Staff Publications

Staff Publications

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

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

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

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

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Variability of the Structure Parameters of Temperature and Humidity Observed in the Atmospheric Surface Layer Under Unstable Conditions
Braam, M. ; Moene, A.F. ; Beyrich, F. - \ 2014
Boundary-Layer Meteorology 150 (2014)3. - ISSN 0006-8314 - p. 399 - 422.
sonic anemometer - water-vapor - aperture scintillometer - heterogeneous surface - local-structure - boundary-layer - sensible heat - fluxes - turbulence - momentum
The structure parameters of temperature and humidity are important in scintillometry as they determine the structure parameter of the refractive index of air, the primary atmospheric variable obtained with scintillometers. In this study, we investigate the variability of the logarithm of the Monin-Obukhov-scaled structure parameters (denoted as log(2s )) of temperature and humidity. We use observations from eddy-covariance systems operated at three heights (2.5, 50, and 90 m) within the atmospheric surface layer under unstable conditions. The variability of log(C2 s ) depends on instability and on the size of the averaging window over which log(C2 s ) is calculated. If instability increases, differences in log(C2s ) between upward motions (large C2 s ) and downward motions (small C2 s ) increase. The differences are, however, not sufficiently large to result in a bimodal probability density function. If the averaging window size increases, the variances of log(C2 s ) decrease. A linear regression of the variances of log(C2 s ) versus the averaging window size for various stability classes shows an increase of both the offset and slope (in absolute sense) with increasing instability. For temperature, data from the three heights show comparable results. For humidity, in contrast, the offset and slope are larger at 50 and 90 m than at 2.5 m. In the end we discuss how these findings could be used to assess whether observed differences in C2 s along a scintillometer path or aircraft flight leg are just within the range of local variability in C2 s or could be attributed to surface heterogeneity. This is important for the interpretation of data measured above a heterogeneous surface.
Measuring H2O and CO2 fluxes at field scales with scintillometry: Part I – Introduction and validation of four methods
Kesteren, A.J.H. van; Hartogensis, O.K. ; Dinther, D. van; Moene, A.F. ; Bruin, H.A.R. de - \ 2013
Agricultural and Forest Meteorology 178-179 (2013). - ISSN 0168-1923 - p. 75 - 87.
small aperture scintillometer - monin-obukhov similarity - temperature-humidity correlation - energy-balance closure - stable boundary-layer - surface-layer - water-vapor - sensible heat - structure parameter - heterogeneous surface
This study introduces four methods for determining turbulent water vapour and carbon dioxide flux densities, the evapotranspiration and CO2 flux respectively. These methods combine scintillometer measurements with point-sampling measurements of scalar quantities and consequently have a faster statistical convergence than the eddy-covariance method. The scintillometer measures the friction velocity and stability averaged over space, allowing the time averaging to be a minute or less in homogenous conditions. This paper aims to thoroughly test the methods by analysing their sensitivity to the variables that go into the method and validate the methods with 30-min eddy-covariance data. Introduced are: the Bowen-variance method, the flux-variance method, the structure-parameter method, and the energy-balance method. Sensitivity analysis shows that each method is sensitive to the turbulence measurements of the scalar quantities that are specific to the method, as well as to the friction velocity. This demonstrates that the accuracy of the flux results from a correct representation of the turbulence variables used by the methods. Furthermore, a 30-min flux validation shows that the methods compare well to the independent eddy-covariance fluxes. We found that the structure-parameter method performs best – a low scatter (the correlation coefficient, r = 0.99) and a 5% underestimation were observed. Also the other methods perform well, although the energy-balance did not close, because storage terms and CO2 flux were neglected. Furthermore, during the night the variance methods were influenced by non-stationarity in the measurement signal. Finally, we suggest using the correlation coefficients between temperature and scalar quantities to acquire the sign of the fluxes. Data for this study were gathered in May–June 2009 over a wheat field near Merken, Germany, in the framework of the TransRegio32 program.
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 (
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
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
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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