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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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    Modelling stable atmospheric boundary layers over snow
    Sterk, H.A.M. - \ 2015
    Wageningen University. Promotor(en): Bert Holtslag, co-promotor(en): Gert-Jan Steeneveld. - Wageningen : Wageningen University - ISBN 9789462572263 - 189
    sneeuw - atmosferische grenslaag - modelleren - modellen - turbulentie - weersvoorspelling - snow - atmospheric boundary-layer - modeling - models - turbulence - weather forecasting

    Thesis entitled:

    Modelling Stable Atmospheric Boundary Layers over Snow

    H.A.M. Sterk

    Wageningen, 29th of April, 2015

    Summary

    The emphasis of this thesis is on the understanding and forecasting of the Stable Boundary Layer (SBL) over snow-covered surfaces. SBLs typically form at night and in polar regions (especially in winter), when radiative cooling at the surface causes a cooler surface than the overlying atmosphere and a stable stratification develops. This means that potential temperature increases with height and buoyancy effects suppress turbulence. Turbulence is then dominated by mechanical origin. If sufficient wind shear can be maintained, turbulence remains active, otherwise it will cease.

    A proper representation of SBLs in numerical weather prediction models is critical, since many parties rely on these forecasts. For example, weather prediction is needed for wind energy resources, agricultural purposes, air-quality studies, and aviation and road traffic. Knowledge on SBLs is also essential for climate modelling. In the Arctic regions, climate change is most pronounced due to stronger changes in near-surface temperature compared to other latitudes. Though this `Arctic amplification' is not yet fully understood, possible responsible processes are the ice-albedo feedback, alterations in cloud cover and water vapour, different atmospheric and oceanic circulations, and the weak vertical mixing in the lower atmosphere. However, many interactions exist between these processes. With positive feedbacks, changes are even further enhanced. This could have worldwide consequences, i.e. due to affected atmospheric circulations and sea level rise with Greenland's melting ice-sheets.

    Scientists try to explain the observed climate changes, as well as provide outlooks for future changes in climate and weather. However, the understanding is hampered by the fact that many model output variables (e.g. regarding the 2 m temperature) vary substantially between models on the one hand, and from observations on the other hand. Modelling the SBL remains difficult, because the physical processes at hand are represented in a simplified way, and the understanding of the processes may be incomplete. Furthermore, since processes can play a role at a very small scale, the resolutions in models may be too poor to represent the SBL correctly. Additionally, there are many different archetypes of the SBL. Turbulence can be continuous, practically absent, or intermittent, and vary in strength which affects the efficiency of the exchange of quantities horizontally and vertically.

    Processes that are considered critical for the SBL evolution are e.g. turbulent mixing, radiative effects, the coupling between the atmosphere and the underlying surface, the presence of clouds or fog, subsidence, advection, gravity waves, and drainage and katabatic flows. In this thesis, the focus is on the first three processes, as these are most dominant for the evolution of the SBL (e.g. Bosveld et al., 2014b).

    In Chapter 3 an idealized clear-sky case over sea-ice was studied based on the GABLS1 benchmark study (e.g. Cuxart et al., 2006), but extended by including radiative effects and thermal coupling with the surface. Hence the following research questions were posed:

    Question 1: What is the variety in model outcome regarding potential temperature and wind speed profiles that can be simulated with one model by using different parametrization schemes?

    Question 2: Which of the three governing processes is most critical in determining the SBL state in various wind regimes?

    Question 3: Can we identify compensation mechanisms between schemes, and thus identify where possible compensating model errors may be concealed?

    From the analysis with different parametrization schemes performed with the WRF single-column model (SCM, Question 1), it followed that quite different types of SBLs were found. Some schemes forecasted a somewhat better mixed potential temperature profile where stratification increased with height, while another scheme produced profiles with the strongest stratification close to the surface and stratification decreased with height. After only 9 h of simulation time, a difference in temperature of almost 2 K was found near the surface. Regarding the wind speed profile, some variation was found in the simulated low-level jet speed and height. Mainly the difference in atmospheric boundary-layer (BL) schemes which parametrize the turbulent mixing are responsible for these model output variations. A variation in long wave parametrization schemes hardly affected the model results.

    Question 2 addresses the problem whether other processes than turbulent mixing may be responsible for a similar spread in model results. A sensitivity analysis was performed where for one set of reference parametrization schemes the intensity of the processes was adjusted. The relative sensitivity of the three processes for different wind regimes was analysed using `process diagrams'. In a process diagram, two physically related variables are plotted against each other, which in this case represent either a time average or a difference over time of the variable. A line connects the reference state with the state for which the process intensity is modified. By comparing the length and orientation of the lines, the relative significance of the individual processes for the different wind speeds can be studied. Overlapping line directions identify possible compensating errors.

    Geostrophic wind speeds of 3, 8 and 20 m s-1 were selected representing low, medium and high wind speeds, capturing the range of wind speeds frequently occurring in the Arctic north of 75oN according to the ERA-Interim reanalysis dataset. Overall, a shift in relative importance was detected for the various wind regimes. With high geostrophic wind speeds, the model output is most sensitive to turbulent mixing. On the contrary, with low geostrophic wind speeds the model is most sensitive to the radiation and especially the snow-surface coupling. The impact of turbulent mixing is then minor, unless when mixing in both boundary layer and surface layer is adjusted. This stresses that proper linking between these two layers is essential.

    Also with one set of parametrization schemes different SBL types were simulated. Potential temperature profiles were better mixed (increasing stratification with height) for high geostrophic wind speeds, and this tended to develop to profiles with the strongest stratification near the surface (decreasing stratification with height) for low geostrophic wind speeds. However, a variety in types was also found when keeping the same wind regime, but by varying the mixing strength. With enhanced mixing, the profile became better mixed, also when the reference profile showed the strongest stratification near the surface. With decreased mixing, profiles with a stronger stratification were found, again shaped with the strongest stratification near the surface. Thus a different mixing formulation has a strong impact on the vertical profiles, even when it may not necessarily strongly affect the surface variables. Therefore, it is recommended that when a model is evaluated and optimized, the vertical structure is also regarded in this process, since near-surface variables may be well represented, strong deviations aloft are still possible.

    Furthermore, the process diagrams showed overlap in sensitivity to some processes. Therefore errors within the parametrizations of these processes could compensate each other and thus remain hidden (Question 3), making the model formulation possibly physically less realistic. This study did not reveal an unambiguous indication for the compensating processes regarding the various sets of variables, though overlap for single variables is seen.

    This study also revealed a non-linear behaviour regarding the 2 m temperature, which is also found in observations (e.g. Lüpkes et al., 2008) and in a model study by McNider et al. (2012). Here the 2 m temperature decreased with enhanced mixing strength and increased with a lower mixing intensity. This counter-intuitive behaviour is explained by that mixing only occurs in a shallow layer close to the surface. Cold air that is mixed up by the enhanced mixing, is insufficiently compensated by the downward mixed relatively warm air. This behaviour was found mostly for low wind speeds or with decreased mixing at the medium wind regime, when the potential temperature profile showed the strongest stratification near the surface.

    The study proceeds with a model evaluation against observations in low wind speed regimes. Three stably stratified cloud-free study cases with near-surface wind speeds below 5 m s-1 were selected with each a different surface: Cabauw in the Netherlands with snow over grass, Sodankylä in northern Finland with snow in a needle-leaf forest, and Halley in Antarctica with snow on an ice shelf.

    Chapter 4 presents the evaluation of the WRF-3D and SCM for these cases. In this study, the WRF-3D model was used to determine the forcings, as often not all the required observations at high resolution in time and space are available. Hence the following questions were formulated:

    Question 4: What is the performance of WRF in stable conditions with low wind speeds for three contrasting snow-covered sites?

    Question 5: How should we prescribe the single-column model forcings, using WRF-3D?

    The standard WRF-3D simulation had an incorrect representation of the snow-cover and vegetation fraction, which deteriorates the conductive heat flux, the surface temperatures and the SBL evolution. Indeed, Chapter 3 highlighted the critical role of the land-surface coupling representation. Adjusting the settings with site specific information, improved model simulations compared with the observations.

    In general, the performance of WRF-3D was quite good for the selected cases, especially regarding the wind speed simulations. The temperature forecast proved to be more challenging. For Cabauw and Sodankylä, 2 m temperatures were strongly overestimated, though a better simulation was seen at higher tower levels. For Halley a better representation of the 2 m temperature was found, though aloft potential temperatures were underestimated. Hence, the three cases had an underestimated modelled temperature gradient in common.

    This study also investigated how the forcing fields for the SCM should be prescribed. Model results for the three study cases all showed a significant deviation from the observed wind field without lateral forcings and time-invariant geostrophic wind speed. Including only a time-varying geostrophic wind speed did not improve the results. Prescribing additional momentum advection did have a positive impact on the modelled wind speed. The results regarding temperature, specific humidity and their stratification improved when temperature and humidity advection was also taken into account. Forcing the SCM field towards a prescribed 3D atmospheric state is not recommended, since unrealistic profiles were found below the threshold forcing height.

    Having established the optimal model set-up, the SCM can be used as a tool to further study the small-scale processes for the three study cases, addressing the following questions:

    Question 6: How do the model results with various process intensities compare with observations?

    Question 7: Are any differences in relative process impacts found for the three contrasting sites?

    Question 8: Does the model sensitivity vary between two different BL schemes?

    The sensitivity analysis was performed with the WRF-SCM and repeated for two BL schemes. In general, the temperature and humidity stratifications intensified by decreasing the process strengths and hence were in better agreement with observations than the reference cases. The wind field was most sensitive to turbulent mixing, with a weaker low-level jet at a higher altitude for enhanced mixing and the opposite for less mixing, while the impact of the other processes was small. Contrary to the temperature profiles, a better agreement with wind observations was found with amplified mixing, except for Halley where results improved with reduced mixing.

    Regarding the surface energy budget, the conductive heat flux was greatly overestimated at Cabauw due to an overestimated snow conductivity, while better agreements were found for the other sites. A revision of the definition for snow conductivity in the model is recommended, because rather large values were assumed for fresh snow, and indeed results improved when the coupling strength was reduced for Cabauw and Sodankylä. For Halley almost the same snow conductivity was modelled as was used to determine the observed conductive heat flux, however, then the temperature gradient through the first soil/snow layer was underestimated leaving the flux too small.

    The net radiation was strongly too negative for the Cabauw and Halley case-studies. This is likely due to an underestimation of the incoming long wave radiation as part of a deficiency in the long wave radiation scheme. For all sites the sensible heat flux was overestimated, and decreased mixing improved the results. However, the eddy covariance measurements may have been made outside the constant flux layer, which hampers the model evaluation.

    Though Question 6 aims to obtain understanding in which processes are most responsible for simulating model results that are in closer agreement with observations, measuring in these cold and dry circumstances is especially challenging. Furthermore, the measurements are mostly point measurements while the model grid represents a larger area, such that the measurements may be influenced by local features which are not captured in the model. These issues hinders a clear comparison of the model results with observations, and the observation uncertainties may be greater than what was represented in the process diagrams.

    When comparing the process sensitivity for the different sites (Question 7), we found some distinct variations in relative process significance. The radiation impact was relatively large at Cabauw and Sodankylä where the specific humidity was higher such that a larger impact on the incoming long wave radiation can be obtained. The snow-surface coupling is more important at Halley. This is related to the higher snow cover at Halley compared to the other sites. Additionally, the conductivity of the underlying medium at Halley is set equal to that of snow. These two factors ensure that the impact of an altered snow conductivity is greater.

    From the comparison of the sensitivity analyses for the two BL schemes (MYJ and YSU, Question 8), it followed that the overall direction of the sensitivity orientation is similar. However, stronger BL temperature stratifications were found with YSU, though between the surface and the first model level stronger stratifications were simulated with MYJ. This is related to the relatively high ratio of mixing in the boundary layer versus the surface layer with MYJ. Therefore the mixing in the BL is relatively more efficient and the surface layer cannot keep up the mixing to keep a smooth profile at the surface-layer / boundary-layer interface. This indicates the importance of a consistent transition between the BL and surface layer, as also pointed out by Svensson and Holtslag (2009). Furthermore, the non-linearity concerning the 2 m temperature behaviour discussed earlier is most profound with YSU, and not as obvious with MYJ due to a stronger implemented minimum diffusivity.

    The results point towards the direction of focus for future research. This could be achieved by e.g. re-evaluating the snow representation, as well as investigating the long-standing problem of the underestimated long wave radiation. Additionally, the mixing seems to be too high in some of the simulations. As such, care should be taken in choosing the BL scheme and its constraints on the mixing, as these may hamper the development of the observed behaviour on non-linear near-surface temperature evolution for example.

    Atmospheric turbulence over crops : confronting theories with observations
    Boer, A. van de - \ 2015
    Wageningen University. Promotor(en): Bert Holtslag, co-promotor(en): Arnold Moene; A. Graf. - Wageningen : Wageningen University - ISBN 9789462572416 - 143
    turbulentie - meteorologie - atmosfeer - gewassen - watergebruiksrendement - transpiratie - modellen - eddy-covariantie - turbulente stroming - turbulence - meteorology - atmosphere - crops - water use efficiency - transpiration - models - eddy covariance - turbulent flow

    Atmospheric turbulence plays a key role in hydrological and carbon cycles, and in weather and climate. Understanding and forecasting turbulence is thereby relevant for human life and environment.

    We deal with some major challenges for studying atmospheric turbulence over crops. Land-atmosphere interactions are specifically complex because of surface heterogeneity. Also, boundary-layer entrainment complicates measuring and studying surface fluxes. Furthermore, the absence of high-frequency observations and of measurements of underlying soil and vegetation processes impedes studying land-atmosphere interactions.

    We show the applicability of analytical footprint models over a heterogeneous land surface, and the validity of Monin-Obukhov similarity theory for a strongly-convective boundary-layer. Moreover, we present improvements on a scheme that can be used to estimate the amount of atmospheric turbulence from single-level weather data. We furthermore suggest to improve the partitioning theory that is used to distinguish soil processes from plant processes in eddy-covariance flux observations.

    Aspects of atmospheric turbulence related to scintillometry
    Braam, M. - \ 2014
    Wageningen University. Promotor(en): Bert Holtslag, co-promotor(en): F. Beyrich; Arnold Moene. - Wageningen : Wageningen University - ISBN 9789461739384 - 151
    turbulentie - meteorologie - atmosfeer - scintillometrie - meteorologische waarnemingen - methodologie - turbulence - meteorology - atmosphere - scintillometry - meteorological observations - methodology

    Aspects of atmospheric turbulence related to scintillometry

    Atmospheric turbulence is the main vertical transport mechanism in the atmospheric boundary layer. The surface fluxes related to this turbulent transport are the sensible () and latent heat fluxes (). The area-averaged values of and are of interest to evaluate mesoscale numerical weather models and in water budget studies. Natural landscapes are often heterogeneous, i.e. and differ among fields.

    The fluxes can be obtained with a scintillometer system, which consists of an electromagnetic beam transmitter at one end of a propagation path and a receiver at the other end. The intensity of the electromagnetic signal at the receiver varies due to fluctuations in the refractive index of air () caused by turbulence along the path. From the magnitude of these fluctuations the structure parameter of temperature () and of humidity () can be derived. Finally, and are used to determine path-averaged and via Monin-Obukhov similarity theory (MOST). The advantage of scintillometry is that the obtained fluxes are path-averaged, which makes scintillometry a more suitable method for obtaining area-averaged fluxes over natural landscapes than traditional point measurements. However, the disadvantage is that the fluxes are not directly measured. Therefore in this thesis four questions are answered related to the applicability of MOST and to the behaviour of structure parameters over heterogeneous surfaces. The latter is important because MOST assumes homogeneous surface conditions. For our studies we used meteorological data measured at three sites (Cabauw; the Netherlands, CASES-99 experiment; Leon; Kansas; USA, LITFASS-2009 and LITFASS-2010 experiments; Lindenberg; Germany) under unstable conditions (day-time).

    MOST is restricted to the part of the atmosphere close to the surface: the atmospheric surface layer (ASL). The depth of the ASL is not constant during the day, and is relatively shallow during the morning. At those moments, the scintillometer observation level can be located outside the ASL, which can question the validity of MOST. Therefore, we proposed and compared two variations in MOST (MOST using local fluxes and MOST using surface fluxes). We found that during the afternoon when both concepts have to be valid, the values of are comparable. During the morning, the data do not unequivocally support one of the two concepts: MOSTl shows the correct temporal behaviour in, but underestimates by a factor of ten.

    The universal function that links the surface fluxes with the structure parameter in MOST needs to be determined empirically. In literature a great variety of these function can be found. Therefore, we investigate to what extent the expression for this function depends on the specific regression approaches, stability range and observation level. First, we found that applying various regression approaches has an impact on the expression. This means that studies always should specify their regression approach, when presenting new functions. We advise to use an orthogonal distance regression method, applied to the logarithmic transformation of both dimensionless groups, and weighted such that unreliable data points have a smaller influence on the fit. Second, we found that the observation height and the stability range have an impact on the coefficients too. This implies that variations found in literature may result from variations in the height and stability ranges among the datasets. Furthermore, application of a given expression on a dataset measured at a different height or within a different stability range has to be done with care.

    In order to investigate whether variations in and along a scintillometer path or aircraft flight leg are within the range of local variability, or could be attributed to surface heterogeneity, we analysed the amount of local variability in the structure parameters at different heights and under different stability regimes. We found that the variability is determined by stability and by the size of the averaging window over which the structure parameters are calculated. If instability increases, differences in structure parameters between upward motions and downward motions increase. If the averaging window size increases, the variance of the logarithmic structure parameters decreases. A rough estimation of this decrease is made by fitting a simple linear regression between this variances and the averaging window size. From this we found that for various stability classes both the offset and slope (in absolute sense) decrease with increasing instability. The offset and slope can be used to quantify the local variability, which in turn can give an indication if variations in the structure parameters along a scintillometer or flight path might be attributed to surface heterogeneity.

    Finally, our last study is an elaboration of the study of Beyrich et al. (2012). They compared obtained with the unmanned meteorological mini aerial vehicle (M2AV) with obtained with the large-aperture scintillometer (LAS) for five flights on one single day during LITFASS-2009 experiment. We investigated if the systematically larger values of the M2AV as observed by them, can be found for other days, and if these differences could be reduced or explained through a more elaborate processing of the data of both instruments. We concluded that the difference can be found for other days during LITFASS-2009 and LITFASS-2010 as well. obtained from the M2AV data is larger, which is not improved by the more elaborate data analysis. Moreover, an exact synchronization of the LAS data with the time intervals of the M2AV data does not eliminate the discrepancy between both datasets.

    All in all, this thesis defines better the borders of MOST and shows the behaviour of the structure parameters in the atmospheric surface layer.

    Beyrich F, Bange J, Hartogensis OK, Raasch S, Braam M, van Dinther D, Gräf D, van Kesteren B, van den Kroonenberg AC, Maronga B, Martin S, Moene AF (2012) Towards a validation of scintillometer measurements: The LITFASS-2009 experiment. Boundary-Layer Meteorol 144:83-112

    Obtaining Crosswind from Single-Aperture Optical Scintillometers
    Dinther, D. van; Hartogensis, O.K. - \ 2010
    - p. 1 - 2.
    windmeters - windsnelheid - monitoring - turbulentie - anemometers - wind speed - monitoring - turbulence
    The goal of this study is to explore a method to obtain the crosswind from single aperture scintillometers through spectral analysis of the raw scintillometer signal.
    How to design single-column model experiments for comparison with observed nocturnal low-level jets
    Baas, P. ; Bosveld, F.C. ; Lenderink, G. ; Meijgaard, E. van; Holtslag, A.A.M. - \ 2010
    Quarterly Journal of the Royal Meteorological Society 136 (2010)648. - ISSN 0035-9009 - p. 671 - 684.
    atmosferische grenslaag - simulatiemodellen - evaluatie - meteorologische waarnemingen - luchtstroming - turbulentie - atmospheric boundary-layer - simulation models - evaluation - meteorological observations - air flow - turbulence - stably stratified conditions - wind-structure variations - energy closure-model - land-surface - diurnal cycle - soil-moisture - stratocumulus - flux - cabauw
    Single-column models (SCMs) are widely employed to evaluate boundary-layer parametrizations under well-controlled conditions. To compare SCM results to observations, these models must be driven by realistic forcings of the three-dimensional (3D) atmospheric state. However, these forcings are inherently uncertain. The central research question is therefore: can observations be used to distinguish between different parametrization schemes in SCM simulations or is the spread due to uncertainties in the forcings too large? This study investigates this question for the nocturnal low-level jet (LLJ) at Cabauw. First, we analyse a single LLJ case that has been used for the third GEWEX Atmospheric Boundary Layer Studies (GABLS3) intercomparison study. To estimate the forcings, for this case a blend of local observations and 3D model output has been used. A sensitivity study to both the forcings and the turbulence formulation is performed by using the SCM version of the ECMWF model. The sensitivity to the turbulence parametrization is largest in the bulk of the stable boundary layer (SBL). The influence of the forcings manifests itself mainly in the upper part of the SBL and above. Second, an ensemble of eight comparable LLJ cases is considered. Using forcings derived from 3D model integrations, SCM results of the separate cases show significant deviations from the observations. However, the mean of the SCM simulations agrees well with the mean of the observations. Based on the eight selected LLJ cases, a composite SCM case is defined from the mean forcings of the 3D model. This improves the signal-to-noise ratio, which enables a better judgement of the quality of boundary-layer parametrizations in a comparison with the observations
    Turbulence and low-level jets in the stable boundary layer
    Baas, P. - \ 2009
    Wageningen University. Promotor(en): Bert Holtslag, co-promotor(en): F.C. Bosveld. - [S.l. : S.n. - ISBN 9789085854463 - 151
    meteorologie - atmosfeer - turbulentie - terrestrische straling - zonne-instraling - turbulente stroming - klimaatfactoren - grenslaagmeteorologie - zwaartekrachtgolven - atmosferische grenslaag - meteorology - atmosphere - turbulence - terrestrial radiation - insolation - turbulent flow - climatic factors - boundary-layer meteorology - atmospheric gravity waves - atmospheric boundary-layer
    It is common knowledge that in the evening the wind close to the ground drops. However, it is less well known that at the same time the wind at a few hundreds of meters height can strongly accelerate. In this case, the vertical wind-profile shows a distinct maximum. This phenomenon is called nocturnal low-level jet (LLJ). Knowledge of the LLJ is relevant for aviation, the generation of wind energy and the distribution of pollutants. This thesis presents a climatology of LLJ characteristics (like the frequency of occurrence and the typical height at which the phenomenon occurs) for observations from the KNMI measurement site at Cabauw. The development of nocturnal LLJs is very sensitive to the amount of turbulent mixing. In this thesis different ways on which turbulence is represented in atmospheric models are tested. Various strategies for model evaluation are discussed. This thesis aims to contribute to the understanding of physical processes and their representation in weather and climate models.
    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.
    Redistribution of velocity and bed-shear stress in straight and curved open channels by means of a bubble screen: laboratory experiments
    Blanckaert, K. ; Buschman, F.A. ; Schielen, R. ; Wijbenga, J.H.A. - \ 2008
    Journal of Hydraulic Engineering 134 (2008)2. - ISSN 0733-9429 - p. 184 - 195.
    waterbouwkunde - kanalen, klein - hydrodynamica - waterstroming - buigen - herverdeling - snelheid - turbulentie - laboratoriumproeven - hydraulic engineering - channels - hydrodynamics - water flow - bending - redistribution - velocity - turbulence - laboratory tests - submerged vanes - secondary flows - alternate bars - topography - bends - rivers
    Open-channel beds show variations in the transverse direction due to the interaction between downstream flow, cross-stream flow, and bed topography, which may reduce the navigable width or endanger the foundations of structures. The reported preliminary laboratory study shows that a bubble screen can generate cross-stream circulation that redistributes velocities and hence, would modify the topography. In straight flow, the bubble-generated cross-stream circulation cell covers a spanwise extent of about four times the water depth and has maximum transverse velocities of about 0.2 ms¿1. In sharply curved flow, it is slightly weaker and narrower with a spanwise extent of about three times the flow depth. It shifts the counter-rotating curvature-induced cross-stream circulation cell in the inwards direction. Maximum bubble-generated cross-stream circulation velocities are of a similar order of magnitude to typical curvature-induced cross-stream circulation velocities in natural open-channel bends. The bubble screen technique is adjustable, reversible, and ecologically favorable. Detailed data on the 3D flow field in open-channel bends is provided, which can be useful for validation of numerical models
    On turbulent exchange processes over Amazonian forest
    Randow, C. von - \ 2007
    Wageningen University. Promotor(en): Bert Holtslag, co-promotor(en): Bart Kruijt. - [S.l.] : S.n. - ISBN 9789085046400 - 166
    bossen - atmosfeer - warmte - vochtigheid - kooldioxide - turbulente stroming - turbulentie - statistiek - amazonia - forests - atmosphere - heat - humidity - carbon dioxide - turbulent flow - turbulence - statistics - amazonia
    This thesis deals with turbulent exchange processes of heat, humidity and carbon dioxide over Amazonian forests. The atmospheric boundary layer overAmazoniafrequently contains slowly-moving large eddies induced by strong convective motions or local circulations related to the heterogeneity of the surface. To better understand their influence, it is studied how turbulence statistics depend on different time scale and spatial scale classes, decomposing the turbulence signals using multi-resolution (wavelets). Largest contributions to measured fluxes occur in turbulent scales (structures with length scales up to 1000 m and time scales up to 15 min). Low-frequency motions (larger eddies and mesoscale motions), however, can contribute with up to 30 % to the total exchange under weak wind conditions.

    The influence of low-frequency motions on similarity and correlations between turbulent signals and the implications for the application of Monin-Obukhov similarity is investigated. For the estimation of heat fluxes by the flux-variance method it is found that reasonable results only occur when the correlation coefficient between vertical wind and temperature ( r wT ) is above 0.5. The latter quantity decreases when the influence of low-frequency motions in the surface layer is high, and in these cases the surface layer is different from the textbook descriptions.

    Additionally, the use of Large Aperture Scintillometry (LAS) overAmazoniais explored, in comparison with the eddy covariance (EC) method. As the LASprovidesa measurement that represents a weighted spatial average of the turbulent eddies along the path, it is not necessary to use a long time period to sample a number of eddies, and the averaging time scale can be reduced. The results show that the EC fluxes are often lower than the LAS. The difference increases with increasing non-stationarity conditions and decreasing correlation coefficient

    In general, the results suggest that one single eddy covariance system is not able to capture quasi-steady large-eddies that significantly contribute to surface-atmosphere exchange over Amazonian forest. Apart from possible horizontal flux divergence at heterogeneous terrain, these factors may explain the failure to close the surface energy balance in complex terrain.
    Exploring Scintillometry in the Stable Atmospheric Surface Layer
    Hartogensis, O.K. - \ 2006
    Wageningen University. Promotor(en): Bert Holtslag, co-promotor(en): H.A.R. Debruin. - Wageningen : S.n. - ISBN 9789085043683 - 228
    atmosfeer - meteorologie - bovenlagen - warmtestroming - meting - warmteuitwisseling - momentum - turbulentie - meteorologische waarnemingen - methodologie - meetapparatuur - scintillometrie - atmosphere - meteorology - surface layers - heat flow - measurement - heat exchange - momentum - turbulence - meteorological observations - methodology - meters - scintillometry
    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).

    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 9789085041719 - 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)
    An updated length-scale formulation for turbulent mixing in clear and cloudy boundary layers
    Lenderink, G. ; Holtslag, A.A.M. - \ 2004
    Quarterly Journal of the Royal Meteorological Society 130 (2004)604. - ISSN 0035-9009 - p. 3405 - 3427.
    grenslaagmeteorologie - wolken - turbulentie - kinetische energie - boundary-layer meteorology - clouds - turbulence - kinetic energy - shallow cumulus convection - large-eddy simulation - atmospheric models - parameterization - scheme - stratocumulus - fluxes - transport - impacts - wind
    A new mixing-length scale is presented for turbulence-closure schemes, with special emphasis on neutral-to-convective conditions in clear and cloudy boundary layers. The length scale is intended for a prognostic turbulent-kinetic-energy closure. It is argued that present-day length-scale formulations may easily fail in one of two limiting situations. Schemes based on a local stability measure (e.g.the Richardson number) display unrealistic behaviour and instabilities in the convective limit. This strongly limits the representation of mixing in cloudy boundary layers. On the other hand, it is shown that non-local parcel methods may misrepresent mixing near the surface. The new length-scale formulation combines local and non-local stability in a new way; it uses vertical integrals over the stability (the Richardson number) in a simple 'parcel' framework. The length scale matches with surface-layer similarity for near-neutral conditions and displays a realistic convective limit. The use of the length-scale formulation can be extended easily to cloudy boundary layers. The scheme is numerically stable and computationally cheap. The behaviour of the length scale is evaluated in a single-column model (SCM) and in a high-resolution limited-area model (LAM). The SCM shows good behaviour in three cases with and without boundary-layer clouds. The prediction of the near-surface wind and temperature in the LAM compares favourably with tower measurements at Cabauw (the Netherlands).
    A review of the relationships describing the signal of a Large Aperture Scintillometer
    Moene, A.F. ; Meijninger, W.M.L. ; Hartogensis, O.K. ; Kohsiek, W. ; Debruin, H.A.R. - \ 2004
    Wageningen : Meteorology and Air Quality (MAQ) (Internal Report 2004/2) - 39
    windsnelheid - meteorologische instrumenten - windmeters - turbulentie - warmtestroming - fluctuaties - wind speed - meteorological instruments - anemometers - turbulence - heat flow - fluctuations
    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 9789058088857 - 164
    meteorologische instrumenten - remote sensing - energiebalans - turbulentie - kalibratie - grenslaag - scintillometrie - meteorological instruments - remote sensing - 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.
    Intermittent turbulence and oscillations in the stable boundary layer over land
    Wiel, B. van de - \ 2002
    Wageningen University. Promotor(en): A.A.M. Holtslag; H.A.R. de Bruin. - S.l. : S.n. - ISBN 9789058087683 - 129
    turbulentie - meteorologische factoren - turbulence - meteorological factors

    As the title of this thesis indicates, our main subject of interest is: "Intermittent turbulence and oscillation in the stable boundary layer over land". As such, this theme connects the different chapters. Here, intermittent turbulence is defined as a sequence of events were 'burst' of increased turbulence activity are followed by relatively quiet periods with low turbulence levels. This intermittent turbulence affects the mean structure of the SBL, in a sense that it may cause alternations on the nocturnal evolution of wind speed and temperature. In this way the time series of these quantities may show an oscillatory-type of behavior, referring to the title. Intermittency is commonly observed, especially in conditions of strong stratification. As such, several observed examples of this intermittent behavior are given in this thesis (chapter 3 and 4). Despite of the fact that it is ubiquitous relatively is known about intermittency: e.g. what physical mechanism causes intermittency? What are its typical statistical characteristics (e.g. regarding time-scales and amplitudes of the turbulent events)? Under what conditions can we expect intermittency to occur?

    From a number of studies with atmospheric column models (e.g. Welch et al., 1986; Lin, 1990, Revelle, 1993; Vukelic and Cuxart, 2000) it appears that an intermittent behavior of turbulence is found in some specific parameter ranges. However, from these studies no general picture explaining the essential physics behind this behavior is available. Furthermore, some of these studies (e.g. Lin, 1990, Revelle, 1993) indicate that different regimes are simulated upon varying the pressure gradient: besides the intermittent regime two non-intermittent regimes emerge. From an observational point of view also, the existence of non-intermittent regimes (e.g. the continuous turbulent regime) is well known. On the other hand, it is not clear what external conditions cause the stable boundary layer to end up in one regime or another.

    The large number of unanswered questions, about the intermittency phenomenon in particular and stable boundary layer dynamics in general, largely motivated the present work. In relation to the problems posed above, the following research questions are addressed in this thesis:

    1. - What is the physical essence of this intermittent behaviour?
    2. - Is it possible to simulate both intermittent and non-intermittent regimes with a simple model?

    3. - What external forcing parameters control the transitions between the different regimes?
    4. - Can we predict the occurrence of intermittent and non-intermittent regimes?

    5. - What regimes are actually observed in the field? Under which conditions do they occur?

    The present work consists of three parts: the first part is a numerical study (chapter 2), the second part an analytical study (chapter 3) and the third part is an observational study (chapter 4).

    The study focuses on an intermittency mechanism first qualitatively described by Turner (1973) and Businger (1973): on clear nights over land in presence of weak winds, strong surface radiation may built up a strong surface inversion, such that turbulence is suppressed effectively. This causes the atmosphere to decouple from the underlying surface. Soon, however, due to the reduced friction, the air in the lower atmosphere will be accelerated by the omnipresent pressure force, until shear is strong enough to break through the stratification. Because of this mixing, shear is reduced largely and soon a new stratification is built up by surface cooling. Thus, the situation has returned to its 'initial' state and the mechanism starts over again, causing intermittent bursts of turbulence.

    In chapter 2 it is shown that the essence of this intermittency mechanism can be captured by a 1D bulk model consisting of three coupled nonlinear differential equations. According to the authors, the bulk model considers the essential elements of the SBL: surface cooling by longwave radiation, supply of mechanical energy by the synoptic pressure gradient, and the limiting effect of stratification on mixing efficiency. In the simplified model structure only direct interaction of the lower atmosphere (first tens of meters) with the vegetation surface was considered, with no interaction with the air aloft. Consequently, this type of assumptions may limit the generality of the results.

    It appears that this bulk model is able to mimic the intermittent behavior described above. Surprisingly (in view of model simplicity), model simulations predict both intermittent and non-intermittent SBL to occur for different external forcings, confirming the results of others with more detailed model configurations (e.g. Lin, 1990, Revelle, 1993). It appears that three regimes occur (two non-intermittent and one intermittent) when the pressure gradient is varied.

    Model results show that intermittent turbulence is most likely to occur over land surfaces with low vegetation under clear sky conditions in presence of a low synoptical pressure gradient. The results indicate that the existence of a vegetation layer has a strong influence on intermittency dynamics: due to its small heat capacity, the vegetation temperature is able to respond quickly to rapid changing conditions. This, in turn, affects the stability of the lower atmosphere, causing an important feedback mechanism (see also: chapter 4). In addition it is found that intermittent behavior in SBL models occurs for various first-order closure schemes with different stability functions (as in Derbyshire, 1999). On the other hand we find that 'broad tail' stability functions that allow turbulent transport beyond the critical Richardson number effectively suppress intermittent/oscillatory behavior. Currently, these types of broad tail stability functions are often used in numerical weather prediction to prevent excessive SBL cooling in very stable conditions. Furthermore it is noted that, strictly speaking, time-averaged flux-profile relationships will not be valid in intermittent flows. In those conditions, average flux-profile relations cannot be unique due to their nonlinear nature.

    The advantage of using a simplified SBL model, as proposed in chapter 2, is that it allows an analytical study of the system. Such analytical study is presented in chapter 3, were the governing equations of the bulk model are studied from a system dynamics point of view. In this way the transition between the different flow regimes is identified as a Hopf bifurcation. At the Hopf bifurcation point the stability of the equilibrium solution of the system changes such that a stable non-oscillatory solution alters in an unstable oscillatory solution (or vice versa). This property is used to derive a dimensionless parameter (denoted as, which is a function of external forcing parameters such as the pressure gradient and the radiative forcing, and of local parameters such as the aerodynamic roughness, heat capacity and bulk conductivity of the vegetation layer. With this dimensionless parameter the equilibrium behavior of the system (i.e. intermittent or non-intermittent) can be predicted exactly. As such this parameter is proposed as a classification tool to predict SBL regimes. The proposed classification parameter provides different information than classical parameters such as z/L and Ri. The main difference lies in the fact that theconsiders the stability of the system as a whole, including feed-backs from the turbulence-, soil heat flux-, and the radiation scheme, whereas z/L and Ri are scaling parameters for turbulence only. Becausehas a rather complicated structure, a less exact but simpler stability criterion is also derived, based on a fixed shear criterion for instability (Derbyshire, 1999). This, more practical criterion allows a clear physical interpretation. It is found that the main cause of instability is the positive feedback between stratification and mixing which occurs under strong stratified conditions. Furthermore it is shown that the heat exchange due to longwave radiation (outside the atmospheric window region) and by the soil heat flux imply strong negative feedbacks counteracting instability. According to the simplified criterion, theparameter can be approximated by two dimensionless groups: a bulk Richardson number and a so-called partitioning parameter. The latter is interpreted as the ratio of the summed radiative and soil heat exchange coefficient compared to the exchange coefficient for turbulent heat transport (or, alternatively, the ratio of fluxes). As such, this partitioning parameter represents the competition between the positive and negative feed-backs described above.

    In chapter 4 SBL classification is studied from an observational point of view. In this chapter observations of the extensive CASES99 field experiment are presented (CASES: Cooperative Atmospheric Surface-Exchange Study). This field experiment, carried out by various groups from the U.S. and Europe, was specially designed to quantify the physical characteristics of the stable boundary layer over land, with a variety of observational tools. It took place in Kansas (U.S.) over a relatively flat area with dry, open prairie-grass, and lasted for a whole month (Oct. 1999), under various meteorological conditions. This makes the experiment very suitable for studying the different SBL regimes in relation to external forcings.

    In chapter 4, first a classification of stable boundary layer regimes is presented based on time-series observations of near surface turbulence during CASES99. It is found that the different nights can be divided in three subclasses: a turbulent regime, an intermittent regime and a radiative regime. The existence of these three regimes is in agreement with the theoretical findings of chapter 2 and 3.

    Secondly, this classification based on flux time series is compared with the theoretical predictions using(based on external parameters). To this end, for the CASES99 nights, thisis evaluated from a detailed analysis of the available data. Such evaluation from real data is not a trivial task, due to the number of assumptions in the equations on whichis based (e.g. the estimation of an effective value for the pressure gradient). The comparison between the theoretical predictions and the actual observed time-series shows generally good agreement. Also, the results are robust and discriminative in a qualitative sense. As such, it is e.g. shown that intermittent turbulence often occurs in clear sky conditions with a moderately weak (effective) pressure gradient. Similarly, in clear sky conditions, radiative and continuous turbulent regimes occur during conditions of very weak pressure gradients and strong pressure gradients respectively. This robustness is explained from the main ingredients of the mechanism described in chapter 2. On the other hand, the quantitative features of the theoreticalclassification are rather sensitive to (often uncertain) local parameter estimations, such as the bulk heat conductance of the vegetation layer. Due to this sensitivity, the relative value offor a certain night compared to other nights at the same location, provides more information about the SBL regime to be expected than a singlevalue by itself.

    As a practical test case also the simplified criterion of chapter 3 is applied to the CASES99 data set. The approximate parameter shows less discriminative than the originalparameter: although the extreme cases are predicted correctly, more subtle cases showed to be less decisive or even incorrect. This is probably caused by the neglection of important feed-backs between wind shear and stratification in the momentum and heat budget equations. Thus apart from its clear conceptual value (chapter 4) its practical value is limited to more extreme cases.

    Generally, we reflect that the analytical approach in this thesis, using a truncated set of equations, has clear advantages: e.g. internal relations between various processes can be made explicit, and equilibrium system behaviour can be expressed a priori in terms of the external forcing parameters. As such, this type of system analysis provides a fruitful way for continuation of the present research on SBL dynamics. Additionally, there is a need for detailed studies on the instability mechanisms that may generate intermittent bursts, using more complex model configurations with higher resolution and less strict assumptions. Such models are useful in simulating individual bursting events selected from observational case-studies. Also, as a continuation of the present observational work, it is interesting to test the proposed classification under different climatological conditions. In this observational respect, there is also a need for long-term measurement campaigns providing an accurate statistical climatology/characterization about the typical time-scales and amplitudes of the intermittent bursts under different conditions. Regarding practical applications, it is shown that the equilibrium solutions presented in the thesis provide a useful starting point for parameterization studies. Because, especially with stable boundary layers, many practical problems are a consequence of its scientific counterparts, future theoretical progress may directly benefit practical applications.

    Shallow cumulus convection = Ondiepe cumulus convectie
    Neggers, R. - \ 2002
    Wageningen University. Promotor(en): A.A.M. Holtslag; A.P. Siebesma. - S.l. : S.n. - ISBN 9789058087744 - 202
    wolken - convectie - atmosfeer - turbulentie - simulatiemodellen - grenslaag - clouds - convection - atmosphere - turbulence - simulation models - boundary layer

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

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

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

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

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

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

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

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

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

    Intermittent turbulence and oscillations in the stable boundary layer: a system dynamics approach
    Wiel, B.J.H. van de; Moene, A.F. ; Hartogensis, O.K. ; Ronda, R.J. ; DeBruin, H.A.R. ; Holtslag, A.A.M. - \ 2002
    In: 15th Symposium on Boundary Layers and Turbulence, 15-19 July 2002, Wageningen, the Netherlands Boston, U.S.A. : American Meteorological Society - p. 477 - 480.
    meteorologie - turbulentie - oscillatie - meteorology - turbulence - oscillation
    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
    Fluxes over a heterogeneous land surface: results and perspectives of the LITFASS program
    Beyrich, F. ; Richter, S.H. ; Weisensee, U. ; Herzog, H.J. ; DeBruin, H.A.R. ; Meijninger, W.M.L. - \ 2002
    In: 15th Symposium on Boundary Layers and Turbulence, 15-19 July 2002, Wageningen, the Netherlands Boston, U.S.A. : American Meteorological Society - p. 653 - 654.
    meteorologie - turbulentie - luchtstroming - atmosfeer - bovenlagen - meteorology - turbulence - air flow - atmosphere - surface layers
    From 1995 till 2001, the German Meteorological Service (DWD) has performed a research project (LITFASS='Lindenberg Inhomogeneous Terrain - Fluxes between Atmosphere and Surface: a Long-term Study') in order to develop and to test a strategy for the determination of the area-averaged turbulent fluxes of heat, momentum, and water vapour over a heterogeneous landscape. The fluxes shall be representative for a horizontal scale of about 10 km (while the typical patch scale is between 10^2 to 10^3 m) corresponding to the size of a grid cell in the present operational numerical weather prediction model of the DWD. The overall project strategy had been successfully tested over a four weeks period in May / June 1998 during the LITFASS-98 field experiment, results from this experiment were presented at the 14th Symposium on Boundary Layer and Turbulence. Over the last two years, the LITFASS measurement program was qualified for continuous long-term operation. The paper will review the current status of the LITFASS field program, and we will present selected results from the operational measurements performed in 2000 and 2001. These include: - the variability of meteorological forcing (radiation budget, rain, wind and temperature profiles) across the area, - the variability of turbulent fluxes depending on different surface characteristics and on the meteorological forcing, - the estimation of area-averaged sensible heat fluxes from the operation of a large-aperture scintillometer. In addition, first modelling results for some case studies will be discussed. Finally, an outlook will be given towards a second field experiment (LITFASS-2003) aimed at the determination of area-averaged evaporation at grid-/pixel scale
    EBEX-2000: the KNMI/WAU contribution
    Kohsiek, W. ; Meijer, E.W. ; Versteeg, P.J.B. ; Hartogensis, O.K. ; DeBruin, H.A.R. - \ 2001
    De Bilt : KNMI (Technical report TR-240) - ISBN 9789036922043 - 11
    meteorologie - meteorologische waarnemingen - stralingsbalans - turbulentie - warmtestroming - meteorology - meteorological observations - radiation balance - turbulence - heat flow
    Eddy covariance and scintillation measurements of atmospheric exchange processes over different types of vegetation
    Nieveen, J.P. - \ 1999
    Agricultural University. Promotor(en): J. Goudriaan; A.F.G. Jacobs. - S.l. : Nieveen - ISBN 9789058080288 - 122
    vegetatietypen - atmosfeer - turbulentie - gasuitwisseling - bladoppervlakte-index - kooldioxide - waterdamp - covariantie - analytische methoden - vegetation types - atmosphere - turbulence - gas exchange - leaf area index - carbon dioxide - water vapour - covariance - analytical methods

    Introduction and objectives

    Good comprehension of the energy and mass cycles and their effect on climate dynamics is crucial to understanding, predicting and anticipating ecological changes due to possible future climate perturbations. Here direct and long-term flux density measurements of greenhouse gases from various ecosystems provide means to supply such fundamental knowledge. For the global water vapour and carbon cycles, however, the interactions between different spatial scales become important, where extrapolating from canopy flux density measurements to global budgets lead to practical and theoretical problems. This thesis focuses on the direct and long-term measurement of surface flux densities and interaction processes at the canopy (< 1 km scale within the framework of the Surface Layer Integration Measurements and Modelling (SLIMM) project. Furthermore, some characteristics and limitations of the scintillation technique are studied in two field experiments in New Zealand.

    As indicated in Chapter 1, the first objective of this project was the direct and continuous long-term measurement of the surface flux densities of radiation, momentum, heat, water vapour and carbon dioxide (CO 2 ) to study the effect of biological and climatic processes that regulate carbon dioxide exchange of this ecosystem at the canopy scale. At the same time these data were used to study the effect of plant related and environmental conditions on the interaction of carbon dioxide and water vapour exchange, to satisfy the second objective of the thesis. The third objective focussed on the prospect of obtaining both the spatial averaged sensible heat flux density and momentum flux density from scintillation measurements.

    Generally, a compromising point measurement of the mean horizontal wind speed or friction velocity is used to calculate the sensible heat flux density from the temperature structure parameter. By using two scintillometers at two heights, point measurements to obtain the atmospheric stability can be omitted. The fourth objective of this thesis was to study the influence of absorption fluctuations on the average sensible heat flux density derived from the scintillation technique.

    Carbon dioxide exchange and the effect of biological and climatic processes

    Carbon dioxide exchange was measured, using the eddy covariance technique, during a one and a half-year period in 1994 and 1995. The measurements took place over a former true raised bog, characterised by a shallow peat layer and tussock vegetation dominated by Molinia caerulea . Peat soils in the Northern Hemisphere's wetlands contain about one third of the worlds carbon pool. Many regions in the arctic tundra, however, have changed from sinks to sources for CO 2 over the past decade but this can not simply be generalised.

    The growing season extended from May until late October, with a maximum LAI in August of 1.7. The carbon balance showed a net release of 97 g CO 2 m -2y -1(265 kg C ha -1y -1) from the peat bog ecosystem to the atmosphere. During June, July and August there was net consumption of CO 2 , while during the rest of the year there was net production of CO 2 . The maximum daytime net exchange rates were about -0.5 mg CO 2 m -2s -1(-11.3μmol CO 2 m -2s -1) with an average peak exchange rate of -0.2 mg CO 2 m -2s -1(-4.5μmol CO 2 m -2s -1), in a period where the LAI ranged between 1 and 1.7. A high vapour pressure deficit (>15 hPa) corresponding with high temperature was found to reduce the net CO 2 exchange rate by on average 50%.

    Apart from these factors, LAI and the soil temperature co-determined the net exchange of CO 2 . The total nocturnal respiration during the growing season was within the same order as the average daytime net photosynthetic rate. Temperature was found to be the main factor controlling soil respiration, with a Q 10 of 4.8.

    The effect of plant related and environmental conditions on the interaction of CO 2 and H 2 O exchange

    The tussock grassland, dominated by Molinea caerulea , was covered with a dense layer of dead organic material from the previous growing seasons. During the summer months, the daytime carbon dioxide uptake often showed a single early morning maximum and a decline in uptake during the rest of the day. Surprisingly, maximum water vapour flux densities were not greatly reduced. The surface cover and the small value of the leaf area index were the main reasons for this phenomenon.

    The layer of dead organic material acted as an insulating blanket to the transport of water vapour from the soil to the atmosphere. Furthermore, the canopy was far from closed with a peak leaf area index of 1.7 in early August. For both low vapour pressure deficit (< 15 hPa) and high vapour pressure deficit (> 20 hPa) at high surface temperatures, the vegetation showed similar behaviour resulting in a clear reduction of the daytime CO 2 uptake. Temperature was therefore inferred to be main the reason for a reduction in CO 2 exchange. The response of the stomata to atmospheric humidity was deduced to be small possibly due to the abundant availability of soil water. Instead transpiration increased with increasing vapour pressure deficit. The latter was stimulated by the surface temperature, which often exceeded the optimum temperature for photosynthesis and led to an increase in the atmospheric evaporative demand.

    The scintillation technique

    An optical or electromagnetic wave propagating through a turbulent atmosphere exhibits fluctuations in intensity known as 'scintillations'. In atmospheric turbulence, fluctuations in temperature, humidity and pressure cause density fluctuation and with it fluctuations in the refractive index ( n ). These refractive index fluctuations cause random refraction and absorption of electromagnetic (EM) radiation passing through the turbulent atmosphere, changing the characteristics of the wave. Scintillation of light is related to these phenomena and is experienced at a receiver as fluctuations in the light intensity caused by interference of refracted light and absorption of the light. Scintillometers measure the turbulent intensity of the refractive index fluctuations of the air from the intensity fluctuations of a received signal expressed in the refractive index structure parameter, C n2.

    The measured C n2value is related to the structure parameters of temperature C T2, humidity C Q2and a covariant term C TQ , respectively. To calculate the sensible heat flux density from C n2compromising point measurements of the Bowen ratio,β, and friction velocity, u * , are necessary. Generally, the deficiency in the available spatial measurement of u * is overcome by using a point measure of the average wind speed, u and surface roughness, z 0 , but the necessity forβoften remains unresolved.

    By using two scintillometers at different heights above the surface, a spatial measurement of the Obukhov length, L o , and u * can be derived without incorporating compromising point measurements of the friction velocity or alternatively the average wind speed combined with a measure of the roughness length. The presumption that such measurements are representative of the entire transect usually holds for homogeneous surface cover but may not be valid for patchwork terrain. The two-scintillometer technique is referred to as the C T2-profile method.

    Refraction is the result of normal and anomalous dispersion. If, however, the frequency of the emitted EM wave is close to a resonance frequency (absorption lines) of atmospheric constituents, like water vapour and carbon dioxide, absorption becomes important. To quantitatively describe the combined effect of refraction and absorption, a complex refractive index structure parameter, C n2, is introduced. Here the phenomenon of absorption is represented by the imaginary part of the refractive index and is solely determined by single absorption lines and their corresponding absorption coefficients (β i ), resulting in a total absorption coefficient for a band of lines (Hill et al. , 1980). The absorption line strength is temperature dependent, while the absorption line width is temperature, humidity and pressure dependent.

    The contribution of absorption fluctuations to C n2is generally neglected, that means to have a real component only. In reality C n2includes both a real part, C nR2, due to refraction and an imaginary part, C nI2, attributable to the absorption mechanism. Any additional source of scintillation such as a contribution from absorption fluctuations could conceivably corrupt the estimation of the sensible heat flux.

    Measuring sensible heat flux density over pasture using the C T2- profile method

    Two large aperture scintillometers were positioned at heights ( z ) of 10 and 1.5 m with beams propagating horizontally over pasture for distances of 3.1 km and 141 m respectively. From each scintillometer a half-hourly average value of the path-averaged, temperature structure parameter ( C T2) was obtained in unstable atmospheric conditions. The result suggested C T2to scale with height as z -2/3. Using the C T2- profile method, a path averaged measure of the Obukhov length ( L o ) was calculated for each half hour period. L o was used to determine the friction velocity and the surface layer temperature scaling parameter, T * . The scintillometer sensible heat flux density, H sc , was then calculated from H sc = -ρC p u *T * . A time series of half-hourly averaged H sc compared to H ec obtained by the eddy covariance method agreed to within 10%, with R 2= 0.67, for a range of unstable conditions (-0.2≤( z/L o )≤-0.01).

    Using a Large Aperture Scintillometer to measure absorption and refractive index fluctuations

    The contribution of refraction and absorption fluctuations to the measured scintillation were observed for a near-infrared absorption region using a NOAA designed large aperture scintillometer. The logarithm amplitude spectra were shown to decay with a frequency as f-8/3for both the absorption and scattering mechanism. For the absorption mechanism this is in line with similar observations made at microwave and infrared frequencies. However, for finite transmitting and receiving apertures, theory predicts a stronger decay of the scattering mechanism due to aperture averaging. The spectral shape is characterised by a region of low frequency absorption, higher frequency refraction separated by a flattish transition zone. The upper observed corner frequency ( f C2 ), compared well with the calculated values using the measured transverse wind speed ( v ) for a known aperture radius. The lower corner frequency ( f C1 ) position was shown to be sensitive to the ratio of the real and imaginary part of the refractive index structure parameter, ( C nR2/C nI2) 3/8and v . The part of the spectrum associated with the absorption scintillations was observed to be much less than that due to refraction until the evening when decreasing C nR2caused C nR2/C nI2to decrease and absorption to become significant. If absorption is ignored, this may have consequences for calculating nocturnal surface heat flux densities. During unstable, daytime conditions the large aperture scintillometer is most sensitive to refractive scintillations despite having an infrared source transmitting in a lossy atmosphere. But also under these conditions, the low frequency absorption part of the spectrum is observable.

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