in 2016. Our simulations show that the spatial distribution of Vibrio spp. in the German North Sea correlates with salinity, while the local temporal patterns correlate with temperature. However, results indicate that transport of Vibrio spp. by coastal river plumes is a crucial driver that favors the presence of Vibrio spp. outside their ecological niche. These spots also include locations of oyster banks and recreational beaches. Finally, we used a published dose-response model to calculate the illnessrisk per swimming event in recreational waters along the German North Sea coast.

Schwichtenberg, F., Goodliff, M., Lorkowski, I., Matthias, V., Nerger, L., Neumann, D., Neumann, T.:
MeRamo – An assimilative biogeochemical model system for the support of public authorities dealing with the Marine Strategy Framework Directive

The European Marine Strategy Framework Directive (MSFD) calls for the establishment of a Good Environmental Status (GES) of the marine environment until 2020. In the current reporting, mostly in-Situ data is used to determine the status of the marine environment. Although close to the real status, in-Situ data are only point measurements and are sparse in time and space when looking on a regional scale. The MeRamo project aims at supporting the public authorities with results and products from an assimilative hydrodynamical-biogeochemical model system (HBM-ERGOM) for the North and Baltic Sea. Thus, a high quality data set can be generated, which is consistent in time and space.

A central part of the project is the implementation of a data assimilation component, which can handle remote sensing data from the European Copernicus initiative. The subsequent assimilation of the diffuse vertical attenuation coefficient (KPAR) enables a more realistic calculation of the vertical light penetration and hence an improved calculation of primary production. Furthermore, a nutrient tagging routine is implemented, which is able to track the fate of specific nutrients depending on its source. Another focus is placed on the effect of shipping emissions on the marine ecosystem by integrating data of an atmospheric chemistry transport model (CMAQ).

As the final step, the model output is transformed into indicators which can be directly used for reporting. Additional forecasting of bottom oxygen and nitrogen from designated sources can support monitoring campaigns. This allows direct usage of different data sets via the operational model system optimized for the reporting for the MSFD.

Shamil Yakubov, Philip Wallhead, Elizaveta Protsenko, Evgeniy Yakushev:
Coupled 1D simulation of ice, water column, and sediment biogeochemistry

Aquatic biogeochemical processes can strongly interact, especially in polar regions, with processes occurring in adjacent ice and sediment layers, yet there are few modelling tools to simulate these systems in a fully coupled manner.  We developed a 1-Dimensional Ice-Pelagic-Benthic transport model (IPBM) for coupled simulation of ice, water column, and upper sediment biogeochemistry. IPBM describes the processes of diffusion and particle sinking in both ice and water, as well sedimentation and bioturbation processes in the sediments.  To describe ice, pelagic, and benthic biogeochemical dynamics (reaction terms), IPBM was partly coupled to the European Regional Seas Ecosystem Model (ERSEM) and partly to the Bottom RedOx Model biogeochemistry module (BROM-biogeochemistry) using the Framework for Aquatic Biogeochemical Models (FABM).

Stanev, E.V., Pein, J., Grashorn, S., Zhang, Y., Schrum, C.:
Dynamics of the Baltic Sea straits. Numerical simulation of exchange flows

The Cross-scale Hydroscience Integrated System Model (SCHISM) which uses unstructured grids is set up for the area of North Sea and Baltic Sea. With a resolution of ~100 m in the narrow straits connecting the two basins, this model resolves accurately the inter-basin exchange. Validation against observations in the straits shows a good skill of the model in simulating the transport and vertical profiles of temperature, salinity and currents. The timing and magnitude of the major inflow in 2014-2015 is also realistically simulated. The analysis is focused on the two-layer exchange, its dependence on the atmospheric forcing, and dominant physical balances. The two-layer flows in the three connecting straits show different dependencies upon the net transport, and the spatial variability of this dependence is also quite pronounced. It was shown that the three straits’ system developed specific dynamics, with time lags and differences between currents in the individual straits during inflow and outflow conditions. Analysis on the impact of resolution indicates that the performance of model changes depending on whether the narrow parts of the straits are resolved with fine resolution of 500 m or with ultra-fine resolution of 100 m. With this ultra-fine resolution gravity flows and variability of salinity in deep layers is more adequately simulated. The paper identifies the needs for more profound analysis of the coupled dynamics of Baltic and North Sea with a focus on the Danish straits.

Wiehle, S., Pleskachevsky, A., Jacobsen, S.:
Coastal modelling topography data by waterline retrieval from Sentinel-1 satellite acquisitions: from method development to a Near Real Time service

The automatic daily processing of coastlines from Sentinel-1 satellite data for updating the bathymetry to support modelling as part of a Near Real Time chain is being developed. In shallow waters, the physical processes caused by the interaction between waves, currents and the sea bottom become important and are crucial for modelling, requiring current and accurate bathymetry data. The spatial properties of sea state and currents change strongly in coastal areas and especially in the Wadden Sea at the Danish, German and Dutch North Sea coast with large areas of intertidal flats. The morphodynamics of seabed structures are significant in such littoral zones; the soft seabed can change within days during severe storms. Data from past measurement campaigns, which are very expensive using ship soundings and airborne LIDAR scanning, deprecates quickly as the positions of islands, sandbanks or tidal inlets change.

Acquisitions from the Sentinel-1 (S1) Synthetic Aperture Radar (SAR) satellites cover most coasts worldwide. With two satellites currently in orbit, the acquisition interval is at most 6 days at the equator and almost daily in our latitudes. The Interferometric Wide Swath (IW) mode covers lands and coastal waters with a swath width of 250km and an image resolution of 10m. In contrast to optical satellites, SAR acquisitions are independent of illumination and cloud cover; hence, they deliver new images very reliably. The DLR satellite ground station in Neustrelitz receives these data and processes and delivers them in Near Real Time (NRT), usually within 20 minutes after acquisition.

We have developed automated algorithms to retrieve the waterline from SAR images. In the tidal flat areas of the Wadden Sea, analyzing time series of acquisitions at similar tidal states allows estimating rates of change and sediment transport; a combination of several images acquired within a short time frame at different tidal states allows estimating the topography. Our algorithms for Sentinel-1 SAR images offer a quick and cheap way to spot ongoing processes, verify modelling results and identify regions of major change for future measurement campaigns.