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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Surface evaporation and water vapor transport in the convective boundary layer
Heerwaarden, C.C. van - \ 2011
Wageningen University. Promotor(en): Bert Holtslag, co-promotor(en): J. Vila -Guerau de Arellano. - [S.l.] : s.n. - ISBN 9789085859130 - 156
evaporatie - aardoppervlak - atmosfeer - grenslaag - evaporation - land surface - atmosphere - boundary layer - cum laude
cum laude graduation (with distinction)
Developments in Scintillometry
Moene, A.F. ; Beyrich, F. ; Hartogensis, O.K. - \ 2009
Bulletin of the American Meteorological Society 90 (2009)5. - ISSN 0003-0007 - p. 694 - 698.
scintillometrie - turbulentie - grenslaag - meteorologische instrumenten - scintillometry - turbulence - boundary layer - meteorological instruments
Thirty scientists from five nations discussed developments in the growing field of scintillometry—the study of wave propagation in the atmospheric surface layer that can be used to measure and decipher low-level turbulence.
Diagnostic equations for the stable boundary-layer height: evaluation and dismensional analysis
Steeneveld, G.J. ; Wiel, B.J.H. van de; Holtslag, A.A.M. - \ 2007
Journal of Applied Meteorology and Climatology 46 (2007). - ISSN 1558-8424 - p. 212 - 225.
grenslaag - vergelijkingen (wiskundig) - prestatieniveau - meteorologische waarnemingen - grenslaagmeteorologie - boundary layer - equations - performance - meteorological observations - boundary-layer meteorology - nocturnal surface inversion - large-eddy simulations - mixing height - equilibrium depth - model - turbulence - formulations - variability - sensitivity - parameters
The performance of diagnostic equations for the stable boundary layer height h is evaluated with four observational datasets that represent a broad range of latitudes, land use, and surface roughness. In addition, large-eddy simulation results are used. Special care is given to data-quality selection.
The performance of diagnostic equations for the stable boundary layer height h is evaluated with four observational datasets that represent a broad range of latitudes, land use, and surface roughness. In addition, large-eddy simulation results are used. Special care is given to data-quality selection. The diagnostic equations evaluated are so-called multilimit equations as derived by Zilitinkevich and coworkers in a number of papers. It appears that these equations show a serious negative bias, especially for It <100 m, and it was found that the parameters involved could not be determined uniquely with calibration. As an alternative, dimensional analysis is used here to derive a formulation for h that is more robust. The formulation depends on the surface friction velocity u(*), surface buoyancy flux B-s, Coriolis parameter, and the free-flow stability N. The relevance of the Coriolis parameter for the boundary layer height estimation in practice is also discussed. If the Coriolis parameter is ignored, two major regimes are found: h similar to u(*)/N for weakly stable conditions and h similar to (vertical bar B-N vertical bar/N-3)(1/2) for moderate to very stable conditions.
Determination of regional surface heat fluxes over heterogeneous landscapes by integrating satellite remote sensing with boundary layer observations
Ma, Y.M. - \ 2006
Wageningen University. Promotor(en): Reinder Feddes; M. Menenti, co-promotor(en): J.M. Wang. - [S.l. ] : S.n. - ISBN 9085044839 - 181
warmtestroming - remote sensing - satellietkarteringen - meteorologische waarnemingen - atmosfeer - landschap - luchtstroming - warmteuitwisseling - grenslaag - aardoppervlak - heat flow - satellite surveys - meteorological observations - atmosphere - landscape - air flow - heat exchange - boundary layer - land surface
On Wind and Roughness over Land
Verkaik, J.W. - \ 2006
Wageningen University. Promotor(en): Bert Holtslag. - Wageningen : S.n. - ISBN 9085043859 - 123
meteorologie - wind - windsnelheid - windresistentie - landgebruik - oppervlakteruwheid - grenslaag - meteorology - wind speed - wind resistance - land use - surface roughness - boundary layer
Water soluble inorganic trace gases and related aerosol compounds in the tropical boundary layer. An analysis based on real time measurements at a pasture site in the Amazon Basin
Trebs, I. - \ 2005
Wageningen University. Promotor(en): J. Slanina. - [S.l.] : S.n. - ISBN 9085041732 - 144
rivierwater - aërosolen - gassen - energiebalans - atmosfeer - amazonia - brazilië - grenslaag - river water - aerosols - gases - energy balance - atmosphere - brazil - boundary layer
The surface energy balance over drying semi-arid terrain in West Africa
Schüttemeyer, D. - \ 2005
Wageningen University. Promotor(en): Bert Holtslag, co-promotor(en): Arnold Moene; H.A.R. Debruin. - [S.l.] : S.n. - ISBN 9085041929 - 154
atmosfeer - land - evapotranspiratie - energiebalans - ghana - grenslaag - atmosphere - evapotranspiration - energy balance - boundary layer
Interactions of the land-surface with the atmospheric boundary layer
Ek, M.B. - \ 2005
Wageningen University. Promotor(en): Bert Holtslag. - [S.l.] : S.n. - ISBN 9085041724 - 210
atmosfeer - energiebalans - simulatiemodellen - nederland - aardoppervlak - grenslaag - atmosphere - energy balance - simulation models - netherlands - land surface - boundary layer
Turbulent dispersion in the Atmospheric Convective Boundary Layer
Dosio, A. - \ 2005
Wageningen University. Promotor(en): Bert Holtslag; P.J.H. Builtjes, co-promotor(en): Jordi Vila-Guerau de Arellano. - [S.l.] : s.n. - ISBN 9085041716 - 173
atmosfeer - turbulentie - convectie - chemische samenstelling - ruimtelijke verdeling - grenslaag - atmosphere - turbulence - convection - chemical composition - spatial distribution - boundary layer - cum laude
cum laude graduation (with distinction)
Surface fluxes over natural landscapes using scintillometry
Meijninger, W.M.L. - \ 2003
Wageningen University. Promotor(en): Bert Holtslag, co-promotor(en): H.A.R. de Bruin. - Wageningen : Wageningen Universiteit - ISBN 9058088855 - 164
meteorologische instrumenten - remote sensing - energiebalans - turbulentie - kalibratie - grenslaag - scintillometrie - meteorological instruments - energy balance - turbulence - calibration - boundary layer - scintillometry
Motivated by the demand for reliable area-averaged fluxes associated with natural landscapes this thesis investigates a relative new measurement technique known as the scintillation method. For homogeneous areas the surface fluxes can be derived with reasonable accuracy. However, fluxes representative for large natural landscapes (comparable to the horizontal grid box size of numerical models or the pixel size of satellite imagers) are more difficult to obtain because at these scales the surface is mostly heterogeneous. At this moment only a few techniques are available that can provide flux information at spatial scales of several kilometres, such as the scintillation method. Based on the propagation statistics of EM radiation that has propagated through the atmosphere over a horizontal path of several kilometres it is possible to derive the surface fluxes of sensible heat, water vapour and momentum. In this study a Large Aperture Scintillometer (LAS) has been developed that can be used over distances up to 5 km. Since the LAS operates at a near-infrared wavelength hence it is primarily sensitive to temperature related scintillations, from which the sensible heat flux can inferred. In this thesis the following aspects regarding the LAS are investigated: The performance of the LAS over heterogeneous land surfaces The reliability of area-averaged water vapour fluxes provided by the LAS and in combination with a radio wave scintillometer over heterogeneous land surfaces Its practical applicability and usefulness in other scientific areas For the derivation of the sensible heat flux from the LAS signal one must rely on the Monin-Obukhov Similarity Theory (MOST). However, MOST requires homogeneous surface conditions. The question arises whether the LAS can be used over distances of several kilometres, since at these scales the surface is mostly heterogeneous. In order to test experimentally the applicability of the LAS over heterogeneous areas and the reliability of the derived fluxes of sensible heat a field campaign was carried out in Flevoland (The Netherlands). The general characteristics of the Flevoland area are as follows: a vast and completely flat area covered by four crops in a chessboard configuration of patches of 500 m ´ 250 m. Based on the horizontal length scale of the patches this landscape is classified as a Type A landscape, meaning that only the lower part of the surface layer is affected by the irregularities. Eddy covariance (EC) measurements were performed over the main types of farmland to provide to aggregation independent area-averaged fluxes. The EC observations reveal that the heterogeneity in the Flevoland area is primarily the result of spatial variations in the thermal properties. The fluxes of two large aperture scintillometers, installed at a height of 11.6 m and 20.4 m, respectively, show a close resemblance with the area-averaged EC fluxes, especially for the upper LAS. The lower LAS shows a slight underestimation of the sensible heat flux of approximately 7%. This underestimation is assessed using a blending height approach and an analytical footprint model for estimating the source areas and the associated fluxes. The blending height is considered as the level above the surface where the influences of the patches gradually decay. It is found, using a heuristic model that the blending height for the Flevoland area varies between 9 m and 14 m. Based on the found blending heights it is concluded that the upper LAS always measured above the blending height, which is consistent with the depicted LAS results. For the lower LAS the situation is more complicated as the individual fields influence the measurements, suggesting that the MOST may be violated. After dividing the Flevoland area into 8 wind-sectors and re-arranging the area-averaged fluxes for the entire area and for the 8 source areas, a closer agreement is found. These results indicate that from a LAS, which measures just below the blending height, still reliable area-averaged fluxes can be derived and that the violation of MOST is small. Next the performance of a combined LAS and radio wave scintillometer (LAS-RWS) over a heterogeneous land surface is studied. Although this scintillation technique, known as the two-wavelength method, provides both the sensible heat flux and the water vapour flux, most attention is focussed at the water vapour flux. The water vapour flux provided by a `stand-alone` LAS is evaluated also. In the latter case the water vapour flux is estimated as the residual of the surface energy balance equation using a simple parameterisation scheme (based on global radiation data) for estimating the area-averaged available energy (i.e. Rn - Gs). The LAS-RWS study is based also on data of the Flevoland experiment. As mentioned before the EC observations collected in Flevoland reveal that the heterogeneity in the area is primarily the result of spatial variations in the thermal properties and likewise in the buoyant production term of MOST. First, the water vapour fluxes from the combined LAS-RWS system are investigated. It was found that these fluxes agree well with the area-averaged water vapour flux aggregated from the in-situ observations. The found scatter is explained to be caused by: closure failure of the energy balance for the EC measurements, the non-linearity between the structure parameters and the inferred fluxes, and low frequency water vapour absorption fluctuations that affect the RWS. Finally, the water vapour fluxes derived from the stand-alone LAS are discussed. These results show that the `stand-alone¿ LAS can provide also reliable estimates of the area-averaged water vapour flux over heterogeneous areas (type A). In order to study the operational aspects of the LAS two LAS devices and a small micrometeorological station were installed in the Gediz Basin near Menemen (Turkey) in 1998 as part of an international experiment. The main objective of this experiment was to compare actual evapotranspiration estimates based on satellite remote sensing methods, hydrological models and field methods. This thesis deals only with the field methods, i.e. the variance method and in particular the LAS. One LAS was set-up over a transect of the valley of the Gediz river basin. For the derivation of the sensible heat flux additional wind speed and temperature data are taken from a nearby meteorological station. In addition a small micro-meteorological station was placed at an irrigated cotton field. The fluxes for this site are inferred from collected temperature fluctuation data using the variance method. Due to experimental problems with a second LAS installed at the same site these data are excluded from this study. The presented time series of 24-hour average fluxes for the valley clearly shows the seasonal trend of the sensible heat flux, including the irrigation events. This time series demonstrates from an operational perspective that the LAS, which was operational during the entire growing season, is a robust and reliable instrument that requires only occasional servicing. Finally, it is investigated whether the LAS fluxes collected in Turkey can be used as `ground-truth` data in other scientific studies such as remote sensing. For that purpose a large number of surface flux maps are generated using the SEBAL remote sensing algorithm, and are compared with the LAS results. In this validation study the in-situ fluxes and radiation measurements of the irrigated cotton field are included also. The SEBAL fluxes are derived from moderate resolution AVHRR visible and thermal-infrared images taken from the NOAA-14 satellite. Both instantaneous and daily average sensible heat fluxes are determined for the entire growing season. It is found that the SEBAL based instantaneous fluxes agree closely with the in-situ fluxes for the cotton site. However, the results for the valley site, i.e. SEBAL versus LAS, reveal a discrepancy. The following reasons are offered: the scaling mismatch between the source area of the LAS and the pixel size of the raw AVHRR images; a possible distortion by the dry slopes in the relative narrow valley compared to the resolution of the AVHRR imager. Therefore a convincing validation of SEBAL for the valley cannot be done. Finally, the daily average sensible heat fluxes for the cotton field are compared. It is found that only during the irrigation period the daily average results agree. Not actually dry reference pixels, which lead to underestimated evaporative fractions, are suggested to be the reason for the observed difference.
Land-surface and boundary layer processes in a semi-arid heterogeneous landscape
Jochum, A.M. - \ 2003
Wageningen University. Promotor(en): Bert Holtslag, co-promotor(en): H.A.R. de Bruin. - Wageningen : S.n. - ISBN 9058088243 - 155
atmosfeer - droge gebieden - droog klimaat - woestijnvorming - wiskundige modellen - spanje - aardoppervlak - grenslaag - atmosphere - arid lands - arid climate - desertification - mathematical models - spain - land surface - boundary layer
Shallow cumulus convection = Ondiepe cumulus convectie
Neggers, R. - \ 2002
Wageningen University. Promotor(en): Bert 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
<font size="2"><p>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.</p><p>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 <em>boundary layer</em> (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 <em>turbulence</em> . 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 <em>shallow cumulus</em> clouds, also known as <em>fair-weather</em> cumulus.</p><p>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 <em>parameterization.</em> 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.</p><p>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 <em>large-eddy simulation</em> . 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.</p><p>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.</p><p>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.</p><p>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.</p><p>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.</p><p>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.
Modeling the exchange of water and energy over natural land surfaces = Het modelleren van de uitwisseling van water en energie over natuurlijke landschappen
Ronda, R.J. - \ 2002
Wageningen University. Promotor(en): Bert Holtslag; H.A.R. de Bruin; B.J.J.M. van den Hurk. - S.l. : S.n. - ISBN 9789058086174 - 119
wiskundige modellen - evapotranspiratie - neerslag - bodem-energie-relaties - grenslaag - mathematical models - evapotranspiration - precipitation - soil energy relations - boundary layer
<p>This thesis deals with the modeling of the surface energy balance and the atmospheric boundary layer over natural land surfaces, on scales of the grid cell of large-scale atmospheric models. In the first part, a model to calculate the canopy conductance as a function of environmental variables evaluated at leaf level is developed and validated. The parameter values of this approach are retained from plant-physiological theory. For a C <sub>4</sub> prairie grass in Kansas and a C <sub>3</sub> soybean crop in southern France, the plant-physiological approach gives better estimates of the canopy conductance, compared to a traditional Jarvis-Stewart approach which relates the canopy conductance to environmental variables at a reference level, using empirical-statistical functions. For a C <sub>3</sub> grassland in the Netherlands, both the plant-physiological approach and the Jarvis-Stewart approach give comparable estimates of the latent heat flux density. In the second part, two approaches to calculate the impact of soil moisture stress on the surface flux densities over natural, heterogeneous areas are compared: a bulk approach where the soil moisture content is assumed to be uniform in a grid cell, and a distributed approach which takes account of the spatial variation of the soil moisture content. In wet conditions, the bulk approach gives larger predictions of the latent heat flux density than the distributed approach. In dry conditions the bulk approach gives lower predictions than the distributed approach. Especially for dry climates the bulk approach predicts during the dry season a severe suppression of the latent heat flux density. In the third part, using three cases that occur frequently in nature it is shown that only a tiling approach can provide estimates of the averaged surface flux densities that are consistent with the averaged temperature difference over the surface layer, the layer of air adjacent to the surface, in all situations.
Heat and water transfer in bare topsoil and the lower atmosphere
Berge, H.F.M. ten - \ 1996
Wageningen : Pudoc (Simulation monographs 33) - ISBN 9789022009611 - 207
luchttemperatuur - analogen - atmosfeer - computersimulatie - energiebalans - milieu - evaporatie - fluctuaties - hydrologie - land - methodologie - modellen - onderzoek - simulatie - simulatiemodellen - bodem - bodemtemperatuur - thermische bodemeigenschappen - bodemwater - oppervlakten - thermische geleiding - grenslaag - aardoppervlak - air temperature - analogues - atmosphere - computer simulation - energy balance - environment - evaporation - fluctuations - hydrology - methodology - models - research - simulation - simulation models - soil - soil temperature - soil thermal properties - soil water - surfaces - thermal conductivity - boundary layer - land surface
This book describes an analysis of processes and factors that affect the energy balance of bare soil, and the associated exchange of heat and moisture at the surface. After a brief treatment of basic transport theory, the processes of soil-atmosphere interaction are expressed in a simulation algorithm. This algorithm provides an instrument to study the 'conditioning' effect of soil on the lower atmosphere, and vice versa. Examples of sensitivity analysis are presented, with emphasis on the behaviour of surface temperature. In remote sensing practice, surface temperature is estimated from thermal imagery; simulation algorithms can assist in its interpretation. Field data of fluxes, state variables and soil properties are extensively discussed and are used to validate appropriate sections of the model
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 - reflection - clouds - microclimate - soil - landscape - boundary layer - land surface - earth
<br/>For many applications in agriculture, hydrology and meteorology simple methods are needed to determine the surface-atmosphere exchange of momentum, heat and water vapour, <em>i.e</em> 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.<p>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.<p>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 <em>z/x</em> = 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 <em>z/x</em> = 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 <em>z/x</em> 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.
Biocatalysis in non-conventional media : kinetic and thermodynamic aspects
Vermuë, M. - \ 1995
Agricultural University. Promotor(en): J. Tramper. - S.l. : S.n. - ISBN 9789054854623 - 177
biokatalyse - enzymen - moleculaire structuur - grenslaag - oppervlakteverschijnselen - biocatalysis - enzymes - molecular conformation - boundary layer - surface phenomena
<p>During the past decade biocatalysis in non-conventional media has gained a lot of interest. Especially in the field of bio-organic synthesis, where poorly water-soluble substrates and products are involved, these media are very attractive.<p>Non-conventional media generally consist of an apolar solvent phase and an aqueous phase. In this thesis, mixtures of water with water-miscible organic solvents, or water- immiscible organic solvents or (near-)supercritical solvents are described. The conventional aqueous phase contains the cellular or enzymic biocatalyst. The aqueous phase can vary from a dilute aqueous solution, with a thermodynamic water activity a <sub><font size="-2">w</font></sub> close to 1, to a dried enzyme particle with only a monolayer of adsorbed water molecules (a <sub><font size="-2">w</font></sub> &lt; 1).<p>In non-conventional media biocatalytic processes are governed by the presence of a phase boundary when two phases are involved. This phase boundary not only influences the rate of the bioconversion (kinetics), but also the yield of the reaction (thermodynamic equilibrium). In this thesis, several factors are described which affect the (kinetics), and thermodynamics of biocatalytic porcessen in non-conventional media.<p>Chapter 2 gives an overview of the recent developments in the field of medium engineering for biocatalysis in non-conventional media. In this chapter a few basic design rules for the rational design are formulated. These rules may serve as useful tools for optimization of biocatalytic processes in non-conventional media.<p>A typical example of a non-conventional reaction medium is the mixture of water and water-immiscible organic solvent. Especially for this type of reaction media the liquid-impelled loop reactor has been developed. This reactor has been used for the bioconversion of tetralin, a very toxic apolar compound. In Chapter 3 the general strategy for the selection of a suitable solvent for the bioconversion of such toxic apolar compounds in the liquid-impelled loop reactor is given, where the tetralin conversion is used as a typical example. The water-immiscible solvents should be non-toxic and nonbiodegradable. Additionally, they should reduce the toxicity of the apolar substrate and they must be practical for use in the liquid-impelled loop reactor. All the steps in the selection procedure proved to be essential. Among the 57 solvents tested, only FC-40 proofs to be suitable for bioconversion of tetralin in the liquid-impelled loop reactor. In addition, the cellular biocatalyst needs to be immobilized, to reduce emulsion formation inside the bioreactor.<p>For the bioconversion of tetralin in the liquid-impelled loop reactor oxygen is needed. Chapter 4 describes the mass transfer of tetralin and oxygen in the liquidimpelled loop reactor from the apolar solvent phase to the aqueous phase, where the bioconversion occurs. It is found that in case of mass-transfer limitation, tetralin is the rate-limiting substrate and not oxygen.<p>One of the selection criteria of a suitable solvent for bioconversion of apolar substates is its non-toxicity for the biocatalyst. The log <em>P</em><sub><font size="-2">octanol</font></sub> , which describes the hydrophobicity of the solvent, is a good measure for the toxicity of the solvent in a twoliquid phase system. The toxicity of a water-immiscible solvent for cellular biocatalyst is caused by two factors, <em>i.e.</em> the presence of a phase boundery (phase toxicity) and by the solvent molecules that are dissolved in the aqueous phase (molecular toxicity). Chapter 5 describes these effects separately. When the solvent concentration in the membrane of the cellular biocatalyst reaches a critical concentration, the solvent becomes toxic. The toxic concentration in the membrane is constant and independent of the solvent used. It is directly related <em>via</em> the partition coefficient over the membrane and water, to the solvent concentration in the aqueous phase. This is in turn directly related to the log <em>P</em><sub><font size="-2">octanol</font></sub> of the solvent. If the critical membrane concentration of a certain microorganism is known, the toxicity of any solvent can be predicted with the<br/>log <em>P</em><sub><font size="-2">octanol</font></sub> .<p>Apart from the log <em>P</em><sub><font size="-2">octanol</font></sub> , also the Hildebrand solubility parameter δcan be used as a measure of the hydrophobicity of the solvent. In Chapter 6 this parameter has been used successfully as an indicator of the solubility of apolar compounds in near-supercritical carbon dioxide (SCCO <sub><font size="-1">2</font></sub> ). In addition, the effect of this parameter on the transesterification rate of Lypozyme in this non-aqueous reaction medium has been studied. The change in δof near-supercritical carbon dioxide hardly influences the reaction rate. The water content of the medium influences the kinetics much more.<p>Water not only affects the kinetics of a synthetic reaction, but it also affects the equilibrium yield of these reactions. When the thermodynamic water activity a <sub><font size="-2">w</font></sub> is decreased, water-dependent side-reactions such as in transesterification reactions are suppressed (Chapter 6). In esterification reactions, a shift in equilibrium towards synthesis is expected upon decreasing the a <sub><font size="-2">w</font></sub> .<p>Chapter 7 describes a new method to control the a <sub><font size="-2">w</font></sub> during esterification reactions. With this a <sub><font size="-2">w</font></sub> -control method the a <sub><font size="-2">w</font></sub> can be maintained at an optimal value, at which the biocatalyst still shows sufficient activity while a high thermodynamic product yield can be obtained.<p>This thesis actually covers two central themes in biocatalysis in non-conventional media: kinetics and thermodynamics. In Chapter 8 a general discussion highlights how thermodynamics can be used as a basic tool to reveal the processes that govern biocatalysis in non-conventional media.
Interface dilation : the overflowing cylinder technique
Bergink - Martens, D.J.M. - \ 1993
Agricultural University. Promotor(en): A. Prins, co-promotor(en): H.J. Bos. - S.l. : Bergink-Martens - ISBN 9789054851288 - 151
vloeistofmechanica - capillairen - oppervlaktespanning - hydrodynamica - vloeistoffen (liquids) - vloeistoffen (fluids) - grenslaag - oppervlakteverschijnselen - fluid mechanics - capillaries - surface tension - hydrodynamics - liquids - fluids - boundary layer - surface phenomena
<p>A pure steady-state dilation of a liquid interface, either liquid-air or water-oil, can be accomplished far from equilibrium by means of the overflowing cylinder technique. The resulting dynamic surface tension data correlate well with characteristic parameters of processes like foaming, emulsification, and the spreading of droplets and thin liquid layers.<p>Fundamental knowledge of the physical mechanism of operation of the overflowing cylinder technique is obtained by analyzing the relation between interface dilation and underlying bulk flow. Upon the addition of a surfactant the interface velocity increases considerably, since the propulsion mechanism changes from driven by the bulk flow to surface tension gradient driven.<p>The surface rheological behaviour of the expanding interface is studied for various surfactant solutions. Generally practical systems give rise to a major increase in surface tension during interface dilation. The results are discussed in terms of the transport of surfactant components.<p>The present findings explain why the overflowing cylinder technique is such a useful too[ for studying many practical processes which imply interface dilation far from equilibrium. Meanwhile, however, they urge a reconsideration of the meaning of the surface dilational viscosity.
Molecular structure and interfacial behaviour of polymers
Lent, B. van - \ 1989
Agricultural University. Promotor(en): G.J. Fleer; J.M.H.M. Scheutjens. - S.l. : van Lent - 104
kunststoffen - industrie - oppervlakten - grensvlak - vloeistofmechanica - capillairen - oppervlaktespanning - polymeren - oppervlakteverschijnselen - grenslaag - moleculaire structuur - plastics - industry - surfaces - interface - fluid mechanics - capillaries - surface tension - polymers - surface phenomena - boundary layer - molecular conformation
<p>The aim of this study was to investigate the influence of the molecular structure on the interfacial behaviour of polymers. Theoretical models were developed for three different systems. All these models are based on the self-consistent field theory of Scheutjens and Fleer for the adsorption of homopolymers.<p>This self-consistent field theory is a lattice model. All possible polymer conformations on the lattice are taken into account. The potential of a conformation is sum of the local potentials of the segments of the molecule. In each layer a mean field approximation is used to calculate the mixing energy. The volume fraction profile is determined by the segmental potentials and vice versa. A numerical method is used to solve the obtained set of equations.<p>In chapter 2 the influence of association of block copolymers on adsorption is considered. In order to model spherical aggregates (micelles), the planar lattice, as used for modelling planar aggregates (membranes) and adsorption on flat surfaces, is replaced by a spherical lattice. The equilibrium solution concentration in a micellar solution is determined by a small system thermodynamics argument. The adsorption of diblock copolymers with long lyophobic and short lyophilic blocks shows strongly cooperative effects. A single molecular layer is present if the lyophobic block adsorbs. The adsorption isotherm shows an S-shape at the onset of adsorption. A strong increase of the adsorbed amount occurs near the cmc and above the cmc the adsorbed amount is almost constant. A bilayer at the surface can be formed if the lyophilic block adsorbs. Adsorption of the lyophilic blocks would expose the insoluble blocks to the solvent. Therefore, a second layer of molecules adsorbs with their lyophobic block towards the molecules attached to the surface. The influence of the interaction energies and the block sizes on these trends is described. The results obtained show good qualitative agreement with experimental results on surfactant adsorption.<p>The adsorption of random copolymers from solution is described in chapter 3. Experimentally, random copolymers are usually very polydisperse, both in chain length and in primary structure. Random copolymers which are only polydisperse in primary structure are considered here. They can be prepared experimentally by random chemical modification of monomer units of monodisperse homopolymers. The sequence distribution of random copolymers is determined by the fractions of the segment types in the polymer and the correlation factors between them. For random copolymers consisting of two different segment types, a blockiness parameter B is defined. The extremes of this parameter are -1 and 1, where the lower limit depends on the fractions of the different segment types. A value of B = -1 represents an alternating copolymer, whereas B = 1 stands for a mixture of two homopolymers. The complete statistical sequence distribution is implemented into the theory. In the results section random copolymers with two different segment types are studied. Chains with a higher than average content of adsorbing segments are preferentially adsorbed from the bulk solution. Only in the first few layers near the surface this preferential effect plays a role. In the remainder of the profile the segment types are more randomly mixed. The adsorption behaviour of these random copolymers is remarkably different from the adsorption of diblock copolymers. In the latter case, the chains have their adsorbing segments mainly in the layers near the surface, whereas further away from the surface long dangling tails of nonadsorbing segments are found. Random copolymers cannot spacially separate their segments so easily. Much higher adsorbed amounts are found for diblock copolymers than for random copolymers with the same fraction of adsorbing segments. The adsorption of random copolymers is less than that of homopolymer of equal length and consisting of the same type of adsorbing segments. Only for very high adsorption energies the adsorbed amounts are essentially the same. An increase in the blockiness parameter of the chains gives an higher adsorbed amount, but it is always below the adsorbed amount of the homopolymer. Analytical expressions have been derived which relate the interaction parameters of purely random copolymer and homopolymer.<p>In chapter 4 the interactions between surfaces coated with grafted polymer (also called hairy plates or soft surfaces) in the presence of nonadsorbing polymer is studied. The interaction free energy between the surfaces is obtained from the partition function. which is rederived for this more general case. For hard plates the interaction is fully determined by the osmotic pressure of the bulk solution and the depletion layer thickness. However. It turns out that In the case of soft surfaces the hairs have an attractive contribution to the free energy of interaction at a plate separation just below twice the hydrodynamic layer thickness of the grafted layer. The hairs mix mutually more easily than with free polymer. At a larger overlap of hairs the interaction becomes repulsive. In contrast with bare planar surfaces, the free energy of interaction between hairy surfaces shows a minimum as a function of the concentration of free polymer in the bulk solution. At a certain (very low) surface coverage the attraction is minimal. For even lower and for larger grafting densities the plates become more attractive. Increasing the repulsion between the hairs and free polymer makes the attraction stronger. The solvencies of grafted and free polymer have a less pronounced effect. Without free polymer, the interaction between the hairy surfaces becomes attractive if the solvency becomes worse than theta conditions.<p>It can be concluded that the self-consistent field theory has been successfully extended to three rather complex but technologically relevant systems. In this way a better understanding of the behaviour of polymers near interfaces has been obtained.
Radiative cooling in the nocturnal boundary layer
Tjemkes, S.A. - \ 1988
Agricultural University. Promotor(en): L. Wartena, co-promotor(en): F.T.M. Nieuwstadt. - S.l. : S.n. - 108
atmosfeer - computersimulatie - infraroodstraling - wiskundige modellen - onderzoek - simulatie - simulatiemodellen - turbulentie - grenslaag - aardoppervlak - optica - atmosphere - computer simulation - infrared radiation - mathematical models - research - simulation - simulation models - turbulence - boundary layer - land surface - optics
<p>In this thesis the transfer of infrared radiation (electromagnetic waves with a wavelength between 3.6 and 100 μm) through a cloudfree nocturnal boundary layer is studied. To simulate the transfer of infrared radiation an accurate narrow band model which simulates the absorption and emission of longwave radiation by water vapor and carbon dioxide is described. The model is tested against 44 cases of surface downward fluxes as well as against independent calculations. From these two comparison studies it is shown that the narrow band model simulates the surface downward flux with 3 W/m <sup><font size="-1">2</font></SUP>and the cooling rate within 0.2 K/day.<p>To analyse the interaction between turbulence and infrared radiation in a horizontally homogeneous cloudless nocturnal boundary layer, the radiation scheme is combined with a turbulence model, in which a prognostic equation for the turbulent kinetic energy is solved together with a diagnostic length scale equation for the turbulence. The performance of this combined radiation turbulence model is compared with detailed observations of the mean thermodynamic and turbulence structure throughout the nocturnal boundary layer.<p>From this comparison study it is shown that the agreement between the calculated and observed profiles of turbulence and mean thermodynamic variables and between the calculated and observed evolution of the surface fluxes is satisfactory. Moreover it is shown that the inclusion of radiative cooling within the boundary layer increases the boundary layer height by about 25%.<p>The combined radiation-turbulence model is also used to study the influence of the geostrophic wind velocity on the surface minimum temperature. From these model calculations a parametrization scheme is derived which is able to reproduce the surface minimum temperature over an almost saturated vegetated clay soil within 2.5 K.<p>The accuracy of the narrow band model used in the above described studies, is at the expense of a considerable amount of computer time. Thus the narrow band model is not recommended for use on a routine base in a boundary layer forecast model. Therefore an alternative radiation code for use in boundary layer models which is presented, which is accurate as well as efficient.
Surface fluxes and boundary layer scaling : models and applications
Holtslag, A.A.M. - \ 1987
Agricultural University. Promotor(en): L. Wartena; H.A.R. de Bruin. - S.l. : Holtslag - 175
atmosfeer - turbulentie - schaalverandering - warmtestroming - fluctuaties - modellen - toepassingen - grenslaag - aardoppervlak - atmosphere - turbulence - scaling - heat flow - fluctuations - models - applications - boundary layer - land surface
This study deals with applied modelling of some Atmospheric Boundary Layer (ABL) features. We use scaling techniques for the description of the turbulent structure in the ABL. A review is given on the different properties of the scaling techniques in stable and unstable conditions. The essential role of the surface fluxes of heat and momentum for the structure in the ABL is discussed.<p>Schemes are proposed for the estimation of the surface fluxes from routine weather data over land. Both for day- and nighttime, hourly values of the surface fluxes are modelled with the aid of the surface radiation and energy balance. Models and parameterizations for the individual components of these balances are compared with observations. During nighttime also the temperature profile up to 80 m is simulated with the modelled surface fluxes. The output of the surface flux schemes can be used for stability determination of the ABL.<p>Subsequently, diabatic wind profiles along the 200 m Cabauw tower are analysed in terms of surface layer similarity. For stable conditions an extension of the profile functions to strong stability is evaluated. Besides, the turning of wind with height up to 200 m is analysed. Together with the flux schemes, the wind speed profile can be estimated from near surface weather data only. It is shown that the agreement between estimates and observations is very good up to at least -100 m in generally level terrain. The methods are applied to simulate the wind frequency distribution and the reversed diurnal variation of the wind at 80 m.<p>Finally, a method for calculating the dispersion of non-buoyant plumes in the ABL is presented. The method is based on the scaling techniques of the ABL. Models are suggested for ground level concentrations of pollutants dispersed from continuous point sources. These models are evaluated with independent tracer experiments over land. The overall agreement between observations and predictions is very good and shown to be better than the skill of the traditional Gaussian plume model.<p>The proposed models and methods are intended for applications in meteorology and hydrology, for wind energy assessment methods and for air pollution dispersion studies.<p><TT></TT>
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