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

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The Effect of Surface Heterogeneity on the Structure Parameters of Temperature and Specific Humidity: A Large-Eddy Simulation Case Study for the LITFASS-2003 Experiment
Maronga, B. ; Hartogensis, O.K. ; Raasch, S. ; Beyrich, F. - \ 2014
Boundary-Layer Meteorology 153 (2014)3. - ISSN 0006-8314 - p. 441 - 470.
convective boundary-layer - monin-obukhov similarity - large-aperture scintillometer - flevoland field experiment - index-structure parameter - heat-flux - sensible heat - turbulence structure - land-surface - small-scale
We conduct a high-resolution large-eddy simulation (LES) case study in order to investigate the effects of surface heterogeneity on the (local) structure parameters of potential temperature C2T and specific humidity C2q in the convective boundary layer (CBL). The kilometre-scale heterogeneous land-use distribution as observed during the LITFASS-2003 experiment was prescribed at the surface of the LES model in order to simulate a realistic CBL development from the early morning until early afternoon. The surface patches are irregularly distributed and represent different land-use types that exhibit different roughness conditions as well as near-surface fluxes of sensible and latent heat. In the analysis, particular attention is given to the Monin–Obukhov similarity theory (MOST) relationships and local free convection (LFC) scaling for structure parameters in the surface layer, relating C2T and C2q to the surface fluxes of sensible and latent heat, respectively. Moreover we study possible effects of surface heterogeneity on scintillometer measurements that are usually performed in the surface layer. The LES data show that the local structure parameters reflect the surface heterogeneity pattern up to heights of 100–200 m. The assumption of a blending height, i.e. the height above the surface where the surface heterogeneity pattern is no longer visible in the structure parameters, is studied by means of a two-dimensional correlation analysis. We show that no such blending height is found at typical heights of scintillometer measurements for the studied case. Moreover, C2q does not follow MOST, which is ascribed to the entrainment of dry air at the top of the boundary layer. The application of MOST and LFC scaling to elevated C2T data still gives reliable estimates of the surface sensible heat flux. We show, however, that this flux, derived from scintillometer data, is only representative of the footprint area of the scintillometer, whose size depends strongly on the synoptic conditions.
Derivation of Structure Parameters of Temperature and Humidity in the Convective Boundary Layer from Large-Eddy Simulations and Implications for the Interpretation of Scintillometer Observations
Maronga, B. ; Moene, A.F. ; Dinther, D. van; Raasch, S. ; Bosveld, F.C. ; Gioli, B. - \ 2013
Boundary-Layer Meteorology 148 (2013)1. - ISSN 0006-8314 - p. 1 - 30.
large-aperture scintillometer - heterogeneous land-surface - flevoland field experiment - index-structure parameter - refractive-index - optical scintillometer - water-vapor - fluxes - model - heat
We derive the turbulent structure parameters of temperature C 2 T and humidity C 2 q from high-resolution large-eddy simulations (LES) of a homogeneously-heated convective boundary layer. Boundary conditions and model forcing were derived from measurements at Cabauw in The Netherlands. Three different methods to obtain the structure-parameters from LES are investigated. The shape of the vertical structure-parameter profiles from all three methods compare well with former experimental and LES results. Depending on the method, deviations in the magnitude up to a factor of two are found and traced back to the effects of discretization and numerical dissipation of the advection scheme. Furthermore, we validate the LES data with airborne and large-aperture scintillometer (LAS) measurements at Cabauw. Virtual path measurements are used to study the variability of C 2 T in the mixed layer and surface layer and its implications for airborne and LAS measurements. A high variability of C 2 T along a given horizontal path in the LES data is associated with plumes (high values) and downdrafts (low values). The path average of C 2 T varies rapidly in time due to the limited path length. The LES results suggest that measured path averages require sufficient temporal averaging and an adequate ratio of path length to height above the ground for the LAS in order to approach the domain average of C 2 T .
Crosswind from a Single Aperture Scintillometer using Spectral Techniques
Dinther, D. van; Hartogensis, O.K. ; Moene, A.F. - \ 2013
Journal of Atmospheric and Oceanic Technology 30 (2013)1. - ISSN 0739-0572 - p. 3 - 21.
windsnelheid - vliegtuigen - scintillometrie - klimaatverandering - wind speed - airplanes - scintillometry - climatic change - flevoland field experiment - heterogeneous surface - wind measurements - turbulence - fluxes - scintillations - profiles - terrain - layer - beam
In this study, spectral techniques to obtain the crosswind from a single large aperture scintillometer (SLAS) time series are investigated. The crosswind is defined as the wind component perpendicular to a path. A scintillometer obtains a path-averaged estimate of the crosswind. For certain applications this can be advantageous, e.g. monitoring the crosswind along airport runways. The essence of the spectral techniques lies in the fact that the scintillation power spectrum shifts linearly along the frequency domain as a function of the crosswind. Three different algorithms are used, which we named the Corner Frequency (CF), Maximum Frequency (MF), and the Cumulative Spectrum (CS) technique. The algorithms track the frequency shift of a characteristic point in different representations of the scintillation power spectrum. The spectrally derived crosswinds compare well with sonic anemometer estimates. The CS algorithm obtained the best results for the crosswind when compared with the sonic anemometer. However, the MF algorithm was most robust in obtaining the crosswind. Over short time intervals (
Towards a Validation of Scintillometer Measurements: The LITFASS-2009 Experiment
Beyrich, F. ; Bange, J. ; Hartogensis, O.K. ; Raasch, S. ; Braam, M. ; Dinther, D. van; Gräf, D. ; Kesteren, A.J.H. van; Kroonenberg, A.C. van den; Maronga, B. ; Martin, S. ; Moene, A.F. - \ 2012
Boundary-Layer Meteorology 144 (2012)1. - ISSN 0006-8314 - p. 83 - 112.
large-aperture scintillometer - heterogeneous land-surface - convective boundary-layer - sensible heat-flux - temperature-humidity correlation - structure-function parameters - eddy-covariance measurements - flevoland field experiment - index-structure parameter
Scintillometry has been increasingly used over the last decade for the experimental determination of area-averaged turbulent fluxes at a horizontal scale of a few kilometres. Nevertheless, a number of assumptions in the scintillometer data processing and interpretation still call for a thorough evaluation, in particular over heterogeneous terrain. Moreover, a validation of the path-averaged structure parameters derived from scintillometer data (and forming the basis for the flux calculations) by independent measurements is still missing. To achieve this, the LITFASS-2009 field campaign has been performed around the Meteorological Observatory Lindenberg – Richard-Aßmann-Observatory of the German Meteorological Service (DWD) in July 2009. The experiment combined tower-based in-situ turbulence measurements, field-scale laser scintillometers, long-range optical (large-aperture) and microwave scintillometers, and airborne turbulence measurements using an automatically operating unmanned aircraft. The paper describes the project design and strategy, and discusses first results. Daytime near-surface values of the temperature structure parameter, CT2C2T, over different types of farmland differ by more than one order of magnitude in their dependence on the type and status of the vegetation. Considerable spatial variability in CT2C2T was also found along the flight legs at heights between 50 and 100 m. However, it appeared difficult to separate the effects of heterogeneity from the temporal variability of the turbulence fields. Aircraft measurements and scintillometer data agreed in magnitude with respect to the temporal variation of the path-averaged CT2C2T values during the diurnal cycle. The decrease of CT2C2T with height found from the scintillometer measurements close to the surface and at 43 m under daytime convective conditions corresponds to free-convection scaling, whereas the aircraft measurements at 54 and 83 m suggest a different behaviour.
Path-average rainfall estimation from optical extinction measurements using a large-aperture scintillometer
Uijlenhoet, R. ; Cohard, J.M. ; Gosset, M. - \ 2011
Journal of Hydrometeorology 12 (2011)5. - ISSN 1525-755X - p. 955 - 972.
multiple-scattering corrections - raindrop size distribution - sensible heat fluxes - heterogeneous land-surface - flevoland field experiment - beer-lambert law - area - attenuation - precipitation - scintillation
The potential of a near-infrared large-aperture boundary layer scintillometer as path-average rain gauge is investigated. The instrument was installed over a 2.4-km path in Benin as part of the African Monsoon Multidisciplinary Analysis (AMMA) Enhanced Observation Period during 2006 and 2007. Measurements of the one-minute-average received signal intensity were collected for 6 rainfall events during the dry season and 16 events during the rainy season. Using estimates of the signal base level just before the onset of the rainfall events, the optical extinction coefficient is estimated from the path-integrated attenuation for each minute. The corresponding path-average rain rates are computed using a power-law relation between the optical extinction coefficient and rain rate obtained from measurements of raindrop size distributions with an optical spectropluviometer and a scaling-law formalism for describing raindrop size distribution variations. Comparisons of five-minute rainfall estimates with measurements from two nearby rain gauges show that the temporal dynamics are generally captured well by the scintillometer. However, the instrument has a tendency to underestimate rain rates and event total rain amounts with respect to the gauges. It is shown that this underestimation can be explained partly by systematic differences between the actual and the employed mean power-law relation between rain rate and specific attenuation, partly by unresolved spatial and temporal rainfall variations along the scintillometer path. Occasionally, the signal may even be lost completely. It is demonstrated that if these effects are properly accounted for by employing appropriate relations between rain rate and specific attenuation and by adapting the pathlength to the local rainfall climatology, scintillometer-based rainfall estimates can be within 20% of those estimated using rain gauges. These results demonstrate the potential of large-aperture scintillometers to estimate path-average rain rates at hydrologically relevant scales.
The use of the scintillation technique for monitoring seasonal water consumption of olive orchards in a semi-arid region
Ezzahar, J. ; Chehbouni, A. ; Hoedjes, J.C.B. ; Er-Raki, S. ; Boulet, G. ; Bonnefonds, J.M. ; Bruin, H.A.R. de - \ 2007
Agricultural Water Management 89 (2007)3. - ISSN 0378-3774 - p. 173 - 184.
large-aperture scintillometer - flevoland field experiment - long-wave-radiation - refractive-index - heterogeneous surface - flux measurements - sonic anemometer - energy fluxes - heat - area
To monitor seasonal water consumption of agricultural fields at large scale, spatially averaged surface fluxes of sensible heat (H) and latent heat (LvE) are required. The scintillation method is shown to be a promising device for obtaining the area-averaged sensible heat fluxes, on a scale of up to 10 km. These fluxes, when combined with a simple available energy model, can be used to derive area-averaged latent heat fluxes. For this purpose, a Large Aperture Scintillometer (LAS) was operated continuously for more than one year over a tall and sparse irrigated oliveyard located in south-central Marrakesh (Morocco). Due to the flood irrigation method used in the site, which induces irregular pattern of soil moisture both in space and time, the comparison between scintillometer-based estimates of daily sensible heat flux (HLAS) and those measured by the classical eddy covariance (EC) method (HEC) showed a large scatter during the irrigation events, while a good correspondence was found during homogenous conditions (dry conditions and days following the rain events). We found, that combining a simple available energy model and the LAS measurements, the latent heat can be reliably predicted at large scale in spite of the large scatter (R2 = 0.72 and RMSE = 18.25 W m¿2) that is obtained when comparing the LAS against the EC. This scatter is explained by different factors: the difference in terms of the source areas of the LAS and EC, the closure failure of the energy balance of the EC, and the error in available energy estimates. Additionally, the irrigation efficiency was investigated by comparing measured seasonal evapotranspiration values to those recommended by the FAO. It was found that the visual observation of the physical conditions of the plant is not sufficient to efficiently manage the irrigation, a large quantity of water is lost (¿37% of total irrigation). Consequently, the LAS can be considered as a potentially useful tool to monitor the water consumption in complex conditions
Comparison of large aperture scintillometer and eddy covariance measurements: Can thermal infrared data be used to capture footprint-induced differences?
Hoedjes, J.C.B. ; Chehbouni, A. ; Ezzahar, J. ; Escadafal, R. ; Bruin, H.A.R. de - \ 2007
Journal of Hydrometeorology 8 (2007)2. - ISSN 1525-755X - p. 144 - 159.
monin-obukhov similarity - sensible heat-flux - frequency-response corrections - flevoland field experiment - atmospheric surface-layer - stable boundary-layer - water-vapor - convective conditions - structure parameter - scintillation measurements
Eddy covariance (EC) and large aperture scintillometer (LAS) measurements were collected over an irrigated olive orchard near Marrakech, Morocco. The tall, sparse vegetation in the experimental site was relatively homogeneous, but during irrigation events spatial variability in soil humidity was large. This heterogeneity caused large differences between the source area characteristics of the EC system and the LAS, resulting in a large scatter when comparing sensible heat fluxes obtained from LAS and EC. Radiative surface temperatures were retrieved from thermal infrared satellite images from the Landsat Enhanced Thematical Mapper+ and Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) satellites. Using these images in combination with an analytical footprint model, footprint-weighted radiative surface temperatures for the footprints of the LAS and the EC system were calculated. Comparisons between the difference in measured sensible heat fluxes and the difference in footprint-weighted radiative surface temperature showed that for differences between the footprint-weighted radiative surface temperatures larger than ±0.5 K, correlations with the difference in measured sensible heat flux were good. It was found that radiative surface temperatures, obtained from thermal infrared satellite imagery, can provide a good indication of the spatial variability of soil humidity, and can be used to identify differences between LAS and EC measurements of sensible heat fluxes resulting from this variability.
Hydrometeorological application of a microwave link: 1. Evaporation
Leijnse, H. ; Uijlenhoet, R. ; Stricker, J.N.M. - \ 2007
Water Resources Research 43 (2007). - ISSN 0043-1397 - 9 p.
flevoland field experiment - sensible heat fluxes - heterogeneous surface - refractive-index - large-aperture - scintillometers - fluctuations - absorption - momentum - basin
A method to estimate areal evaporation using a microwave link (radio wave scintillometer) in combination with an energy budget constraint is proposed. This radio wave scintillometry-energy budget method (RWS-EBM) is evaluated for its applicability in different meteorological conditions and for its sensitivity to various variables (the structure parameter of the refractive index of air C n 2, the total available energy R n - G, the wind velocity u, the effective average vegetation height h 0, and the correlation coefficient between the temperature and humidity fluctuations r TQ ). The method is shown to be best suited for use in wet to moderately dry conditions, where the latent heat flux is at least a third of the total available energy (i.e., Bowen ratio =2). It is important to accurately measure the total available energy and the wind velocity as the RWS-EBM is most sensitive to these variables. The Flevoland field experiment has provided the data, obtained with a 27-GHz radio wave scintillometer (over 2.2 km), a large-aperture scintillometer (also 2.2 km), and four eddy covariance systems, which are used to test the RWS-EBM. Comparing 92 daytime measurements (30-min intervals) of the evaporation estimated using the RWS-EBM to that determined in alternative manners (eddy covariance and two-wavelength scintillometry) leads to the conclusion that the method provides consistent estimates (coefficient of determination r 2 = 0.85 in both cases) under relatively wet conditions.
Area-averaged surface fluxes over the litfass region based on eddy-covariance measurements
Beyrich, F. ; Leps, J.P. ; Mauder, M. ; Bange, J. ; Foken, T. ; Huneke, S. ; Lohse, H. ; Ludi, A. ; Meijninger, W.M.L. ; Mironov, D. ; Weisensee, U. ; Zittel, P. - \ 2006
Boundary-Layer Meteorology 121 (2006)1. - ISSN 0006-8314 - p. 33 - 65.
flevoland field experiment - water-vapor fluxes - aircraft measurements - turbulent fluxes - boundary-layer - boreal forest - hapex-mobilhy - heterogeneous surface - large-aperture - land-surfaces
Micrometeorological measurements (including eddy-covariance measurements of the surface fluxes of sensible and latent heat) were performed during the LITFASS-2003 experiment at 13 field sites over different types of land use (forest, lake, grassland, various agricultural crops) in a 20 × 20 km2 area around the Meteorological Observatory Lindenberg (MOL) of the German Meteorological Service (Deutscher Wetterdienst, DWD). Significant differences in the energy fluxes could be found between the major land surface types (forest, farmland, water), but also between the different agricultural crops (cereals, rape, maize). Flux ratios between the different surfaces changed during the course of the experiment as a result of increased water temperature of the lake, changing soil moisture, and of the vegetation development at the farmland sites. The measurements over grass performed at the boundary-layer field site Falkenberg of the MOL were shown to be quite representative for the farmland part of the area. Measurements from the 13 sites were composed into a time series of the area-averaged surface flux by taking into account the data quality of the single flux values from the different sites and the relative occurrence of each surface type in the area. Such composite fluxes could be determined for about 80% of the whole measurement time during the LITFASS-2003 experiment. Comparison of these aggregated surface fluxes with area-averaged fluxes from long-range scintillometer measurements and from airborne measurements showed good agreement
Scintillometer-Based Turbulent Fluxes of Sensible and Latent Heat Over a Heterogeneous Land Surface : a Contribution to Litfass-2003
Meijninger, W.M.L. ; Beyrich, F. ; Lüdi, A. ; Kohsiek, W. ; Debruin, H.A.R. - \ 2006
Boundary-Layer Meteorology 121 (2006)1. - ISSN 0006-8314 - p. 89 - 110.
large-aperture scintillometer - flevoland field experiment - refractive-index - structure parameter - water-vapor - optical scintillation - fluctuations - layer - area - temperature
The performance of a combined large aperture scintillometer (LAS) and a millimetre wave scintillometer (MWS) for estimating surface fluxes of sensible and latent heat over natural landscape is investigated, using data gathered during LITFASS-2003. For this purpose the LAS¿MWS system was installed in a moderate heterogeneous landscape over a path length of 4.7 km with an effective beam height of 43 m. The derived surface fluxes have been compared with aggregated eddy-covariance (EC) measurements. The fluxes of sensible and latent heat from the LAS¿MWS combination, as well as sensible heat fluxes of the single LAS, agreed fairly well with the EC-based fluxes, considering the uncertainties of the similarity stability functions and observed energy imbalance
Structure parameters for temperature and humidity from simultaneous eddy-covariance and scintillometer measurements
Beyrich, F. ; Kouznetsov, R. ; Leps, J.P. ; Lüdi, A. ; Meijninger, W.M.L. ; Weisensee, U. - \ 2005
Meteorologische Zeitschrift 14 (2005)5. - ISSN 0941-2948 - p. 641 - 649.
large-aperture scintillometer - flevoland field experiment - refractive-index - surface-layer - heterogeneous surface - land-surface - fluxes - scintillation - heat - turbulence
Line-averaged values of the structure parameters of temperature and humidity, CT2 and Cq2, were estimated from simultaneous measurements with an optical and a microwave scintillometer over a path of 4.7 km length at the Meteorological Observatory Lindenberg during the LITFASS-2003 experiment. By cross-correlating the detected signals of the two scintillometers, the temperature-humidity structure parameter, CTq, and the temperature-humidity correlation, rTq, were also derived directly from the measurements. Comparison with corresponding values obtained from local measurements with an eddy-covariance system on a meteorological tower show a consistent behaviour in time (with some exceptions especially for Cq2 and rTq during nighttime). The deviations are of a magnitude between 20 % and 35 % of the typical daytime values of the structure parameters.
Derivation of an effective height for scintillometers: La Poza Experiment in Northwest Mexico
Hartogensis, O.K. ; Watts, C.J. ; Rodriguez, J.C. ; Bruin, H.A.R. de - \ 2003
Journal of Hydrometeorology 4 (2003)5. - ISSN 1525-755X - p. 915 - 928.
large-aperture scintillometer - flevoland field experiment - sensible heat fluxes - refractive-index - heterogeneous surface - area - scintillation - fluctuations
The large-aperture scintillometer (LAS) is by now a generally accepted device for routinely obtaining the area-averaged sensible heat flux, H, on a scale of up to 10 km. It is an optical instrument that consists of a transmitter and receiver. In practice, the LAS beam height often varies along the path due to a variety of reasons. This study will explain what effective height to use in such situations, when analyzing scintillometer data to derive H. Several aspects are covered: a slanted path over flat terrain, structured terrain, and varying path height due to the curvature of the earth's surface. To test the derived effective height formulation the authors present LAS data taken in September and October 1996 at a rangeland site in Sonora, Mexico. In experiment 1, the LAS was set up over a slant path, ranging roughly between 10 and 45 m above the surface over a 3200-m path. In experiment 2, a horizontal LAS path was used at approximately 30 m over a pathlength of 1100 m. The resulting sensible heat fluxes were compared with eddy-covariance data and show satisfactory results for both the full and one of the approximate formulations of the effective height.
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