Showing papers on "Scintillometer published in 2014"
TL;DR: In this article, an interpretation of the triangular space has been applied over a heterogeneous area in central Spain, using Landsat5-TM, Envisat-AATSR/MERIS and MSG-SEVIRI images.
48 citations
TL;DR: In this article, a comparison of eddy covariance and scintillometry measurements from a reservoir in southeast Queensland, Australia, was conducted to determine whether important theoretical assumptions required for both techniques are valid in the complex environment of a small reservoir.
Abstract: Accurate quantification of evaporation from small water storages is essential for water management and planning, particularly in water-scarce regions. In order to ascertain suitable methods for direct measurement of evaporation from small water bodies, this study presents a comparison of eddy covariance and scintillometry measurements from a reservoir in southeast Queensland, Australia. The work presented expands on a short study presented by McJannet et al. (2011) to include comparisons of eddy covariance measurements and scintillometer-derived predictions of surface energy fluxes under a wide range of seasonal weather conditions. In this study, analysis was undertaken to ascertain whether important theoretical assumptions required for both techniques are valid in the complex environment of a small reservoir. Statistical comparison, energy balance closure, and the relationship between evaporation measurements and key environmental controls were used to compare the results of the two techniques. Reasonable agreement was shown between the sensible heat flux measurements from eddy covariance and scintillometry, while scintillometer-derived estimates of latent heat flux were approximately 21% greater than eddy covariance measurements. We suggest possible reasons for this difference and provide recommendations for further research for improving measurements of surface energy fluxes over small water bodies using eddy covariance and scintillometry. Key Points Source areas for Eddy covariance and scintillometry were on the water surface Reasonable agreement was shown between the sensible heat flux measurements Scintillometer estimates of latent heat flux were greater than eddy covariance
35 citations
TL;DR: In this paper, a short field experiment with Distributed Temperature Sensing (DTS) in a fibre optic cable having 13 levels was undertaken, in which a dry and a wetted section of a fiber optic cable were suspended on a 6 m high tower installed over a sugar beet trial near Pietermaritzburg (South Africa).
Abstract: The Bowen ratio surface energy balance method is a relatively simple method to determine the latent heat flux and the actual land surface evaporation. Despite its simplicity, the Bowen ratio method is generally considered to be unreliable due to the use of two-level sensors that are installed by default in operational Bowen ratio systems. In this paper we present the concept of a new measurement methodology to estimate the Bowen ratio from high resolution vertical dry and wet bulb temperature profiles. A short field experiment with Distributed Temperature Sensing (DTS) in a fibre optic cable having 13 levels was undertaken. A dry and a wetted section of a fibre optic cable were suspended on a 6 m high tower installed over a sugar beet trial near Pietermaritzburg (South Africa). Using the DTS cable as a psychrometer, a near continuous observation of vapour pressure and temperature at 0.20 m intervals was established. These data allows the computation of the Bowen ratio with a high precision. By linking the Bowen ratio to net radiation and soil heat flux, the daytime latent heat flux was estimated. The latent heat flux derived from DTS-based Bowen ratio (BR-DTS) showed consistent agreement (correlation coefficients between 0.97 and 0.98) with results derived from eddy covariance, surface layer scintillometer and surface renewal techniques. The latent heat from BR-DTS overestimated the latent heat derived with the eddy covariance by 4% and the latent heat derived with the surface layer scintillometer by 8%. Through this research, a new window is opened to engage on simplified, inexpensive and easy to interpret in situ measurement techniques for measuring evaporation.
35 citations
TL;DR: In this article, a low cost customised system for continuously measuring the local meteorological data is developed and presented in a field test scintillometer setup is established for a link range of 0.5 km at an altitude of 15.25 m.
Abstract: The major factor that limits the performance of Free Space Optical Communication is atmospheric turbulence which fluctuates over time in accordance with the variations in local meteorological parameters. Estimating the atmospheric turbulence strength C n 2 with the measurement data becomes significant to find the data rate the system is capable of operating under different outdoor local environmental conditions. Hence, a low cost customised system for continuously measuring the local meteorological data is developed and presented in this study. A field test scintillometer setup is established for a link range of 0.5 km at an altitude of 15.25 m. Specialised sensors are interfaced to the digital architectures to acquire the real-time data corresponding to atmospheric changes. The accuracy and performance of the measurement system are tested against standard instruments and the maximum correlation coefficients of 99.92, 99.63, 99.73 and 99.88% are achieved for wind speed, temperature, relative humidity and pressure, respectively. An experimental model to estimate C n 2 using measured meteorological data is developed and the atmospheric turbulence strength is estimated. The validations of the estimated results with the scintillometer measurement are also analysed. The weather profile and corresponding C n 2 variations at our test field for different seasons in 1 year period are presented and the results are analysed.
29 citations
TL;DR: In this paper, the effects of surface heterogeneity on the structure parameters of potential temperature and specific humidity in the convective boundary layer (CBL) of large-eddy simulation (LES) were investigated.
Abstract: 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 $$C_T^2$$
and specific humidity $$C_q^2$$
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 $$C_T^2$$
and $$C_q^2$$
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, $$C_q^2$$
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 $$C_T^2$$
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.
25 citations
TL;DR: The measurement uncertainty of optical flow velocity measurements suffers from distortions caused by refractive index fluctuations in the imaging path as mentioned in this paper, e.g. due to a contamination of the optical access or temperature and pressure gradients within the flow.
18 citations
21 Oct 2014
TL;DR: In this paper, a field trial was performed to compare different measurement techniques analyzing the diurnal formation of the vertical distribution of optical turbulence up to a height of 16 m above ground.
Abstract: Optical turbulence represented by the structure function parameter of the refractive index C n 2 is a relevant parameter for the performance of electro-optical systems and characterization of the atmospheric influence on imaging. It was investigated during a field trial above an Highveld grassland in the atmospheric surface layer at the Rietvlei Nature Reserve close to Pretoria in South Africa from 18th June to 30th June 2013. This campaign was performed to compare different measurement techniques analyzing the diurnal formation of the vertical distribution of optical turbulence up to a height of 16 m above ground. The chosen time period was characterized by a pronounced diurnal cycle of the meteorological conditions, i.e. low variations from day to day. Ultra sonic anemometers were used to measure high frequency time series (50 Hz) of temperature at single points. From the statistical analysis of these time series C n 2 was derived. Three instruments were mounted at a portable mast in the center of slant path measurements over a horizontal distance of 1000 m using large aperture scintillometers (Boundary layer scintillometer BLS 900). Averaging over a time period of 5 minutes, the results of both methods are compared. The agreement in the results of optical turbulence is quite good. Discrepancies and agreement are analyzed with respect to the atmospheric stability and other meteorological parameters. Lowest values of C n 2 at 4.6 m above ground amount to about 8*10 -17 m -2/3 , daily maxima to 6*10 -13 m -2/3 . Additional to the nearly constant meteorological conditions in the diurnal cycle, the uniformity of the terrain let the results of this measurement campaign an ideal data set for investigating methodological questions regarding a comparison of single point measurements with integrated measurements over a horizontal distance. Four stability regimes were identified in the diurnal cycle and investigated. These are convective conditions during the day, neutral conditions about sunrise and sunset, and two different stable regimes at night.
9 citations
TL;DR: The accuracy in determining sensible heat flux (H) of three Kipp and Zonen large aperture scintillometers (LAS) was evaluated with reference to an eddy covariance (EC) system over relatively flat and uniform grassland near Timpas (CO, USA).
Abstract: The accuracy in determining sensible heat flux (H) of three Kipp and Zonen large aperture scintillometers (LAS) was evaluated with reference to an eddy covariance (EC) system over relatively flat and uniform grassland near Timpas (CO, USA). Other tests have revealed inherent variability between Kipp and Zonen LAS units and bias to overestimate H. Average H fluxes were compared between LAS units and between LAS and EC. Despite good correlation, inter-LAS biases in H were found between 6% and 13% in terms of the linear regression slope. Physical misalignment was observed to result in increased scatter and bias between H solutions of a well-aligned and poorly-aligned LAS unit. Comparison of LAS and EC H showed little bias for one LAS unit, while the other two units overestimated EC H by more than 10%. A detector alignment issue may have caused the inter-LAS variability, supported by the observation in this study of differing power requirements between LAS units. It is possible that the LAS physical misalignment may have caused edge-of-beam signal noise as well as vulnerability to signal noise from wind-induced vibrations, both having an impact on the solution of H. In addition, there were some uncertainties in the solutions of H from the LAS and EC instruments, including lack of energy balance closure with the EC unit. However, the results obtained do not show clear evidence of inherent bias for the Kipp and Zonen LAS to overestimate H as found in other studies.
8 citations
TL;DR: In this article, the authors evaluate scintillation fluctuations on optical communication lasers and evaluate potential system improvements to reduce scintillillation effects using a 4-detector board array.
Abstract: The purpose of this research is to evaluate scintillation fluctuations on optical communication lasers and evaluate potential system improvements to reduce scintillation effects. This research attempts to experimentally verify mathematical models developed by Andrews and Phillips [1] for scintillation fluctuations in atmospheric turbulence using two different transmitting wavelengths. Propagation range lengths and detector quantities were varied to confirm the theoretical scintillation curve. In order to confirm the range and wavelength dependent scintillation curve, intensity measurements were taken from a 904nm and 1550nm laser source for an assortment of path distances along the 1km laser range at the Townes Laser Institute. The refractive index structure parameter (Cn2) data was also taken at various ranges using two commercial scintillometers. This parameter is used to characterize the strength of atmospheric turbulence, which induces scintillation effects on the laser beam, and is a vital input parameter to the mathematical model. Data was taken and analyzed using a 4-detector board array. The material presented in this paper outlines the verification and validation of the theoretical scintillation model, and steps to improve the scintillation fluctuation effects on the laser beam through additional detectors and a longer transmitting wavelength. Experimental data was post processed and analyzed for scintillation fluctuations of the two transmitting wavelengths. The results demonstrate the benefit of additional detectors and validate a mathematical model that can be scaled for use in a variety of communications or defense applications. Scintillation is a problem faced by every free space laser communication system and the verification of an accurate mathematical model to simulate these effects has strong application across the industry.
7 citations
TL;DR: In this paper, the crosswind is determined from the time-lag correlation function [r12(τ)] measured by a dual large-aperture scintillometer; U⊥ is defined as the wind component perpendicular to a path.
Abstract: In this study the crosswind (U⊥) is determined from the time-lag correlation function [r12(τ)] measured by a dual large-aperture scintillometer; U⊥ is defined as the wind component perpendicular to a path—in this case, the scintillometer path. A scintillometer obtains a path-averaged U⊥, which for some applications is an advantage compared to other wind measurement devices. Four methods were used to obtain U⊥: the peak method, the Briggs method, the zero-slope method, and the lookup table method. This last method is a new method introduced in this paper, which obtains U⊥ by comparing r12(τ) of a measurement to r12(τ) of a theoretical model. The U⊥ values obtained from the scintillometer were validated with sonic anemometer measurements. The best results were obtained by the zero-slope method for U⊥ 2 m s−1. The Briggs method also showed promising results, but it is not always able to obtain U⊥. The results showed that a high parallel wind component...
6 citations
TL;DR: In this article, the sensitivity of scintillometer-derived sensible heat fluxes to uncertainties in spatially distributed topographic measurements is analyzed and a two-dimensional variable terrain sensitivity function is developed for quantitative error analysis.
Abstract: . Scintillometer measurements allow for estimations of the refractive index structure parameter Cn2 over large areas in the atmospheric surface layer. Turbulent fluxes of heat and momentum are inferred through coupled sets of equations derived from the Monin–Obukhov similarity hypothesis. One-dimensional sensitivity functions have been produced that relate the sensitivity of heat fluxes to uncertainties in single values of beam height over flat terrain. However, real field sites include variable topography. We develop here, using functional derivatives, the first analysis of the sensitivity of scintillometer-derived sensible heat fluxes to uncertainties in spatially distributed topographic measurements. Sensitivity is shown to be concentrated in areas near the center of the beam path and where the underlying topography is closest to the beam height. Relative uncertainty contributions to the sensible heat flux from uncertainties in topography can reach 20% of the heat flux in some cases. Uncertainty may be greatly reduced by focusing accurate topographic measurements in these specific areas. A new two-dimensional variable terrain sensitivity function is developed for quantitative error analysis. This function is compared with the previous one-dimensional sensitivity function for the same measurement strategy over flat terrain. Additionally, a new method of solution to the set of coupled equations is produced that eliminates computational error.
21 Oct 2014
TL;DR: In this article, the authors introduce the concept of scintillation imaging, when unlike the conventional turbulence imaging techniques the variance of the series of images of the scene is calculated and used to gain information either about the object or about the turbulence on the propagation path.
Abstract: Fluctuations in the images of scenes viewed over large distances are the most obvious manifestation of the turbulence effects on the imaging of the incoherent objects. While the average or long-exposure imaging is arguably the most well studied topic of the optical propagation in turbulence, and substantial progress was also made in understanding the average short-exposure imaging, the image scintillations for complex extended scenes are not well understood. We discuss some available results of the image scintillation theory and report on some recent progress. We introduce the concept of the scintillation imaging, when unlike the conventional turbulence imaging techniques the variance of the series of images of the scene is calculated and used to gain information either about the object or about the turbulence on the propagation path. The third constraint in the turbulent PSF [1] plays a critical role in the scintillation imaging making scintillation images insensitive to the constant background and emphasizing the areas with higher local contrast. The bilinear structure of the Object-to-Variance (O2V) maps makes it impossible to use the analogues of the PSF or MTF for scintillation images and precludes development of the general theory of scintillation imaging. We discuss the fundamental properties of the O2V kernel and discuss four examples of scintillation images of simple objects. We present the measurement data where colored scintillation images of the edge were obtained. The variance distributions are normalized using the traditional long-exposure images to remove dependence on the object brightness. In this case scintillations are concentrated near the edge and carry information about the turbulence on the imaging path. The amplitude and width of these variance distributions are sensitive to the turbulence level and can be used as passive scintillometer without the need to deploy the laser source and receiver at both ends of the propagation path. Variance images of the object with sinusoidal brightness distribution consists of the uniform background and doublefrequency sinusoidal oscillations. It has the features consistent with turbulent super-resolution originally described in [2]. Namely, for unresolved object oscillating components disappears while the background persevere.
TL;DR: In this paper, an artificial neural network-based approach was proposed to estimate optical turbulence near the land surface using routinely measured meteorological variables (e.g., temperature, wind speed).
Abstract: The focus of this paper is on the estimation of optical turbulence (commonly characterized by C 2 n ) near the land-surface using routinely measured meteorological variables (e.g., temperature, wind speed). We demonstrate that an artificial neural network-based approach has the potential to be effectively utilized for this purpose. We use an extensive scintillometer-based C 2 n dataset from a recent field experiment in Texas, USA to evaluate the accuracy of the proposed approach.
Journal Article•
TL;DR: In this article, the effect of turbulence on underwater imaging system and image restoration is simulated by computer fluid dynamics, which contains the pressure data that influences refractive index distribution, when the pressure value is conversed to refractive indices, the refractive image distribution can be received with the refraction formula.
Abstract: In order to research the effect of turbulence on underwater imaging system and image restoration, the underwater turbulence model is simulated by computer fluid dynamics. This model is obtained in different underwater turbulence intensity, which contains the pressure data that influences refractive index distribution. When the pressure value is conversed to refractive index, the refractive index distribution can be received with the refraction formula. In the condition of same turbulent intensity, the distribution of refractive index presents gradient in the whole region, with disorder and mutations in the local region. With the turbulence intensity increase, the holistic variation of the refractive index in the image is larger, and the refractive index change more tempestuously in the local region. All the above are illustrated by the simulation results with he ray tracing method and turbulent refractive index model. According to different turbulence intensity analysis, it is proved that turbulence causes image distortion and increases noise. Copyright © 2014 IFSA Publishing, S. L.
TL;DR: The characterization of atmospheric effects on a propagated laser beam is important to applications ranging from free-space optical communications to high-energy laser systems for ship defense.
Abstract: The characterization of atmospheric effects on a propagated laser beam is important to applications ranging from free-space optical communications to high-energy laser systems for ship defense. These applications are frequently developed for a dynamic propagation environment in which either one or both ends of the optical link are moving. The instruments are often constrained by size, weight, and power limitations due to the platforms on which they will be installed. The dynamic nature of the optical link induces several difficulties in link-path instrumentation: turbulence statistics on a continuously changing path are hard to interpret, and the optical instruments must be designed to maintain a high-quality link between beacon and receiver. We will review some of the scintillometer designs and we examine the associated data produced by these different instruments.
13 Oct 2014
TL;DR: In this paper, the use of an eye-safe laser rangefinder at 1.5 μm and TV/IR imaging to obtain information on atmospheric properties at various paths close to the sea surface was investigated.
Abstract: The present paper investigates the use of an eye-safe laser rangefinder at 1.5 μm and TV/IR imaging to obtain
information on atmospheric properties at various paths close to the sea surface. On one day active/passive imaging NIR
and SWIR systems were also used. The paper will describe the experimental equipment and the results from
measurements of atmospheric backscatter as well as TV and IR images of test targets along a 1.8 km path close to the
Baltic Sea. The site also contained a weather station and a scintillometer for logging weather and turbulence parameters.
Results correlating the lidar attenuation with the imaging performance will be given and compared with models.
TL;DR: The Bendersky, Kopeika, and Blaunstein (BKB) model of measuring the refractive structure index parameter, Cn² has proven to be a reliable, well-used means of quantifying and characterizing the atmospheric turbulence in a given environment as discussed by the authors.
Abstract: The alternative Bendersky, Kopeika, and Blaunstein (BKB) model of measuring the refractive structure index parameter, Cn² has proven to be a reliable, well-used means of quantifying and characterizing the atmospheric turbulence in a given environment. This model relies on various meteorological parameters such as temperature, wind speed, relative humidity, and time of day in order to procure the resulting Cn² quantity. Using experimentally confirmed results from a desert environment, the utility of this model may be extended to other climates by adapting temporal hour weights used within the model. The adaptation of these weighted parameters are shown to have a relationship with the unique weather conditions of a given region which are demonstrated by data points collected from two testing ranges located in Florida in addition to archived weather data. The resulting extended model is then compared to commercial scintillometer data for validation. Keywords: BKB Model, Scintillation, Scintillometer, Atmospheric Turbulence, Refractive Index Structure Parameter
13 Jul 2014
TL;DR: In this paper, a target-in-the-loop atmospheric sensing concept is proposed for the development of a new type of scintillometer that does not require precise transmitter/receiver alignment and can operate over extended-range distances.
Abstract: We discuss a target-in-the-loop atmospheric sensing concept that offers a possibility for the development of a new type of scintillometer that does not require precise transmitter/receiver alignment and can operate over extended-range distances
01 Jan 2014
TL;DR: In this article, the authors used remote sensing technology to capture surface information at various spatial scales, including large and continuous spatial coverage within a few minutes, less cost when same spatial information is required, and its benefit in ungauged areas.
Abstract: As one of the most significant components in the hydrological processes (accounting for approximately 60 % of the mean precipitation globally), land surface evapotranspiration (ET, or latent heat flux, LE, in W/m2) controls the water and energy transfer in the interface between land surface and atmosphere (Idso et al. 1975; Brutsaert 1986; Su 2002; Oki and Kanae 2006). Nowadays, measuring directly the actual ET remains unresolved over large heterogeneous areas in practical applications (Brutsaert 1986). Conventional ET measurements (i.e., sap flow, weighing lysimeter, pan measurement, Bowen ratio system, eddy correlation system, and scintillometer) are generally of limited use because of their spatial unrepresentativeness (with a fetch of 100–103 m) and have difficulties in spatial extrapolation due to the significant surface heterogeneity. To date, remote sensing technology has been recognized as the most effective means to capture surface information at various spatial scales. Advantages in the remote sensing technology over the conventional “point” measurements include (1) its large and continuous spatial coverage within a few minutes, (2) its less cost when same spatial information is required, and (3) its benefit in ungauged areas (Engman and Gurney 1991; Rango 1994). Different combinations of the remote sensing spectral information from visible and near-infrared bands to mid and thermal infrared bands can produce surface characteristic parameters that are indispensable to ET models. These parameters consist of Normalized Difference Vegetation Index (NDVI), Leaf Area Index (LAI), fractional vegetation coverage (Fr), surface albedo, surface emissivity, and radiometric surface temperature (Mauser and Schadlich 1998).
25 Nov 2014
TL;DR: In this paper, a technique for determination of the structure characteristic of refractive index of air from data of a coherent Doppler wind lidar was proposed for atmospheric experiments and tested in atmospheric experiments.
Abstract: A technique is proposed for determination of the structure characteristic of refractive index of air from data of a coherent Doppler wind lidar. The proposed technique is tested in atmospheric experiments. Time profiles of the structure characteristic of refractive index in the atmospheric surface layer are obtained and compared with the time profiles of the dissipation rate of the kinetic energy of turbulence obtained from the same lidar data. It is shown in this way that coherent lidars can be used for investigation of not only wind turbulence, but also temperature turbulence.
01 Jan 2014
TL;DR: In this paper, a novel approach is tested, which uses satellite leaf area index (LAI) data retrieved by the Moderate Resolution Imaging Spectroradiometer (MODIS) to inform the FAO-56 Penman-Monteith equation for calculating reference evaporation (ET₀) of vegetation phenological activity.
Abstract: Recent advances in satellite sensor technology and micrometeorological instrumentation for water flux measurement, coupled with the expansion of automatic weather station networks that provide routine measurements of near-surface climate variables, present new opportunities for combining satellite and ground-based instrumentation to obtain distributed estimates of vegetation water use over wide areas in South Africa. In this study, a novel approach is tested, which uses satellite leaf area index (LAI) data retrieved by the Moderate Resolution Imaging Spectroradiometer (MODIS) to inform the FAO-56 Penman-Monteith equation for calculating reference evaporation (ET₀) of vegetation phenological activity. The model (ETMODIS) was validated at four sites in three different ecosystems across the country, including semi-arid savanna near Skukuza, mixed community grassland at Bellevue, near Pietermaritzburg, and Groenkop, a mixed evergreen indigenous forest near George, to determine potential for application over wider areas of the South African land surface towards meeting water resource management objectives. At Skukuza, evaluated against 170 days of flux data measured at a permanent eddy covariance (EC) flux tower in 2007, the model (ETMODIS) predicted 194.8 mm evapotranspiration relative to 148.9 mm measured fluxes, an overestimate of 31.7 %, (r² = 0.67). At an adjacent site, evaluated against flux data measured on two discrete periods of seven and eight days in February and May of 2005 using a large aperture scintillometer (SLS), ETMODIS predicted 27.4 mm and 6.7 mm evapotranspiration respectively, relative to measured fluxes of 32.5 and 8.2 mm, underestimates of 15.7 % and 18.3 % in each case (r² = 0.67 and 0.34, respectively). At Bellevue, evaluated against 235 days of evapotranspiration data measured using a surface layer scintillometer (SLS) in 2003, ETMODIS predicted 266.9 mm evapotranspiration relative to 460.2 mm measured fluxes, an underestimate of 42 % (r² = 0.67). At Groenkop, evaluated against data measured using a SLS over three discrete periods of four, seven and seven days in February, June and September/October respectively, ETMODIS predicted 9.7 mm, 10.3 mm and 17.0 mm evapotranspiration, relative to measured fluxes of 10.9 mm, 14.6 mm and 23. 9 mm, underestimates of 22.4 %, 11.2 % and 24.1 % in each case (r² = 0.98, 0.43 and 0.80, respectively). Total measured evapotranspiration exceeded total modelled evapotranspiration in all cases, with the exception of the flux tower site at Skukuza, where evapotranspiration was overestimated by ETMODIS by 31.7 % relative to measured (EC) values for the 170 days in 2007 where corresponding modelled and measured data were available. The most significant differences in measured versus predicted data were recorded at the Skukuza flux tower site in 2007 (31.7 % overestimate), and the Bellevue SLS flux site in 2003 (42 % underestimate); coefficients of determination, a measure of the extent to which modelled data are able to explain observed data at validation periods,…
01 Jan 2014
TL;DR: In this article, the authors investigated the effect of local variability in the structure parameters at different heights and under different stability regimes on the area-averaged fluxes over natural landscapes, and found that the variability is determined by stability and by the size of the averaging window over which structure parameters are calculated.
Abstract: 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 (M 2 AV) 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 M 2 AV 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 M 2 AV 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 M 2 AV 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, Graf 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
Posted Content•
TL;DR: In this paper, the authors developed an analysis of the sensitivity of displaced-beam scintillometer derived sensible heat fluxes to uncertainties in spacially distributed topographic measurements, showing that the sensitivity is concentrated in areas near the center of the beam and where the underlying topography is closest to the beam height.
Abstract: Displaced-beam scintillometer measurements of the turbulence inner-scale length $l_o$ and refractive index structure function $C_n^2$ resolve area-average turbulent fluxes of heat and momentum through the Monin-Obukhov similarity equations. Sensitivity studies have been produced for the use of displaced-beam scintillometers over flat terrain. Many real field sites feature variable topography. We develop here an analysis of the sensitivity of displaced-beam scintillometer derived sensible heat fluxes to uncertainties in spacially distributed topographic measurements. Sensitivity is shown to be concentrated in areas near the center of the beam and where the underlying topography is closest to the beam height. Uncertainty may be decreased by taking precise topographic measurements in these areas.
13 Jul 2014
TL;DR: In this article, the authors apply the quasi-wavelet model to the turbulent refractive index, then the first-order phase fluctuations are calculated, and explore its potential by estimating the angle-of-arrival variance.
Abstract: We apply the novel quasi-wavelet model to the turbulent refractive index, then the first-order phase fluctuations are calculated. We explore its potential by estimating the angle-of-arrival variance. We found a good correspondence with previous models.
TL;DR: In this article, the sensitivity of scintillometer-derived sensible heat fluxes to uncertainties in spatially distributed topographic measurements has been analyzed and a two-dimensional variable terrain sensitivity function has been developed for quantitative error analysis.
Abstract: Scintillometer measurements allow for estimations of the refractive index structure parameter $C_n^2$ over large areas in the atmospheric surface layer. Turbulent fluxes of heat and momentum are inferred through coupled sets of equations derived from the Monin--Obukhov similarity hypothesis. One-dimensional sensitivity functions have been produced that relate the sensitivity of heat fluxes to uncertainties in single values of beam height over flat terrain. However, real field sites include variable topography. We develop here, using functional derivatives, the first analysis of the sensitivity of scintillometer-derived sensible heat fluxes to uncertainties in spatially distributed topographic measurements. Sensitivity is shown to be concentrated in areas near the center of the beam path and where the underlying topography is closest to the beam height. Relative uncertainty contributions to the sensible heat flux from uncertainties in topography can reach 20\,\% of the heat flux in some cases. Uncertainty may be greatly reduced by focusing accurate topographic measurements in these specific areas. A~new two-dimensional variable terrain sensitivity function is developed for quantitative error analysis. This function is compared with the previous one-dimensional sensitivity function for the same measurement strategy over flat terrain. Additionally, a~new method of solution to the set of coupled equations is produced that eliminates computational error.
01 Aug 2014
TL;DR: In this paper, an overdetermined interferometric setup is proposed to track the variations of the refractive index of air on-line directly in the line of the measuring beam and to compensate for the fluctuations.
Abstract: We present an interferometric technique based on differential interferometry setup for measurement in the subnanometer scale in atmospheric conditions. One of the important limiting factors in any optical measurement are fluctuations of the refractive index of air representing a source of uncertainty traditionally compensated when the index is evaluated indirectly from the physical parameters of the atmosphere. Our proposal is based on the concept of overdetermined interferometric setup where a reference length is derived from a mechanical frame made from a material with very low thermal coefficient on the 1∗E-8 level. The technique allows to track the variations of the refractive index of air on-line directly in the line of the measuring beam and to compensate for the fluctuations. The optical setup consists of three interferometers sharing the same beam path where two measure differentially the displacement while the third evaluates the changes in the measuring range acting as a tracking refractometer. The principle is demonstrated on an experimental setup and a set of measurements describing the performance is presented.
21 Oct 2014
TL;DR: In this paper, the horizontal and vertical line spread function (LSF) was measured with a camera with 5 μrad resolution using a standard BLS scintillometer over a nearby range.
Abstract: For users of Electro-Optical (EO) sensors at sea, knowledge on their resolution is of key operational importance for the prediction of the obtainable classification ranges. Small targets may be located at ranges of 20 km and more and the present day sensor pixel size may be as small as 10 μrad. In this type of scenarios, sensor resolution will be limited by blur, generated by atmospheric turbulence, easily being greater than 30 μrad (at 20 km range). Predictions of the blur size are generally based upon the theory, developed by Fried [1]. In this theory, the turbulence strength is characterized by the structure parameter for the refractive index Cn 2, of which data are assumed to be available from secondary instruments. The theory predicts the atmospheric Modulation Transfer Function (MTF), which can be incorporated into the total system MTF, used in range performance predictions, as described by Holst [2]. Validation of blur predictions by measurements is a complex effort due to the rapid variations of the blur with time and the problems associated with the simultaneous acquisition of proper Cn 2 data. During the FATMOSE trial, carried out over a range of 15.7 km in the False Bay near Simon’s Town (South Africa) from November 2009 to October 2010, these data were collected in a large variety of atmospheric conditions [3]. In stead of the atmospheric MTF, the horizontal and vertical line spread function (LSF) was measured with a camera with 5 μrad resolution. Various methods for the determination of the LSF and the associated problems are discussed in the paper. The width of the LSF is via its Fourier transform directly related to the MTF. Cn 2 data were collected with a standard BLS scintillometer over a nearby range. Additional Cn 2 data were obtained via conversion of the scintillation data from the same camera and from a high speed transmissometer, collecting data over the same range. Comparisons between blur and Beam Wander predictions and measurements from the FATMOSE campaign are discussed in the paper as well as their impact on the range performance of present day sensors at sea.