The Effect of a New Calibration Procedure on the Measurement Accuracy of Scintec's Displaced-Beam Laser Scintillometer
Kesteren, A.J.H. van; Hartogensis, O.K. ; Kroonenberg, A.C. van den - \ 2014
Boundary-Layer Meteorology 151 (2014)2. - ISSN 0006-8314 - p. 257 - 271.
atmospheric surface-layer - stable boundary-layer - optical scintillation - inner scale - structure parameter - flux measurements - refractive-index - sensible heat - turbulence - spectrum
We describe a new calibration procedure included in the production process of Scintec’s displaced-beam laser scintillometers (SLS-20/40) and its effect on their measurement accuracy. The calibration procedure determines the factual displacement distances of the laser beams at the receiver and transmitter units, instead of assuming a prescribed displacement distance of 2.70 mm. For this study, four scintillometers operated by Wageningen University and the German Meteorological Service were calibrated by Scintec and their data re-analyzed. The results show that significant discrepancies may exist between the factual and the prescribed displacement distances. Generally, the factual displacement is about 0.1 mm smaller than 2.70 mm, but extremes varied between 0.04 and 0.24 mm. Correspondingly, using non-calibrated scintillometers may result in biases as large as 20 % in the estimates of the inner-scale length, l0, the structure parameter of the refractive index, Cn2, and the friction velocity, u*. The bias in the sensible heat flux was negligible, because biases in Cn2 and u* cancel. Hence, the discrepancies explain much of the long observed underestimations of u * determined by these scintillometers. Furthermore, the calibration improves the mutual agreement between the scintillometers for l 0 , but especially for Cn2. Finally, it is noted that the measurement specifications of the scintillometer do not expire and hence the results of the calibration can be applied retroactively
Test of Scintillometer Saturation Correction Methods Using Field Experimental Data
Kleissl, J. ; Hartogensis, O.K. ; Gomez, J.D. - \ 2010
Boundary-Layer Meteorology 137 (2010)3. - ISSN 0006-8314 - p. 493 - 507.
large-aperture scintillometer - atmospheric surface-layer - optical scintillation - sonic anemometer - strong turbulence - inner-scale - fluxes - temperature - algorithms - calibration
Saturation of large aperture scintillometer (LAS) signals can result in sensible heat flux measurements that are biased low. A field study with LASs of different aperture sizes and path lengths was performed to investigate the onset of, and corrections for, signal saturation. Saturation already occurs atC2n˜ 0.074D5/3¿1/3L-8/3, where C2n is the structure parameter of the refractive index, D is the aperture size, ¿ is the wavelength, L is the transect length, which is smaller than theoretically derived saturation limits. At a transect length of 1 km, a height of 2.5 m, and aperture ˜0.15 m the correction factor exceeds 5% already at C2n= 2 × 10-12m-2/3, which will affect many practical applications of scintillometry. The Clifford correction method, which only depends on C2 n and the transect geometry, provides good saturation corrections over the range of conditions observed in our study. The saturation correction proposed by Ochs and Hill results in correction factors that are too small in large saturation regimes. An inner length scale dependence of the saturation correction factor was not observed. Thus for practical applications the Clifford correction method should be applied
Regional spore dispersal as a factor in disease risk warnings for potato late blight: A proof of concept
Skelsey, P. ; Kessel, G.J.T. ; Holtslag, A.A.M. ; Moene, A.F. ; Werf, W. van der - \ 2009
Agricultural and Forest Meteorology 149 (2009)3-4. - ISSN 0168-1923 - p. 419 - 430.
phytophthora-infestans sporangia - atmospheric surface-layer - gaussian plume model - maize pollen - soybean rust - survival - forecast - validation - simulation - cultivars
This study develops and tests a novel approach for including regional risk factors in operational disease risk warnings against potato late blight. The central premise is that fungicide inputs can be reduced by omitting applications on days when conditions are unsuitable for the atmospheric transport of viable sporangia. The decision support system first decides whether a specific day is `high risk¿ (suitable for disease development in planta). Simulation studies revealed that on such high risk days, the capacity of the atmosphere to transport sporangia viably over relevant distances varies widely. An additional rule assesses this capacity, which is high when weather conditions allow a large number of spores to be released from the canopy and transported viably over long distances. When this capacity is high the original spray advice is followed, and when it is low a no-spray advice is given. The concept is implemented using the published decision support system SIMCAST complemented with models for spore release from sporangiophores, spore escape from the canopy, a newly developed model for spore dispersal and dry deposition of spores, spore survival during transportation, and weather forecast data from the mesoscale meteorological model MM5. Cultivar resistance was also incorporated into spray advice. The concept was tested in a field experiment in 2007 with three cultivars, representing a range in resistance to potato late blight from susceptible to highly resistant, and compared to a `stand-alone¿ version of SIMCAST. In a period with normal `infection pressure¿ (risk of disease) one third of the spray recommendations made by SIMCAST alone were modified and negated by the new system for the highly resistant cultivar. These savings came on top of a reduced, resistance-level dependent dose rate of Shirlan (a.i. Fluazinam). The results demonstrate the feasibility of including dispersal modeling and forecasted meteorology in disease warnings against Phytophthora infestans, even if the whereabouts of sources is unknown. The principles can be used in many decision contexts, but further work is needed to test and refine the method before it can be used in practice.
Development and validation of a quasi-Gaussian plume model for the transport of botanical spores
Skelsey, P. ; Holtslag, A.A.M. ; Werf, W. van der - \ 2008
Agricultural and Forest Meteorology 148 (2008)8-9. - ISSN 0168-1923 - p. 1383 - 1394.
phytophthora-infestans sporangia - atmospheric surface-layer - ground-level source - vertical dispersion - maize pollen - release - parameters - ascospores - diffusion - survival
Aerial dispersal of inoculum is the primary means of movement for many plant diseases. One of the challenges of modern decision support for plant health is to provide predictions of the influx of viable pathogen inoculum from sources outside a crop. Such prediction in a practical setting requires prediction tools that have modest computing and input requirements, yet provide sufficiently accurate predictions. In this paper a hybrid dispersion model is developed, combining Taylor's statistical theory of diffusion for horizontal dispersal with the eddy diffusion theory as implemented in the Lagrangian similarity diffusion model of [van Ulden, A.P., 1978. Simple estimates for vertical diffusion from sources near the ground. Atmos. Environ. 12, 2119¿2124] and [Gryning, S.E., van Ulden, A.P., Larsen, S.E., 1983. Dispersion from a continuous ground-level source investigated by a K model. Q. J. Roy. Meteor. Soc. 109, 355¿364]. The model is extended with a dry deposition method and an effective source strength. Model results are compared with experimental data for the transport of artificially released spores of Lycopodium clavatum above a potato canopy. The numerical results are in close agreement with the experimental data, which cover distances up to 100 m. Numerical predictions are compared to those produced by two alternative model versions and a previously published Gaussian plume model for the transport of spores above potato canopies. The potential for practical implementation of atmospheric dispersion models in plant disease decision support systems is discussed
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.
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.
Evaluating parameterizations of aerodynamic resistance to heat transfer using field measurements
Liu, S. ; Lu, L. ; Mao, D. ; Jia, L. - \ 2007
Hydrology and Earth System Sciences 11 (2007)2. - ISSN 1027-5606 - p. 769 - 783.
atmospheric surface-layer - flux-profile relationships - boundary-layer - tibetan plateau - evapotranspiration - stability - models
Parameterizations of aerodynamic resistance to heat and water transfer have a significant impact on the accuracy of models of land - atmosphere interactions and of estimated surface fluxes using spectro-radiometric data collected from aircrafts and satellites. We have used measurements from an eddy correlation system to derive the aerodynamic resistance to heat transfer over a bare soil surface as well as over a maize canopy. Diurnal variations of aerodynamic resistance have been analyzed. The results showed that the diurnal variation of aerodynamic resistance during daytime (07:00 h-18:00 h) was significant for both the bare soil surface and the maize canopy although the range of variation was limited. Based on the measurements made by the eddy correlation system, a comprehensive evaluation of eight popularly used parameterization schemes of aerodynamic resistance was carried out. The roughness length for heat transfer is a crucial parameter in the estimation of aerodynamic resistance to heat transfer and can neither be taken as a constant nor be neglected. Comparing with the measurements, the parameterizations by Choudhury et al. (1986), Viney (1991), Yang et al. (2001) and the modified forms of Verma et al. (1976) and Mahrt and Ek (1984) by inclusion of roughness length for heat transfer gave good agreements with the measurements, while the parameterizations by Hatfield et al. (1983) and Xie (1988) showed larger errors even though the roughness length for heat transfer has been taken into account.
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.
Monin-Obukhov Similarity Functions of the Structure Parameter of Temperature and Turbulent Kinetic Energy Dissipation Rate in the Stable Boundary Layer
Hartogensis, O.K. ; Debruin, H.A.R. - \ 2005
Boundary-Layer Meteorology 116 (2005)2. - ISSN 0006-8314 - p. 253 - 276.
atmospheric surface-layer - aperture scintillometer test - convective conditions - sonic anemometer - fluxes - scintillation - algorithms - cases-99 - validity - models
The Monin-Obukhov similarity theory (MOST) functions fepsi; and fT, of the dissipation rate of turbulent kinetic energy (TKE), ¿, and the structure parameter of temperature, CT2, were determined for the stable atmospheric surface layer using data gathered in the context of CASES-99. These data cover a relatively wide stability range, i.e. ¿ = z/L of up to 10, where z is the height and L the Obukhov length. The best fits were given by f¿ = 0.8 + 2.5¿ and fT= 4.7[1 + 1.6(¿)2/3], which differ somewhat from previously published functions. ¿ was obtained from spectra of the longitudinal wind velocity using a time series model (ARMA) method instead of the traditional Fourier transform. The neutral limit f¿ = 0.8 implies that there is an imbalance between TKE production and dissipation in the simplified TKE budget equation. Similarly, we found a production-dissipation imbalance for the temperature fluctuation budget equation. Correcting for the production-dissipation imbalance, the 'standard' MOST functions for dimensionless wind speed and temperature gradients (Øm and Øh) were determined from f¿ and compared with the Øm and Øh formulations of Businger and others. We found good agreement with the Beljaars and Holtslag [J. Appl. Meteorol. 30, 327-341 (1991)] relations. Lastly, the flux and gradient Richardson numbers are discussed also in terms of f¿ and fT.
Wind forces and related saltation transport
Leenders, J.K. ; Boxel, J.H. van; Sterk, G. - \ 2005
Geomorphology 71 (2005)3-4. - ISSN 0169-555X - p. 357 - 372.
atmospheric surface-layer - boundary-layer - sediment transport - sea
The effect of several wind characteristics on sand transport was studied in three experiments in north Burkina Faso, West Africa. The first experiment is used to analyse the relation between wind speed and shear stress fluctuations across height. The second experiment is used to study the relation of these wind characteristics with saltation transport for fourteen convective storms, registered during the rainy seasons of 2002 and 2003. The effect of sampling time is studied for two of these convective storms. The third experiment relates the turbulent structures of four convective storms to saltation transport. Wind speed measurements were undertaken with two sonic anemometers and sediment transport was measured by two saltiphones. The sampling frequency was either 8 or 16 Hz. The sonic frame of reference was rotated according to a triple rotation. Horizontal fluctuations showed a (fairly) good correlation with height because the wind speed at both sensors was affected by the same vortices. The correlation coefficients ranged from 0.42 (when the distance between the sensors was 1.75 m) to 0.92 (when the distance was 0.25 m). The instantaneous Reynolds' stress had the weakest correlation (correlation coefficient of 0.05 at 1.75 m between the sensors and 0.56 at 0.25 m between the sensors), because the momentum at 2 m above the soil surface is transported by different eddies than those close to the ground. This also explains the fairly good correlation coefficients between the horizontal components of the wind and saltation compared to the poor correlations between instantaneous Reynolds' stress and saltation. An increase in sampling time did not have much impact on these correlation coefficients up to sampling periods of about 30 s. However, this sampling interval would be too coarse to describe the vertical wind component adequately. The classification of the moments of shear stress into the turbulent structures, sweeps, ejections, inward and outward interactions, showed that the mean saltation flux is higher at sweeps and outward interactions than at ejections and inward interactions. Also, saltation occurred more often during sweeps and outward interactions than during ejections and inward interactions.