Student's research projects

 

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.

 

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).

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

1. How well is infrared radiation captured by radiative transfer models?

Water vapor is the most important greenhouse gas in the Earth's atmosphere. The downward infrared radiation received on the Earth's surface is increased if more water vapor is present in the atmosphere. The Dutch Atmospheric Large-Eddy Simulation Model, a high resolution turbulence model that is widely used by the Dutch research community, uses the Rapid Radiative Transfer Model for General Circulation Model (RRTMG).

We wish to establish the skill with which RRTMG calculates infrared radiation. To this end the RRTMG model will be run with vertical profiles of temperature and humidity, both of which will be based on a compilation of observations and reanalysis data.  The results will be compared with direct observations of infrared radiative fluxed collected at Cabauw, Netherlands. There will also be an opportunity to use observations of infrared radiation that were measured with a newly developed instrument by the Delft company Hukseflux.

 

 

2. The effect of radiative trapping on the Urban Heat Island
In the summer, during periods of weather with clear skies and weak winds, cities may become much warmer than their surrounding rural environment. This phenomenon is called the Urban Heat Island effect and is frequently observed both during the day and the night. During the day multiple reflections of solar radiation by vertical walls cause an overal enhancement of its absorption, while during the night buildings strongly diminish the escape of infrared radiation to space thereby allowing high air temperatures to persist. This radiative trapping effect does depend on the geometry of streets as quantified by the height to width ratio (H/W).
In this project we wish to use data from the Algemene Hoogtebestand Nederland (AHN) to make a map of H/W ratios. The next question is to which extent this is correlated to observed warm temperature anomalies. Finally, calculations of the radiative trapping effect will be performed to study which H/W ratios favor a maximum trapping effect.
3. Large Eddy Simulation of a city
The Dutch Large Eddy Simulation Model (DALES) is a widely used tool to study turbulence in the atmosphere. However, at present it does not include the capacity to include the effect of obstacles such as buildings on the air flow. The urban terrain has very different surface properties and a very different surface energy balance as compared rural areas. Since there is an increasing demand for detailed flow simulations in urban areas (i.e. air quality, wind energy and temperature), this strongly motivates to adapt DALES to be able to capture buildings.
The project aims to include the effect of buildings by implementing the so-called immersed boundary condition technique. Runs will be made both for cases with and without a vertical stratification of the temperature. A comparison will be made with results as obtained with  the Reynolds Averaged Navier Stokes Model URBSIM developed by our colleagues Patrick Schrijvers and Dr. Sasa Kenjeres from Applied Physics.
4. Can radiation drive flow in the nocturnal stable boundary layer?
Under conditions of weak winds and clear skies turbulence in the atmosphere tends to weaken strongly. Theoretical considerations predict that turbulence vanishes completely, which would occur is the air just above the ground surface becomes sufficiently cold to offset the potential of the wind to generate turbulence. However, observations indicate that some vertical transport does occur even under very stably stratified condtions. A pilot study has demonstrated that moisture, and its effect on radiative cooling, is a potential driver of air flow, even under very stable conditions.
The project aims to assess the importance of moisture heterogeneity on air flow in the nocturnal boundary layer via its effect on radiative cooling. To this end a suite of simulations with the Dutch Atmospheric Large Eddy Simulation (DALES) model will be performed. In particular the effect of the magnitude of moisture fluctuations and the vertical temperature stratification on air flow will be explored. The initialization of the model will need to be guided by observations such as collected at the KNMI measurement platform at Cabauw.

Ongoing and finished Master End (MEP) projects

 

Anda Knol: 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).

 

Wouter Stiphout: Is it possible to retrieve cloud droplet concentration numbers from measurements of solar radiation and vertically integrated liquid water?

 

 

Amber Nusteling: Assessment of horizontal turbulent fluxes of heat, moisture and momentum

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.

The question

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.

Research tool

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.

Relevance

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.

 

 

Maurice van Tiggelen: 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.

Jiyunting SunDevelop a LES-based air quality model by nesting DALES in LOTOS-EUROS, 2016 (co-supervision with Dr. Tim Vlemmix).


Pim van Dorp: Wind turbine wakes: the effect of atmospheric stability (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.

 

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.

 

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.

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.

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.

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).

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.

 

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.

 

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.

 

Johan Flikweert - A parcel model for shallow cumulus clouds

Additional supervisor: Prof. Pier Siebesma (TUD/KNMI)

In large-scale models that are used for weather predictions or the global climate the dynamics of shallow cumulus clouds are computed using a so-called mass-flux approach. The temperature and humidity in the cloud are affected by lateral mixing of clear environmental air across the cloud boundaries, and the intensity of the horizontal mixing rate is controled by the lateral entrainment factor. This rather simple model is currently being elaborated by the inclusion of a vertical velocity equation. In that case the in-cloud velocity is driven by a positive buoyancy, and is damped by a drag term. The budget equation for the vertical velocity has recently been investigated by BEP student Yoerik de Voogd, who provided some useful information concerning parameterization constants to be used for the buoyancy and drag term. In this research the mass-flux equations will be coded to study the sensitivity of the results as a function of the parameter constants used. For example, how are quasi steady-state solutions controled by the lateral entrainment factor?

 

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

Eddy-diffusivity and mass-flux models are two different branches of parameterization schemes that are used to calculate vertical turbulent transport in weather forecast and climate models. The eddy-diffusivity approach is used for homogeneous turbulence in the clear and stratocumulus-topped boundary layer, and mass flux models are applied to shallow cumulus cloud fields. Recently a new branch of eddy-diffusivity mass flux (EDMF) models have been developed that combine both schemes in order to capture all these different boundary layer regimes in a unified framework (Neggers et al. 2009). The schemes are typically well calibrated with results from well defined cases for a single cloud type. Recent results from a set of model intercomparison studies of stratocumulus-to-cumulus cloud transitions show that many a scheme used in weather forecast and climate models have difficulties in accurately representing the transition regime. This project is dedicated to diagnose and validate quantities from LES results that are key in parameterization schemes, like the eddy-diffusivity and mass flux profiles.

 

 

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.


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.

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.

 

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.

 

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.

 

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.

 

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.

 

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

 

Lisette Kloosterboer - Effects of water vapor on turbulence in the noctural stable boundary layer.

Kevin Schuurman - 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.

Gijs van OuwerkerkAnalysis of the horizontal momentum budget in the surface layer at Cabauw

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

 

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

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.

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

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

Jesse van Sluis - Solar Radiation Transfer: Effects of water vapor in the clear atmosphere, 2017.

Stefan Wigman - The effect of large-scale subsidence on the vertical temperature profile over the Antarctic Plateau, 2016.

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

Ivo van Kreveld - Introducing OpenACC TO DALES, 2016.

Mirja Dooren - Impact of aerosols on stratocumulus occurence, 2016, co-supervision with Prof. Peter Hoogeboom.

Annette Duetz - Large-Eddy Simulations of horizontal wind streaks in a stable boundary layer, 2016.

Thijs Bon - Analysis of single wind-turbine wake dynamics with large-eddy simulation, 2016.

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

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 diff erent 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.

Victor Vertregt - Radiative fluxes in an urban heat canyon. 

In this study the problem of the Urban Heat Canyon (UHC) has been addressed. The Urban Heat Canyon is a simplified model of an urban area, modelled as a two dimensional model of a street with uniform walls and an uniform street. This model is used to investigate the radiative component of heat balance and absorption, or the ratio between absorbed and incident radiation, of an urban area. The radiation balance in urban areas, modelled with the UHC, is such an interesting topic, because in urban areas, the walls absorb reflected solar radiation and emitted heat radiation from the street, and thus trap radiation. Several papers have been written already on the subject, in which the UHC has been researched with real life measurements, algebraic solutions and models on scale. Here the UHC has been addressed by using the Monte Carlo simulation, in order to simulate the statistics of a large number of photons, the quanta of radiation. The research did focus on finding the influences of the geometry, defined as the height to width ratio of the walls and street, on the absorption and the heat output, and the influence of the zenith angle, on the absorption. The main conclusions were that there is a positive relation between height to width ratio and the absorption, and a negative relation between the radiation output and the height to width ratio. Finally, a negative relation was found between the absorption of an Urban Heat Canyon and the zenith angle for direct radiation.

 

 

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.

 

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.

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.

 

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.


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.

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

 

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).

 

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 φh and φm 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

Benjamin ter Kuile – 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.