1.
Cai, Yiting, 1994
(författare)
The impact of loading-induced stress variation on the seismicity of active faults
2023
Licentiatavhandling (övrigt vetenskapligt/konstnärligt) abstract
Surface loads such as the ocean, atmosphere, and continental water, constantly modify the stress field of the Earth’s crust. Most earthquakes occur at tectonic plate boundaries and such stress perturbations on the active faults at the plate boundaries may trigger earthquakes. Several previous studies reported that tides or hydrological loading could modulate seismicity in some areas. We elaborate on this idea and further investigate the accumulative effect of various loadings. In this work, we compute the total Coulomb stress change created by hydrological loading, atmospheric loading, and non-tidal ocean loading from 2011 to 2016 in the Kamchatka-Kuril Islands-Japan region, and then compare it to the background earthquakes in the same period. This thesis contributes to our understanding of the complex interactions between surface loading and induced seismicity, which has important implications for earthquake hazard assessment and risk mitigation.
2.
Nylund, Amanda, 1989
(författare)
Turbulent ship wakes and their spatiotemporal extent
2021
Licentiatavhandling (övrigt vetenskapligt/konstnärligt) abstract
Shipping activities occur in almost every part of the global oceans and in intensely trafficked shipping lanes there can be up to one ship passage every ten minutes. All these ships impact the marine environment in different ways through pollution or physical disturbance. This thesis is focused on the turbulent ship wake, a physical disturbance from ships and previously overlooked as an environmental impact. When a ship moves through water, the turbulence induced by the propeller and hull, will create a turbulent wake that remains and expands after the ship passage. The turbulence in the wake will govern the spread of contaminants and affect gas exchange in the wake water, physically perturb plankton, and potentially impact local biogeochemistry through increased vertical mixing. To be able to assess the environmental impact of ship-induced turbulence in areas with intense ship traffic, knowledge of the spatiotemporal extent and development of the turbulent wake is necessary. The aim of this thesis is to increase that knowledge, by conducting in situ observations of turbulent ship wakes, which can be used to estimate the spatiotemporal extent of the turbulent wake. By using a collection of methods, the thesis work has resulted in a first estimate of the spatiotemporal extent of the turbulent ship wake, based on more than 200 field observations of different real-size ships in natural conditions. The observed turbulent wakes showed large variation in their spatiotemporal extent, and further studies are needed to fully disentangle how environmental conditions and vessel specifications affect the intensity and extent of the turbulent wake. The results and experiences gained from the in situ observations, give an indication of the complexity entailed in characterising the development of the turbulent wake, and provide valuable input regarding the relevant parameters and spatiotemporal scales to include in future studies. The work of this thesis constitutes the first step in addressing the knowledge gap regarding the environmental impact of ship-induced turbulence and can be used as a road map for further studies within the field.
3.
Elyouncha, Anis, 1978
(författare)
Sea Surface Current Measurements Using Along-Track Interferometric SAR
2018
Licentiatavhandling (övrigt vetenskapligt/konstnärligt) abstract
Ocean currents affect the weather, the climate and the marine ecosystem. Observing ocean currents is important for understanding the upper-ocean layer dynamics and its interaction with the other components of the climate system. In-situ measurements are sparse and their deployment and maintenance is costly. Satellite remote sensing with large spatial coverage offers a good complement to the in-situ observations. In this work we have studied the spaceborne Along-Track Interferometric SAR (ATI-SAR) for measuring sea surface currents. The measurement principle is based on the fact that the phase difference between two SAR acquisitions is directly related to radial (line-of-sight) velocity of the illuminated surface. Previous studies based on similar systems were carried out in areas with well defined and strong tidal currents ( 1 -3 m /s). In this work we demonstrate the capability of ATI-SAR, through several study cases, in areas with weak currents ( <0.5 m /s). This is challenging for the satellite measurements of surface currents because it requires very accurate processing and retrieval algorithms. In addition, it has been found that wave motion contribution, systematically dominates the measured ATI-SAR radial velocity in these weak current areas. Estimation of the wave motion contribution relies on high-resolution and accurate wind data. Thus, a wind speed retrieval algorithm from SAR is needed to support the ATI-SAR current retrieval. We have shown that with an appropriate processing of the ATI-SAR phase and with applying the necessary corrections to the measured velocity a good agreement with ocean circulation models is achieved (rmse = 0.1 m/s). These corrections include phase calibration and wind compensation to correct for instrument and geophysical systematic errors, respectively. Finally, a novel method for removing the wind direction ambiguity, based on the ATI-SAR phase, is presented. In previous methods, the wind ambiguity removal was based on external information, e.g. an atmospheric model or on visual observation of wind shadows.
4.
Strandberg, Joakim, 1991
(författare)
Inverse modelling of GNSS multipath signals
2017
Licentiatavhandling (övrigt vetenskapligt/konstnärligt) abstract
Measuring the world around us is necessary to observe and understand the changes that occur in our environment. A widely distributed network of measurement stations can help us to understand ongoing and predict future climate change. GNSS reflectometry has the capacity of providing data from all over the world, as there are already many GNSS stations established and operated for navigational and meteorological purposes. This thesis presents a new way of retrieving environmental data from GNSS signal-to-noise ratio measurements which has the capability to provide new types of measurements. The method is based on inverse modelling of the signal-to-noise ratio in order to retrieve physical parameters of reflecting surfaces around GNSS installations. It is successfully demonstrated that the method improves the precision of the GNSS reflectometry derived sea surface height measurements significantly. By using the signal-to-noise ratio pattern, it is also — for the first time — demonstrated that it is possible to use GNSS reflectometry to detect coastal sea ice.
5.
Wu, Lichuan
(författare)
Impact of surface gravity waves on air-sea fluxes and upper-ocean mixing
2016
Licentiatavhandling (övrigt vetenskapligt/konstnärligt) abstract
Surface gravity waves play a vital role in the air-sea interaction. They can alter the turbulence ofthe bottom atmospheric layer as well as the upper-ocean layer. Accordingly, they can affect themomentum flux, heat fluxes, as well as the upper-ocean mixing. In most numerical models, waveinfluences are not considered or not fully considered. The wave influences on the atmosphereand the ocean are important for weather forecasts and climate studies. Here, different aspects ofwave impact on the atmosphere and the ocean are introduced into numerical models.In the first study, a wave-state-dependent sea spray generation function and Charnock co-efficient were applied to a wind stress parameterization under high wind speeds. The newlyproposed wind stress parameterization and a sea spray influenced heat flux parameterizationwere applied to an atmosphere-wave coupled model to study their influence on the simulationof mid-latitude storms. The new wind stress parameterization reduces wind speed simulationerror during high wind speed ranges and intensifies the storms. Adding the sea spray impacton heat fluxes improves the model performance concerning the air temperature. Adding the seaspray impact both on the wind stress and heat fluxes results in best model performance in allexperiments for wind speed, and air temperature.In the second study, the influence of surface waves on upper-ocean mixing was parameter-ized into a 1D k − ε ocean turbulence model though four processes (wave breaking, Stokes driftinteraction with the Coriolis force, Langmuir circulation, and stirring by non-breaking waves)based mainly on existing investigations. Considering all the effects of surface gravity waves,rather than just one effect, significantly improves model performance. The non-breaking-wave-induced mixing and Langmuir turbulence are the most important terms when considering theimpact of waves on upper-ocean mixing. Sensitivity experiments demonstrate that vertical pro-files of the Stokes drift calculated from 2D wave spectrum improve the model performancesignificantly compared with other methods of calculating the vertical profiles of the Stokes drift.Introducing the wave influences in modelling systems, the results verified against measure-ments. Concluding from these studies for the further model development, the wave influencesshould be taken into account to improve the model performance.
6.
Beltrán-Abaunza, José M., 1971-
(författare)
Uncertainty measurements and validation of ocean colour data in optically complex waters
2013
Licentiatavhandling (övrigt vetenskapligt/konstnärligt) abstract
The thesis addresses validation activities associated with the use of in situ and satellite-based radiometers to assess water quality parameters, such as chlorophyll-a (Chl-a), total suspended matter (TSM) and coloured dissolved organic matter (CDOM- also known as yellow substances, YEL) in CDOM-dominated waters. In paper I, an inter-comparison of in situ radiometers is presented. Here, a new hand-held radiometer, the Water Insight Spectrometer (WISP-3) was tested and evaluated for routine water monitoring against other common radiometers used for validation. The WISP-3 measures the reflectance at the surface, and thus also works in shallow depths. The WISP-3 is designed for validation in places where other radiometers are difficult to deploy. As it is hyperspectral it can be used to develop in-water algorithms for the retrieval of water-quality information. In paper II, satellite data from the 3rd reprocessing archives of the MEdium Resolution Imaging Spectrometer (MERIS) sensor on board of ENVISAT is evaluated. MERIS level 2 reflectance and water products are assessed against in situ data. This assessment is required in order to ensure the reliability of monitoring systems based on MERIS data such as the Swedish coastal and lake monitoring system www.vattenkvalitet.se. The evaluation of reflectance data is based on a pair-wise comparison of the standard MEGS processor and three coastal processors that are provided as source free plug-ins for the VISAT BEAM software; all pairs are compared to data measured in situ. The derived water products are evaluated both on a pair-wise comparison of processors as well as on an individual comparison of some processors to sea-truthing data. The studied processors improved the retrievals of MERIS reflectance when used the latest MERIS FR 3rd reprocessing, equalized and “smile” corrected and a land adjacency effects were corrected using the improved contrast between ocean and land (ICOL). The blue spectral bands remain problematic for all processors. Chlorophyll was retrieved best using FUB with an overestimation between 18% - 26.5% (MNB) dependent on the compared pairs. At low chlorophyll < 2.5 mg m-3, random errors dominates the retrievals of MEGS. MEGS showed lower bias and random errors when deriving suspended particulate matter (SPM) with an overestimation in the range 8-16% (MNB). All processors failed to retrieve CDOM correctly, but FUB could at least resolve variations in CDOM, however with a systematic underestimation that may be corrected for by using a local correction factor . MEGS has shown already potential to be used as operational processor in the Himmerfjärden bay and adjacent areas, but it requires further improvement of the atmospheric correction for the blue bands and better definition at relatively low chlorophyll concentrations in presence of CDOM.
7.
Berg, Anders, 1983
(författare)
Spaceborne SAR in Sea Ice Monitoring: Algorithm Development and Validation for the Baltic Sea
2011
Licentiatavhandling (övrigt vetenskapligt/konstnärligt) abstract
The usefulness of new spaceborne synthetic-aperture radar (SAR) sensors at L-, C-, and X-band for operational sea ice monitoring in the Baltic Sea has been evaluated. It is concluded that the information content in C-band and X-band data is largely equivalent, although larger spatial coverage is an advantage of the current C-band satellites. L-band data provide complementary information and is especially useful to distinguish ice ridges, shear zones, and other deformation features. It is also concluded that cross-polarized data adds to the interpretation for wind conditions that make separation of open water and sea ice difficult in co-polarized data. A sea ice concentration algorithm has been developed that processes wide swath SAR images to estimate the ice concentration. The method is based on spatial autocorrelation of the images, and makes use of a neural network which is trained against operational sea ice chartsproduced by the Swedish Ice Service at the Swedish Meteorological and Hydrological Institute (SMHI). The algorithm is able to classify pixels as either open water or sea ice with an accuracy of about 90 percent. The sea ice concentration is determined with a root-meansquareerror of less than 7 percentage points for a uniform distribution of sea ice concentrations after spatial averaging. A sea ice drift algorithm published by M. Thomas in 2008 has also been evaluated. The ice drift is computed from SAR images over the Bay of Bothnia and the result was comparedwith wind data from meteorological stations along the Swedish coastline. The evaluation was made with HH-polarized C-band data from the satellites ENVISAT and RADARSAT-2. The direction of the ice drift was in agreement with the flow direction of the wind. An improvement of the algorithm was suggested that will make tracking more robust in the vicinity of the shoreline, where the motionless solution often is favoured. A field campaign has been conducted in the Bay of Bothnia in spring 2010. An ice buoy was deployed in central Bay of Bothnia and its position was tracked. The drift data werecompared with results from a further developed version of the sea ice drift algorithm using SAR images from ENVISAT. While still lying on the ice, the buoy successfully measured in situ ice drift between eleven image pairs. Thermometers mounted on the buoy provided basic data that made it possible to distinguish between water and ice underneath the buoy.
8.
Carpman, Nicole
(författare)
Marine Current Resource Assessment : Measurements and Characterization
2015
Licentiatavhandling (övrigt vetenskapligt/konstnärligt) abstract
The increasing interest in converting energy from renewable resources into electricity has led to an increase in research covering the field of marine current energy, mainly concerning tidal currents and in-stream tidal turbines. Tides have the advantage of being predictable decades ahead. However, the tidal resource is intermittent and experiences local variations that affect the power output from a conversion system. The variability is mainly due to four aspects: the tidal regime, the tidal cycle, bathymetry at the site and weather effects. Each potential site is unique, the velocity flow field at tidal sites is highly influenced by local bathymetry and turbulence. Hence, characterizing the resource requires careful investigations and providing high quality velocity data from measurement surveys is of great importance. In this thesis, measurements of flow velocities have been performed at three kinds of sites.A tidal site has been investigated for its resource potential in one of all of the numerous fjords in Norway. Measurements have been performed to map the spatial and temporal variability of the resource. Results show that currents in the order of 2 m/s are present in the center of the channel. Furthermore, the flow is highly bi-directional between ebb and flood flows. The site thus have potential for in-stream energy conversion. A model is proposed that predicts peak current speed from information on tidal range at the site. A corresponding model can be set up and implemented at other similar sites affected by tides, i.e. fjord inlets connecting the ocean to a fjord or a basin.A river site serves as an experimental site for a marine current energy converter that has been designed at Uppsala University and deployed in Dalälven, Söderfors. The flow rate at the site is regulated by an upstream hydrokinetic power plant nearby, making the site suitable for experiments on the performance of the vertical axis turbine in its natural environment. The turbine has been run in uniform flow and measurements have been performed to characterize the extent of the wake.An ocean current site was a target of investigation for its potential for providing utilizable renewable energy. A measurement campaign was conducted, mapping the flow both spatially and temporally. However, the site was shown to not be suitable for energy conversion using present technique.
9.
Carstens, Christoffer, 1981-
(författare)
In the Pipe or End of Pipe? : Transport and Dispersion of Water-borne Pollutants and Feasibility of Abatement Measures
2012
Licentiatavhandling (övrigt vetenskapligt/konstnärligt) abstract
Eutrophication is one of the key environmental problems of today, both in terms of complexity and magnitude. For the Baltic Sea (BS), eutrophication is an acute problem, leading to hypoxic conditions at the bottom; a situation that is sustained and amplified, when phosphorus is released from hypoxic sediments. Reducing nutrient loading is a top political priority but the present situation is believed to require active measures within the catchments and recipients to reduce both loading and adverse effects. Implementation of effective and cost-efficient abatement methods requires understanding of natural processes in watersheds, streams and recipients as well as technological expertise in order to compare the effects of measures of different kinds and locations. This thesis tries to combine process understanding of catchment transport behaviour, especially in coastal zones, and feasibility of certain technologies for reducing nutrient loading and effects of eutrophication in-situ. The over-arching theme is the fate of the individual contaminant, from injection to removal. Transport and dispersion in catchments are investigated, combining physically-based, distributed, numerical groundwater models with Lagrangian stochastic advective reactive solute (LaSAR) transport modelling. The approach is powerful in the sense that it incorporates catchment structural, geomorphological dispersion in the numerical model with hydrodynamic and sub-scale dispersion as well as uncertainty in the LaSAR framework. The study exemplifies the complex nature of transport time distributions in catchments in general and when varying source size and location, importance of dispersion parameters and retention due to molecular diffusion. It is shown that geomorphological control on dispersion is present even for relatively heterogeneous systems and that neither the mean residence time nor a statistical distribution may provide accurate representations of hydrological systems. To combat internal loading of P from sediments in-situ, large-scale aeration of deep waters, halocline ventilation, has been suggested. This study further investigates the feasibility of wave-powered devices to meet the energy demands for such an operation. It is shown that the required amount of oxygen needed to keep the sediments at oxic conditions could be provided, cheaply and efficiently, through the use of wave power.
10.
Carvajal, Gisela, 1983
(författare)
Retrieval of Wind Velocity Vectors Over the Ocean Surface with Spaceborne Synthetic Aperture Radar
2011
Licentiatavhandling (övrigt vetenskapligt/konstnärligt) abstract
Wind over the ocean (normally defined at 10 meter height) is one of the main parameters affectingsea state, generating waves and driving currents. Wind field maps with large area coverageand high resolution therefore represent valuable information for a wide range of applications such asinformation for shipping, which carries 80% of the world’s trade, oil platforms and other maritime infrastructures,modeling, analysis and validation of weather models, improving performance of sensorsonboard maritime infrastructures, etc.Spaceborne Synthetic Aperture Radar (SAR) is a promising sensor for providing wind informationwith global coverage and high resolution irrespective of weather conditions. This thesis presentsresearch carried out on the implementation and evaluation of an algorithm for retrieval of wind parameters(wind speed and direction) over the ocean surface with SAR data.The implemented algorithm uses C-band (frequency 4-8GHz) SAR data to retrieve wind speedand direction information at 10 meters above the ocean surface. The wind direction retrieval is basedon the local gradient method that extracts streak features in the SAR image aligned with the wind.The wind speed is inverted by providing the extracted wind direction to the CMOD-IFR2 geophysicalmodel function.A quality assessment of the wind retrievals by comparison with data from a numerical weathermodel (NWM) and a spaceborne scatterometer has been performed for various SAR images takenat different locations showing a diversity of features. The wind retrievals agree very well with thereference (wind speed bias smaller than 0.5 m/s and root-mean-square difference smaller than 2 m/sfor the wind speed and about 20º for the wind direction) when the wind presents a smooth behavior.It was found that for data with high spatial variability the SAR retrievals agree better with the scatterometerretrievals than the NWM data. Future work requires further quality assessment of the windretrievals against in-situ measurements and scatterometer data.
