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Thesis symposium 1 July 2020

13:20-14:00 Thesis talk Michelle Bindels

The impact of sea level rise on tides, waves and tidal sand ridges in the North Sea

Tidal sand ridges are large-scale bedforms that occur on continental shelves with strong tidal currents and an abundance of sand. The length of these features is in the order of several kilometres and the height in the order of tens of meters. They evolve on decadal and centennial time scales. As yet, it is not fully understood how they respond to the rise in sea level caused by climate change. Especially when both the tide and the waves are also affected by the rising sea level.
This study uses an available numerical morphodynamic model (Delft3D) to investigate the effects of sea level rise (SLR) on tides, waves and tidal sand ridges in a coastal area that mimics the Belgian shelf. Different scenarios for SLR were modelled, using the current SLR rate and different SLR rates from IPCC projections. The range in SLR rates is between 2 and 11 mm/yr.
The results presented will show how tides in the North Sea respond to future SLR, as well as how the wave climate changes due to SLR, assuming that the wind climate remains unchanged. Finally, the evolution of tidal sand ridges will be studied for the different scenarios of SLR. It will be demonstrated that SLR has a profound influence on these ridges.

 

14:00-14:40 Thesis talk Reint Fischer

Lagrangian Analysis of small-scale reef organism networks

Changing global ocean conditions stress many tropical marine ecosystems and coral reefs. Affected reefs in turn change the local ocean conditions in many ways. Local changes in bottom friction, nutrient availability, oxygen consumption and spreading of spawn and diseases determine the transient response of these ecosystems to changing global conditions. After 40 years of monitoring the benthic community changes at specific sites of the fringing reef off the coast of Curaçao by NIOZ researchers, more detailed models of the complex turbulent flow over the reef seafloor are starting to be developed. This study aims to develop Lagrangian tools to analyze the small-scale advection-based processes and the network of benthic organisms, which enable studying the local mechanisms driving past and future ecosystem changes.

Using Parcels virtual particles are released and tracked in model output of a Direct Numerical Simulation of the unsteady turbulent flow over a reef seafloor. This study presents a novel approach to defining the boundary conditions for particles in an A-grid, important for an accurate representation of the trajectories around complex solid objects. Subgrid-scale repelling of particles away from the solid walls is achieved by separating the normal and tangential velocity components relative to the nearest wall.

The decrease of coral organisms observed in the Curaçao reef is shown to increase entrainment of water higher up in the water column by turbulent eddies that become larger as the spacing between large coral organisms increases. By defining a network of sedentary organisms based on the advection of water between them, the local exchange of metabolites and other particles can be studied in more detail.

 

15:00-15:40 Thesis talk Deborah Bassatto

Bayesian inference of plastic sources by back-tracking particles in the Black Sea

The aim of this project was to establish a new tool to help understanding the pathways of plastic particles in the ocean, by using Bayesian inference to back-track their most likely source. In fact, because of the diffusion effects, running a simulation backward in time to find these particles origins is not precise enough. Instead, the Bayes’ theorem is applied such as:

P(A\B) = P(BA)P(A)
                       P(B)

Or

P(Source\Sampled location) = P(Sampled Location\Source)P(Source)
                                                                     P(Sampled Location)

With the P(source) found in the literature and the P(sampled location|source) found by running simulations. This project mainly focused on the Black Sea for different reasons : I personally worked on sampled plastic particles from it and because there is only a few documentation on this region concerning plastic pollution, even if it might be one of the most polluted basin in the world. Hence, this ‘lack’ of information is used to demonstrate how easily adaptable this method to infer plastic source is. The results of this work are probability maps, on which one can see the probability that plastic particle comes from a specific source at each point of the basin. Around this main goal, a couple ‘side-projects’ have been done, such as, for instance, assessing the quantity and on which coasts plastics strand.

 

15:40-16:20 Thesis talk Raf Antwerpen

The role of bare ice extent and albedo variability on runoff from the Greenland ice sheet

Mass loss from the Greenland ice sheet (GrIS) has accelerated over the past decades, mainly due to enhanced surface melting and liquid water runoff in response to atmospheric warming. The majority of runoff from the GrIS originates from bare ice exposure in the ablation zone. Since surface albedo plays a critical role in modulating surface melt, it is imperative to understand the processes governing albedo variability to accurately project future mass loss from the GrIS. Bare ice albedo is spatio-temporally variable and contingent on non-linear feedbacks and presence of surface-darkening impurities. Due to lack of understanding of these processes, the full extent of bare ice albedo variability can not yet be represented properly in climate models. In this study, I compare regional climate model output with satellite imagery, focusing on bare ice extent and albedo. The findings suggest that bare ice albedo is overestimated by the climate model by ~0.05 on average during 2000-2018, with respect to the observed bare ice albedo. This bare ice albedo bias results in an underestimation of total runoff over the entire GrIS of 128 Gton during 2000-2018, or ~7 Gton/year.

 

16:20-17:00 Thesis talk Simon Brouwer

Implementing and improving a meltwater plume parametrisation for defining sub-shelf melt rates

Inside this thesis we look at ways to better describe sub-shelf melt inside an ice model. To explore this topic we used IMAU-ICE, an hybrid SIA+SSA ice sheet + shelf model, and an ice melt plume parametrisation done by Lazeroms. This plume parametrisation tries to explain sub-shelf melt not only as a function of ambient water temperatures but also as a function of ice shelf geometry. The original plume model and Lazeroms´ parametrisation is designed for a 1D plume which travels along a section of shelf, some work has been done to map 2D geometries to this 1D model and thereby acquiring subshelf melt rates, however, little work has yet been done into implementing this melt scheme inside a full model where we have dynamic feedback between geometry and melt.

So the main goal and aim we set for ourselves in this thesis was to try and implement this melt parametrisation and improving upon the mapping the 2D shelf geometry to input data for the 1D melt model. To do this we used a simple test geometry where sub-shelf melt can have pronounced effect on shelf dynamics and throughout our tests with different mapping schemes we have shown that we can indeed improve on the quality of melt fields by improving the already existing mapping procedures.

 

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