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.

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    Area-Averaged Fluxes from Field to Kilometer Scale with Optical and Microwave Scintillometers
    Hartogensis, O.K. ; Bruin, H.A. ; Meijninger, W.M.L. ; Kohsiek, W. ; Beyrich, F. ; Moene, A.F. - \ 2007
    - 1 p.
    Scintillometry has proven to be a suitable method to obtain surface fluxes over heterogeneous areas over spatial scales of up to 10 km. We will present two of many field-studies conducted by the Meteorology and Air Quality Group of Wageningen University to illustrate this point. Different scintillometer types have been tested. Optical scintillometers yield the structure parameter of temperature, CT2, for long-path Large Aperture Scintillometers (LAS) and both CT2 and the dissipation rate of turbulent kinetic energy, e, for short-path laser scintillometers. CT2 and e are related to the surface fluxes of heat, H, and momentum, t, by virtue of Monin-Obukhov similarity theory. For the LAS - that provides CT2 only - t is obtained from additional wind speed measurements and an estimate of the roughness length. An optical scintillometer in combination with a millimeter-wave scintillometer (MWS) yields both CT2 and Cq2, the structure parameter of humidity, from which the sensible and the latent heat flux can be determined. The following two scintillometer field experiments will be discussed: EVAGRIPS, Lindenberg, Germany 2003. This study deals with a LAS and a MWS (94 GHz) installed over path length of 5 km at 45 m height over a heterogeneous flat agricultural terrain consisting of a mix of lakes, forest and agriculture fields over undulating terrain. The concept of an effective scintillometer height will be introduced, which needs to be applied when the scintillometer height is not constant over the path. RAPID, Idaho, USA, 1999. This study deals with the estimation of evapotranspiration using a LAS and laser scintillometer installed at field scale (~500m) over irrigated alfalfa in an area affected by advection of warm and dry desert air. In these conditions the sensible heat becomes negative and the water vapor deficit is increased, both enhancing evapotranspiration. References: De Bruin, H.A.R.: 2002, 'Introduction, renaissance of scintillometry', Boundary-Layer Meteorol. 105, 1-4. De Bruin, H.A.R., Hartogensis, O.K., Allen, R.G., and Kramer, J.W.J.L., 2004: 'Note on the Regional Advection Perturbations in an Irrigated Desert (RAPID) Experiment', Theor. Appl. Climatol. 80, 143-152. Hartogensis, O.K., De Bruin, H.A.R., Van De Wiel, B.J.H.: 2002, 'Displaced-Beam Small Aperture Scintillometer Test. Part II: Cases-99 Stable Boundary-Layer Experiment', Boundary-Layer Meteorol. 105, 149-176. Meijninger, W.M.L.; Beyrich, F.; Lüdi, A.; Kohsiek, W.; De Bruin, H.A.R.: 2005, 'Scintillometer-Based Turbulent Fluxes of Sensible and Latent Heat Over a Heterogeneous Land Surface : a Contribution to Litfass-2003 Boundary-Layer Meteorol. 121, 89-110.
    The energy balance experiment EBEX-2000. Part III: Behaviour and quality of the radiation measurements
    Kohsiek, W. ; Liebethal, C. ; Foken, T. ; Vogt, R. ; Oncley, S.P. ; Bernhofer, C. ; Debruin, H.A.R. - \ 2007
    Boundary-Layer Meteorology 123 (2007)1. - ISSN 0006-8314 - p. 55 - 75.
    calibration - pyrgeometers - radiometry
    An important part of the Energy Balance Experiment (EBEX-2000) was the measurement of the net radiation and its components. Since the terrain, an irrigated cotton field, could not be considered homogeneous, radiation measurements were made at nine sites using a variety of radiation instruments, including pyranometers, pyrgeometers and net radiometers. At several of these sites multiple instruments were employed, which enabled us to compare instruments and assess accuracies. At all sites the outgoing longwave and shortwave radiation and the net radiation were measured, while the incoming radiation was supposed to be uniformly distributed over the field and was therefore measured at three sites only. Net radiation was calculated for all sites from the sum of its four components, and compared with the direct measurement of net radiometers. The main conclusions were: (a) the outgoing shortwave radiation showed differences of up to 30 W m-2 over the field; the differences were not clearly related to the irrigation events; (b) the outgoing longwave radiation showed differences of up to 50 W m-2; the differences increased during the periods of irrigation; (c) the net radiation showed differences of several tens of W m-2 across the field, rising to 50 W m-2 or more during the periods of irrigation; (d) the net radiation is preferably to be inferred from its four components, rather than measured directly, and (e) attention should be paid to the characteristics of pyranometers that measure the outgoing radiation, and thus are mounted upside down, while they are commonly calibrated in the upward position. The error in the net radiation at EBEX-2000 is estimated at max (25 W m-2, 5%) per site during the day and 10 W m-2 at night
    The Energy Balance Experiment EBEX-2000. Part I: Overview and energy balance
    Oncley, S.P. ; Foken, T. ; Vogt, R. ; Kohsiek, W. ; Debruin, H.A.R. ; Bernhofer, C. ; Christen, A. ; Gorsel, E. van; Grantz, D. ; Feigenwinter, C. ; Lehner, I. ; Liebethal, C. ; Liu, H. ; Mauder, M. ; Pitacco, A. ; Ribeiro, L. ; Weidinger, T. - \ 2007
    Boundary-Layer Meteorology 123 (2007)1. - ISSN 0006-8314 - p. 1 - 28.
    atmospheric surface-layer - frequency-response corrections - relaxed eddy accumulation - water-vapor transfer - flux measurements - sonic anemometer - heat-flux - sensible heat - land-surface - exchange
    An overview of the Energy Balance Experiment (EBEX-2000) is given. This experiment studied the ability of state-of-the-art measurements to close the surface energy balance over a surface (a vegetative canopy with large evapotranspiration) where closure has been difficult to obtain. A flood-irrigated cotton field over uniform terrain was used, though aerial imagery and direct flux measurements showed that the surface still was inhomogeneous. All major terms of the surface energy balance were measured at nine sites to characterize the spatial variability across the field. Included in these observations was an estimate of heat storage in the plant canopy. The resultant imbalance still was 10%, which exceeds the estimated measurement error. We speculate that horizontal advection in the layer between the canopy top and our flux measurement height may cause this imbalance, though our estimates of this term using our measurements resulted in values less than what would be required to balance the budget.
    The energy balance experiment EBEX-2000. Part II: Intercomparison of eddy-covariance sensors and post-field data processing methods
    Mauder, M. ; Oncley, S.P. ; Vogt, R. ; Weidinger, T. ; Ribeiro, L. ; Bernhofer, C. ; Foken, T. ; Kohsiek, W. ; Bruin, H.A.R. de; Liu, H. - \ 2007
    Boundary-Layer Meteorology 123 (2007)1. - ISSN 0006-8314 - p. 29 - 54.
    frequency-response corrections - water-vapor transfer - flux measurements - sonic anemometer - flow distortion - boundary-layer - heat
    The eddy-covariance method is the primary way of measuring turbulent fluxes directly. Many investigators have found that these flux measurements often do not satisfy a fundamental criterion¿closure of the surface energy balance. This study investigates to what extent the eddy-covariance measurement technology can be made responsible for this deficiency, in particular the effects of instrumentation or of the post-field data processing. Therefore, current eddy-covariance sensors and several post-field data processing methods were compared. The differences in methodology resulted in deviations of 10% for the sensible heat flux and of 15% for the latent heat flux for an averaging time of 30 min. These disparities were mostly due to different sensor separation corrections and a linear detrending of the data. The impact of different instrumentation on the resulting heat flux estimates was significantly higher. Large deviations from the reference system of up to 50% were found for some sensor combinations. However, very good measurement quality was found for a CSAT3 sonic together with a KH20 krypton hygrometer and also for a UW sonic together with a KH20. If these systems are well calibrated and maintained, an accuracy of better than 5% can be achieved for 30-min values of sensible and latent heat flux measurements. The results from the sonic anemometers Gill Solent-HS, ATI-K, Metek USA-1, and R.M. Young 81000 showed more or less larger deviations from the reference system. The LI-COR LI-7500 open-path H2O/CO2 gas analyser in the test was one of the first serial numbers of this sensor type and had technical problems regarding direct solar radiation sensitivity and signal delay. These problems are known by the manufacturer and improvements of the sensor have since been made.
    Evaporation over a heterogeneous land surface - The EVA-GRIPS project
    Mengelkamp, H.T. ; Beyrich, F. ; Heinemann, G. ; Ament, F. ; Bange, J. ; Berger, F. ; Bosenberg, J. ; Foken, T. ; Hennemuth, B. ; Heret, C. ; Huneke, S. ; Johnsen, K.P. ; Kerschgens, M. ; Kohsiek, W. ; Leps, J.P. ; Liebethal, C. ; Lohse, H. ; Mauder, M. ; Meijninger, W.M.L. ; Raasch, S. ; Simmer, C. - \ 2006
    Bulletin of the American Meteorological Society 87 (2006)6. - ISSN 0003-0007 - p. 775 - 786.
    hydrologic-models - flux measurements - drainage-basin - energy-balance - boundary-layer - scale - parameters - area - landscape - climate
    The representation of subgrid-scale surface heterogeneities in numerical weather and climate models has been a challenging problem for more than a decade. The Evaporation at Grid and Pixel Scale (EVA-GRIPS) project adds to the numerous studies on vegetation-atmosphere interaction processes through a comprehensive field campaign and through simulation studies with land surface schemes and mesoscale models. The mixture of surface types in the test area in eastern Germany is typical for larger parts of northern Central Europe. The spatial scale considered corresponds to the grid scale of a regional atmospheric weather prediction or climate model and to the pixel scale of satellite images. Area-averaged fluxes derived from point measurements, scintillometer measurements, and a helicopter-borne turbulence probe were widely consistent with respect to the sensible heat flux. The latent heat flux from the scintillometer measurements is systematically higher than the eddy covariance data. Fluxes derived from numerical simulations proved the so-called mosaic approach to be an appropriate parameterization for subgrid heterogeneity.
    Saturation of the large aperture scintillometer
    Kohsiek, W. ; Meijninger, W.M.L. ; Debruin, H.A.R. ; Beyrich, F. - \ 2006
    Boundary-Layer Meteorology 121 (2006)1. - ISSN 0006-8314 - p. 111 - 126.
    scintillometrie - meteorologische waarnemingen - verzadiging - scintillometry - meteorological observations - saturation - atmospheric surface-layer - scintillation measurements - optical scintillometer - strong turbulence - inner-scale - spectrum - fluxes
    The saturation aspects of a large aperture (0.3 m) scintillometer operating over a 10-km path were investigated. Measurements were made over mainly forested, hilly terrain with typical maximum sensible heat fluxes of 300-400 W m(-2), and over flat terrain with mainly grass, and typical maximum heat fluxes of 100-150 W m(-2). Scintillometer-based fluxes were compared with eddy-correlation observations. Two different schemes for calculating the reduction of scintillation caused by saturation were applied: one based on the work of Hill and Clifford, the other based on Frehlich and Ochs. Without saturation correction, the scintillation fluxes were lower than the eddy-correlation fluxes; the saturation correction according to Frehlich and Ochs increased the scintillometer fluxes to an unrealistic level. Correcting the fluxes after the theory of the Hill and Clifford gave satisfying results.
    Scalar spectra in the near-dissipation range derived from structure functions
    Moene, A.F. ; Dijk, A. van - \ 2006
    In: 17th Symposium on Boundary Layers and Turbulence, 22-25 May 2006, San Diego, California. - Boston : American Meteorological Society - p. P2.1 - P2.1.
    Laser scintillometers (both single and double beam) are sensitive to temperature (and humidity) fluctuations with sizes between inertial range and dissipation range (Hartogensis et al., 2002). Furthermore, large aperture scintillometers become sensitive to fluctuations at those small scales when the signal becomes saturated (Kohsiek et al., 2006). Despite their importance for the interpretation of scintillometer measurements, the exact shapes of the spectra of temperature and humidity are still a point of debate. In state-of-the-art scintillometry (Hartogensis, 2002), one uses Hill¿s (1978) model spectrum to interpret the measured samples. This model is pragmatic and leads to a fair match with experimental data for the scalar spectra. We present an alternative method to estimate the scalar spectrum, relying only on assumptions that can be experimentally verified. The method, which has been inspired by the recent work of Tatarskii (2005), is based on the simultaneous solution of the differential equations for the second-order structure functions of temperature (DTT) and longitudinal velocity (Duu). The third-order structure functions occurring in those differential equations are closed via the second-order structure functions and the validity of these closure relations is checked against experimental evidence. Following Obukhov and Yaglom (1951) we first assume the closure relations to be independent of separation. Secondly, we use recent DNS results of Watanabe and Gotoh (2004) to incorporate the separation-dependence. The resulting spectrum exhibits a similar bump as the solution of Hill's model (scalar variance piles up in the spectrum where turbulence does no longer break up the patches of scalar variance into even smaller bits). The exact location and height of the bump depends on the two closure parameters. We assess their values from the inertial subrange in the spectra of field data. Using the natural spread in the two closure parameters and our model for the temperature spectrum, the natural spread in the shape and location of the bump can be assessed. Consequently, it is shown how the variation in the assumed temperature spectrum influences the outcomes of flux calculations of a displaced-beam scintillometer.
    Validation of fluxes of an extra large aperture scintillometer at Cabauw using Sky Arrow aircraft flux measurements
    Moene, A.F. ; Meijninger, W.M.L. ; Kohsiek, W. ; Gioli, B. ; Miglietta, F. ; Bosveld, F.C. - \ 2006
    In: 17th Symposium on Boundary Layers and Turbulence, 22-25 May 2006, San Diego, California. - Boston : American Meteorological Society - p. 7.2 - 7.2.
    An extra-large aperture scintillometer (XLAS) has been operated at the Cabauw tower for several years over a path of 9.8 kilometers, at an average height of 43 meters (Kohsiek et al., 2002). This yields a long term record of the area-averaged sensible heat flux. During the RECAB summer campaign on July 27 2002, the Sky Arrow flux aircraft was flown close to the path of the scintillometer at approximately 70 meters (and two more levels) at four times of the day (Vilà-Guerau de Arellano et al., 2004). First the aircraft data are used to validate the fluxes obtained from the XLAS. Since the aircraft measures the local flux at flying altitude, whereas the scintillometer estimates the surface flux, the flux divergence below the plane needs to be taken into account. This flux divergence is estimated from the temperature profile along the Cabauw mast. The fluxes of the scintillometer and the aircraft match very well, within their respective accuracies. Secondly, the variability of the aircraft-derived flux along the path is investigated. Despite the rather homogeneous land use, some -statistically significant- differences could be found when the flight-lag was subdivided into five parts. Some of this variability was constant between different times of the day, and should thus be linked to properties of the underlying surface, whereas other variations appeared to be random. Finally, the spatial variability of the structure parameter of the refractive index along the flight lag is investigated using the aircraft data, in order to study how well the scintillometer represents the path-average structure parameter.
    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
    An extra large aperture scintillometer for operational long range applications
    Kohsiek, W. ; Meijninger, W.M.L. ; Debruin, H.A.R. - \ 2005
    Scintillometer fluxes fluxes of sensible and latent heat over a heterogeneous land surface
    Meijninger, W.M.L. ; Beyrich, F. ; Lüdi, A. ; Kohsiek, W. ; Debruin, H.A.R. - \ 2005
    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
    Energy and water vapor fluxes over a heterogeneous land surface: the LITFASS-2003 experiment
    Beyrich, F. ; Bange, J. ; Berger, F.H. ; Bernhofer, Ch. ; Foken, T. ; Hennemuth, B. ; Huneke, S. ; Kohsiek, W. ; Leps, J.P. ; Lohse, H. ; Lüdi, A. ; Mauder, M. ; Meijninger, W.M.L. ; Mengelkamp, H.T. - \ 2004
    In: 16th Symposium on Boundary Layers and Turbulence, 9-13 August 2004, Portland, Maine / McGillis, W.R., Bach, Jr., W.D., Boston : American Meteorological Society - p. 9.1 - 9.1.
    Scintillometer fluxes of sensible and latent heat over a heterogeneous area - a contribution to LITFASS-2003
    Meijninger, W.M.L. ; Beyrich, F. ; Kohsiek, W. ; Bruin, H.A.R. de; Lüdi, A. - \ 2004
    In: 16th Symposium on Boundary Layers and Turbulence, 9-13 August 2004, Portland, Maine / McGillis, W.R., Bach, Jr., W.D., Boston, USA : American Meteorological Society - p. 9.2 - 9.2.
    Oxygen sensitivity of krypton and Lyman-alpha hygrometers
    Dijk, A. van; Kohsiek, W. ; Bruin, H.A.R. de - \ 2003
    Journal of Atmospheric and Oceanic Technology 20 (2003)1. - ISSN 0739-0572 - p. 143 - 151.
    The oxygen sensitivity of krypton and Lyman-¿ hygrometers is studied. Using a dewpoint generator and a controlled nitrogen/oxygen flow the extinction coefficients of five hygrometers associated with the third-order Taylor expansion of the Lambert¿Beer law around reference conditions for oxygen and for water vapor were measured. Latent heat flux corrections for cross-talk of the sensible heat flux are given as a function of the Bowen ratio. The spread observed in oxygen sensitivities calls for individual oxygen calibrations for each apparatus. It is found that the separation between the tubes is the crucial parameter with respect to the oxygen sensitivity. The oxygen-related nonlinearities in the Lambert¿Beer law are stronger than those associated with water vapor. This leads to a strong reduction of the oxygen sensitivity at a more or less constant water vapor sensitivity when the separation between the tubes is doubled from 1.3 to 2.6 cm. This characteristic is supported by a sensitivity analysis of the theoretical response relations of both types of hygrometers, based on multiple emission and absorption lines. The findings of Tanner et al. are confirmed¿the results of a previous study strongly overestimate the oxygen sensitivity of today's krypton hygrometers
    Experimental determination of turbulent fluxes over the heterogeneous LITFASS area: Selected results from the LITFASS-98 experiment
    Beyrich, F. ; Richter, S.H. ; Weisensee, U. ; Kohsiek, W. ; Lohse, H. ; Bruin, H.A.R. de; Foken, T. ; Gockede, M. ; Berger, F. ; Vogt, R. ; Batchvarova, E. - \ 2002
    Theoretical and Applied Climatology 73 (2002)39479. - ISSN 0177-798X - p. 19 - 34.
    sensible heat - land-surface - structure parameter - natural conditions - momentum - scintillation - inhomogeneity - aircraft - exchange - forest
    During the LITFASS-98 experiment, local flux measurements were performed over five different types of underlying surface (grass, barley, triticale, pine forest, water) in a heterogeneous landscape using eddy covariance and profile techniques over a three week time period in June, 1998. Estimates of the area-integrated sensible heat flux during daytime were obtained from continuous measurements with a large aperture scintillometer (LAS) along a 4.7¿km path. The calculation of a mean diurnal cycle of the fluxes during the experiment revealed significant differences between the main land use classes. A land-use weighted average of the sensible heat flux was found to be in good agreement with the LAS based estimate, which in turn was supported by other regionally integrated flux estimates from budget considerations and aircraft measurements for a few case studies. The profiles of turbulent quantities measured along a 99¿m-tower significantly deviate from ¿idealised¿ profiles measured over homogeneous terrain. Peculiarities in the profile structure could be attributed to the heterogeneity of the terrain, namely to the differences in the surface characteristics of the footprint areas for the different tower levels
    The applicability of the scintillation method over heterogeneous areas
    Meijninger, W.M.L. ; Kohsiek, W. ; DeBruin, H.A.R. - \ 2002
    In: 15th symposium on boundary layers and turbulence, 15-19 July 2002, Wageningen, The Netherlands American Meteorological Society - p. 8 - 11.
    meteorologie - warmtestroming - bovenlagen - nederland - meteorology - heat flow - surface layers - netherlands
    Surface fluxes at a scale of several kilometers are required in many meteorological studies. The scintillation technique is one of the few methods that can provide fluxes at these scales (1 - 10 km). Since usually the earth's surface is heterogeneous at these scales the question arises whether the scintillation method, which is based on the Monin-Obukhov Similarity Theory (MOST), can be applied. To test the applicability of the method over a heterogeneous area an experiment was carried out in Flevoland (The Netherlands). The patchy area consisted of many rectangular plots (500 x 250 m) where four crops were grown namely, sugar beet, potatoes, wheat and onions. Each crop covered 25 % of the area independent of the wind direction (i.e. isotropic conditions). Eddy covariance instruments were placed at four plots, each covered by a different crop, to provide independant surface flux measurements. Based on the eddy covariance measurements it was found that the heterogeneity in the area was caused by variations in thermal properties (i.e. H and LvE). No variation in the surface roughness for momentum was observed. Two Large Aperture Scintillometers (LAS) were placed on two windmills at a height of 11.6 and 20.4 m. The path length of both scintillometers was 2.2 km. A good resemblance was found between the sensible heat fluxes derived from the upper LAS and the area averaged sensible heat fluxes derived from the aggregated in-situ eddy covariance measurements. The slightly lower fluxes from the LAS at 11.6 m could be assessed by using a blending height and a footprint model. After accounting for the spatial distribution of the surface fluxes of the crops in the source area of the LAS the results agreed fairly well. The results have demonstrated that the scintillation method is applicable over heterogenous areas. Also when the scintillometer is measuring below the blending height the violation of the MOST relationship between path averaged structure parameters and fluxes is small and reasonable fluxes can be obtained
    Surface fluxes, dissipation rate of TKE and temperature structure parameter determined from scintillometers
    DeBruin, H.A.R. ; Meijninger, W.M.L. ; Hartogensis, O.K. ; Beyrich, F. ; Kohsiek, W. ; Moene, A.F. - \ 2002
    In: Proceedings European Geophysical Society Nice, France : EGS
    Application of surface layer similarity theory to carbon dioxide, moisture and temperature
    Vilà-Guerau de Arellano, J. ; Hartogensis, O.K. ; Vermeulen, A.T. ; Bosveld, F.C. ; Kohsiek, W. ; 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, 2002 - p. 532 - 534.
    Long range scintillometry
    Kohsiek, W. ; Meijninger, W.M.L. ; DeBruin, H.A.R. - \ 2002
    In: 15th Symposium on Boundary Layers and Turbulence, 15-19 July 2002, Wageningen, the Netherlands Boston, U.S.A. : American Meteorological Society - p. 125 - 128.
    meteorologie - warmtestroming - meteorologische waarnemingen - scintillometrie - meteorology - heat flow - meteorological observations - scintillometry
    In the past years there has been a renewed interest in the use of scintillometers for the measurement of sensible and latent heat flux. This interest is partly invoked by the need to infer the energy fluxes over areas that match in size with satellite pixels or the grid of a numerical model. In this respect it is of importance to exploit the path limits of a scintillometer. One of the problems encountered in extending the path length is saturation of scintillation. To avoid saturation one may use large apertures, or choose a longer wavelength such as microwave or radiowave, or install the scintillometer at a higher level above the ground. Here our experiences with an extra large aperture scintillometer (XLAS) operating over a path of 9.8 km are reported. The instrument has transmitter and receiver apertures of 31 cm and uses a 0.94 m, 100 mW light emitting diode as light source. The height of the beam over the surface is 40 m. The transmitter was installed in the TV tower near the town of IJsselstein and the receiver in the KNMI meteorological tower near Cabauw, both at 40 m height. The path crosses mainly pasture land and some low built-up area, mainly situated near the transmitter. The scintillometer was first installed in August 2000, dismantled in October of that year, and re-installed in October 2001 with the aim to get a full year of data. From publications in the literature it can be concluded that in our set-up saturation gets significant at a sensible heat flux of 100 Wm-2 or more. Corrections may be made for higher heat fluxes. We compare the scintillometer fluxes with eddy correlation fluxes obtained near the receiver end of the optical path. In doing so, one faces the issue of comparing a local measurement with an area-averaged one. To gain insight in the degree of homogeneity of the area, we will analyse thermal pictures of the surface beneath the scintillometer path. In this way we hope to get better to grips with the performance of the scintillometer as a device for measuring area-averaged heat fluxes
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