Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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Why does rain exist?
Dijksma, R. - \ 2017
Wageningen :
weather - rain - clouds
We always talk about the weather and are afraid of the clouds. What is the function of the different clouds we see?
Chemical and physical drivers of the evolution of organic aerosols over forests
Janssen, R.H.H. - \ 2013
Wageningen University. Promotor(en): Pavel Kabat, co-promotor(en): Jordi Vila-Guerau de Arellano; Laurens Ganzeveld. - [S.l. : S.n. - ISBN 9789461735102 - 148
aërosolen - bossen - atmosferische grenslaag - meteorologische factoren - wolken - klimaat - aerosols - forests - atmospheric boundary-layer - meteorological factors - clouds - climate

Diurnal evolution of organic aerosol over boreal and tropical forests

The first research question of this thesis is: how do local surface forcings and large-scale meteorological forcings shape the evolution of organic aerosol over the boreal and tropical forest? This question is dealt with in Chapters 3 and 4 in case studies for the boreal and tropical forest, respectively. To answer this question a modeling tool (MXLCH-SOA) is developed, which represents land surface conditions and dynamical and chemical processes that influence the evolution of organic aerosol (OA) in a balanced way. The novelty of our approach is that it combines the dynamics of a convective boundary layer (BL) with a reduced gas-phase chemistry mechanism and a module for gas/particle-partitioning of semi-volatile organic species. The principles and governing equations of this modeling tool are described in Chapter 2 and in the subsequent chapters the simplified chemical reaction schemes are presented to calculate secondary organic aerosol (SOA) formation from terpenes (Chapter 3 and 4) and from isoprene (Chapter 4).

Despite its simplicity, MXLCH-SOA is able to satisfactorily reproduce the main observed characteristics of dynamics, gas-phase chemistry and gas/particle partitioning for the two studied forest ecosystems and it enables us to explain the temporal variability of the concentrations of organic aerosol and its precursors as a function of the various processes. In short, the results show that the diurnal evolution of organic aerosol in a boreal and a tropical forest is the net result of land surface conditions, boundary layer dynamics, chemical transformations and gas/particle partitioning. In the case study for the boreal forest, the entrainment term of the background OA dominates the OA tendency, while in the tropical forest case it is the interplay of several local and large scale processes that shape the diurnal evolution of OA. A sensitivity analysis for the boreal forest case further shows that the OA concentration is sensitive to both volatile organic compound (VOC) emissions and the partitioning of the surface energy budget into a latent and a sensible heat flux. We have identified two regimes, based on which of the two studied land surface drivers dominates: one in which OA is mainly driven by SOA formation from the emitted VOCs and another in which dilution due to entrainment, as driven by the surface energy fluxes, determines the OA concentration. A background OA to fresh SOA ratio is introduced to facilitate the interpretation of this analysis and is used to quantify the contributions of both fresh and background components to the total OA concentration. One main difference between the two case studies is that in the boreal forest entrainment appears to dominate the diurnal cycle, which leads to a decreasing OA concentration during the day, while in the tropical forest the formation of SOA from both isoprene and terpenes leads to increasing OA concentrations during day time. The MXLCH-SOA framework therefore shows the need to represent all these biochemical and physical processes simultaneously in order to understand the diurnal evolution of OA.

As the boundary layer dynamics-chemistry system is not a closed system, it is necessary to further study the influence of external forcings on the diurnal evolution of OA, besides the surface forcings. Two types of large-scale meteorological forcings and their effects on OA evolution through their impact on BL dynamics have been studied: subsidence due to the presence of a high pressure system and advection of relatively cool air. In Chapter 3 a theoretical sensitivity analysis is given of OA evolution to subsidence, which is applied to the tropical forest case study in Chapter 4. Subsidence has a rather counter-intuitive effect on OA concentrations: even though it suppresses the growth of the BL and consequently decreases the mixing volume for chemical species, it leads to decreased OA concentrations. The reason for this is that entrainment is strongly enhanced in case of subsidence due to thermodynamic effects, which results in a stronger dilution of OA. This knowledge is applied in the case study for the tropical forest in Chapter 4, since results from a large-scale model show subsiding air motions over the measurement site and surroundings at Borneo.

In addition to subsidence, the advection of cool air is needed to reproduce the observed boundary layer dynamics at Borneo: only if subsidence and advection of relatively cool air are accounted for, the observed low BL height can be reconciled with the large observed surface sensible and latent heat flux. This cool air suppresses BL growth and entrainment. Consequently, the aerosol is trapped in a shallower layer, which leads to an increased concentration compared to the case without advection of cooler air. In conclusion, the large-scale meteorological forcings subsidence and advection of cool air have opposing effects on the diurnal evolution of OA, even though both suppress BL growth. These findings show the utility of our method in identifying effects that should be accounted for in large-scale chemistry transport models.

The second research question is whether recently discovered pathways of isoprene chemistry are the key to closing the gap between measured and modeled organic aerosol concentrations in tropical forests and other high isoprene environments. To address this issue, several mechanisms which may affect SOA formation from isoprene are implemented in MXLCH-SOA and discussed in Chapter 4. The hydroxyl radical (OH), the main oxidant of isoprene, is thought to be regenerated in the oxidation of isoprene. We find that for the tropical forest case study, we cannot reconcile the modeled concentrations of VOCs, OH and OA with their observed concentrations and fluxes both for cases with and without OH recycling. Therefore, we conclude that the issue of recycling of the OH radical in the oxidation of isoprene has to be solved before its effect on SOA formation can be determined.

The formation of SOA from isoprene involves multiple generations of oxidation and due to this complex chemistry there is no single mechanism which can explain SOA formation from isoprene under all conditions. To gain understanding in this issue, we have implemented different pathways through which isoprene SOA is known to form, although we do not explicitly account for the detailed isoprene oxidation chain. A central aspect of this branching approach is whether the isoprene peroxy radical chemistry follows the low- or the high-NOx pathway. We find that the latter channel dominates in our case study. For SOA formed through the high-NOx channel, we further account for the effect of the NO2/NO ratio on SOA yields. In the presented case study this has little effect as this ratio is low, it but could be more important in regions with slower photochemistry or higher emissions of anthropogenic pollution. In the low-NOx regime, isoprene epoxides (IEPOX) are important intermediate gas-phase species in the formation of isoprene SOA. Even though the low-NOx pathway is only a minor one here, the amount of IEPOX SOA formed is likely substantial, although a better understanding of the exact mechanisms for its formation is needed to confirm this. However, as in previous studies we systematically underestimate the organic aerosol concentration in a tropical forest even though we incorporate the state-of-the-art knowledge on isoprene SOA formation in MXLCH-SOA. Nevertheless, we advocate accounting for NOx regime specific chemical pathways when modeling isoprene SOA formation. As this field is rapidly evolving in terms of the development of new measurement techniques and the discovery of chemical mechanisms, we strongly recommend the intensive use of our modeling system to gain further understanding of the diurnal variability of OA and for testing new hypotheses under atmospheric conditions.

Satellite observations of cloud droplet concentration over the boreal forest

The final objective of this thesis is to understand how aerosols and meteorological factors influence cloud droplet concentration over the boreal forest. This is a first step in translating the process understanding such as addressed in the previous chapters to larger spatio-temporal scales. Since this objective considers different temporal and spatial scales, a different method is applied in Chapter 5 than in the foregoing chapters. Observations of cloud properties by the MODIS instrument onboard the Terra satellite are combined with a model that contains the microphysics and thermodynamics of a single-layered water cloud to obtain a seasonal cycle of cloud droplet number concentrations, averaged over 9 years of data. This seasonal cycle in cloud droplet concentration is compared with aerosol concentrations at the surface and meteorological fields from ECMWF reanalysis. We find that the cloud droplet number concentration is related to the potential temperature gradient in the boundary layer, a measure for the strength of convection, while it shows no clear relationship with the cloud active aerosol concentration at the surface. From this we conclude that the convective transport of the aerosols from the surface to cloud base is the limiting factor for their activation as cloud droplets. However, convection will also influence the formation of clouds from a thermodynamic perspective. Therefore, it is likely that convection, as driven by land surface conditions, regulates both transport of aerosols to cloud base and the height of the cloud base, defined as the height at which water vapor reaches its saturation pressure. To ultimately understand the effect of the boreal forest on cloud properties, the effects of aerosols and thermodynamics should be studied simultaneously.

Model experiments with HARMONIE – Fog and low clouds
Steeneveld, Gert-Jan - \ 2010
climatic change - airports - weather data - clouds - fog - meteorological observations
Cloud physical properties retrieval for climate studies using SEVIRI and AVHRR data = Afleiden van fysische eigenschappen van wolken voor klimaatstudies met behulp van SEVIRI en AVHRR gegevens
Roebeling, R.A. - \ 2008
Wageningen University. Promotor(en): Bert Holtslag, co-promotor(en): A.J. Feijt; P. Stammes. - [S.l.] : S.n. - ISBN 9789085048923 - 158
wolken - fysische eigenschappen - optische eigenschappen - meteorologie - meteorologische waarnemingen - satellieten - klimatologie - clouds - physical properties - optical properties - meteorology - meteorological observations - satellites - climatology
Accurate and long term information on the physical properties of clouds is required to increase our understanding on the role of clouds in the current climate system, and to better predict the behavior of clouds in a changing climate. This thesis investigates if retrievals of cloud physical properties from satellite imagers can be used to prepare time series of these properties for monitoring climate change, and to evaluate parameterizations of cloud processes in weather and climate prediction models.

An algorithm for retrieval of Cloud Physical Properties (CPP) from visible and near-infrared reflectances of the AVHRR instrument onboard NOAA and the SEVIRI instrument onboard METEOSAT is presented. This algorithm retrieves cloud optical thickness, effective radius, and liquid water path, whereas a cloud model is used to simulate cloud geometrical thickness and droplet number concentration. Due to the large differences found between the reflectances from the different instruments (up to 25%), a recalibration procedure is developed that successfully reduces the retrieval differences to less than 5%. The uniqueness of the SEVIRI cloud property retrievals is in its unprecedented sampling frequency (15 minutes) that ensures the statistical significance of the dataset. One year of cloud liquid water path retrievals is validated against simultaneous Cloudnet microwave radiometer observations over Europe. The results show that during summer the agreement is very good while during winter an overestimation of about 20% is observed. Possible reason for this overestimation is the plane-parallel assumption in the CPP algorithm used to simulate real clouds. For single-layer stratocumulus days, a sub-adiabatic cloud model is used to obtain cloud geometrical thickness and cloud droplet number concentration. During these days good agreement is found between geometrical thickness simulations and Cloudnet lidar and radar observations, and cloud liquid water path retrievals and Cloudnet microwave radiometer observations. The simulated droplet concentration is found to vary independently from liquid water path and the geometrical thickness, which suggests possible interactions between aerosols and clouds. This shows potential in our dataset for studies of the indirect aerosol effect.

The SEVIRI dataset of cloud property retrievals is used to evaluate the Regional Climate Model (RACMO) over Europe during a six-month period. The results show that RACMO represents the spatial variations of cloud amount and cloud liquid water path realistically, but underpredicts cloud amount by 20% and overpredicts liquid water path by 30%. Examination of the diurnal cycle shows that the RACMO maximum liquid water path occurs two hours earlier than that observed by SEVIRI, while the RACMO maximum cloud amount agrees reasonably well with SEVIRI’s amount. The largest differences in the diurnal cycle between RACMO and SEVIRI are found in regions of alternating stratiform and convective regimes where RACMO has difficulty representing the transition between these regimes. The SEVIRI dataset of cloud physical properties proves to be a powerful tool for evaluating parameterizations of cloud and precipitation processes in weather and climate prediction models, and thus helps increase the confidence in these models.

An updated length-scale formulation for turbulent mixing in clear and cloudy boundary layers
Lenderink, G. ; Holtslag, A.A.M. - \ 2004
Quarterly Journal of the Royal Meteorological Society 130 (2004)604. - ISSN 0035-9009 - p. 3405 - 3427.
grenslaagmeteorologie - wolken - turbulentie - kinetische energie - boundary-layer meteorology - clouds - turbulence - kinetic energy - shallow cumulus convection - large-eddy simulation - atmospheric models - parameterization - scheme - stratocumulus - fluxes - transport - impacts - wind
A new mixing-length scale is presented for turbulence-closure schemes, with special emphasis on neutral-to-convective conditions in clear and cloudy boundary layers. The length scale is intended for a prognostic turbulent-kinetic-energy closure. It is argued that present-day length-scale formulations may easily fail in one of two limiting situations. Schemes based on a local stability measure (e.g.the Richardson number) display unrealistic behaviour and instabilities in the convective limit. This strongly limits the representation of mixing in cloudy boundary layers. On the other hand, it is shown that non-local parcel methods may misrepresent mixing near the surface. The new length-scale formulation combines local and non-local stability in a new way; it uses vertical integrals over the stability (the Richardson number) in a simple 'parcel' framework. The length scale matches with surface-layer similarity for near-neutral conditions and displays a realistic convective limit. The use of the length-scale formulation can be extended easily to cloudy boundary layers. The scheme is numerically stable and computationally cheap. The behaviour of the length scale is evaluated in a single-column model (SCM) and in a high-resolution limited-area model (LAM). The SCM shows good behaviour in three cases with and without boundary-layer clouds. The prediction of the near-surface wind and temperature in the LAM compares favourably with tower measurements at Cabauw (the Netherlands).
Influence of Soil Moisture on Boundary Layer Cloud Development
Ek, M.B. ; Holtslag, A.A.M. - \ 2004
Journal of Hydrometeorology 5 (2004). - ISSN 1525-755X - p. 86 - 99.
wolken - atmosferische grenslaag - bodemwater - aardoppervlak - clouds - atmospheric boundary-layer - soil water - land surface - surface parameterization schemes - land-surface - daytime evolution - relative-humidity - solar-radiation - energy-balance - pine forest - eta-model - simulation - water
The daytime interaction of the land surface with the atmospheric boundary layer (ABL) is studied using a coupled one-dimensional (column) land surface–ABL model. This is an extension of earlier work that focused on modeling the ABL for 31 May 1978 at Cabauw, Netherlands; previously, it was found that coupled land–atmosphere tests using a simple land surface scheme did not accurately represent surface fluxes and coupled ABL development. Here, findings from that earlier study on ABL parameterization are utilized, and include a more sophisticated land surface scheme. This land surface scheme allows the land–atmosphere system to respond interactively with the ABL. Results indicate that in coupled land–atmosphere model runs, realistic daytime surface fluxes and atmospheric profiles are produced, even in the presence of ABL clouds (shallow cumulus). Subsequently, the role of soil moisture in the development of ABL clouds is explored in terms of a new relative humidity tendency equation at the ABL top where a number of processes and interactions are involved. Among other issues, it is shown that decreasing soil moisture may actually lead to an increase in ABL clouds in some cases.
The daytime interaction of the land surface with the atmospheric boundary layer (ABL) is studied using a coupled one-dimensional (column) land surface-ABL model. This is an extension of earlier work that focused on modeling the ABL for 31 May 1978 at Cabauw, Netherlands; previously, it was found that coupled land atmosphere tests using a simple land surface scheme did not accurately represent surface fluxes and coupled ABL development. Here, findings from that earlier study on ABL parameterization are utilized, and include a more sophisticated land surface scheme. This land surface scheme allows the land-atmosphere system to respond interactively with the ABL. Results indicate that in coupled land-atmosphere model runs, realistic daytime surface fluxes and atmospheric profiles are produced, even in the presence of ABL clouds (shallow cumulus). Subsequently, the role of soil moisture in the development of ABL clouds is explored in terms of a new relative humidity tendency equation at the ABL top where a number of processes and interactions are involved. Among other issues, it is shown that decreasing soil moisture may actually lead to an increase in ABL clouds in some cases.
Shallow cumulus convection = Ondiepe cumulus convectie
Neggers, R. - \ 2002
Wageningen University. Promotor(en): A.A.M. Holtslag; A.P. Siebesma. - S.l. : S.n. - ISBN 9789058087744 - 202
wolken - convectie - atmosfeer - turbulentie - simulatiemodellen - grenslaag - clouds - convection - atmosphere - turbulence - simulation models - boundary layer

Clouds play an important role in the earth's climate. Firstly, they are important in the radiative energy budget of the global atmosphere. Clouds absorb and reflect ultraviolet solar radiation, and emit infrared radiation depending on their temperature. Secondly, an important part of the vertical transport of heat, moisture and momentum in the atmosphere is associated with the relatively strong vertical motions inside certain types of clouds, also called convective clouds. These clouds often produce intense precipitation, and play an important role in the global water cycle. In the tropics near the equator, these clouds act as the engine for whole large-scale atmospheric circulations.

This thesis is concerned with clouds in the lowest few kilometers of the atmosphere surrounding the earth. This sphere is also known as the atmospheric or planetary boundary layer (PBL), defined as the part of the atmosphere directly influenced by the proximity of the surface of the earth. In this layer the exchange takes place of heat and moisture between the earth and the atmosphere. Basically the boundary layer is formed and maintained by vertical motions of air, known as turbulence . The turbulence is driven by heating of air close to the surface, and by the drag on the horizontal winds by the roughness of the earth's surface. The resulting turbulent eddies mix heat, moisture and momentum throughout the boundary layer. As rising air cools adiabatically, some eddies can get cooled so much in certain situations that water droplets form inside them, forming cumuliform clouds. This type of clouds is the main subject of this thesis. More specifically, the research is focused on shallow cumulus clouds, also known as fair-weather cumulus.

Because of their important role in the earth's radiative budget and in the vertical transport of air, it is essential for weather and climate prediction modelling to know where, when and to what extent cumuliform clouds occur. Since a few decades ago numerical models for the general circulation are used to make weather and climate predictions. Despite the rapid developments in supercomputing, the typical spatial and temporal resolutions used in state of the art models are still as large as 30 to 50km. These grid-spacings are still much too large to realistically resolve shallow cumulus clouds, as their dimensions are in the order of a few kilometers at maximum. Nevertheless, their strength lies in their numbers, as these clouds typically occur in whole populations covering large areas of the globe. In order to represent the impact of these clouds on the general atmospheric circulation which is to be resolved by the models, it is necessary to implement simplified formulas which mimic the presence of shallow cumulus clouds. This technique is known as parameterization. These formulations typically are dependent on a few relevant meteorological parameters. Due to the complexity of this problem much effort has already been put in the scientific research on cumulus convection.

In parameterizations for cumulus it is custom to separate the modeling of turbulent transport in the cloud layer and in the dry air below the clouds. This often leads to unwanted interactions of the modeling of these mechanisms in the lower atmosphere. The purpose of the research project behind this thesis was twofold: firstly to comprehend and model the exchange of air between the subcloud layer and the cumulus clouds, and secondly to quantify and model the mixing of this ventilated air over the cloud layer. Associated with this approach is a study of the typical turbulent and geometrical variability of cumulus cloud populations. The research is performed using observations of natural cumulus clouds by aircraft, surface-based meteorological instrumentation, remote sensing devices on satellites and cloud radar. To supplement these datasets which are often scarce and incomplete on important points, use is also made of high-resolution numerical models for atmospheric flow, also known as large-eddy simulation . These models simulate a domain of ten by ten by five kilometers, including whole populations of cumulus clouds. These simulated fields are used as a virtual laboratory to study cumulus convection.

Large-eddy simulation results on shallow cumulus convection are directly evaluated against detailed cloud observations in Chapter 3, using aircraft-measurements of the Small Cumulus Microphysics Study (SCMS) as well as high-resolution Landsat images. The results show that given the correct initial and boundary conditions the LES concept is capable of realistically predicting the bulk thermodynamic properties of temperature, moisture and liquid water content of the cumulus cloud ensemble as observed in SCMS. Furthermore the vertical component of the in-cloud turbulent kinetic energy and the cloud size distribution in LES were in agreement with the observations. Several hypotheses which make use of conditionally sampled fields were tested on the SCMS data. The magnitudes and the decrease with height of the bulk entrainmen t rate following from the SCMS data confirm the typical values first suggested by Siebesma and Cuijpers (1995) using LES results on the Barbados Oceanic and Meteorological Experiment (BOMEX). An alternative formulation of the lateral entrainment rate as a function of the liquid water content and the mean lapse rate agrees well with the original form based on the conserved variables. Applying the simplified equation for the cloud vertica l velocity by Simpson and Wiggert (1969) to the aircraft-measurements results in a reasonably closed budget. These results support the credibility of cloud statistics as produced by LES in general, and encourage its use as a tool for testing hypotheses and developing parameterizations of shallow cumulus cloud processes.

The geometrical variability of shallow cumulus cloud populations is assessed in Chapter 4 by means of calculating cloud size densities. We find a power-law scaling at the small cloud sizes and the presence of a scale break. The corresponding functional parameters have values which are typical for observed populations. The scale-break size appears to be the relevant length-scale to non-dimensionalize the cloud size, as this causes a data-collapse of the cloud size densities over several different cumulus cases. These findings suggest that a universal functional form exists for the cloud size density of shallow cumulus. A better understanding of the scale-break size is essential for for a complete definition this function. The scale-break co-determines the cloud size density, and defines the intermediate dominating size in the mass flux and cloud fraction decompositions. Its intermediate position between the largest clouds and the grid-spacing in LES implies that the clouds which do matter are resolved well by LES.

In Chapter 5 the (thermo)dynamic variability of shallow cumulus is visualized by means of conserved variable diagrams, showing the joint pdfs of the conserved thermodynamic variables and (vertical) momentum. This approach inspired the formulation of a multi parcel model, meant to at least partially reproduce the joint pdfs. A new conceptual model for the lateral mixing of such an updraft-parcel is presented, based on an adjustment time-scale for the dilution of the excess of the conserved properties of this updraft parcel over its environment. A statistical analysis of many LES clouds showed that this adjustment time-scale is constant in all clouds, which implies a lateral mixing rate which is inversely proportional to the vertical velocity. This dynamical feedback between thermodynamics and vertical momentum is shown to be capable of reproducing the cloud population-average characteristics as well as the increase of the in-cloud variances with height.

Chapter 6 deals with the cloud-subcloud coupling, which manifests itself in many aspects of shallow cumulus topped boundary layers, not in the last place in the turbulent variability. The parameterization of the transport properties of the simplified top-hat pdf is expressed in the mass flux model, of which the closure at cloud base represents this cloud-subcloud interaction. Three closure methods for shallow cumulus are critically examined for the difficult case of a diurnal cycle of shallow cumulus over land. First the various closures are diagnostically evaluated in a large-eddy simulation of a diurnal cycle. Subsequently they are implemented in an offline 1D model to study their impact on the development of the modelled cloudy boundary layer. Significant moistening occurs in the subcloud mixed layer in the first hours after cloud onset in LES, which makes the boundary-layer equilibrium closure Tiedtke (1989) substantially overestimate the mass flux at cloud base. As a result the boundary layer deepens unrealistically rapid at that stage in the single column model. The adjustment closure on the convective available potential energy (CAPE) of Fritsch and Chappell (1980) fails at the early and final stages of the diurnal cycle, when the cloud base transport is controlled by subcloud layer properties. The subcloud convective velocity scale closure of Grant (2001) is promising, as it reproduces the timing of both the maximum and the final decrease of the cloud base mass flux in LES. Apparently this closure catches the coupling between the two layers at cloud base. As a consequence the development of the thermodynamic structure of the boundary layer in the 1D model strongly resembles that in LES.

The validation of global weather and climate models with observations in general shows that in many situations the characteristics of clouds are not represented well. Especially concerning low convective clouds it has become clear that existing parameterizations for important meteorological parameters such as cloud cover and occurrence of different type of clouds do not always give realistic results. Misrepresentations of these parameters can lead to serious deviations in the modelled circulation and climatology. It is clear that further research and development is required in this field of meteorology. The results as presented in this thesis have contributed to this in several ways. The thermodynamic variability and population statistics of cumulus cloud fields has been further charted and quantified. The interaction between shallow cumulus cloud layers and subcloud layers has been analyzed and the performance of several well-known conceptual models for this interaction has been compared. The dynamics of mixing between cumulus clouds and their environment has been studied and captured in a coneptual model. Finally, it has been shown that the cloud populations as produced by LES models have realistic cloud size statistics and thermodynamic properties.

Representation of the seasonal hydrological cycle in climate and weather prediction models in West Europe
Dolman, A.J. ; Stricker, J.N.M. ; Feddes, R.A. ; Holtslag, A.A.M. ; Ronda, R.J. ; Bruin, H.A.R. de; Soet, M. ; Hurk, B.J.J.M. van den - \ 2001
Bilthoven : RIVM - ISBN 9789058510624 - 126
klimatologie - weersvoorspelling - bodemwater - waterbalans - ruimtelijke verdeling - regen - wolken - evapotranspiratie - hydrologische factoren - modellen - west-europa - hydrologische cyclus - aardoppervlak - climatology - weather forecasting - soil water - water balance - spatial distribution - rain - clouds - evapotranspiration - hydrological factors - models - western europe - hydrological cycle - land surface
Time series of satellite data: development of new products
Roerink, G.J. ; Menenti, M. - \ 2000
Delft : Netherlands Remote Sensing Board (BCRS), Programme Bureau, Rijkswaterstaat Survey Department - ISBN 9789054112990 - 101
satellietkarteringen - gegevensanalyse - statistische analyse - tijdreeksen - computer software - wolken - klimaat - vegetatie - weersvoorspelling - fourieranalyse - wavelets - satellite surveys - data analysis - statistical analysis - time series - clouds - climate - vegetation - weather forecasting - fourier analysis
Quantitative cloud analysis using meteorological satellites = [Kwantitatieve analyse van wolken met meteorologische satellieten]
Feijt, A.J. - \ 2000
Agricultural University. Promotor(en): A.A.M. Holtslag; A.C.A.P. van Lammeren. - S.l. : S.n. - ISBN 9789058083159 - 186
wolken - kwantitatieve analyse - satellieten - satellietbeelden - meteorologie - meteorologische instrumenten - clouds - quantitative analysis - satellites - satellite imagery - meteorology - meteorological instruments

This thesis is about observations of clouds from satellite and ground based instruments. The aim is to reconstruct the three dimensional cloud distributions. This information is used both in climate research and operational meteorological applications. In climate research, cloud observations provide a reference to atmospheric models, which enables optimization of cloud parameterizations. For operational meteorologists clouds are symptoms of atmospheric conditions. Cloud observations therefore are helpful in understanding the current weather (nowcasting) and improve the estimate of how of the atmospheric conditions will evolve (forecasting).

In order to obtain cloud field characteristics, analysis environments were developed for the interpretation of meteorological satellite measurements in terms of cloud properties. A large effort was put in the evaluation of the results with synoptic observations and measurements from two measurement campaigns. As a result this thesis comprises three major research topics: Meteosat analysis, AVHRR analysis and combined analysis of ground and satellite observations.

Meteosat analysis

A new cloud detection scheme was developed that includes the use of the surface temperature fields of a Numerical Weather Prediction (NWP) model as a threshold value to distinguish cloudy and cloud free areas. It is shown that also for cloud free conditions, the equivalent black body temperature as measured from satellite is different from the model surface temperature. An innovative part of the scheme is the quantification of this temperature difference, which is used to improve the skill of the cloud detection method. The improvement of the detection efficiency was quantified over land and ocean for 1997 on a 3 hourly basis in a semi-operational setting. As the method optimizes the use of the infrared information it is relatively insensitive to changes of insolation conditions with time of day, location, or season.

AVHRR analysis

The NWP model surface temperatures are also used in the AVHRR analysis environment. For the interpretation of the 0.6μm channel reflectivities, extensive radiative transfer calculations were done with the Doubling-Adding KNMI (DAK) radiative transfer code. The results were put in Look-up tables (LUT). The LUTs are used to obtain the following cloud field properties: cloud cover fraction, cloud top temperature, optical thickness and liquid water path. In order to assess the quality, the retrieved properties were compared to measurements from two campaigns: the Tropospheric Budget Experiment, TEBEX, and the Clouds and Radiation intensive measurement campaigns, CLARA. The comparison shows that the retrieval algorithms yield results that agree with independent ground based measurements for the cases studied.

Combined analysis of satellite and ground observations

Combined analysis of satellite and ground based observations from the TEBEX and CLARA data sets yields information on the quality of the satellite retrieval, but also on the merits of the ground based remote sensing instruments. The study shows that both observational sets have strong points, but a combination is preferred to obtain a good definition of the cloud field. In all comparisons the problem of collocation occurs. The ground based instruments measure continuous in time at one location, while satellites measure a spatial distribution at one moment in time. When comparing ground and satellite derived cloud products it is always questionable which part of the time series corresponds to which part of the spatial distribution. This correlation is studied by comparing variance spectra of time series and spatial distributions of liquid water path derived from microwave radiometer and AVHRR data respectively. It is shown that for two cases with different scaling properties the variance spectrum is similar for a part of the time-series and for a part of the AVHRR image.

This thesis contributes to quantitative use of meteorological satellite data in meteorology and climate research. Furthermore, it advances combined analysis of space born and ground based remote sensing measurements of clouds for routine applications.

Cloud forming properties of ambient aerosol in the Netherlands and resultant shortwave radiative forcing of climate
Khlystov, A. - \ 1998
Agricultural University. Promotor(en): J. Slanina; H.M. ten Brink. - Alkmaar : Khlystov - ISBN 9789054858706 - 302
atmosfeer - aërosolen - samenstelling - stof - luchtverontreiniging - luchtkwaliteit - wolken - straling - klimaatverandering - paleoklimatologie - Nederland - atmosphere - aerosols - composition - dust - air pollution - air quality - clouds - radiation - climatic change - palaeoclimatology - Netherlands

This thesis discusses properties of ambient aerosols in the Netherlands which are controlling the magnitude of the local aerosol radiative forcing. Anthropogenic aerosols influence climate by changing the radiative transfer through the atmosphere via two effects, one is direct and a second is indirect. Due to the scattering of solar light on aerosol particles the Earth surface receives less radiation and thus cools, which is called the direct aerosol effect.

The indirect effect includes processes by which aerosols influence the radiation balance indirectly - via clouds. The indirect effect includes mechanisms by which anthropogenic aerosol particles enhance the reflectivity and prolong the life time of marine stratoform clouds by increasing their droplet number concentration. Anthropogenic aerosols were demonstrated to have a considerable cooling effect on climate, comparable in magnitude to that of the greenhouse gases, but opposite in sign. However, calculation of the aerosol radiative forcing is much more complex than that for the green house gases. This results in high uncertainties in the estimates of the effects of anthropogenic aerosols on climate.

Both types of the aerosol radiative forcing and their magnitude, as estimated by the Intergovernmental Panel on Climate Change (IPCC), are discussed in Chapter 1. The aerosol radiative forcing is a regional phenomenon because of the limited life time of aerosols in the atmosphere. Thus, the aerosol effects should be assessed locally.

In Chapter 2 chemical composition of local ambient aerosols in the size range relevant to the radiative forcing is discussed. It is shown that ammonium nitrate, which is omitted by the IPCC, is a major contributor to the direct, and perhaps to the indirect forcing. The contribution of this compound to the local direct forcing is equal to that of anthropogenic sulfate aerosols. Measurements of ammonium nitrate content in submicrometer aerosol are subject to artifacts due to the volatility of this compound. Automated techniques which were developed for artifact-free measurements of chemical composition of ambient aerosols are described.

Aerosol particles composed of water-soluble salts increase their size at elevated relative humidities. The increase in size enhances light scattering by aerosol particles which in turn enhances the direct radiative forcing. Hygroscopic properties of aerosol particles also play a central role in cloud droplet formation and thus are also relevant for the indirect aerosol effect. Chapter 2 discusses the hygroscopic size changes and characteristic growth times. It is also demonstrated that hygroscopic growth may substantially enhance dry deposition of aerosol particles to forests.

According to the IPCC the indirect aerosol effect is the most uncertain factor in the anthropogenic perturbation of the climate, with an uncertainty of the same magnitude as the total radiative effect by the greenhouse gases. One of the major uncertainties in the indirect effect is the unknown relation between the concentration of anthropogenic aerosol particles and the increase in droplet number concentration in clouds. In Europe only few measurements of the relation between the number of particles and the number of cloud droplets have been performed.

It should be also noted that those measurements were performed with instruments that did not span the full range of aerosol sizes that is relevant for cloud formation. For this reason, the major part of the thesis was devoted to an experimental assessment of the cloud forming properties of ambient aerosol in the Netherlands. The indirect effect as well as factors controlling cloud droplet formation are discussed in Chapter 3.

Chapter 4 describes the large flow-through cloud chamber in which cloud forming properties of ambient aerosol were studied. The advantage of a cloud chamber over in-situ measurements is the use of instrumentation which is too delicate for use in aircraft and the possibility to compare, simultaneously, the aerosol before and after cloud processing. The aerosol was characterized both in terms of its number concentration and size as well as its chemical composition with the emphasis on the amount of soluble material. The unique feature of the cloud chamber is its reproducible stable low supersaturations (around 0.1%) common for marine clouds.

Aerosol monitors were used that measure the concentration of particles in the full range of sizes relevant for cloud formation. The unique large size and the high throughput flow of the chamber allowed unperturbed use of conventional cloud instrumentation for sizing and counting of droplets. Also high-flow cascade impactors for chemical analysis of aerosol were used. The cloud chamber was calibrated with artificially generated ammonium sulfate aerosol. This aerosol was selected because in anthropogenically influenced air in Western Europe most particles should consist of this compound. Experiments were done at different total aerosol number concentrations to check its influence on the number of resulting cloud droplets. A sub-linear relation between the aerosol and the droplet number concentrations was observed. The results of the tests with the reference aerosol were used for the interpretation of experiments with ambient aerosols.

The cloud activation tests on ambient aerosol are discussed in Chapter 5. The cloud forming efficiency of the aerosols was assessed by comparing the number concentration of droplets formed in the cloud chamber with that found during the tests with the reference ammonium sulfate aerosol at the same number concentration. Similarly to the reference sulfate aerosol, a sub-linear relation between the aerosol and droplet number concentrations was observed. However, the deviation from a linear relation was stronger in the ambient aerosol. This suggested that about one third of the (anthropogenic) particles was water-insoluble. The measurements of the composition supported this hypothesis. This demonstrates the importance of hygroscopicity for cloud formation. It also shows that measurements of the solubility of (individual) particles should be done along with conventional measurements of aerosol size and number.

The results of experiments discussed in Chapter 5 were used for an estimate of the local indirect aerosol radiative effect (Chapter 6). The difference between the droplet number concentration in clean marine air and the average droplet number concentration in polluted air was used to estimate the corresponding increase in cloud reflectivity which, in turn, was translated into local indirect aerosol effect. The data on the aerosol light scattering discussed in Chapter 2 are used to estimate the local direct radiative forcing. The estimated total (direct plus indirect) local aerosol radiative forcing is about 4 times higher than the forcing by the anthropogenic greenhouse gases but opposite in sign.

Op zoek naar de meest zuinige schermregeling
Kempkes, F.L.K. ; Bloemhard, C.M.J. - \ 1997
Groenten en Fruit. Vakdeel glasgroenten 7 (1997)48. - ISSN 1380-3573 - p. 20 - 21.
thermische schermen - leveringen - vochtigheid - wolken - neerslag - meteorologie - glastuinbouw - thermal screens - supplies - humidity - clouds - precipitation - meteorology - greenhouse horticulture
De ingestelde luchtvochtigheid bij het gebruik van een energiescherm verschilt enorm. Daardoor is op het Proefstation een proef uitgevoerd om de meest energiezuinige combinatie van schermkier en raamstand te vinden. Door meer te schermen bij een hogere luchtvochtigheid wordt er meer energie bespaard
Betere schermregeling bespaart energie
Kempkes, F.L.K. ; Bloemhard, C.M.J. - \ 1997
Groenten en Fruit. Vakdeel glasgroenten 7 (1997)48. - ISSN 1380-3573 - p. 28 - 29.
thermische schermen - kassen - klimaat - energie - efficiëntie - energiegebruik - brandstofverbruik - vochtigheid - wolken - neerslag - meteorologie - thermal screens - greenhouses - climate - energy - efficiency - energy consumption - fuel consumption - humidity - clouds - precipitation - meteorology
Een gesloten scherm bespaart energie. Maar deze besparing blijft beperkt doordat het scherm opengaat bij een te hoge relatieve luchtvochtigheid. Op het IMAG-DLO en het proefstation is een schermkierregeling gesimuleerd die beter is dan de huidige regelingen. Met de verbeterde regeling kan vocht worden afgevoerd en meer energie worden bespaard dan met de huidige regelingen
A model of greenhouse humidity suitable for control of crop processers
Stanghellini, C. ; Jong, T. de - \ 1996
In: Proceedings Mathematical and Control Applications in Agriculture and Horticulture / Day, W., Young, P.C., - p. 125 - 131.
kassen - klimaat - vochtigheid - wolken - neerslag - meteorologie - transpiratie - evapotranspiratie - modellen - onderzoek - computersimulatie - simulatie - simulatiemodellen - glastuinbouw - greenhouses - climate - humidity - clouds - precipitation - meteorology - transpiration - evapotranspiration - models - research - computer simulation - simulation - simulation models - greenhouse horticulture
A physical model of humidity within a greenhouse was developed, whereby ambient vapour concentration resulted from the balance of three fluxes: crop transpiration, ventilation and condensation at the cover. The present paper describes the application of such a model in the greenhouse climate control. Two examples are shown in order to demonstrate that it is possible to deduce set points for climate actuators from desired levels of humidity-related crop processes such as transpiration or dew forming. Within the selected range, the final choice of set points can be based on cost-benefit analysis. This could prevent useless ventilation and heating and consequently save energy. Accordingly, modern greenhouse climate management could be largely improved by incorporating physical models in order to deduce set points for actuators (mainly temperature and ventilation) directly from the desired level of a crop process.
The structure of the atmospheric surface layer subject to local advection
Bink, N.J. - \ 1996
Agricultural University. Promotor(en): L. Wartena; H.F. Vugts; L.J.M. Kroon. - S.l. : Bink - ISBN 9789054855132 - 206
atmosfeer - luchttemperatuur - fluctuaties - zonnestraling - albedo - reflectie - wolken - microklimaat - bodem - landschap - grenslaag - aardoppervlak - aarde - atmosphere - air temperature - fluctuations - solar radiation - albedo - reflection - clouds - microclimate - soil - landscape - boundary layer - land surface - earth

For many applications in agriculture, hydrology and meteorology simple methods are needed to determine the surface-atmosphere exchange of momentum, heat and water vapour, i.e to determine the fluxes of momentum, heat and water vapour. Most methods to calculate these fluxes are only valid for horizontal, homogeneous terrain with sufficient large dimensions. It is thus assumed that so-called advective effects can be neglected because wind speed, temperature and humidity do not change in the horizontal direction. In practice, the surface is hardly ever homogeneous. It was the objective of this study to investigate the effects of advection of heat and moisture on the fluxes. It appeared that only a few sets related to advection were available, and the available sets yielded contradictive results. Therefore an experiment was carried out in La Crau, France around a step-change from a dry and bare terrain to irrigated grass in order to measure the influence of advection on the structure of the flow and the exchange processes near the earth's surface.

Before this study there was confusion about the behaviour of the flux-gradient ratios or eddy diffusivities under conditions of local advection. The flux-gradient ratios or eddy diffusivities were estimated using the calculated surface fluxes and the gradients from the profile measurements. It was found that the flux-gradient ratio for heat was smaller than that for water vapour in the lower part of the surface layer after the step-change. This was in agreement with the ratio of the observed transfer efficiencies. Higher up in the surface layer after the step-change and for weak advective conditions it was found that the flux-gradient ratio for heat was larger than that for water vapour.

Also, flux determination methods were tested using a second-order closure model which was found to compare favourably with the measurements. It was found that for situations similar to that in the Crau, the so-called gradient Bowen ratio can be used at fetch-to-height ratios up to z/x = 0.02 which is high compared to what was accepted untill now. It was also found that the difference between the Bowen ratio at the surface and at some level above the surface is compensated for by the ratio of the eddy diffusivities at the height where the gradient is measured. For the Bowen ratio from standard deviations in the thermally stable surface layer the error was found to be below 10%, up to z/x = 0.02. The ratio of the transfer efficiencies was below unity which compensated the error due to the flux divergence. For the unstable surface layer z/x must be below 0.008 to achieve the same accuracy because now the ratio of the transfer efficiencies amplifies the error due to flux divergence. For the cases studied here, a number of factors may have cooperated in a favourable manner due to which the error sources cancel out. However, it is significant to note that the model can be used to forecast which conditions are favourable and which are not for the application of micro- meteorological methods to determine the surface fluxes under conditions of local advection.

A simple surface radiation budget model for a point in snow covered mountainous terrain
Uijlenhoet, R. - \ 1992
Wageningen : Landbouwuniversiteit Wageningen (Rapport / Landbouwuniversiteit, Vakgroep Waterhuishouding 20) - 125
albedo - atmosfeer - wolken - gebergten - straling - reflectie - sneeuw - zonnestraling - temperatuur - aarde - atmosphere - clouds - mountains - radiation - reflection - snow - solar radiation - temperature - earth
Waterdamp in varkensstallen met diepstrooisel
Klooster, C.E. van 't; Greutink, G.J. - \ 1992
Rosmalen : Proefstation voor de Varkenshouderij (Proefverslag / Proefstation voor de Varkenshouderij P1.85) - 20
huisvesting, dieren - dierenwelzijn - gebouwen - klimaat - wolken - milieubeheersing - vochtigheid - meteorologie - varkensstallen - neerslag - regulatie - animal housing - animal welfare - buildings - climate - clouds - environmental control - humidity - meteorology - pig housing - precipitation - regulation
Waterdamp in varkensstallen met diepstrooisel
Klooster, K. van 't; Greutink, T. - \ 1992
Praktijkonderzoek varkenshouderij 6 (1992)6. - ISSN 1382-0346 - p. 23 - 25.
gebouwen - klimaat - wolken - milieubeheersing - vochtigheid - ligstro - meteorologie - varkensstallen - neerslag - regulatie - buildings - climate - clouds - environmental control - humidity - litter - meteorology - pig housing - precipitation - regulation
Water verdampen uit de mest is één van de doelen in diepstrooiselstallen, anders wordt het strooisel te nat. Lukt het om het water te verdampen en wat betekent dit voor de ventilatie?
Effect of interrupted leaf wetness periods on pustule development of Puccinia recondita f. sp. tritici on wheat.
Stuckey, R.E. ; Zadoks, J.C. - \ 1989
Netherlands Journal of Plant Pathology 95 (1989)Suppl. 1. - ISSN 0028-2944 - p. 175 - 185.
wolken - distributie - epidemiologie - hexaploïdie - vochtigheid - meteorologie - plantenziekten - plantenziekteverwekkende schimmels - plantenplagen - neerslag - puccinia - triticum aestivum - pucciniales - tarwe - clouds - distribution - epidemiology - hexaploidy - humidity - meteorology - plant diseases - plant pathogenic fungi - plant pests - precipitation - wheat
Kiemplanten van het vatbare tarweras Rubis werden geinoculeerd met uredosporen van bruine roest. Gezorgd werd voor bladnatperioden met varierende tijdsduur, met of zonder onderbreking. Het aantal verkregen puistjes op het blad werd geteld. De epidemiologische betekenis van onderbrekingen van de bladnatperiode wordt besproken
The energy balance of the earth's surface : a practical approach
Bruin, H.A.R. de - \ 1982
Landbouwhogeschool Wageningen. Promotor(en): L. Wartena, co-promotor(en): H. Tennekes. - Wageningen : De Bruin - 177
luchttemperatuur - albedo - atmosfeer - wolken - evaporatie - fluctuaties - warmte - hydrologie - land - mechanica - meteorologie - reflectie - bodem - bodemtemperatuur - zonnestraling - structuur - oppervlakten - temperatuur - theorie - thermische eigenschappen - thermodynamica - water - waterbalans - aarde - hydrologische cyclus - aardoppervlak - air temperature - albedo - atmosphere - clouds - evaporation - fluctuations - heat - hydrology - land - mechanics - meteorology - reflection - soil - soil temperature - solar radiation - structure - surfaces - temperature - theory - thermal properties - thermodynamics - water - water balance - earth - hydrological cycle - land surface
This study is devoted to the energy balance of the earth's surface with a special emphasis on practical applications. A simple picture of the energy exchange processes that take place at the ground is the following. Per unit time and area an amount of radiant energy is supplied to the surface. This radiation originates partly from the sun, but an~ other fraction is coming from the atmosphere (= infra-red radiation emitted by clouds, water vapour and CO 2 ). From these gain terms the following losses must be subtracted: (a) the reflected solar radiation and (b) the infra-red radiation emitted by the surface itself. The final result is that a net amount of radiant energy is received by the surface, simply denoted as net radiation. At the ground net radiation is used to heat the ground (soil heat flux), to evaporate liquid water (evaporation), and to heat the atmosphere (sensible heat flux). In this simple picture we have neglected minor terms such as the energy used by the plants for their photosynthesis.

Due to the high value of the latent heat of vaporization, the energy needed for evaporation is often an important term in the energy balance. In addition the energy balance of the earth's surface is linked with the water budget of both the atmosphere and the earth's surface, through the evaporation at the ground.

Several practical questions in agriculture, hydrology and meteorology require information m the energy balance of the surface. It is the purpose of this study to find solutions for some of these problems.

In hydrology one is mainly concerned in evaporation averaged over 1 day or more on a regional scale. Generally, this refers to land surfaces, but the evaporation of inland lakes or reservoirs is also of interest. In this context we also mention the problem of thermal pollution of open water bodies by industry or power plants. For this the so-called natural water temperature must be known, which is the temperature of the water in the hypothetical case that there is no artificial heating. It appears that this temperature depends mainly m the energy balance at the surface. In Chapter VI a model dealing with this problem is discussed.

In agriculture one is interested also in evaporation. Now time intervals ranging from half an hour to several days are of interest.

The relation between evaporation an the one side and plant diseases and pest control an the other can be mentioned.

Furthermore, the yield of several agricultural crops is the greatest when the evapotranspiration is potential (= a maximum under the given weather conditions). When the crop transpires less than the potential rate, because the soil is too dry, the yield can be augmented by artificial precipitation. For applications such as these cheap and simple techniques are required for measuring the actual and potential evaporation. This applies also to agricultural research projects, e.g. to determine yield-water use relationships.

In Chapter II simple measurement techniques are considered.

Recent developments in meteorology have led to an increase of the interest in the energy balance of the earth's surface, especially in the input of heat and humidity at ground level into the atmosphere. Examples are models for the atmospheric boundary layer and related models for short range weather forecasts (12-18 h ahead). These models require simple parameterizations of the surface fluxes. This applies also to weather forecast models on a medium time range (3-10 days ahead).

Since the height of the boundary layer is related to the heat input at the ground information an the surface energy balance is needed also for air pollution problems.

In Chapter III a simple parameterization for evaporation and sensible heat flux is described that can be used for these type of problems.

Usually, the only available data are standard weather observations. For that reason, many of the practical questions, mentioned above, can be formulated as: "How can the surface energy balance be estimated from standard weather data only'?" In Chapters III and VI possible answers to that question are discussed.

Chapter II is devoted to simple measuring techniques that, in principle, can be used on an operational base. These methods will be compared with the so-called energy-balance method, using Bowen's ratio.

In Chapter III two models for evaporation and sensible heat flux during daytime are compared. Both require standard weather data as input and an indication of the surface wetness. The first model needs more data, but contains more physics. The second is less complete, but requires less input data.

Chapter IV has a mainly theoretical character. A model is presented that couples the evolution of the atmospheric boundary layer to the surface energy balance. It describes the course of the height, temperature and humidity of the boundary layer, together with the surface fluxes, when the initial profiles of temperature and humidity the radiative forcing and the surface wetness are known. It is restricted to convective conditions. Model output will be compared with observations.

In Chapter V an empirical evaporation model for open water is considered. Comparisons with observations of evaporation of the former Lake Flevo will be made; the annual and the diurnal cycle will be considered.

In Chapter VI a model for the (natural) temperature and energy balance of inland lakes and water reservoirs is discussed that requires standard weather data only. A comparison between the calculated and measured water temperature will be given. This concerns two adjacent water reservoirs, which have about the same size, but which differ in depth (5 and 15 m). This is of importance, since the water temperature also depends on water depth.

At some places we made new modifications, but most of the theoretical concepts applied in this study are adopted from literature. This is inherent in our practical approach. Some of the theories used have been available for many years. But, e.g. because no suitable instruments were available, they were not usefull for practical applications. Recent developments in the field of instrumentation and data handling have changed the situation-to our advantage. A good example is the temperature fluctuation method for measuring the sensible heat flux (discussed in 11.4). The theoretical basis for this approach was given by Prandtl already in 1932. But for an experimental verification we had to wait until the sixties and early seventies. In that period instruments were developed to measure turbulent surface fluxes and fast temperature fluctuations, while also the data handling techniques were improved significantly. Finally) the method wouldn't be operationally until quite recently.

For the verification of the parameterizations, measuring techniques and models treated in this study, we used data collected at the 200 m mast at Cabauw, and at the nearby micrometeorological field, of the Royal Netherlands Meteorological Institute.

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