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Alle steden in Nederland, groot en klein, zijn kwetsbaar voor de effecten van klimaatverandering. De mate van kwetsbaarheid varieert nogal binnen het stedelijk gebied. Dat betekent dat het vergroten van de klimaatbestendigheid van steden het meest efficiënt kan gebeuren door het nemen van veel relatief kleine en lokale maatregelen. Veel daarvan kunnen tegelijkertijd met groot onderhoud of renovaties worden uitgevoerd. Daarvoor is wel samenwerking met veel en diverse partijen nodig.

Simultaneous scintillometer measurements at multiple wavelengths (pairing visible or infrared with millimetre or radio waves) have the potential to provide estimates of path-averaged surface fluxes of sensible and latent heat. Traditionally, the equations to deduce fluxes from measurements of the refractive index structure parameter at the two wavelengths have been formulated in terms of absolute humidity. Here, it is shown that formulation in terms of specific humidity has several advantages. Specific humidity satisfies the requirement for a conserved variable in similarity theory and inherently accounts for density effects misapportioned through the use of absolute humidity. The validity and interpretation of both formulations are assessed and the analogy with open-path infrared gas analyser density corrections is discussed. Original derivations using absolute humidity to represent the influence of water vapour are shown to misrepresent the latent heat flux. The errors in the flux, which depend on the Bowen ratio (larger for drier conditions), may be of the order of 10%. The sensible heat flux is shown to remain unchanged. It is also verified that use of a single scintillometer at optical wavelengths is essentially unaffected by these new formulations. Where it may not be possible to reprocess two-wavelength results, a density correction to the latent heat flux is proposed for scintillometry, which can be applied retrospectively to reduce the error.

Simultaneous scintillometer measurements at multiple wavelengths (pairing visible or infrared with millimetre or radio waves) have the potential to provide estimates of path-averaged surface fluxes of sensible and latent heat. Traditionally, the equations to deduce fluxes from measurements of the refractive index structure parameter at the two wavelengths have been formulated in terms of absolute humidity. Here, it is shown that formulation in terms of specific humidity has several advantages. Specific humidity satisfies the requirement for a conserved variable in similarity theory and inherently accounts for density effects misapportioned through the use of absolute humidity. The validity and interpretation of both formulations are assessed and the analogy with open-path infrared gas analyser density corrections is discussed. Original derivations using absolute humidity to represent the influence of water vapour are shown to misrepresent the latent heat flux. The errors in the flux, which depend on the Bowen ratio (larger for drier conditions), may be of the order of 10%. The sensible heat flux is shown to remain unchanged. It is also verified that use of a single scintillometer at optical wavelengths is essentially unaffected by these new formulations. Where it may not be possible to reprocess two-wavelength results, a density correction to the latent heat flux is proposed for scintillometry, which can be applied retrospectively to reduce the error.

The goal of this study is to test an alternative method for determining turbulent H2O and CO2 fluxes, which has a faster statistical convergence than the classical eddy-covariance method. This enables determining turbulent fluxes during strongly non-stationary conditions, e.g. in the intermittent stable boundary layer or rapidly changing cloud-cover. In our new method, we suggest a hybrid setup that combines a point-sensor for scalar H2O and CO2 with a dual-beam laser scintillometer (DBLS). We used a LiCor7500 open-path fast-response H2O/CO2 sensor. The H2O/CO2 sensor forms the basis for estimating the turbulent exchange scale for H2O and CO2. The DBLS yields the friction velocity and stability. The data presented were gathered in May-June 2009 over a wheat field near Merken, Germany in the framework of the TransRegio32 program. With the DBLS installed in homogeneous areas, turbulence is averaged both in time and space, allowing short averaging flux intervals down to a couple of seconds. When determining the averaging-time-dependent systematic and random errors in the H2O and CO2 fluxes, we found that eddy-covariance is not apt to measure 1-minute averaged fluxes, whereas our new method is. Even for 1-minute averaging intervals, our new method does not have a averaging-time-dependent systematic error and the random error is only halve that of the eddy-covariance method. Thus, we demonstrate that our new method reproduces reliable estimates of H2O and CO2 fluxes for 1-minute averaging intervals and show that not only individual plants, but also at field scales wheat responds rapidly to changes in solar radiation.

Guided and constrained by a complete data set of surface and upper-air observations taken during the fifth Intensive Observational Period (IOP-05, 25th June 2011) of the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) experiment, we reconstruct the evolution of the atmospheric boundary layer (ABL) using mixed-layer theory. The model results are corroborated by UHF radar and multiple profiles done by unmanned and manned aircraft platforms as well as by radiosondes. During the first morning hours, the ABL is mainly controlled by surface forcing, reaching a depth between 500 and 600 meter. At these levels, large-scale subsidence motions become similar in magnitude to the entrainment velocity and the ABL-growth becomes nearly zero. This equilibrium is maintained until the afternoon transition where mixed-layer model results and observations show a decrease of the boundary layer depth in spite of well mixed observed vertical profiles of heat and moisture. We discuss potential explanations to the development of subsidence motions associated to mesoscale flows driven by the proximity of the Pyrenees mountain range and to large-scale forcing. The heat and moisture budgets are also further analyzed with the mixed layer model. We find strong indications of the active role of heat and moisture advection required to reproduce the observed diurnal variability in temperature and specific humidity. By quantifying the budget terms, we are able to obtain a first estimation of the contribution of advection to the heat and moisture budgets. To complete the analysis, we extend the study to examine the turbulent characteristics of the ABL by using aircraft measurements combined with large-eddy simulations. In so doing, we are able to study the role of wind (directional) shear in the development and maintenance of the ABL. IOP05 was characterized by easterly flows within the ABL and westerly winds aloft. Our findings stress the important role of large scale forcing in understanding the ABL development at various stages during BLLAST. The proposed method can also be very useful to support the further interpretation of observations and mesoscale model experiments.

The focus of Knowledge for Climate (KfC) research in Hotspot Schiphol Mainport is to optimize the contribution of meteorological services to a sustainable operation and reliable operation of Schiphol airport. Three research projects have been carried out in the first tranche of the Knowledge for Climate programme on climate change and local critical weather conditions at Mainport Schiphol. Two projects (IMPACT and WindVisions) will be continued in the second tranche of the programme, in the Knowledge for Climate consortium High-Quality Climate Projections (Theme 6). This report presents a summary of the main results of the first tranche projects, and an outlook on the expected end results of the second tranche research at the end of the programme in 2014.

It is the objective of this project to develop a Wind and Visibility Monitoring System (WindVisions) at Mainport Schiphol. WindVisions will consist of a crosswind scintillometer, which is a horizontal long range wind and visibility sensor, and a SODAR (Sound Detecting And Ranging), a vertical scanning wind sensor. The area of interest to monitor is the landing and take-off course of airplanes ranging from the surface to about 300m height along a runway.

Mixed-layer theory and large-eddy simulations are used to analyze the dynamics of the boundary layer on two intensive operational periods during the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) campaign: 1st and 2nd of July 2011, when convective boundary layers (CBLs) were observed. Continuous measurements made by several remote sensing and in situ instruments in combination with radiosoundings, and measurements done by unmanned aerial vehicles and an aircraft probed the vertical structure and the temporal evolution of the boundary layer. Mixed layer theory (Tennekes and Driedonks, 1981) and the Dutch Atmospheric Large-Eddy Simulation model (DALES, Heus et al., 2011) are set to reproduce and analyze the dynamics of the atmosphere during these two days. The initial vertical profiles of potential temperature, specific humidity and wind, and the temporal evolution of the surface heat and moisture fluxes prescribed in the model runs are inspired by the observations taken at the two supersites that concentrated most of the instrumentation during the campaign. For DALES the initial profile (at 7 UTC on 1st July and 5 UTC on 2nd July) takes into account the existence of a residual layer above the nocturnal stable layer observed during the early morning. The mixed layer model is initialized when a well-developed convective boundary layer was observed, from 10 UTC. Due to the surface heterogeneity of the area, the models were run separately to analyze the boundary layer conditions at the two different supersites. First, the research focuses on the role-played by the residual layer (RL) on the evolution of the boundary layer. By using DALES, we show the importance of the dynamics of the boundary layer during the previous night to the development of the boundary layer at the morning. DALES, which takes into account the residual layer, is capable to model the observed sudden increase of the boundary layer depth and of the potential temperature occurred during the morning transition. There are different sources, which can be the responsible of the overshooting for instance surface and entrainment fluxes or large-scale phenomena. Analyzing the entrainment buoyancy heat flux, a large increase is obtained by the simulation when the residual layer is incorporated in the mixed layer by the growing convective boundary layer. Consequently, a precise definition of the characteristics of the residual layer is fundamental even though it is complex because the evolution of the main variables in the residual layer during the previous night depends on different factors such as radiation divergence or advection. Once the boundary layer is fully developed around midday, both models are used to study the dynamics of the boundary layer during the studied days. The models are able to reproduce the evolution of the boundary layer depth, potential temperature and specific humidity from late morning. They are also used to analyze the role played by subsidence and large-scale advection during the afternoon transition. During 1st of July, heat and moisture advection are negligible contributions to the heat or moisture budget, respectively, but subsidence plays an important role in the turbulence decay during the afternoon when a clear decrease of boundary layer depth was observed. On 2nd of July the main boundary layer variables follow similar evolution to the previous day, but with a lower maximum of the boundary layer depth. However, in the last part of the day, a front was approaching advecting moisture over the area and increasing the cloud cover. To summarize, the residual layer and large-scale phenomena play a crucial role in the development of the boundary layer during some days of the BLLAST campaign.

The goal of this study is to test an alternative method for determining turbulent H2O and CO2 fluxes, which has a faster statistical convergence than the classical eddy-covariance method. This enables determining turbulent fluxes during strongly non-stationary conditions, e.g. in the intermittent stable boundary layer or rapidly changing cloud-cover. In our new method, we suggest a hybrid set-up that combines a point-sensor for scalar H2O and CO2 with a dual-beam laser scintillometer (DBLS). We used a LiCor7500 open-path fast-response H2O/CO2 sensor. The H2O/CO2 sensor forms the basis for estimating the turbulent exchange scale for H2O and CO2. The DBLS yields the friction velocity and stability. The data presented were gathered in May-June 2009 over a wheat field near Merken, Germany in the framework of the TransRegio32 program. With the DBLS installed in homogeneous areas, turbulence is averaged both in time and space, allowing short averaging flux intervals down to a couple of seconds. When determining the averaging-time-dependent systematic and random errors in the H2O and CO2 fluxes, we found that eddy-covariance is not apt to measure 1-minute averaged fluxes, whereas our new method is. Even for 1-minute averaging intervals, our new method does not have an averaging-time-dependent systematic error and the corresponding random error is only halve that of the eddy-covariance method. Thus, we demonstrate that our new method reproduces reliable estimates of H2O and CO2 fluxes for 1-minute averaging intervals and show that not only individual plants, but also at field scales wheat responds rapidly to changes in solar radiation.

BLLAST (Boundary Layer Late Afternoon and Sunset Turbulence) aims at better understanding the thermodynamical processes that occur during the late afternoon in the lower troposphere. In direct contact with the Earth surface, the atmospheric boundary layer is governed by buoyant and mechanical turbulence, with a strong diurnal cycle. The late afternoon transition, from the daytime dry convection to the night-time stable boundary layer, still raises a lot of issues and is poorly represented in the meteorological models. Yet, it plays an important role in the transport and diffusion of trace gases, like water vapour, carbon dioxide, pollutants, dusts... How does the afternoon decay in the lower troposphere happen, when the surface sensible heat flux start to sharply decrease ? How do the scales of the motions and transfers change ? What is the impact on the chemical closure and transport of trace gases and aerosols ? How to properly represent those processes in the meteorological models ? An international group is working on those issues by use of observations and numerical simulations, in order to improve our understanding and representation of the turbulent processes of the boundary-layer late afternoon transition. The roles of surface heterogeneity, entrainment at the boundary layer top, large scale subsidence, radiative effects, advection and gravity waves are studied. Due to the large lack of observations during this phase, a field campaign was organized in the vicinity of a 60-m instrumented tower of Laboratoire d'Aérologie, near the Pyrenees ridge in Southwest France, from 14 June to 8 July 2011. This experiment puts together complementary observation resources, in order to obtain an exhaustive description of the boundary-layer dynamical processes, its vertical structure, and the spatial variability related to surface heterogeneity. Continuous measurements (UHF radar and sodar wind profilers, lidars, ground stations), and intensive observations with aircraft, unmanned aerial vehicles, tethered balloons and radiosoundings were used. Particular emphasis was placed to bridge the different spatial scales with an integrated analysis of the observations and a complete numerical model hierarchy. This presentation will give an overview of the field experiment, with an emphasis on specific and innovative instrumental aspects, and with some preliminary results, introducing various contributions of BLLAST participants that will be shown along the 20th BLT conference.

The turbulent exchange of heat and water vapour are essential land surface – atmosphere interaction processes in the local, regional and global energy and water cycles. Scintillometry can be considered as the only technique presently available for the quasi-operational experimental determination of area-averaged turbulent fluxes at a horizontal scale of a few kilometres needed to validate the fluxes simulated by regional atmospheric models or derived from satellite images. It has thus been increasingly used over the last decade. The scintillometer principle is based on the quantitative evaluation of intensity fluctuations of electromagnetic radiation propagating across the turbulent atmosphere over distances up to several kilometres. Although the successful application seems to justify the broad use of this technique, a number of assumptions in the scintillometer data processing and interpretation still call for a thorough evaluation of the scintillometer principle, in particular over heterogeneous terrain. These open issues include the spatial variability and corresponding aggregation rules for structure parameters, the structure parameter vertical profiles, and the temperature-humidity correlation. Moreover, a validation of the path-averaged structure parameters of temperature and humidity derived from scintillometer data by independent measurements is still missing. To answer these questions, the LITFASS-2009 field experiment has been performed in July 2009 around the Meteorological Observatory Lindenberg – Richard-Aßmann-Observatory of the German Meteorological Service (DWD). It combined tower-based in-situ turbulence measurements, field-scale laser scintillometers, long-range optical large-aperture scintillometers, and airborne turbulence measurements using an autonomous unmanned aircraft. Questions studied included the effect of tower vibrations on scintillometer measurements, the determination of the spatial (horizontal and vertical) and temporal variability of structure parameters (underlying the scintillometer principle), and the validation of scintillometer measurements over moderately heterogeneous terrain by airborne measurements. The paper will present exemplary results from LITFASS-2009. Suggestions will be derived for a continuation of this type of studies.

To study turbulent flows in the atmospheric boundary layer dimensional analysis is widely applied. For flows in the lower part of the atmospheric boundary layer the cornerstone of analysing experiments and modelling is Monin-Obukhov Similarity Theory (MOST). The central assumption is that once all relevant variables are identified, the number of dimensionless groups can be determined. Those dimensionless groups should be related in a universal way, irrespective of the magnitude of the individual (dimensional) variables. For MOST the list of relevant variables is limited, which is both its strength and its weakness. The main assumptions needed to limit the number of relevant variables are stationarity, horizontal homogeneity and irrelevance of processes outside the atmospheric surface layer. These conditions are easily violated in realistic situations. Although violations of MOST conditions in relation to non-universality of the MOST relationships are widely discussed in literature, this is nearly always done in a qualitative sense only. Here we will focus on the similarity relationships for standard deviations of temperature and humidity (sT and sq) and study how they deviate from universal behaviour due to heterogeneity of the underlying surface. The spatial scale of the heterogeneity we address is the scale of contrasts between individual fields (order of 100 meter). In the analysis we will use the framework presented in Moene and Schüttemeyer (2008). This study presents a detailed analysis of a dataset gathered during the BLLAST field campaign near Lannemezan (Southern France) in the summer of 2011. Three eddy-covariance systems were set up: one in a meadow (green throughout the campaign), one in a field with a mixture of cereals and peas (ripening during the campaign) and one right at the edge between the two fields ('edge station'). This setup enables us to obtain surface fluxes and other turbulence statistics both for two contrasting landuse types with homogeneous fetch as well as for a station with a heterogeneous fetch. The objective is to quantify the impact of surface heterogeneity on the dissimilarity between the similarity relationships of different scalars. The degree to which the edge station experiences a heterogeneous fetch depends on two aspects. The potential heterogeneity is determined by the contrast in surface fluxes between the two fields: the mid-day Bowen ratio of the meadow is approximately constant at 0.3, whereas the Bowen ratio of the cereal field increases from 1 to 1.5 over the course of the experiment. The midday CO2 flux is -7·10-7 kg m-2s-1 for the meadow, whereas for the cereals it increases from -2·10-7 kg m-2s-1 to +0.6·10-7 kg m-2s-1. The actual heterogeneity is determined by the wind direction, or more exactly, the location of the source area. Only if the source area of the observations is located in different fields, then the contrast in fluxes between fields will be actually relevant and lead to non-universal behaviour of MOST relationships. In order to quantify the relative contribution of the two fields in the signal of the edge station the footprint model of Hsieh et al. (2000), augmented with the crosswind distribution of Kormann and Meixner (2001), is used. The development of the fluxes of both surfaces indicates that the potential heterogeneity indeed increases in time. Furthermore, it turns out that for a significant part of the time footprint contributions come from both fields. Hence, the actual heterogeneity increases in time as well. This we express in a new heterogeneity parameter that combines the flux contrast between the fields and the heterogeneity of the land use in the footprint. The actual heterogeneity is clearly visible in the midday correlation coefficient between temperature and humidity (rTq) which decreases from 0.75 to 0.5 in the course of the experiment (rTq is of the order of 0.8 for both homogeneous fields). The mean Bowen ratio measured at the edge station is generally below 0.5, which implies that, according to Moene and Schüttemeyer (2008), one would expect that sT/¿* > sq/q* (equivalent to a relative transport efficiency ¿=rwT/rwq less than 1). This is indeed what is observed. Midday values for ¿ gradually decrease from 0.9 to 0.7. Apart from the similarity relationships for sT and sq, the relationships for CO2 standard deviation will also be discussed briefly.

Scintillometry has become a generally accepted technique to obtain area-averaged turbulent fluxes at the 0.1 – 10 km scale. Optical large aperture scintillometers (LAS) that yield the sensible heat flux (H) have been tested under a wide range of circumstances (De Bruin, 2002) and are commercially available. With a LAS in combination with a millimeter-wave scintillometer (MWS) the evapotranspiration (LvE) can be determined. Our aim is to develop such a combined Optical Millimeter Wave Scintillometer (OMS) system for operational use. In this study we will present results of the two OMS systems that were operated in the summer of 2009 as part of the LITFASS2009 campaign at the Meteorological Observatory Lindenberg, Germany. Although the final goal of the OMS system is to obtain LvE, first a number of instrumental issues have to be dealt with, such as scintillations caused by water vapor absorption, direct observation of the correlation between atmospheric temperature and humidity fluctuations, saturation of the LAS signal for long paths and the sensitivity of the OMS to mast vibrations. To this end all raw signals of the scintillometers were measured and stored at 500Hz, providing maximum freedom in the data processing, e.g. allowing spectral filtering. In addition, 2D-acceloremeter measurements were taken on the sensors to evaluate the impact of mast-vibration.

The Weather Research and Forecasting Model (WRF) and the The Weather Research and Forecasting Model (WRF) and the Regional Atmospheric Mesoscale Model System (RAMS) are frequently used for (regional) weather, climate and air quality studies. This paper covers an evaluation of these models for a windy and calm episode against Cabauw tower observations (The Netherlands), with a special focus on the representation of the physical processes in the atmospheric boundary layer (ABL). In ad-dition, area averaged sensible heat flux observations by scintillometry are utilized which enables evaluation of grid scale model fluxes and flux observations at the same horizontal scale. Also, novel ABL height observations by ceilometry and of the near surface longwave radiation divergence are utilized. It appears that WRF in its basic set-up shows satisfactory model results for nearly all atmospheric near surface variables compared to field observations, while RAMS needed refining of its ABL scheme. An important inconsistency was found regarding the ABL daytime heat budget: Both model versions are only able to correctly forecast the ABL thermodynamic structure when the modeled surface sensible heat flux is much larger than both the eddy-covariance and scintillometer observations indicate. In order to clarify this discrepancy, model results for each term of the heat budget equation is evaluated against field observations. Sensitivity studies and evaluation of radiative tendencies and entrainment reveal that possible errors in these variables cannot explain the overestimation of the sensible heat flux within the current model infrastructure.

The main objective of this thesis is to investigate observation methods of heat and momentum exchange and key variables that characterise turbulence in the atmospheric stable surface layer (SSL), a layer defined as the lower part of the stable boundary layer (SBL) where surface fluxes do not change significantly with height. The SBL is often confined to a shallow layer above the surface and is often intermittent, i.e. quiescent periods with almost laminar flow are interchanged with turbulent bursts. These conditions complicate surface flux measurements considerably, since ideally these then need to take place close to the surface and over short flux averaging intervals. Scintillometers, unlike traditional flux measurement techniques such as eddy covariance (EC), can be operated just above the surface (< 1 m) and over short flux averaging intervals (< 1 minute). These features have led us to explore in more detail the applicability of scintillometers in the SSL.Two types of scintillometers will be considered, notably the displaced-beam small-aperture scintillometer (DBSAS) and the large-aperture scintillometer (LAS) deployed in three field campaigns we contributed to as part of this thesis: RAPID in Idaho, USA, (1999), CASES-99 in Kansas, USA (1999) and BBC in Cabauw, the Netherlands (2001). In addition, an old data-set is analysed with LAS data gathered during the La Poza experiment in Sonora, Mexico (1996).The DBSAS and the LAS are optical instruments that consist of a transmitter and a receiver. The receiver records intensity fluctuations of the light beam emitted by the transmitter, which are caused by refraction of the beam upon its passage through the turbulent surface layer. These intensity fluctuations are a measure of the structure parameter of temperature,C T2. The DBSAS obtains also the dissipation rate of turbulent kinetic energy,e, from the correlation between the two displaced beams. In itself, these quantities are important properties of turbulence. Moreover, when the flow is turbulent they are related to the turbulent fluxes of sensible heat, H , and momentum,t, usually expressed by the velocity scale u * , by virtue of Monin-Obukhov similarity theory (MOST).The DBSAS is the most suitable scintillometer to be used in the SBL, since it gives a measure of the mechanically induced turbulence (i.e.e), which is the only turbulence generating mechanism in stable conditions. For the LAS - that does not measuree- the mechanical turbulent transport is usually included using wind speed measurement and an estimate of the roughness length.Several detailed aspects of the application of scintillometry and EC in obtaininge, C T2 , H andtare discussed. The most general aspects presented are the following. For CASES-99 and BBC we compared the DBSAS performance against EC in obtaininge, C T2 , H andtover a wide range of stable conditions and conclude that the DBSAS is superior in obtaining turbulence information over short intervals with remarkably little scatter, but that the derived parameters contain systematic errors. When corrected for the systematic errors (using ad-hoc solutions) the DBAS appears to provide accurate C T2 ,eand resulting H , andtfor short time intervals and close to the ground. In addition, for the BBC we also investigated the LAS and combinations of LAS and DBSAS to jointly solveeand C T2 for both stable and unstable conditions. Furthermore, for CASES-99 we derived new MOST relations foreand C T2 and show how these can be used to evaluate the MOST relations for dimensionless wind speed and temperature gradients. Also, alternative scaling parameters based oneand C T2 are introduced. Last, we investigated an important practical aspect of the scintillometer application, i.e. what effective height to use to calculate H when the beam-height of the instrument varies along the path. This is done based on a data-set from the La Poza experiment in Sonora, Mexico (1996).

In this paper a classification of stable boundary layer regimes is presented based on observations of near-surface turbulence during the Cooperative Atmosphere–Surface Exchange Study-1999 (CASES-99). It is found that the different nights can be divided into three subclasses: a turbulent regime, an intermittent regime, and a radiative regime, which confirms the findings of two companion papers that use a simplified theoretical model (it is noted that its simpliflied structure limits the model generality to near-surface flows). The papers predict the occurrence of stable boundary layer regimes in terms of external forcing parameters such as the (effective) pressure gradient and radiative forcing. The classification in the present work supports these predictions and shows that the predictions are robust in a qualitative sense. As such, it is, for example, shown that intermittent turbulence is most likely to occur in clear-sky conditions with a moderately weak effective pressure gradient. The quantitative features of the theoretical classification are, however, rather sensitive to (often uncertain) local parameter estimations, such as the bulk heat conductance of the vegetation layer. This sensitivity limits the current applicability of the theoretical classification in a strict quantitative sense, apart from its conceptual value.

In this study we investigated the performance of a displaced-beam small aperture scintillometer (DBSAS) - operated over a path length of 112 m - under stable conditions using data gathered during the CASES-99 experiment in Kansas, USA. The DBSAS has the advantage over the eddy covariance method that it can determine fluxes of sensible heat and momentum close to the surface and/or over short (< 1 minute) averaging intervals. Both aspects are of importance in the often shallow and non-stationary stable boundary layer (SBL). From raw DBSAS measurements the dissipation rate of turbulent kinetic energy and the structure parameter of temperature can be deduced, which follow Monin-Obukhov similarity theory (MOST) to give the friction velocity, u* the temperature scale, T* and subsequently the sensible heat flux, H. All these variables were compared with eddy covariance data for 10-minute time averages. Systematic errors were found for the DBSAS u*; i.e. overestimation for low u* values and underestimation for high u* values. It seems these errors cannot be attributed to the use of MOST, since they are already present in the dissipation rate data. In determining the dissipation rate, a form of the temperature spectrum in the dissipation range has to be assumed. The way the DBSAS weights this spectrum will be discussed. Furthermore, the sensitivity of the DBSAS method to small off-sets in instrumental parameters will be discussed. It was found that an adjustment of the beam displacement distance, d that is within the accuracy at which d is determined, removes some of the systematic errors. This adjustment is presented as a working hypothesis, not a general solution

The stable boundary layer (SBL) is often characterised by turbulence which is not continuous in space and time. This socalled intermittent turbulence may affect the whole depth of the SBL. In this study intermittent turbulence is studied from both theoretical and experimental point of view. The study is restricted to the type of intermittency which is caused by the so-called Businger-Blackader mechanism. According to this mechanism the following picture emerges: During clear and stable nights often stability develops faster than shear due to the strong surface radiation. This causes the Richardson number to increase, leading to cessation of turbulence. As a consequence air becomes decoupled from the surface. Soon however air will be accelerated by the omnipresent pressure force until shear is strong enough to break down the stability causing a turbulence burst. Because of strong mixing shear is rapidly reduced and stability takes over. Now the situation has returned to its begin and the mechanism starts over again, causing intermittent bursts of turbulence. In this study we seek for a theoretical foundation and an experimental validation of this mechanism. In the theoretical part of the study, the mechanism described above is simplified to its physical essence. For a certain parameter range the outcome from the numerical runs shows intermittent behaviour. Furthermore this model is studied analytically from a system-dynamics point of view. By doing so a dimensionless parameter is found which determines the equilibrium behaviour of the model (e.g. intermittent or non-intermittent behaviour). This critical parameter is merely a function of external ‘forcings’ such as pressure gradient, cloud cover and soil roughness. The experimental part of our study on intermittent turbulence was tackled during an extensive cooperative nocturnal boundary layer experiment( CASES99) in Kansas, USA. Apart from conventional eddy correlation systems, two types of scintillometers were used; a large aperture and a split-beam laser scintillometer. These instruments provide directly an areally averaged flux which has the main advantage of interchanging of space over time averaging. This allows shorter averaging times of fluxes, which is a major advantage in the non-stationary conditions encountered of the SBL. Our results indicate that the intermittency mechanism described above is indeed a likely candidate to explain intermittent turbulence in the stable boundary layer over land

Sonic anemometers are used to determine the vertical fluxes of momentum and scalars in the atmospheric surface layer. In practice the coordinate systems of the sonic anemometer and the surface will not be perfectly aligned so that a tilt-correction for these small deviations of the anemometer coordinates is needed. Three options are available: (1) to use known (e.g. measured) angles, (2) to impose requirements on the means and stresses within an averaging period (the 'classic method'), and (3) to assume that the mean flow over a certain period, as observed by the sonic, is in a plane parallel to the surface (planar fit method). Data from five 3-D sonic anemometers (three different types) collected in June 2001 have been used to assess these three methods. The main questions were: -are the rotation angles of the 'classic' method significant (relative to their tolerance)? -do the rotation angles correlate among the five systems? -does the planar fit method give results that are more consistent among the five systems, in terms of the measured shear stress? It is shown that for all five systems the run-to-run variation of classic pitch angles is significant relative to its tolerance but the variation shows no correlation among adjacent instruments, suggesting that there is no meteorological cause for the variation. The mean and run-to-run variation of the roll angle are in most cases insignificant.Comparison of the correlation of the shear stress between different systems, shows that day-by-day (modified) planar fit rotation increases the correlation for most combinations of systems. To our opinion this enhanced consistency shows the superiority of the (modified) planar-fit method to the classical tilt-corrections in the context of surface-flux estimation