This thesis investigates the implementation of three-dimensional nudging into large-eddy simulation (LES) to assimilate observed atmospheric data into an LES model. 3D-nudging 'pushes' the thermodynamic fields in a simulation towards the desired observed fields. The aim is to test if such a method is useful in improving solar forecasts of stratocumulus-topped boundary layers. For this purpose 3D-nudging LES solar forecasts are compared to persistence forecasts and conventional LES-based forecasts. As a proxy for observations, exact thermodynamic fields from LES were used in this research. Using LES fields is advantageous as it provides full 3D thermodynamic fields but also dynamic fields for checking the turbulence in the different methods. Results show that 3D-nudging is quite capable of replicating the desired thermodynamic fields. Unfortunately, nudging comes with a penalty as it causes the turbulence built up in a simulation to be flawed. This effect is mitigated by the design of variations on the nudging technique, the most promising of which is multiple time fields nudging, which nudges the thermodynamic fields in a simulation to subsequent desired fields every 10 minutes during the nudging period. Solar forecasts found by this method are found to be more accurate than the persistence and regular LES methods on forecast horizons of 30 minutes and larger. Approaches proposed in this study to approximate thermodynamic fields from observational data estimate thermodynamic fields to a reasonable accuracy but are far from perfect, and thus it should be noted that solar forecast accuracy of the discussed methods will be less accurate when applied to real observations. Further research is recommended to focus on the use of the 3D-nudging methods in more LES case studies, and on devising better methods for the estimation of thermodynamic fields from observations.

In this research, a model to simulate the urban surface energy balance (SEB) model is developed. Urban geometry plays an important role in the SEB and the resulting surface temperatures. A well-known effect of how building geometry affects radiation is shadowing and trapping of LW radiation. The goal of the energy balance model is to quantify the difference in SEB for different urban geometries and thereby guide the choice of building-resolving methods for turbulence models. In order to capture the surface geometry, two algorithms are developed to calculate the sky view factor (SVF) and shadow factor (SF). The SVF represents how much of the sky is visible for a surface and influences diffuse radiation. The SF determines whether a surface receives direct solar radiation, based on the solar trajectory. The surface energy balance is simulated for both a 0.5m resolution AHN data grid and LES-averaged grids, which usually have larger grid cells. The LES method considers 3D grid cells, where a grid cell has volume ΔxLES ⋅ ΔyLES ⋅ ΔzLES. An LES grid cell is considered part of a building if the volume is more than half filled, and empty if it is less than half filled. The two different grids result in large differences in SVF and SF values, which result in temperature differences up to around 6 K for the simulations run in this study. The simulations are affected by all simulation parameters, of which the aerodynamic resistance, outside temperature and building temperature, are discussed. From the results, we conclude that the choice of grid has a significant impact on surface temperature and energy fluxes.

In the atmospheric boundary layer, when surface heat flux is small and mean wind shear is strong, horizontal convective rolls that are elongated along the wind shear are formed. This study attempts to explain the asymmetry of rolls in terms of turbulence using large-eddy simulations. A pressure gradient in the north-south y direction is applied, which results in an east-west geostrophic wind. It is shown that the turbulent kinetic energy components in the x and y directions are not equal when rolls develop. In addition, a countergradient regime is present for vertical momentum flux in the y direction in convective boundary layer with rolls. In the countergradient regime, the wind variance in the y direction is destroyed, contrary to being produced in the x direction. The presence of a countergradient regime for v'w' but not u'w' suggests that the eddy viscosity in the x and y directions would be rather different, and even become negative for v'w'. Thus, the existing parameterization scheme in global atmospheric models may need to be modified. However, the design of an improved parameterization scheme is non-trivial as the countergradient regime is non-stationary when stability decreases, while it does not exist in neutral or stable boundary layers with rolls.

The Earth’s climate is changing, due to global warming, impacting the ocean circulation around the world. As the ocean circulation distributes large amounts of energy around the world, this can alter climate drastically if changed. The Atlantic Meridional Overturning Circulation (AMOC) is a fundamental ocean component to comprehend climate change and further investigation enhances our capacity to predict it. The AMOC plays a pivotal role in regulating the ocean heat transport within the North Atlantic Ocean, influencing the climates of North America and Europe. This study centers its attention on the Sub-Polar Gyre (SPG), a critical region where the AMOC activity peaks. Within this region, this study aims to get a better understanding of the overturning dynamics of the SPG, on a seasonal and annual time scale. To achieve this, the reanalysis model GLORYS12 is used, which offers a detailed simulation of ocean dynamics spanning the period from 1993 to 2020. With its high-resolution, eddy-resolving capabilities, GLORYS12 is particularly well-suited for capturing the nuanced small-scale overturning processes associated with the AMOC. From these model data, the overturning is calculated from alongstream changes in boundary current transport divided in density classes. The analysis is performed for the entire SPG by dividing it into its major basins: the Iceland Basin, Irminger Sea, and Labrador Sea. Subsequently, the boundary currents of the SPG are further subdivided into seventeen individual segments, providing insights into how overturning dynamics vary along the SPG. The results reveal that the mean overturning strength in the SPG for 1993-2020 is 23.8 Sverdrups (106 m3/s (Sv)). The distribution in overturning strength between the basins is 41%, 29%, and 30% for the Iceland Basin, Irminger Sea, and Labrador Sea respectively. Furthermore,
the results shows overturning occurs at increasingly higher densities, the further west you go. Each basin displays a pronounced seasonal pattern, with maximum overturning occurring in March and the minimum in September. On an inter-annual time scale, the overturning strength in both the Iceland Basin and Irminger Sea exhibits a decreasing trend of -0.04 and -0.02 Sv/year respectively, whereas the Labrador Sea has an increasing trend of 0.02 Sv/year over 1993-2020. A further division in shorter segments yields large spatial differences in overturning, both in overall strength and the distribution over density classes. However, these outcomes are less robust as flows are highly variable and numerical errors associated with the overturning calculations become more prominent. This also raises questions about the reliability of the assessment
of overturning along segments from observations to determine the local overturning dynamics. In conclusion, this study leverages GLORYS12 for a detailed basin and segmented analyses to offer a comprehensive understanding of the AMOC within the SPG. The findings provide valuable insights into the AMOC’s long-term behavior, seasonal variations, annual trends, and high spatial variability. Using this increased understanding, future research can improve on why the AMOC behaves in the observed way, by analyzing the overturning dynamics sensitivity to oceanic and atmospheric conditions

We aim to make solar power forecasts using regression for grid optimization. We forecast from 1 to 6 hours ahead. The forecasts prove to be accurate with a skill score of 20% compared to persistence for a lead time of one hour.

As the intensity and frequency of urban heat wave increases in Europe, evaporative cooling from watering pavements has been considered as a promising strategy to regulate urban temperature. The aim of this research is to include evaporative cooling modelling and enhance the surface temperature model SURF-TEMP, which is in development at the Swiss Federal Institute of Aquatic Science and Technology (Eawag), to predict the optimal use of water resources towards mitigating urban heat. In order to test various evaporation models found in literature, we first developed a simple Python model to evaluate ten evaporation models. Differences among the simulated evaporative heat fluxes and surface temperature reduction can be around two folds under the same simulation condition for different evaporative cooling models. Besides the evaporation models, three equations to obtain the convective heat transfer coefficient were tested. We tested their uncertainties and determined the equation that provides the most neutral estimation on convective heat transfer coefficient. We proceeded to conduct various simulations to understand the sensitivity of these evaporation models with respect to four factors (wind speed, relative humidity of air, initial surface temperature, and initial water height). These factors influence the evaporative cooling efficiency and therefore the optimal watering rate. Two evaporative models that represent the higher and lower limit of the simulated evaporative heat flux are incorporated into SURF-TEMP. Results show that the model that estimates the highest evaporative heat flux is in good agreement with the lab measurement published by (Parison et al.,2020) when watering rate is equal or above 1 mm per hour. When watering rate falls below 0.75 mm per hour, the models studied underestimate the evaporative heat flux and surface temperature reduction. Discrepancies during low watering rate are caused by the absence of modelling of water conduction and infiltration with the pavement, non-linear relation of evaporation rate with vapor pressure difference, and the inherent error between simulation and experimental results even under dry condition.

In this thesis, the foundations are explored for establishing two-dimensional depth-integrated morphodynamical equilibria for a long embayment with two tidal inlets connected to the open sea. Governing water motion, suspended sediment transport and bed evolution equations had to be rederived from the underlying balances, where the tidal inlets are prescribed by M2 forcing, and the bedload transport is primarily due to the slope gradient. The equilibrium solutions are determined by an asymptotic expansion in a small parameter and applying a temporal Fourier series up to the leading order term. From this, a solvable system of six partial differential equations is derived. Using finite-difference discretization, this system of partial differential equations is turned into a discrete lexicographically ordered system of nonlinear constraints. The Newton-Rapsons method is used in a newly developed code to be able to produce morphodynamical equilibrium solutions. When the cross-sectional discretization contains only one interior point, the solutions are in accordance with the cross-sectional model. In particular, the more complex bifurcation structure arises after variation of the tidal phase, amplitude and inlet depth. This strengthens the recent findings of complex bifurcation structures. These width-averaged equilibria seem to stay accurate equilibria in the cross-sectional expanded model even for the more complex behaviour. The question of these equilibria are stable cannot be answered as firmly as some unexplained behaviour arose during the analysis of the eigenvalues. However, these results hint that the width-averaged equilibria remain stable until the width of the channel is half its length. Further research is needed to prove this.

Airport operations are highly dependent on safe weather conditions for their daily operations. Especially the visibility conditions are crucial. Low Visibility Procedures (LVP phase M, A, B and C) are defined at Schiphol Airport, resulting in capacity and runway use restrictions. These Low Visibility Pro­cedures are based on a post­processing model of a Numerical Weather Prediction (NWP): the Terminal Aerodrome Forecast Guidance (TAFG). This model provides a probability for the occurrence of a given LVP phase, based on thresholds of the Runway Visual Range (visibility including back luminescence at airports) and the ceiling (cloud base height). Currently, the TAFG is operational for three NWP models (HARMONIE, HIRLAM and ECMWF). This research found that the HIRLAM TAFG has a better performance than the HARMONIE TAFG. This result is caused by the larger training data­set that is used for the HIRLAM TAFG (20 years) compared the training data­set of for the HARMONIE TAFG (3 years). The ECMWF TAFG has a better performance for the severe LVP phases B and C. This is remarkable, since ECMWF is not often used for airport operations. However, the standard error of the performance indicator of the ECMWF TAFG is higher compared to the other models and, therefore, a larger data­set should be studied to confirm these findings.

Er wordt geschat dat 80% van al het zuurstof op aarde geproduceerd wordt door fytoplankton. Dit is een soort eencellige plant die voorkomen in wateren over de hele wereld. Deze kleine planten worden voornamelijk gegeten door zoöplankton, kleine organismen bestaand uit meestal enkele cellen. Afsterving of overbevolking van fytoplankton heeft ook verregaande gevolgen voor de rest van het maritieme ecosysteem. Fyto- en zoöplankton hebben populaties die een oscillerend patroon kunnen volgen. De groei van de ene beïnvloed het voorkomen van de ander en zo ontstaat een interessante wisselwerking. Er zijn vele modellen die dit gedrag proberen te simuleren om zo inzicht te krijgen in de verschillende patronen die zichtbaar zijn in planktonpopulaties. Een van deze modellen is het model van Steele en Henderson uit 1992. Dat model is echter gemaakt voor wateren zonder ruimtelijke structuren. Om het model van Steele en Henderson ook toe te passen op stromend water in een rivier wordt in dit onderzoek een model beschreven voor planktonpopulaties in rivieren, waarbij de volgende onderzoeksvragen zijn gesteld:
- Welke patronen zijn er te vinden in het model van Steele en Henderson zonder convectie en diffusie?
- Wat is het effect van convectie in een rivier op de uitkomsten van het plankton model van Steele en Henderson?
- Wat is het effect van diffusie in een rivier op de uitkomsten van het plankton model van Steele en Henderson?
- Wat is het effect van convectie én diffusie

In this thesis the hydrodynamic behaviour of an idealised relatively short tidal estuary is studied using a cross-sectionally averaged model, i.e. a one dimensional model. The geometry of the estuary is simplified and only the hydrodynamic processes are considered. This model is used to gain insight in the behaviour of currents and water levels influenced by a movable barrier situated in the estuary. It is investigated whether shifting the motion of the barrier in time influences the sedimentation in the embayment.

Estuaries support a unique hydrodynamic system due to the fresh water flowing into a salt water sea. This leads to stratification which can significantly alter internal tidal dynamics in the estuary. A model has been developed by Winant (2010) to describe the water motions of a two-layer system in tidal basin. The aim of this thesis is provide an extension to this model by parameterizing bottom stress using a partial slip parameter and viscosity. Also level variations of the bed are taken into account in this thesis. A width-averaged model of an idealized stratified tidal basin is developed. With increased bottom stress, the velocity near the bed is significantly slowed down, whereas increasing the viscosity slows down the velocity further away from the bed. The interface fluctuates more when the viscosity is low, the wavelength of these fluctuations is decreased with increasing viscosity. Resonance characteristics in the hydrodynamics are dampened severely by increased viscosity, and less by bottom stress. Increasing the slope of the bed results in a lag of tidal dynamics at the end of the embayment compared to at the embayment entrance. A sloped bed also dampens the resonance characteristics.

The present study aims to assess and advance the prediction skill of solar radiation in high resolution weather forecasts by large-eddy simulations (LES). The GPU-Resident Atmospheric Simulation Platform (GRASP) was used to simulate the atmosphere around the Cabauw experimental site for atmospheric research (CESAR) in the Netherlands. Large-scale boundary conditions were provided by coupling the simulation to a general circulation model (GCM). Radiative tendencies were calculated using two di_erent implementations of the Rapid Radiative Transfer Method for GCMs (RRTM-G): one runs in advance of the simulation using pre-calculated atmospheric _elds, the other employs dynamically updated _elds during the simulation. Both con_gurations generated simulations of every day in 2016, which were compared to each other and validated using observations from the Baseline Surface Radiation Network (BSRN). This study revealed that the implementation of interactive radiation altered cloud representation in the simulations, in most cases causing clouds to rise. This process correlated with an increase in turbulence kinetic energy of up to 2 m2/s2 locally. The clouds that were raised tended to break up more often between 5 and 7 km altitude, leading to a decrease in average cloud fraction and increase in short-wave down-welling radiation. The results suggest that the implementation of interactive radiation enabled the development of cloud top entrainment instabilities, which could be responsible for the cloud breakup in these cases. Regardless of the chosen implementation of the radiative transfer method, large errors are made in the prediction of surface solar radiation. GRASP produced root mean squared errors (RMSE) of 122.4 W/m2 and 115.7 W/m2 using prescribed and interactive radiation, respectively, while the large-scale model used to provide the initial and boundary conditions to the simulation produced an RMSE of 87.3 W/m2. The large error in GRASP's prediction of surface solar radiation can partly be attributed to conversion errors made during GRASP's initialization of the thermodynamic state, which lead to erroneous diagnoses of the liquid water content in the atmosphere.

A freshening of the boundary current (Labrador current and West Greenland Current) supresses deep convection in the Labrador Sea. Furthermore, the fresher water contributes to the ``strength'' of Irminger Rings and increases their life time. This leads to a larger spread of high EKE into the interior of the Labrador Sea and confines the deep convection region more towards the west.

Tropical cyclones have the ability to very quickly increase in strength. This process is called rapid intensification and as a result, tropical cyclones can transform into hurricanes. Rapid intensification is related to the availability of heat and the amount of negative feedback of the ocean on the tropical cyclone. Negative feedback results in the weakening of the tropical cyclone. Cyclones passing over a warm ocean anomaly have access to more heat and due to the relatively high temperatures, the amount of negative feedback is reduced considerably. A necessary condition for rapid intensification is therefore the presence of a warm ocean anomaly, often being warm core eddies. This paper relates the rapid intensification of tropical cyclone Matthew to the presence of warm core eddies in the track of Matthew. Results show that there is no extensive evidence found for the presence of a warm core eddy before rapid intensification took place. Although maps of the sea surface height and sea surface temperature indicate the possible existence of a warm core eddy, surface velocities do not show the characteristic rotation flow of an eddy. The enthalpy flux is considerably large just before the rapid intensification of Matthew indicating that the negative feedback by the ocean is reduced and heat is available for transport. The rapid intensification of Matthew might be linked to other physical mechanisms that have been overlooked. Possible mechanisms identified are the Amazon-Orinoco river plume and La Ni˜na. Further studies on the rapid intensification of tropical cyclone Matthew should therefore take into account these mechanisms and study their influence on rapid intensification.

This report proposes improvements of the land surface model (LSM) used in the turbulence-resolving Dutch Atmospheric Large-Eddy Simulation model (DALES). Important changes include the infiltration of precipitation, the parametrization of the soil hydraulic functions and the formulation of the soil water extraction by vegetation roots. The performance of the improved LSM version is validated using offline simulations and observations from the CESAR meteorological observatory in Cabauw (Netherlands), during the year of 2015. An optimal parameter set for this location set is obtained from both satellite retrievals and in-situ observations. In particular, the seasonal dependence of both the Leaf Area Index (LAI) and the roughness lengths for heat and momentum is prescribed to the model.

The offline simulations determined that the improved version of the LSM is able to model the surface fluxes during the year of 2015 with an accuracy of 10 W/m2. This accuracy was found to be strongly dependent on the chosen land surface parameters such as the LAI, the roughness lengths, the soil textures and the vegetation root profiles. Furthermore, the lower boundary for soil water transfer, set to free drainage, was found to cause too much vegetation stress after an extended simulation time.

Finally, the LSM is tested in a fully-coupled mode during three different meteorological events observed at the CESAR observatory. Firstly, stratocumulus simulations confirm the importance of the surface evaporation on the development of the stratocumulus layer, which is considerably reduced compared to simulations that assume the surface to be saturated with water. Secondly, the growth of the clear convective boundary layer was found to be greatly affected by the partitioning of the surface fluxes, as the sensible heat flux plays a key role in entrainment. Although the modelled sensible heat flux was found to be in agreement with the observations, the growth of the boundary layer was still substantially underestimated. Lastly, it was found that it is possible to model the main development stages of radiation fog in DALES, despite the fact that the modelled fog layer appears to be too persistent.

The wind energy industry is growing more than ever before and wind energy as a renewable energy source has shown quite a potential over the years. Unfortunately, the power yield of a wind farm can fluctuate largely over time, which originates from fluctuating wind speed magnitudes. Convection of air, turbulence, humidity and radiation of heat are processes in the atmospheric boundary layer, the lowest region of the troposphere above Earth’s surface, that are responsible for these fluctuations. This region is characterized by a diurnal cycle in which two examinable cases can be found, namely the convective boundary layer (CBL) case during the daytime period and the stable boundary layer (SBL) case during the nighttime period of the diurnal cycle. In the SBL case, the earlier mentioned physical processes result in a cool surface layer and relatively little turbulence is present, whereas in the CBL case a relatively warm surface layer and high turbulence level are present. The Dutch Atmospheric Large Eddy Simulation (DALES) model is used in which wind turbines are implemented with the help of former TU Delft master student P.A. van Dorp. With DALES, a simulation is performed for each case and the output data of the two simulations are analysed and compared to find the optimal case in terms of total power yield of a configuration of two wind turbines with the second fully in the wake produced by the one in front. An arbitrary distance of 600m between the wind turbines is chosen, which corresponds to 7.5 times the diameter length of the wind turbines, and a hub height of 80m is chosen. First, the simulations are validated by examining the turbulent kinetic energy profiles. Then, the wind speed profiles over the domain are analysed, corresponding to the turbulent kinetic energy profiles and showing that the average wind speed at hub height for the SBL case is larger than for the CBL case. Furthermore, the wake profiles behind the turbines are displayed and compared for the two cases, showing little difference. Finally, the total power yield of this specific wind turbine configuration is calculated. With an average power yield of 1.71MW for the SBL case compared to 0.90MW for the CBL case, the SBL case is shown to be the optimal atmospheric boundary layer case in terms of power production for this specific wind turbine configuration.

The clouds that form in the atmospheric region close to the Earth surface work as a parasol for our planet. Their primary effect is to reflect the incoming solar radiation and in this way to cool the Earth. Tlie more extensive and the thicker they are, the more radiation is reflected back to space. The extension is generally quantified by the cloud cover. Stratocumulus is a low-level cloud type that presents a particularly high cloud cover (40%-60% compared to the global average value of 30%). To measure their thickness, the amount of water in liquid phase in the whole cloud, the liqidd water path, is considered. @en