If you are interested to study turbulence, clouds, radiation or wind  in the atmosphere you are always welcome to talk about a possible research project. We are more and more switching to using our modeling tools to make predictions of solar radiation and wind velocities as input for PVs and wind turbines. To give you a flavour of the range of topics that can be studied, you may want to read the summaries of some current and previous student's research projects.

A nice introduction paper about stratocumulus is written by Bennetts et al. from the UK Meteorological Office. A good manual of the LES model is written by Thijs Heus, Chiel van Heerwaarden and Johan van der Dussen. You may also want to read the Dales manual that has been written for the course "From Field Observations to Modeling". If you are going to report your results, you may want to take a look at "Writing a Paper" by George Whitesites. A thorough overview of many aspects of "Clouds and Climate" was presented by various lecturers at the EUCLIPSE Summer School in Les Houches, France. The syllabus 'Clouds' was originally written for a course on clouds at Utrecht University, and a draft syllabus focusing on atmospheric boundary layers is available for TU Delft students.

Students who have just started their project

Documentation about running DALES on the local system (by Pim van Dorp).

Information about all the DALES namoptions file (by Thijs Heus, Chiel van Heerwaarden, Johan van der Dussen, Huug Ouwersloot, August 21, 2015)

Documentation about the local IT infrastructure (by Erwin de Beus). Example of a job script to run Dales at the VRLab.

Version control with git (by Thijs Heus and Chiel van Heerwaarden, September 27, 2009)

A handy tip to move files between the VR lab environment and your Windows machine (by Lisette Kloosterboer, 2 Nov 2018)

For GRS/EE students: please find all relevant forms here (Faculty) and here (GRS)

Proposals for Master End (MEP) projects

Ongoing and finished Master End (MEP) projects

45. Gijs Henstra (Applied Physics), 2023. How do buildings heights and widths optimize the urban heat island effect?

44. Caspar Jungbacker (GRS), 2023. Speeding up model simulations with DALES using OpenACC as an accelerator. Project in collaboration with Pedro Costa (3ME) and Fredrik Jansson (GRS).

43. Eric de Gruijter (Applied Physics), 2023. Efficiency of local heat sources to batlle surface frost under stable nighttime conditions. During conditions of nighttime frost some farmers attempt to protect their future harvest by producing local fires. Eric is investigating the impact of local heat sources on the 3D temperature distribution. In particular, he questions whether it benefits to use more but smaller fires, at smaller intermediate distances. This is motivated by the fact that warm air tends to rise upwards whereby heat can potentially be distributed more efficiently in the vertical direction than in the horizontal directions, thereby opposing the main aim to heat the air close to the ground surface.

42. Mats Steerneman (Applied Physics), 2023. Improving stratocumulus cloud predictions with a novel nudging technique applied to large-eddy simulations.

41. Annelot Broerze (Applied Physics), 2023, collaboration with the KNMI (Dr Natalie Theeuwes and Dr. Bert van Ulft) and Erasmus MC. The Netherlands is preparing for climate change, which includes land-use changes. For example, more space for open water bodies are planned. This is an important measure to mitigate potential flooding conditions in case of extreme precipitation. However, the presence of water may change the atmospheric conditions in terms of humidity and temperature. Such a scenario is of interest as it may impose possible changes in the risk of the spreading of vector-borne diseases.

40. Lydia Mak (GRS), 2023, additional thesis, Dr. Louise Nuijens as 2nd supervisor. Countergradient momentum flux in the presence of rolls in the atmospheric boundary layer.

39. Pam Witsenboer (2023, Applied Physics) Modelling the dispersion of air pollutants in Rotterdam using DALES

38. Rosalie Kievits (2023, Applied Physics) To improve the representation of building geometry for turbulence modelling in urban areas

37. Steven Lin (2022), Environmental Engineering, co-supervision with Dr. Alejandro Figueroa, Swiss Federal Institute of Aquatic Science and Technology and Dr. Marie-Claire ten Veldhuis). Additional Thesis entitled 'Modelling pavement evaporative cooling to mitigate heat wave'.

36. Suzanne Huijskes (Applied Physics), 2021-2022: The closure of the momentum budget at Cabauw. Observations of the full momentum budget (tendencies, large-scale advection, pressure and Coriolis forces and the turbulent momentum flux) that were provided by Pim van Dorp of Whiffle company reveal a systematic error in the budget. Suzanne has applied and studied data obtained from both observations and the KNMI HARMONIE weather forecast model to study the full momentum budget including large-scale horizontal advection, the horizontal pressure force, and turbulent fluxes.

35. Mark van Venrooij (Applied Physics), 2021-2022: Mark has analysed satellite data as obtained from MSG (Meteosat Second Generation, kindly provided by Dr. Jan Fokke Meirink). Mark explored whether these satellite data can be used to improve the initial conditions (cloud base and top heights) in weather forecast models including large-eddy simulation models. A comparison of ground-based observations indicated that stratocumulus cloud layers in MSG have too high cloud tops.

34. Wouter Koks (Applied Physics), 2021-2022: How does the partitioning between the sensible heat flux and evaporation control the onset of deep convection? In collaboration with Prof. Jordi Vila of Wageningen University (WUR). The ground soil conditions determine which fraction of the net radiative energy is used for producing sensible and latent (evaporation) heat fluxes. Wouter has used large-eddy simulation to show that the onset of shallow cumulus clouds is highly dependent on this partitioning. Surprisingly, shallow cumulus cliouds appear the earliest in cases of low surface evaporation, thereby corroborating findings in previous literature like early cloud formation over cities. Wouter has used the LES results to improve a conceptual cloud model used by the WUR.

33. Yuri Bonnana (Applied Physics), 2021: Speeding up the radiative transfer calculations in DALES (co-supervision with Prof. Pier Siebesma and in collaboration with Howard Barker).

32. Noud Jaspers (2022, TBM), Spatio-temporal solar power forecasts via regression (with Dr. Remco Verzijlberg as a main supervisor)

31. Bart Warmerdam (Applied Physics), 2021:  Atmospheric Flow over Hills: A Comparison of Large-Eddy Simulation results with measurements from the Bolund Experiment. Bart has investigated whether the Immersed Boundary Method can be used to represent the effect of topography in a large-eddy simulation model. To this end he has implemented the 'Bolund' hill in DALES. He has performed simulations for neutral stratifications and has compared the modeling results with observations. As a key founding Bart has found that the wake effect in the simulations is too weak. This is possibly due to the parameterized friction by obstacles that stems from small-scale laboratory experiments, and which tends to become negligibly small for typical atmospheric boundary layer applications.

30. Steef de Baat (Aerospace Engineering, co-supervision by Dr. Daphne Stam), 2021: Marine cloud brightening.  Low stratocumulus clouds strongly reflect downward solar radiation back to space. Their reflectivity is enhanced if more cloud condensation nuclei are present as this increases the cloud droplet number concentration. The latter tends to enhance the cloud reflection because it leads to a larger total cross section (more surface area) of the cloud droplets such that more photons will hit cloud droplets. One has been speculating that spraying salt sea water in the air, thereby enhancing the number of cloud condensation nuclei, may aid to reduce the global warming effect. However, because processes like rain formation and turbulence are also strongly dependent on the cloud droplet number concentration dynamical feedbacks occur which potentially can lead to a thinning of the cloud, thereby opposing the desired effect of more reflection of solar radiation. Steef has explored this for a few selected cases of subtropical marine stratocumulus clouds. One of the key findings is that entrainment mixing of relatively clean air from above the thermal inversion that caps the cloud may oppose the input of cloud condensation nuclei from the surface.

29. Soraya van Beek (Environmental Engineering), 2021: The visibility at Schiphol is key for the number of aircraft operations at this airport. Short-term predictions as obtained from ECMWF, HIRLAM and HARMONIE are analysed to assess their skill.

28. Hannah Herbschleb (Environmental Engineering), 2021: Modeling air pollution concentrations in the city of Eindhoven with the Dutch Atmospheric large-eddy simulation model. This project is performed in close collaboration with TNO (Dr. Ruud Jansen). Hannah has used data from the Algemene Hoogtekaart Nederland to implement the geometry of buildings as well as a highway in DALES. The immersed boundary method is used to capture the blocking effect of buildings on the flow.

27. Giovanni Negro (IMAU, Utrecht University, with Dr. Aarnout van Delden as a co-supervisor), 2021: During cold night turbulence may vanish. However, if infrared radiative cooling is non-homogeneous,  air parcels may obtain different temperature (buoyancy) perturbations, which in turn leads to different vertical accelerations. Giovanni has found that different radiative cooling rates will be induced by variations in the water vapor concentrations. However, the effect on driving turbulence is found to be very weak.

26. Anda Knol (Applied Physics), 2020: The effect of turbulence in low clouds on the evolution of the wind and wake behind a wind turbine. This project is in collaboration with Pim van Dorp of Whiffle (company that focuses on detailed wind predictions with aid of a GPU based large eddy simulation model).

25. Wouter Stiphout (Applied Physics), 2020: Is it possible to retrieve cloud droplet concentration numbers from measurements of solar radiation and vertically integrated liquid water?

24. Michael Koene (Applied Physics), 2020: Michael has implemented the immersed boundary method as developed by Jasper Tomas in Dales. Thanks to Michael's work DALES can now be run with obstacles like buildings.

23. Amber Nusteling (Applied Physics): Diagnosis of turbulent fluxes in the grey zone from large-eddy simulations of convective boundary layers.

The problem: Up to some years ago weather forecast models applied grid size meshes that were too large to resolve turbulent flow. This set up requires the need to parameterize the effect of turbulent fluxes of heat, moisture and momentum. Traditionally it has been assumed that the effect of horizontal fluxes on the mean state evolution is negligibly small. However, due to  the ever increasing computational power, finer and finer horizontal resolutions can be applied, which makes the latter assumption questionable. We would like to estimate the contribution of horizontal fluxes to the evolution of the mean state from three-dimensional fields as obtained from a large-eddy simulation model. More precisely, we would like to quantify the magnitude of the horizontal transports as a function of the horizontal grid size. Amber works with the Dutch Atmospheric Large-Eddy Simulation (DALES) model.  The model allows the output of 3D fields of wind velocity, temperature and humidity.This project is aiming at improving the representation of turbulent fluxes in the so-called Grey Zone (Wyngaard, 2004, 'Toward numerical modeling in the Terra Incognita'), a topic which is of interest to any weather or climate institution that operates its model at a horizontal grid mesh size smaller than ~ 10 km.

22. Maurice van Tiggelen (Geosciences and Remote Sensing): Towards improving the land-surface-atmosphere coupling in the Dutch Atmospheric Large-Eddy Simulation model, 2018. After his MSc project he started with a Phd project at Utrecht University.

This study 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 momentumis prescribed to the model.

21. Sander Guissepin (Applied Physics), 2018: Influence of shear and buoyancy on the length scales in the atmospheric boundary layer

20. Jiyunting Sun (Geosciences and Remote Sensing) - Develop a LES-based air quality model by nesting DALES in LOTOS-EUROS, 2016 (co-supervision with Dr. Tim Vlemmix).

19. Pim van Dorp: Large-eddy simulation of wind farms in clear and cloud-topped boundary layers (2016). Pim has continued  this work at the company Whiffle.

The vertical profile of the horizontal wind is strongly controlled by the vertical stability of the atmosphere as quantified by the vertical gradient of the temperature. During the day the ground surface is heated by the sun which leads to the subsequent triggering of convective plumes, whereas during clear nights often calm wind conditions are observed. This has a pronounced effect on the evolution of the wake structure behind wind turbines. In this project a non-uniform actuator disk model is implemented in the  Dutch Large-Eddy Simulation model, and the effect of the turbulence on the wake structure is investigated.

18. Floris Heuff: Stratocumulus prediction with LES. After this project Floris switched topic to a Phd project on geodesy.

Weather prediction models have great difficulties with the representation of stratocumulus clouds. This is due to the rather coarse vertical resolution which is close to the depth of the cloud layer itself. It is therefore tempting to investigate whether a better prediction can be made with an LES model. Emphasis will be put on the used initial conditions and the effect of large-scale advection.

17. Victor Vertregt: Modification of the LES subgrid model and its application to stable boundary layers (SBLs). A part of this work has been published in de Roode, S. R., H.J.J. Jonker, B. J. H. van de Wiel, V. Vertregt and V. Perrin, 2017: A diagnosis of spurious mixing in Smagorinsky type subfilter-scale turbulent kinetic energy models for stably stratified conditions, J. Atmos. Sci., 74, 1495-1511. Victor is currently working at Shell.

The development of stable boundary layers during night-time causes the sizes of turbulent eddies to become very small. To simulate SBLs a very high spatial resolution needs to be used in order to properly resolve the flow. If not, the solution will be fully determined by the TKE subgrid model. In this research a method originally proposed by Peter Sullivan will be implemented in the LES mode and will be tested for SBLs.

16. Laurens Wester: Convective radiative equilibrium (CRE). He currently works at Liander.

In the tropics deep convective clouds are continuously forced by a relatively warm sea surface and a destabilization of the atmosphere by radiative cooling. In this project an LES model is used to study CRE, with an emphasis on the the energy balance of the atmopheric column and the role of clouds in the system. Laurens' work was important for performing similar runs for the paper "Clouds and Convective Self-Aggregation in a Multimodel Ensemble of Radiative-Convective Equilibrium Simulations" published in 2020 by Wing et al.

15. Sander Giuseppin: Vorticity budgets of convective boundary layers.

During strong surface driven convection so-called dust devils can develop. On the other hand, if turbulence is driven by a strong horizontal wind street-like roll structures are observed. In this project the precise role of buoyancy versus wind-shear driven turbulence on the development of convective structures is investigated by means of a LES vorticity budget study.

14. Chiel Donkers: Solar radiative transfer in fields of broken clouds, Co-supervision with Harm Jonker. Chiel continued working in the area of radiation at Hukseflux Thermal Sensors.

Numerical models apply radiative transfer codes to vertical atmospheric columns. Because direct solar radiation does not travel in the vertical direction, in this project the LES model domain is tilted to compute radiative transfer according to the tilted independent pixel approximation (TIPA) as proposed by Varnai and Davies (1999).

13. Louise Vlot: A two-layer model for stratocumulus clouds.

Mixed-layer models consider the stratocumulus-topped boundary layer as a single slab. The model only requires the specification of the surface and top fluxes of heat and moisture, in addition to possible sink terms like radiation or precipitation. These models have pushed forwards our understanding of stratocumulus dynamics as well as the influence of large-scale conditions like the sea surface temperature on the stratocumulus cloud amount. However, its strength in handling the atmosphere as a single slab layer is also a weakness in the sense that it does not allow for a distinct two-layer structure with a warmer and drier cloud layer as compared to the layer below. The fact that the latter structure is more in line with observations motivates to build a new mixed-layer including a separate subcloud and cloud layer. The model will be used to study equilibrium states of stratocumulus as well as lagrangian experiments in which an air mass is advected from the subtropics towards the tropics.

12. Marijke Schuurbiers: High-resolution predictions for stratocumulus and a comparison with operation weather forecast model results.

In this study simulation of stratocumulus under a strong thermal inversions and in regions of subsidence with a Large-Eddy Simulation (LES) model has been researched. With use of observational data covering the year 2011 and 2012, stratocumulus cases are identified at Cabauw, in the Netherlands. With use of the selection, the climatology of stratocumulus at Cabauw is studied. It was found that the occurrence of stratocumulus under a strong thermal inversions and in regions of subsidence is about 5 % Stratocumulus is most likely to occur in combination with a south-west or north-east wind direction, in wintertime and in the morning or evening hours. A study of the representation of stratocumulus in a mesoscale model (RACMO) has shown that the model has difficulties with predicting stratocumulus. Subsequently simulations with a LES model have been performed on the basis of data assimilation. It is found that with use of observational data and LES the prediction of stratocumulus has potential. Especially predictions of the down welling solar radiation in the first 3 to 6 hours after initialization have shown to be improved.

11. Thomas Frederikse: Cold air outbreak: An LES study. Thomas obtained a Phd in Geodesy (cum laude) and is currently working at JPL.

Accompanying paper: S.R. de Roode, T. Frederikse, A. P. Siebesma, A. S. Ackerman, J. Chylik, P. R. Field, J. Fricke, M. Gryschka, A. Hill, R. Honnert, S. K. Krueger, C. Lac, A. T. Lesage and L. Tomassini, Turbulent Transport in the Gray Zone: A Large Eddy Model Intercomparison Study of the CONSTRAIN Cold Air Outbreak Case, 2019, J. Adv. Modeling Earth Systems.

Additional supervisor: Prof. Dr. Pier Siebesma (TUD/KNMI), Prof. Dr. Harm Jonker

If the wind is blowing from Greenland towards the North Sea area beautiful convective cloud structures are frequently observed. The typical length scales of the cloud structures are larger than the horizontal grid resolution used by state-of-the-art weather forecast models. This means that some key assumptions made in such models are violated. For example, the subgrid models are designed to take into account all turbulent transport. If the horizontal resolution is refined to ~1 km, some part of the turbulent transport will be resolved by the model. The aim is to use the Dutch Large-Eddy Simulation model to simulate a cold air outbreak, and to study its turbulence characteristics along its trajectory towards the North Sea area. Emphasis will be put on a length scale analysis using the Fast Fourier Transformation technique. This project will be done as part of an international collaboration between weather forecast institutions (UKMO, ECMWF, KNMI) and universities.

10. Johan Flikweert - A parcel model for shallow cumulus clouds

9. Arjan van Leeuwen – Evaluation of cloudy boundary-layer parameterization schemes from 3D fields obtained from large-eddy simulations of stratocumulus-to-shallow cumulus transitions.

8. Jasper Sival – Stratocumulus cloud response to changing large-scale forcing conditions 

Additional supervisor: Jerome Schalkwijk

The mixed-layer model (MLM) study by Melchior van Wessem showed a regime with unphysical stratocumulus solutions. In particular, for a relatively weak inversion stability the MLM tends to produce a stable stratified surface layer. This is the regime where the key assumption of a MLM breaks down, namely that turbulence tends to generate vertically well mixed profiles for conserved quantities like the total specific humidity and the liquid water specific humidity. In reality it is likely that shallow cumulus will dominate. Inspired by the CGILS project initiated by Minghua Zhang and Chris Bretherton, this project aims to study equilibrium states of stratocumulus close to this transition regime with a large-eddy simulation model. As runs with this model are computationally expensive, an GPU-LES (GALES) version that is recoded by Jerome Schalkwijk, Frits Post (TUD-EWI) and colleagues  will be used. The research will focus on the following questions: 1. can we find a criterion that distinguishes stratocumulus from cumulus solutions 2. what is the effect of including the diurnal cycle of solar radiation. Part of this work has been published in De Roode, S. R., A. P. Siebesma, S. Dal Gesso, H. J. J. Jonker, J. Schalkwijk, and J. Sival, 2014: A mixed-layer study of the stratocumulus response to changes in large-scale conditions. J. Adv. Model. Earth Syst., 6, DOI: 10.1002/2014MS000347.

7. Melchior van Wessem – Equilibrium states of the stratocumulus-topped boundary layer.  

The evolution of a stratocumulus cloud layer is determined by the turbulent fluxes of heat and moisture at the surface and the top (entrainment) of the boundary layer, in addition to radiative effects and loss of moisture by precipitation. This research attempts to assess whether equilibrium solutions of cloud layer depth are possible. To this end, the cloudy boundary layer will be treated as a single slab layer, and analytical solutions will be sought. In particular the effect of parameters like the sea-surface temperature and the large-scale divergence on the cloud layer depth will be investigated.

A part of Melchior's work has been published in Dal Gesso, S., A. P. Siebesma, S. R. de Roode, and J. M. van Wessem, 2014: A mixed-layer model perspective on stratocumulus steady-states in a perturbed climate. Q. J. R. Meteor. Soc., 140, 2119-2131, DOI:10.1002/qj.2282.

6. Steven de Boer – Scale dependence of parameterizations for shallow cumulus clouds.

Additional supervisors: Prof. Dr. A. P. Siebesma and Drs. W. de Rooij (KNMI) .

The grid size of numerical weather prediction models continues to become smaller. Some regional weather forecast models do use a horizontal grid size of 2 km. Therefore the key assumption made in such models, namely that all turbulent fluctuations are fully subgrid, becomes flawed. This study uses the LES model to analyse the scale dependence of quantities like the lateral entrainment and detrainment rates and variances of (thermo-) dynamic quantities in shallow cumulus clouds.

5. Huug Ouwersloot – Study of the flux imbalance problem in Cabauw tower observations

Additional supervisors: Dr. H.J.J. Jonker, P.S. Kroon (ECN) and Dr. F. C. Bosveld (KNMI)

Huug is interested in the eddy-correlation technique used to measure turbulent fluxes of heat, moisture, momentum and greenhouse gases. A key assumption made is that the mean vertical velocity is zero. In this study it is shown that this assumption is typically violated in the observations. In addition, time scales of turbulence are calculated from measurements collected along the 200 m tall tower in Cabauw, Netherlands. This work has been presented at the Boundary-Layers and Turbulence conference, Keystone, CO, USA, 2-6 August 2010.

4. Johan van der Dussen – The transition of stratocumulus to cumulus during ASTEX: An LES study .

In 1992 a giant field campaign was organized near the Azores to study the transition of stratocumulus to broken shallow cumulus clouds. Johan is studying this observed transition with the Dutch Atmospheric Large Eddy Simulation model. The question is whether the LES model is capable to reproduce the observations. Therefore the results of turbulent fluxes of heat and moisture from the model will be critically compared against observations. In addition, the entrainment rate will be diagnosed and interpreted by means of a mixed layer model. This work has been presented at the CFMIP-GCSS Meeting, Vancouver, Canada, 12 June 2009, and published in van der Dussen, J. J. , S. R. de Roode, A. S. Ackerman, P. N. Blossey, C. S. Bretherton, M. J. Kurowski, A. P. Lock, R. A. J. Neggers, I. Sandu, and A. P. Siebesma, 2013: The GASS/EUCLIPSE Model Intercomparison of the Stratocumulus Transition as Observed During ASTEX: LES results. J. Adv. Model. Earth Syst., 5, 1-17, doi:10.1002/jame.20033.

3. Ruud van der Ent – A new perspective on continental moisture recycling  (co-supervision), 2009.

Supervisors: Prof. Dr. H. H. G. Savenije, Dr. B. Schaefli and Dr. S. C. Steele-Dunne (TU Delft, Faculty of Civil Engineering and Geosciences)

The importance of moisture feedback between continental precipitation and evaporation, referred to as moisture recycling, is still under debate. A new accounting procedure based on ERA-40 re-analysis is used to calculate moisture recycling ratios.

2. Girmaw Gezahegn - Representation of nocturnal marine stratocumulus cloud by means of a mesoscale model (MM5), 2006 (co-supervision).

Supervisor: Dr. J. Vila (Wageningen University)

The regional weather forecast model MM5 is used to study the representation of stratocumulus near the Azores. It is found that the model typicallly underestimates stratocumulus cloud amount. A part of this bias is due to too small values of the eddy diffusivities used  in the model.

1. Simon Axelsen - The role of humidity on shallow cumulus dynamics: results from a Large-Eddy Simulation model, 2005.

This study used the LES model to perform simulations of the BOMEX shallow cumulus case. Small perturbations of humidity and temperature were introduced while maintaining an identical virtual potential temperature. As such the amount of CAPE was not changed. The simulation results clearly showed that the specific humidity is very important for the development of cumulus. In a slightly drier environment clouds grow less high and are less turbulent. It is concluded that the humidity should be considered to improve cumulus parameterizations. This work has been presented at the GCSS Working Group 1 meeting, Athens, May 2005.

Bachelor's (BEP) research projects

30. Teun Slob (2023, Civil Engineering), "Cooling down the Middle East evaporation of sea water with wind turbines"

29. Barbara Huijskes (2023, Applied Physics) - Barbara has applied DALES to confirme that the growth of a dry convective boundary layer  by entrainment is hardly affected if infrared radiatiive cooling of the thermal inversion layer is take into account.

28. Femke Brouwer (2021, Applied Physics) - The effect of an idealized sinusoidally perturbed sea surface temperature on turbulence and shallow cumulus cloud organisation

27. Lisette Kloosterboer (2021, Applied Physics) - Effects of water vapor on turbulence in the noctural stable boundary layer.

26. Kevin Schuurman (2020, Applied Earth Sciences) - An assessment of the spatial variability of large wind speed fluctuations from observations collected over the North Sea. The data were measured from a tower that was set up to study the wind climatology for wind energy.

25. Gijs van Ouwerkerk - Analysis of the horizontal momentum budget in the surface layer at Cabauw

24. Pim Jacobs - Using the COSMO-REA2 dataset to assess wind resource over the North Sea (co-supervisor, with Dr. Sukanta Basu), 2017.

23. Joey Smit - Observed power production in a wind farm, 2017.

22. Huib van Rijt - Optimization of droplet concentration of stratocumulus clouds and optical thickness of aerosols for RRTMG and calculation of the effective droplet radius at CESAR, 2017.

21. Julia Weiffenbach - A comparison of large-eddy simulation results of deep convection over India with observations, 2017.

20. Koen Steinebach - Simulation of the effect of atmospheric stratification on the power production of a wind farm, 2017.

19. Jesse van Sluis (Applied Physics) - Solar Radiation Transfer: Effects of water vapor in the clear atmosphere, 2017.

18. Stefan Wigman (Applied Earth Sciences) - The effect of large-scale subsidence on the vertical temperature profile over the Antarctic Plateau, 2016.

17. Caroline van Calcar (TBM) - Het effect van de concentratie condensatiekernen in diepe bewolking op albedo en neerslag , 2016.

16. Ivo van Kreveld (Civil Engineering) - Introducing OpenACC TO DALES, 2016.

15. Mirja Dooren (Applied Earth Sciences) - Impact of aerosols on stratocumulus occurence, 2016, co-supervision with Prof. Peter Hoogeboom.

14. Annette Duetz (Applied Physics) - Large-Eddy Simulations of horizontal wind streaks in a stable boundary layer, 2016.

13. Thijs Bon (Applied Physics) - Analysis of single wind-turbine wake dynamics with large-eddy simulation, 2016.

12. Magaly Doggen - The visibility of stratocumulus on weather radar (co-supervision with Prof. P. Hoogeboom), 2016

11. Marijn Mur - Long-term wind power prediction with the ECMWF model (co-supervision with Dr, R. Verzijlbergh, TBM, TU Delft)

This Bachelors thesis investigates the usability of the ECMWF model as a wind power output prediction model validated with measurements done with the meteorological mast at Cabauw, The Netherlands. Two different approaches for estimating the wind speed at 80 meters are compared to the 80 meter winds as measured at Cabauw. We found that extrapolating the 10 meter wind speed from the model with the simple logarithmic wind prole produces a large error in the prediction of the wind during the night. Interpolating between the wind speeds from two pressure levels from the model yields a 80 meter wind that has a Root Mean Square Error, Standard Deviation and correlation in the same order of magnitude as the extrapolated wind, but it predicts the wind at night better. The output power, Pm, is predicted with a power curve applied to the interpolated 80 meter wind. As reference output power, Po, we computed the power with the measured wind speeds using the same power curve. We found an average critical time of 65 hours with a predicted average power output error of 16% of the installed power.

10. Victor Vertregt - Radiative fluxes in an urban heat canyon. 

9. Evert Merkx (2013) An assessment on the scale dependency of horizontal turbulent tansport during a cold air outbreak

8. Corine de Winter - Persistent structures in a shallow cumulus cloud field. 

During the last decades, clouds have received a lot of attention in the climate community. The interaction of many processes (turbulence, condensation and evaporation of water, radiation) on a large range of scales (millimeters to hundreds of km) make the accurate simulation of clouds challenging. Large scale weather forecasting or climate models do not have a sufficiently high resolution to resolve all of those processes. Models that do have a high enough resolution to resolve all of the turbulence, on the other hand, are too computationally demanding to simulate entire clouds. The best of both worlds is offered by the so-called Large Eddy Simulation models, which resolve the larger turbulence scales (from several meters up to a few km), where most of the turbulent transport occurs. The processes with typical length scales smaller than the grid-size of the model are modelled. A LES model, called the “Dutch Atmospheric LES” (DALES), was used to simulate an observed transition of a stratocumulus cloud field (associated with a high cloud cover up to 100%) to a thin, broken stratocumulus layer penetrated from below by cumulus clouds. This transition is based on aircraft observations done during a large field campaign, which spanned almost 2 days. Large 3D fields of the most important variables, obtained with this model, can be used to investigate the processes controlling the formation of the cumulus clouds. Large scale, persistent updrafts, which have been observed in measurements and in simulations may play an important role in this.

7. Noud Brasjen - Steady-state solutions of Arctic stratus

One of the well-known effects of global warming is the decrease of the amount of sea ice in the Arctic Ocean. Besides an indicator of climate change, it is a source of warming in the polar regions as well. The so-called ice-albedo feedback describes the mechanism in which melting of sea ice due to warming of the surface leads to more warming due to the reduced surface albedo. This feedback system does not account the effects the changing surface has on cloud formation. The key of this thesis is to quantify whether the cloud formation increases or decreases the positive ice-albedo feedback. In order to answer this question a Large-Eddy Simulation experiment is performed in order to find equilibrium solutions of an observed boundary layer cloud in the Arctic Ocean. In this experiment only the surface description concerning the surface albedo and turbulent transfer coefficient is altered for a case with sea ice conditions and a case with open water conditions. The study shows that the presence of open water conditions leads to cloud thickening and thus more absorption of shortwave radiation in the cloud layer. However, the change in surface albedo proves to dominate the radiative budget, leading to the conclusion that clouds dampen the ice-albedo feedback, but the overall feedback remains positive.

6. Coen Hennipman - Quantification of the vertical turbulent transport in a 'decoupled' cloudy boundary layer.  

During daytime the stratocumulus cloud layer can become stably stratified with respect to the subcloud layer below due to absorption of solar radiation. As a result, surface-driven thermals can not easily penetrate the cloud layer anymore. A clear fingerprint of such a decoupled situation is given by the vertical velocity variance which will exhibit a double peak structure in this situation. A local minimum is typically found near the cloud base, at which level the buoyancy flux has a relatively large negative value. The question is how much heat and moisture is transported by the few scattered cumuli that tend to develop at the top of the subcloud layer. This will be quantified with aid of LES results of stratocumulus to cumulus transitions.

5. Joost Klip - Clear or cloudy boundary layers - that's the question, 2011. 

A mixed layer model is used to simulate the evolution of clear convective boundary layers as they are advected over the oceans from the subtropics towards the tropics by the trade-winds. The question is whether clouds develop, and if so, to calculate the time scale that predicts their formation. The free parameters we are interested in are the magnitude of the horizontal wind, the large-scale subsidence rate, and the strength of the inversion.

4. Arno Nederlof – Inversion stability and boundary layer cloud transitions, 2010.

In the subtropical parts of the Hadley cell large stratocumulus fields tend to develop. As they are transported equatorwards by the trade-winds these cloud fields gradually break up and are replaced by shallow cumulus clouds. The large-eddy simulation model is used to simulate cloud transitions for different initial inversion stabilities. Observations collected during the ASTEX First Lagrangian are used as a reference case to initialize the model, and additional simulations were performed in which inversion jumps of total water and temperature were systematically varied.

3. Yoerik de Voogt – The vertical velocity budget computed from a Large Eddy Simulation of shallow cumulus clouds, 2009.

Additional supervisor: Dr. A. P. Siebesma

In the literature different values are proposed for constants to be used for the vertical velocity equations applied to shallow cumuli. In this study Yoerik summarizes the literature findings and he diagnoses the values of some key constants from LES results. This work has been published in De Roode, S. R., A. P. Siebesma, H. J. J. Jonker, Y. de Voogd, 2012: Parameterization of the vertical velocity equation for shallow cumulus clouds. Monthly Weather Review. 140, 2424-2436, doi: 10.1175/MWR-D-11-00277.1

2. Tim Frijnts – The inversion structure above stratocumulus cloud top: A comparison between observations and Large Eddy Simulations of the DYCOMS RF01 case, 2008.

The Dutch Atmospheric LES model typically performed poorly for stratocumulus cases which inversion stability satisfies the buoyancy reversal criterion. Whereas observations show that stratocumulus can persist even for very dry inversion layers, the cloud layer in the LES model tended to rapidly evaporate. As part of their PhD projects Thijs Heus and Chiel van Heerwaarden have completely modified the model. This study investigates if the simulations of the observed DYCOMS II stratocumulus cloud layer are improved. To this end a detailed analysis of the simulated cloud top structure is made and compared to observations presented by Gerber et al. (2005).

1. Vincent Perrin – The effect of surface heterogeneities on the turbulence structure of the stable boundary layer, 2008.

As the name of the model suggests, a Large Eddy Simulation model is designed to compute large eddies in a turbulent flow. However, these models are also applied to stable boundary layers where the turbulence is very weak and eddies are relatively small. For simulations of the GABLS1 case the solution of the Dutch LES model results is fully subgrid. This study shows that the subgrid turbulent kinetic energy (TKE) solution is identical to the Smagorinsky solution, despite the fact that the LES model uses a prognostic subgrid TKE model. The value of the turbulent Prandtl number is critiziced. In particular its effect on diagnosed similarity laws for the flux-gradient relations φ_h and φ_m is discussed. This work has been presented at the 8th Annual Meeting of the European Meteorological Society, 29 September-3 October 2008, Amsterdam, The Netherlands.

Internships

Zelin Xu (GRS), 2021, internship at Royal Haskoning-DHV (Ronald Groen): "Comparison and validation to Sentinel-2 based urban albedo derivation methods".

Frans Liqui Lung (GRS), 2020, Application of DALES to simulate forest fires (with Roland Stull, University of British Colombia, Canada)

Anne Blecourt (Applied Physics), 2016.

Jorine Dreef (GRS), 2018, internship at Infoplaza (Michiel Severin), "Verbetering van motregen visualisatie"'

Benjamin ter Kuile (Applied Physics) – The relation between temperature and precipitation, KNMI, 2009.

Supervisor: Dr. G. Lenderink (KNMI)

It is expected that the intensity of the global hydrological cycle will change due to an enhanced greenhouse gas forcing. In particular, the magnitude of extreme precipitation is expected to increase for increasing sea surface temperature (SST). Invoking the Clausius-Clapeyron relation leads to an estimate for the rate of change of precipitation of about 7% per Kelvin. However, observations collected at De Bilt, Netherlands, suggest a higher change for temperatures above 15⁰C, namely ~ 14% per Kelvin (Lenderink and Van Meijgaard, Nature, 2008). This research combines precipitation measurements and weather type characterizations by meteorological observers to show that convective clouds exhibit a steeper precipitation increase for higher SSTs than rain in large-scale synoptic systems.