Optimal control of wind farms requires accurate, yet computationally efficient models to capture the complex interactions between turbines. These interactions are dependent on atmospheric turbulence, wind direction changes, but also turbine properties which can be controlled. Existing control strategies often 1) neglect atmospheric effects such as wind veer, 2) rely on steady-state simulations or 3) do not optimise turbine actuation costs. This thesis addresses these limitations through the development of an optimisation-based control algorithm in a two-step process.

Firstly, the dynamic wind farm modelling and control package FLORIDyn is extended with veer-capable wake, turbulence and power calculation models. This enables the evaluation of the farm-wide power time traces, under veered conditions, as a function of yaw control inputs. These model additions are validated with Large Eddy Simulation data in single-, double-, and farm configurations. These comparisons show reasonable agreement with the high-fidelity simulations, and dependence of prediction accuracy on turbulence level estimation is highlighted. Secondly, a sparse control parametrisation is developed and integrated, making use of the new model in FLORIDyn. This optimisation-based approach jointly solves for the optimal actuation times and turbine yaws, making use of weather forecast information. Real-time numerical performance is achieved by using an efficient genetic algorithm as optimiser. Up to 6% in power gains are observed in strongly waked dynamic scenarios. When moving to veered conditions, these gains drop to 2%, quantifying the impact of veer on yaw control for power optimisation in wind farms.

Overall, this thesis establishes a framework for real-time wind farm optimisation that supports realistic atmospheric behaviour while remaining computationally tractable ... Read more

Large Eddy Simulation (LES) is a mathematical technique for performing simulations of turbulent flows, such as those found in the Earth’s atmosphere. Compared to traditional numerical weather and climate models, LES is more accurate in representing turbulent processes and cloud dynamics. The computational burden of LES, however, have histor- ically limited its application to relatively small domain sizes. In this work, part of the DALES atmospheric LES model was ported to Graphics Processing Units (GPUs) using the OpenACC programming model. GPUs, originally designed for accelerating computations related to 3D computer graphics, excel at parallel computations, which are abundant in LES models. The performance of the GPU port of DALES was measured on an NVIDIA RTX 3090 in a desktop workstation and an NVIDIA A100 in the Snellius supercomputer and compared to the existing CPU implementation. For the BOMEX intercomparison case, a speedup of 11.6 was achieved versus 8 CPU cores on the desktop system, while on Snellius a speedup of 3.9 was observed compared to 128 CPU cores. Furthermore, the existing MPI parallelization of DALES was adapted such that multiple GPUs can be used simultaneously. This thesis represents a step towards the enhancement of the scalability of DALES, enabling simulations on larger domains at higher resolutions. While a substantial acceleration of DALES was achieved, further efforts are needed to port more components of the model to the GPU to facilitate the simulation of increasingly realistic meteorological phenomena. ... Read more

During a solar eclipse, sunlight incident on the Earth is reduced due to the (partial) shadow of the Moon. Atmospheric trace gas concentrations which are influenced by the amount of available sunlight, such as nitrogen dioxide (NO₂), may be affected due to the disrupted photolysis processes. Large-scale observations of the increased NO₂ concentrations caused by the solar eclipse would improve our understanding of the sensitivity of NO₂ in the atmosphere to short-term variations in sunlight. Spaceborne
measurements can provide valuable information about the large-scale spatial distribution of NO₂, which is provided daily by the TROPOMI instrument aboard the Sentinel-5 Precursor satellite by measuring and retrieving locally reflected sunlight. However, the TROPOMI NO₂ retrieval is unable to derive reliable concentrations during a solar eclipse, as solar eclipses are not taken into account in its retrieval algorithm. In this research, we have adjusted the NO₂ retrieval of TROPOMI such that it can handle solar eclipses and study the large-scale response of NO₂ during two solar eclipses over Europe in 2021 and 2022. We found a large-scale increase of NO₂ in the adjusted measurements, which linearly correlated with the degree of obscuration. We compared the measured NO₂ increase with the values from the atmospheric chemistry model TM5 including an applied eclipse implementation and we found a close agreement in most areas that are not highly polluted. Our measurements and model predict a NO₂ increase of 60%±12% and 70%±7% for an obscuration fraction of 1, respectively. More advanced chemistry modelling work is needed to explain the measurements in highly populated areas. We conclude that our results demonstrate that the TROPOMI algorithm is capable of correctly measuring NO₂ after an adjustment of the NO₂ retrieval. We have shown that it is possible to adjust an atmospheric trace gas retrieval for the influence of a solar eclipse. Moreover, we are the first to provide evidence for an increase in NO₂ during a solar eclipse using space-based measurement techniques and to quantify this increase on a large scale with the same instrument. Our measurements can be used to test atmospheric chemistry models, possibly improving their sensitivity to solar eclipses but also artificial shadows on the Earth induced by sunlight-intercepting geoengineering approaches. ... Read more

Interferometric Synthetic Aperture Radar (InSAR) is a geodetic technique that is capable of monitoring surface displacements up to millimeter-level of precision. The end products from conventional InSAR processing are application-agnostic, which means that they are not optimized for any particular application. InSAR products could be more beneficial if tailored for a relevant application, particularly if expert users can tune the products according to their monitoring requirement. Here, we develop tools for application-aligned monitoring by means of the selection of application-relevant arcs between scatterers in InSAR. We are interested in the use of local (short) arcs between point scatterers, as these arcs are more likely to be better suited for monitoring localized differential deformation, and may provide observations of better quality due to the fact that they are less prone to atmospheric noise.

We first compare the time series of local arcs and conventional time series w.r.t.\ a common reference point based on their deformation behavior. The comparison reveals that the time series of local arcs are capable of providing additional information on deformation behavior over the conventional method. However, the quality of observations in local arcs in general is found to be more variable, and often even worse than those from the conventional method. Most likely, the reason for this is the absence of noise reduction in local arcs in comparison to the time series from the conventional method which optimizes the selection of the common reference point to reduce noise in the time series.

In addition, to optimize the arc selection for a given application, we propose an arc tuning strategy, where criteria can be set based on arc parameters, i.e., the length, the elevation difference (between point scatterers) and the azimuth of the arc. We also introduce the arc clustering method as an exploratory data analysis algorithm for general-purpose monitoring using local arcs. Both of these methods are demonstrated on test scenarios over the quay walls along the canal network of Amsterdam. The demonstration on arc tuning shows that arc setting criteria on arc geometry parameters are adequate to select arcs with certain orientations, and the selection can be further aided by estimating displacement parameters with multiple hypothesis testing. The results from the arc clustering show the potential of detect instability over a certain area using arcs without knowing the motion of the specific object.

This study contributes to monitoring deformation where the InSAR data can be optimally attuned based on a particular application. In order to convey information on selected arcs effectively, a visualization tool based on an interactive map is created in a jupyter notebook environment.
... Read more

A phenomenon that compromises the health of city inhabitants is the urban heat island (UHI) effect. This effect is mainly driven by thermal radiation and designates the typically higher air temperature in cities compared to the surrounding rural areas. Over 55% of the world’s population lives in cities, and with this percentage set to rise to more than 65% by 2050, it is vital to model the urban climate to be able to mitigate the UHI-effect. In this study, the surface energy balance (SEB) is modeled in urban environments using a one-way coupled RANS 𝑘 − 𝜖 approach in order to gain fundamental insights into the influence of thermal radiation on the UHI-effect. The SEB model is built mechanism-by-mechanism, adding in different physical mechanisms one at a time and validating the model at each step with literature- or theoretical values. The SEB model showed a good agreement with experimentally found values for the effect of surface irregularity of an urban structure on the absorption and reflection of incident solar radiation. A good agreement was also found with theoretical values for the longwave radiation and conductive heat flux part of the SEB model. Subsequently, a study was done on a 2D street canyon to compare the model with literature results. This study showed a good agreement for the radiative fluxes, but a lesser agreement for the conductive- and sensible heat fluxes. A more complex scenario of an intersection between buildings was studied. It was shown that the longwave trapping effect is highly correlated with building height. Finally, the effect of wall heating on flow characteristics in a 2D street canyon was examined. For homogeneously heated walls, the SEB model showed a relatively good agreement with literature results. A study was also done with non-homogeneously heated walls. This study showed less realistic results and should be further investigated in further research. ... Read more

The Atlantic Meridonial Overturning Circulation (AMOC) in the North Atlantic Ocean (NAO) plays a major role in earth’s climate and climate change. A key element of the AMOC is deep convection, which is still not fully understood. One of the unknowns is where water is exchanged between the boundary current and the regions where deep convection can occur. This is important for models to know where deep waters are formed and where they are transported to. This study focuses on the Irminger Sea (IRS), a sub-sea of the NAO. The interior of this sub-sea is a known area where deep convection can occur. Using data from the Argo Float Program, a analysis was conducted to investigate exchanges of water between the boundary current of the IRS and the area where deep convection can occur. The entries and departure locations of the Argo floats are collected and statistically compared. Furthermore, seasonality difference between winter and summer months are compared using the Mann-Whitney U-Test. Lastly, the internal pathways water takes within the interior area are analysed, by tracking where a float enters the interior area and where it afterwards leaves the area. The results show water takes many different pathways in and out of the interior area and the pathways taken within the area show the expected cyclonic pattern. There were no clear differences between summer and winter months, except in the northern part of the interior area, where in winter a clear south-western current is present, but not in summer. Future studies on the exchange between the boundary current and the interior area can use these results as an indication that the exchange happens all around the area, but the water does follow a cyclonic pattern. ... Read more

As the environment is changing temperatures are changing, becoming more extreme. This is expected to affect the oceans and its transport, specifically the Atlantic Meridional Overturning Circulation (AMOC). The Labrador Sea is a part of the AMOC, where overturning in depth and density space occurs, due to deep convection. Deep convection is the process of seawater losing its heat to the atmosphere, due to atmospheric cooling during the winter. This causes the seawater to become colder and denser, and it therefore sinks towards the bottom of the basin. Deep convection is previously studied extensively as it is a unique and important process of the global ocean circulation system. The key process that causes the AMOC water to overturn, is due to buoyant eddies shedding from the boundary current into the interior. The buoyant eddies exchange their buoyant boundary current water with the dense interior water, causing the boundary current (and in extension the AMOC water) to cool down.

Previous studies have shown that the properties of the boundary current water are strongly dependent on the eddy exchange, and therefore on the surface heat loss. However, it is not known how consecutive strong winters impact the dynamics of the Labrador Sea on various timescales, which will therefore be the focus of this thesis. Data for this research will be obtained by using an idealised model configuration of the Labrador Sea, where the hydrostatic primitive equations of motion are solved by the MIT general circulation model (MITgcm). Different types of scenarios are defined to analyse different effects on the dynamics. These scenarios are analysed by looking into how the mean basin temperature changes, how the eddy kinetic energy (EKE) and mixed layer depth (MLD) develop, and how the properties through a transect of the basin change. The effects of these interactions are then studied by looking at how the transport of water throughout the boundary current, per density class and per vertical layer change.

The thesis mainly shows that the mixed layer depth in the interior increases during a strong winter. As a result, the eddy kinetic energy increases significantly in the boundary current, as the horizontal density gradient increases, thus causing an increase in boundary current velocity in the downstream direction. Additionally, more and denser interior water accumulates, depending on how many consecutive strong winters occur. This deep convected water in the interior partly remains near the bottom of the basin. In the next winter, it is mixed again due to deep convection, consequently a positive feedback loop occurs. Meaning, that the number of consecutive winters positively impacts the interactions in the basin, as the horizontal density gradient increases, and thus the velocity and eddy kinetic energy increase as well, in respect to the previous winter. The effect of the strong winters persists in the years afterwards, as the interior remains relatively cold. Additionally, a part of the accumulated convected interior water resides too deep in the basin to be exchanged by the eddy exchange and therefore flows near the bottom out of the basin, due to a pressure difference. The flow near the bottom is a negative feedback loop, as the volume of dense convected water decreases and can therefore not be further cooled during consecutive strong winters. Finally, the properties and the transport of the boundary current water are directly related to the interior water and eddy exchange. As the MLD in the interior and eddies in the BC are still relatively large in the years after the additional surface heat loss, the export of boundary current water therefore also remains affected. In conclusion, the effect of wintertime surface heat loss on the Labrador Sea Water in the short term has the most influence on the MLD and EKE, however the influence of the MLD and EKE remains and therefore in the long term affects the export through the BC. These conclusions can help to better interpret the limited available measurements of the Labrador Sea Water. ... Read more

Recent reports show that deforestation in Latin-America has been severe over the last decades. Especially Brazil is subject to an alarming rate of forest loss, which will remain a factor in the coming decades. In addition to deforestation, there is an increasing amount of hazards like floods and mudslides. These hazards result in major damage to human life and nature. This research analyzes the relation between deforestation and river discharge for observed data, a simple conceptual model and complex model WFLOW for 30 catchments subject to deforestation in Brazil. These three studies use satellite data provided by \cite{GEE} to determine annual cumulative deforestation per catchment. In the study with observed data the relation between deforestation and data from CAMELS-BR \citep{CAMELS2020} is researched by analyzing annual values of two discharge driven parameters: Runoff coefficient and recession coefficient. The second study consists of a simple self-made conceptual model. In this second study, the relation between recession coefficient $\alpha$ and annual deforestation is analyzed by calibrating the model per year for parameter $\alpha$. Lastly, a study is conducted on the performance of WFLOW in catchments subject to deforestation. During this study, several input parameters are changed to observe the response of hydrological processes in WFLOW.

The study with observed data shows both increasing and decreasing trends in coefficients for several degrees of deforestation. Literature shows that landcover type after deforestation is a major factor in the interpretation of these results. However, a lack of quality annual landcover data prevents better research in the non-masked impact of deforestation on discharge. The results of the simple model study show no significant relation between deforestation and recession coefficient $\alpha$. The simplicity of this self-made conceptual model is the weak and strong point of this sub-study. The simplicity makes the results less suitable for analyzing the exact impact of deforestation on discharge, but it is useful for observing general signals and is easily scalable to different catchments. The simulated discharge by WFLOW show a steep overestimation of discharge during peak flow in comparison to observed discharge by CAMELS-BR. Therefore it is not possible to analyze how WFLOW reacts to deforestation. Instead, an in-depth analysis on the cause of this poor performance is conducted by analyzing timeseries for hydrological factors like unsaturated zone depth. The results of this analysis indicate the overestimation of discharge is caused by a lack of outflow from soil layers. In addition, the difference between the Budyko framework of different data sets used in this research, show that uncertainty in quality of input data is a plausible factor on the output of WFLOW.

In conclusion it is observed that deforestation does not necessarily lead to higher runoff coefficients and recession coefficients for measured data in this study area. In addition, the conclusion of the simple model study is that no significant relation between deforestation and recession coefficient $\alpha$ is observed for this conceptual model. Finally, the performance of WFLOW is considered too poor to analyze the impact of deforestation on discharge. The root of this poor performance is considered to be a combination between lack of groundwater modelling and uncertainty on the quality of input-data. ... Read more

With the growing offshore wind industry, ramp events, i.e., significant variation in wind speed (or power) within a short time period, have become of critical importance to the end-users. The analysis of wind power ramp events from the perspective of grid operators or energy traders is prevalent in the literature. Nonetheless, this area of research is still evolving. The criterion of a minimum of turbines and its effect on the detection of ramp events, thresholds, or smoothing of power differences have not been considered so far. Moreover, little research has been conducted on analyzing the impact of ramp events on ultimate wind turbine loads. To the best of the authors’ knowledge, the results have not yet been validated with measurements. Therefore, the primary objective of this report is to investigate the impact of wind-induced ramp events on offshore wind farm power and ultimate wind turbine loads by using offshore measurements.

At first, a wind power ramp detection algorithm is developed by considering the suitable power normalization technique and the criterion of a minimum number of turbines. The detected ramp events followed a decreasing trend in a number of events with the increasing wind speed, turbulence intensity, ramp duration, and wind direction changes. This can be because of the strong dependence of the number of ramp events on the non-linearity of the power curve. However, the available measurement data sets were found to be insufficient to capture the correlation and travel time of ramp events within two wind farms. Therefore, detailed meteorological information with the high spatial and temporal resolution is necessary for thorough analysis.

Following that, extreme wind speed fluctuations (ramp-like events) were found to be extreme load drivers for wind speed bins that fall under the inactive pitch or above rated wind speed region. The measured loads are further compared with the simulation results, which are obtained by following the International Electrotechnical Commission (IEC) guidelines of extreme turbulence load cases. In general, the measured loads associated with the extreme fluctuations did not outperform the simulations, except for the tower side-side moments. Besides, extreme loads around rated wind speed exceed the simulations, but the values are not related to the extreme fluctuations. Therefore, this study examined the high-frequency time series for the most interesting blade root flap-wise moments. Based on the investigation, extreme loads over the wind speed range, except near rated wind speed, were governed by extreme turbulence or ramp events involving a sudden pitch transition from the inactive to the active region. The extreme loads around rated wind speed, which exceeded the simulations, were associated with the standard deviations equivalent to the normal turbulence but with comparatively higher fluctuation frequency. In conclusion, for the ultimate load analysis, the wind speed time series should include a sudden transition of a pitch angle from the inactive to the active region.
... Read more

Due to the insufficient worldwide efforts to reduce greenhouse gas emissions and related global warming, marine cloud brightening (MCB) is gaining interest as an instrument to artificially lower Earth's temperature. MCB is based on exploiting the aerosol-cloud effects to enhance the albedo of stratocumulus clouds and prolong their lifetime by injecting clouds with sea salt aerosols. As there are still many uncertainties in the expected output of MCB due to the unresolved questions related to cloud processes and feedback mechanisms, this thesis aims to assess the efficacy of MCB to enhance solar radiation reflection of stratocumulus clouds using the turbulence-resolving Dutch Atmospheric Large-Eddy Simulation model. This assessment is made by investigating which cloud properties and processes that determine the cloud's radiative forcing are affected by the aerosol injection for different meteorological conditions and injection strategies in 30-hour simulations. The simulation domain has a horizontal size of 25.2 $\times$ 25.2 km$^2$ and 2 km in the vertical, with a mesh size of 100 m horizontally and 20 m vertically. The two studied cloud cases are based on the measurements of the first and second research flights of the DYCOMS-II field experiment and are characteristic for a shallow stratocumulus-topped boundary layer (STBL). The investigated surface aerosol injection strategies are a horizontally uniform source and a point source that is restricted to a single grid cell. In addition, an assessment is made of the effects of differences in aerosol number concentrations between the STBL and the free troposphere on the cloud's aerosol number concentration and radiative properties. This is done in 6-hour simulations with and without a uniform aerosol source, based on the second research flight of DYCOMS-II. The simulated horizontal domain is 3.2 $\times$ 3.2 km$^2$ and 2 km in the vertical, with a mesh size of 25 m horizontally and 20 m vertically. Our simulations showed that boundary layers with relatively low background aerosol concentrations were most effective in generating a negative radiative forcing, compensating slightly less than half of the forcing related to the CO$_2$ doubling in the atmosphere. For boundary layers with an average background aerosol concentration, the radiative forcing approaches a quarter of this value, which diminishes to negligible effects for boundary layers with relatively high background aerosol concentrations. For precipitating boundary layers, the enhanced radiative forcing is mainly caused by its suppressing effect on precipitation. For non-precipitating boundary layers, the reduction in cloud droplet size showed to be the primary source of the enhancement. No pronounced differences in efficacy were found when using a homogeneously distributed aerosol source or a point source. For the two studied cloud cases, investigation of the contributions to changes in the liquid water path showed that to exploit the effect of aerosol injection on the liquid water path, it is most effective to suppress precipitation and enhance cloud cover. The differences in aerosol number concentration between the STBL and the free troposphere significantly and consistently altered the aerosol number concentration in the cloud layer due to the entrainment of air from just above the cloud-top into the cloud layer. This indicates that these differences in aerosol number concentration need to be considered when modelling MCB, as they change the intended aerosol number concentration enhancement and thereby affect their cloud-modifying effects. ... Read more

Tidal energy has a large potential to contribute to achieving the sustainability goals in the Netherlands, due to the long coastline and many estuaries. The investment costs for the implementation of offshore tidal energy are however, still a drawback. A possibility to lower the investment costs for tidal energy is to implement tidal energy extraction into existing coastal infrastructure. An example of this is the current pilot project in the Eastern Scheldt barrier where five Tocardo turbines are attached to the barrier's geometry. The flow passing the turbines in the barrier is contracted by the barrier's geometry and the weir of the foundation of the barrier. Deltares executed an environmental impact assessment for the pilot project with a measurement campaign and a computationally expensive but accurate blade resolved numerical model. In this thesis, a cheaper computational model is added to the these assessment tools. To do so, a turbine parameterization is introduced in the finite element model FinLab. The relatively cheaper computational model should make it possible to execute more extensive sets of simulations of the pilot project in the future. FinLab solves the non-hydrostatic three-dimensional incompressible Navies-Stokes equations in an unstructured mesh with an optional moving free-surface. The used meshes are refined in certain areas of complex flow, such as the turbine parameterization, near the bed, the recirculation zone of the weir and the wake of the turbine. An extra source term is added to the Navier-Stokes equations on a chosen number of turbine integration points, to implement the turbine into the mesh. The placement of these integration points in a two-dimensional mesh is rather simple, the integration points are evenly distributed over the diameter of the turbine. In a three-dimensional mesh, a Fibonacci series is used to implement an actuator disc (AD) into the model domain. A benefit of the Fibonacci circle is that each turbine integration point represents a similar area. The force in the turbine integration points is determined by three methods: an actuator disc with a uniform thrust distribution (uniform AD), a rotational averaged actuator disc blade element momentum method (AD-BEM) and a non-rational averaged actuator line blade element momentum method (AL-BEM). In the uniform AD method, a measured thrust force is divided equally over all turbine integration points. In the BEM methods axial and tangential forces are determined for each turbine integration point based on the local flow speed and experimental lift and drag coefficients. With these methods, it is possible to estimate the generated power and thrust by the turbine. A tip-correction is added to the AD-BEM method to achieve accurate results of the simulations. Furthermore, in both the AD-BEM and AL-BEM methods a small actuator disc is applied in the nacelle region to reproduce the nacelle's blockage of the flow. The uniform AD, AD-BEM and AL-BEM parameterizations are validated with measurement data of flume experiments of a single turbine, flume experiments with different turbine-weir geometries and flume experiments with multiple turbines. The combination of these last two sets of experiments does very well represent the situation in the Eastern Scheldt barrier. As stated above, the AD-BEM and AL-BEM methods are able to estimate the turbine performance. The results of the AD-BEM simulations show a relative error of the turbine's generated power of within 20% in comparison with the flume experiments. The accuracy of the thrust fluctuates more. The AD-BEM method shows to be able to accurately estimate the location of optimum power harvesting in a combined turbine-weir geometry. Both the power and thrust accuracy depend on the mesh resolution and the turbine's rotational speed. Accurate results can be obtained by fine-tuning these settings. The AL-BEM method should be able to produce at least similar similar accuracy to the AD-BEM method when it comes to power and thrust. The AL-BEM method in this thesis is however, not yet able to do so. The results of the simulations with the three methods are also compared with time-averaged velocity measurements of the flume experiments. In the near wake, the uniform AD method does not imply any wake rotation. The rotation of the wake is represented accurately in a time-averaged sense by the AD-BEM method. The AL-BEM method adds transient features such as the downstream trailing tip-vortices to the flow. The velocity shear and turbulent eddies in the near wake as caused by the different methods influence the TKE in the near wake and the recovery of the far wake. The uniform AD under-estimates the wake recovery. The extra velocity shear in the AD-BEM method improves the accuracy but still under-estimates the recovery rate. Resolving the transient features of the near wake in the AL-BEM method further improves the accuracy of the wake recovery. In the simulations with the weir, the geometry seems to be dominant and the wake recovery rate is more accurate than in the simulations without the weir. The AD-BEM method is applied to the Eastern Scheldt field case. Five turbines are introduced in a mesh which represents Roompot 7 to 9 in a simplified manner. Due to the simplification of the mesh, the resistance of the barrier on the flow appeared to be too low. Therefore, the simulations are executed with an upstream discharge boundary. This discharge boundary makes the model less useful to estimate the environmental impact of the turbines in the barrier and at the moment not yet an alternative for the blade resolved model by Deltares when it comes to estimating these effects. With the discharge boundary, the AD-BEM model predicts the average thrust over the five turbines with a relative error of 3% and the average power with a relative error of 12%. This is only slightly less accurate than the results by the blade resolved model by Deltares, while the computational costs of the Eastern Scheldt field case simulations are only a fraction of the earlier executed blade resolved simulations by Deltares. ... Read more

As the efficiency of silicon (Si) photovoltaic (PV) moves ever closer to the theoretical limit, 2-junction PV becomes increasingly interesting. Since PV cells are tested under standard test conditions (STC), but real world working conditions differ, it is interesting to see how 2-junction PV performs worldwide in different climates. Worldwide spectra where simulated in SMARTS using data from NASA’s Global Land Data Assimilation System (GLDAS), Clouds and the Earth’s Radiant Energy System (CERES) and Socioeconomic Data and Applications Center (SEDAC) and from the Joint Institute for the Study of the Atmosphere and Ocean (JISAO). SMARTS provides only clear-sky spectra, to account for cloudiness, the BRL model is used. The top absorber is 1.72 eV Perovskite and the bottom absorber is 1.12 eV Si. Because Si is an indirect bandgap material, a limiting Efficiency model by Richter et al. is used that takes both Auger and radiative recombinations into account. Because this model relies on variables and constants only given for Si, and the top absorber is a direct bandgap material, the detailed limit model by Shockley and Queisser is used to calculate the performance of Perovskite. The distribution of high and low local yearly average irradiance is overall realistic, except for the north of Africa and the Middle East. Here aerosol optical depth (AOD) values are elevated. The elevated AOD can be explained by dust events, still these areas should be among the ones with the highest irradiance. The AOD effects the blue side of the spectrum mostly. All spectra where normalised using CERES’ irradiance data. The Richter model is thickness dependent. Thus the optimal thickness of Si is determined to find the optimal efficiency. To distinguish between the effects of the top layer and local climate effects on the thickness, firstly the optimal thickness of single junction Si is calculated and analysed. The optimal thickness ranges from 70 to 870 µm globally. The average is 205 µm and the irradiance weighted average is 165 µm. For double junction Perovskite - Si current matching was used to find the optimum. The thickness of Si ranges from 10 to 4500 µm. But only nine locations (of 597) have a thickness above 500 µm. The average thickness is 77 µm and the irradiance weighted average is 81 µm. Locations with a high optimal thickness had a blue-er spectrum than the locations with a low thickness. Using the optimal thickness of single junction Si and current matching, the optimal top-bandgap was determined around the world. The optimal bandgap ranges between 1.55 and 1.76 eV, the average is 1.655 eV and the irradiance weighted average is 1.663 eV. The relation between the top bandgap and optimal Si thickness was determined under STC. High bandgaps had the lowest optimal thicknesses and the thickness increased with decreasing top-bandgap. Alterations in spectrum and temperature where applied as well to find the effect on that relation. Decreasing temperature and airmass both resulted in an increase in optimal thickness per bandgap vise versa. Changing the spectrum has a greater effect than the temperature change. ... Read more

Wind energy is becoming an important renewable energy source. An increased number of offshore wind farms are constructed due to the relatively higher wind speeds. Besides, compared with the land, the ocean areas offer more empty space for the installation of wind turbines. In recent years, several governments in Europe have the plan to expand their countries’ wind farms over the North Sea area. With this surge in the development of offshore wind farms, extreme weather events over the sea pose threats to the installations. A waterspout is one of such phenomenon of concern.

In this study, we simulated and characterized the atmospheric conditions associated with two waterspout events observed recently over the North Sea. These cases were selected from the European Severe Weather Database. Various types of observational data, including radiosondes, radar reflectivities, satellite imageries, lighting maps, and floating liar-based wind profiles, were utilized for detailed characterization. Atmospheric circulation patterns associated with waterspouts were deduced from surface-level and upper-air synoptic charts. A mesoscale model, called the Weather Research and Forecasting model, was used for simulations with a high spatial resolution of 1 km. We used five different parameterizations of varying complexities to quantify the sensitivity of the simulated results with respect to cloud microphysics. A number of meteorological variables and indices (e.g., thermodynamic indices, wind shear, vertical velocity, reflectivity) are extracted from the simulations and compared with the observational data. In general, our results are in agreement with the findings from previous studies. For instance, we have found that a double moment microphysics parameterization produces more realistic results in comparison with a single moment one. However, we have noticed that our simulated results fall outside the range specified by the so-called Szilagyi waterspout nomogram. This nomogram was initially proposed based on observational data from the Great Lakes region and is widely used by the operational meteorologists. Based on the results, updating this nomogram is needed with additional observational and simulated data from the North Sea region. ... Read more

In this thesis the water motion and sediment dynamics are investigated in a periodically closed and opened estuary. The water motion in an estuary is mainly driven by the semi-diurnal tide with an period of 12h25m and river discharge. An example of such an estuary is the Ems-Dollard estuary. Recent observations show an increase in tidal range (height difference between high and low tide), suspended sediment concentration and the depletion of oxygen levels (consequently harming the ecosystem). A possible solution, periodically closing and opening the estuary, is investigated. The water motion in a periodically opened and closed estuary is described by the linearised cross-sectionally averaged equations which give the sea-surface elevations and tidal velocity when solved with the eigenfunction expansion method. It was found that the sea-surface elevations and tidal velocity for a periodically opened and closed estuary are again 12h25m periodic. For the sediment transport, when no overtide is considered the residual sediment transport is seaward directed if the estuary is closed at low water and landward directed if the estuary is closed at high water. The barrier location determines the magnitude of the residual sediment. When overtide is included in the forcing of the system no relation is found
between the direction of the residual sediment transport and the closing height and closing position. The location of the barrier and closing height both determine the magnitude of the residual sediment transport and direction. By introducing a barrier that periodically closes and opens we intended to achieve a seaward directed residual sediment transport in the Ems-Dollard estuary. The results suggest that this is not possible. Further research is needed with more
extensive models to confirm this. For future Research I recommend to extend the model to a two-dimensional model with the eigenfunction expansion method. Other possibilities may be to consider a spatial dependent erodible bed. ... Read more

In current study, several fundamental and inherent problems in original Deardorff subgrid model are identified under stably stratified condition. It is found that the mixing length parameterization in this subgrid model is at the root of a long trouble problem of grid size sensitivity in large-eddy simulation (LES). A new formulation of mixing length is proposed under the consideration of some basic elements including the presence of surface, the dependence of grid size Δ and a smoothing interpolation. The performance of this modified scheme is remarkable regarding the improvement of the simulation quality and accuracy. In other words, not only is the convergence of the simulated results from a range of grid size achieved but also in the precise intensity of physical variables are modelled. The only discrepancies display in the variance of temperature in the middle of boundary layer and high turbulent kinetic energy near the surface.

To further experiment the performance of the new scheme under different scenarios, the cases of different stability condition, an independent LES code with same modification, the cases of different advection schemes and different prescribed parameters are explored. In very stable condition, the first order variables from the modified scheme are in reasonable range but with some spreads compared to the results from a dynamic code. The deviation of second order statistics shows that the proposed formulation of mixing length meets limitations due to the complex interaction between the surface and turbulent flow in shallower boundary layer. The modified scheme is model system independent based on the similar improvement of simulation results in an independent LES code system. The sensitivity of advection schemes is surprisingly hardly found in new proposed SGS model. The cases of tested parameters further verifies the limitation of original Deardroff subgrid model. ... Read more

In this paper we introduce a setup to investigate aeolian saltation and surface dynamics on a centimetre spatial resolution and a sub second temporal resolution. We develop a Lagrangian saltation model and a high-resolution surface model, which we couple to each other and to a turbulence resolving large eddy simulation model. The simulated transport takes place primarily in the form of aeolian streamers, bursts of elongated transport structures parallel to the wind field, which result in a mass flux signal that is highly heterogeneous both in space and time. The temporal frequency responses up to 1 Hz of the mass flux and wind field share the same characteristics, which indicates a coupling between the two. The system can be in equilibrium, during which the stress profiles induced by the particles, the turbulent fluxes and the imposed large scale pressure gradient balance each other. A bimodal shape is found in the mass flux profile, in which we can distinguish an upper and lower saltation layer. The upper layer is associated with a transitional phase between transport by saltation and suspension that exists in the aeolian streamers. Furthermore, The setup is able to simulate ripples, although future research is needed to investigate the mechanisms that influence the final shape of the ripples. ... Read more

Climate change causes temperatures to rise worldwide. Up until now it is unclear what effect this has on the global water cycle. In this study the output of two GFDL model experiments were thoroughly investigated and used in the moisture tracking model WAM-2layers, accounting for model runs in a past case in the end-20th century and future case in the end-21st century, based on RCP8.5. This is done in order to acquire knowledge on what will happen to the global as well as regional water cycle in the future and to find out what processes provide climate change to have an effect on the water cycle. Changes in precipitation and evaporation rates are spatially highly dissimilar, therefore regional differences can be distinguished. Past studies have suggested that a DDWW paradigm - where dry regions dry out further and wet regions get wetter - will take place with climate change. Even though in some regions this might happen, results show that on land this is not necessarily the case - at least, if precipitation rate is the benchmark for a region to get drier or wetter. Processed GFDL data concludes that dry region Western Sahara gets wetter while dry region Middle East gets drier and wet region Indonesia gets wetter while wet region Amazonia gets drier. The DDWW paradigm covers another aspect as well, that there will be a larger spatial variability for variables characterising a region to be wet or dry. However, while everywhere on the globe the temperatures will rise according to the GFDL data, the spread of the yearly mean temperatures over the globe will actually decrease, meaning that mean yearly temperature in the coldest place will lie closer to the mean yearly temperature in the warmest place. This is also the case for mean yearly precipitation and evaporation rates. The mean yearly precipitation and evaporation rate in the driest place will lie closer to the mean yearly precipitation and evaporation rate in the wettest place in the future case. Continents show divergent effects in water cycle due to climate change. The continents with relatively larger sources of terrestrial moisture - North America, Europe and Asia - have an increased water cycle with higher precipitation and evaporation rates in the future. The increases in precipitation rates on these continents will originate from terrestrial evaporation and a higher percentage of evaporation will return as precipitation on land. South America shows a distinct effect, different than all other continents. This continent shows a decreased water cycle with lower precipitation and evaporation rates in the future. Of this precipitation a lower percentage comes from land and the evaporated moisture will return less on land. Africa and Oceania show another pattern. Both these continents will experience more precipitation in the future case, but this moisture will come from oceanic evaporation. Two case study areas are examined in more depth by looking at seasonality effects. A case study in the Amazon forest shows a distinct dry season in the future case, the effect of possible land use change in the Amazon forest. Less evaporation that is an effect of deforestation of the rain forest gives less moisture for clouds to form and precipitate. Moisture evaporates from the oceans on the East of Amazon region but moves over the region before it can precipitate. A case study in Western Africa shows a magnified rain season. The analyses show that during the wet season in the future case there in an increased amount of moisture coming from the oceans West and South of Western Africa and meanwhile there was still moisture coming in from the tropical rain forest East of the area. ... Read more

The non-linear relation between Evapotranspiration (ET) and the atmospheric moisture demand in terms of vapor pressure deficit (VPD) has been widely reported. Observations point towards a diverse range of potential drivers; however, without uncovering why the identified factors are found to control diurnal ET-VPD hysteresis. Modelling efforts have laid a theoretical foundation to unravel the compound effect of surface and atmospheric states on this hysteresis. However, so far these models have been unable to realistically incorporate the non-linear feedbacks between atmosphere and surface, and lack a vegetation representation that reflects the underlying biological mechanisms. To unravel in what manner the characteristics of ET-VPD hysteresis are controlled, multi-scale observations spanning biology, meteorology, and hydrology from the CloudRoots campaign are combined with modelling in a novel manner. For the first time a proof-of-concept is delivered for reproducing ET-VPD hysteresis with a model that incorporates both surface-atmosphere feedbacks as well as a mechanistic vegetation component. Observations are used for parameter initialization and evaluation to ensure that the modelled hysteresis curve and underlying processes represent a realistic case. Next, this calibrated model is used for a sensitivity analysis. The results show that both the early morning state of the surface and the state of the atmosphere influence the characteristics of the hysteresis loop with respect to its width, height, and initial slope. Via control of the stomatal aperture, soil moisture stress and the vegetation’s capacity to assimilate CO2 for photosynthesis affect the height and width of the curve. The height of the hysteresis loop is significantly affected by entrainment of warm dry air, while its width is minorly impacted. The characteristic fingerprint of entrainment is distinctly different to that of soil moisture stress and the capacity for CO2 uptake. The sensitivity analysis applied to our observationally inspired case, appears to facilitate quantification of the strongest model responses to changing external forcings and parameters. Therefore, the presented approach offers a promising tool, allowing further research into the controlling mechanisms of ET-VPD hysteresis. ... Read more

Accurate predictions of the extinction and scattering properties of the atmosphere are important for climate research and interpreting satellite data. This study introduces a model (called the H-model) that calculates the scattering coefficients and scattering enhancement factors based on in situ measurements of the dried ambient aerosol. A disadvantage of using dried aerosol measurements is that they do not correspond with the ambient conditions, as they are measured at a relative humidity below 40% and thus the particles are assumed to contain no water. Measurements of aerosol chemical composition do not contain water mass concentrations and measurements of the particle size distribution do not include water. To solve this problem, the H-model uses ISORROPIA, a thermodynamic equilibrium model, to estimate the expected amount of aerosol water content and growth factor g(RH) of aerosol particles for any given temperature and relative humidity (RH). With this information, the conversion between dry and enhanced relative humidity can be made. The chemical composition measurements can be complemented with the estimated aerosol water concentrations and the particle size distribution can be recalculated based on the growth factor for any given RH. In addition, the growth factor is also calculated by using k-Köhler theory and compared to the results of ISORROPIA. The findings of this sub-study show that the growth factors calculated by both approaches (ISORROPIA and k-Köhler theory) are similar as they significantly correlate. ISORROPIA, however, is more sensitive to small chemical changes which makes it more appropriate for the H-model. The calculated growth factors are used in the H-model to estimate changes in the chemical composition and particle size distribution of the aerosol particles at enhanced relative humidity. Subsequently, the H-model uses MIE theory to estimate the scattering properties of the particles at a specific relative humidity. By doing so, the scattering properties can be calculated at dry and enhanced RH, making it possible to calculate scattering enhancement factors. Finally, the H-model is validated by comparing the calculated scattering properties to measured scattering properties of a (humidified) nephelometer. To do so, in situ measurements from the CINDI campaign in 2009 and the TROLIX campaign in 2019 at Cabauw are used. The findings of this validation show that the results from the H-model do not yet accurately match the measurements. That being said, a strong correlation is observed between the calculated and the measured scattering properties. This shows that the H-model is able to capture changes in the particle size distribution and chemical composition while calculating the enhancement factors. It can be concluded that the results from the H-model are promising but need further work to close the gap between the calculations and measurements. The H-model makes multiple simplifications and assumptions which could be improved upon, thereby increasing the precision of the results as well. Furthermore, to fully conclude the findings of this study, the measurements of the SMPS and the nephelometers should be calibrated. A better statement can then be made about the accuracy of the comparison between the scattering properties calculated by the H-model and measured by the nephelometers. ... Read more

As a key component to the bottom limb of the Atlantic Meridional Overturning Circulation (AMOC), the Labrador Sea is one of the regions where deep ocean convection takes place. This convection is driven by atmospheric cooling during winter, which brings the surface water into the intermediate and deep layers by uniformizing water mass properties. This homogeneous layer is called Mixed Layer (ML). As a result of this convection, stratification is no longer maintained, and the Mixed Layer Depth (MLD) deepens. During this deepening, an enormous amount of potential energy is converted to kinetic energy, and meso- and sub-mesoscale instabilities develop. After wintertime, the MLD starts to shallow again. Atmospheric-induced convection ceases or decreases significantly and physical components return to stratified conditions. Baroclinic instabilities grown to mesoscale or geostrophic scale play a role in restratifying the ML through the formation of coherent ocean eddies. This chain of processes follows a seasonal cycle that strongly depends on the imbalance between horizontal and vertical buoyancy gradients. A practical way to quantify this imbalance is the use of the Ertel potential vorticity or a derived magnitude as the Richardson angle, which allow to infer the existence of instabilities and to classify them respectively.
This study analyzes the physical processes behind the MLD seasonal variability in the Labrador Sea. To this end, high-resolution model data (1/12° × 1/12°) from a global simulation has been used. An evaluation of spatial and temporal patterns of the MLD and energy conversion is provided, and the dominant types of instabilities are determined. It is hypothesized that these instabilities drive the energy conversion and the growth of coherent mesoscale eddies, which can modify the MLD and restratify the ocean. Finally, the sequential interactions among the processes are investigated to provide better understanding about seasonal MLD variability. This study shows that the density-based MLDs with a threshold of 0.03 kg m^-3 are the most credible values, and the spatial and temporal patterns of energy conversion and gravitational/symmetric instabilities are in phase with the MLD variability. The energy conversion is investigated by means of the available potential energy (APE), kinetic energy (KE) and Energy Ratio (ER) which is introduced in this study, and a large amount of gravitational and/or symmetric instabilities is found within ML, especially in the upper ocean layers. The role of baroclinic instabilities is investigated with the Eady growth rate, while the presence of coherent mesoscale eddies is inferred from the Okubo-Weiss parameter and the Eddy Kinetic Energy, whose size is limited by the internal Rossby radius. This study shows that the MLD variability is the result of changes in the conversion between the available potential energy (APE) and kinetic energy (KE) as well as of the competition between ravitational/symmetric and baroclinic instabilities. The former favoring MLD deepening, and the latter favoring MLD shallowing. ... Read more