Showing papers on "Scintillometer published in 2006"

Area-Averaged Surface Fluxes Over the Litfass Region Based on Eddy-Covariance Measurements

Abstract: Micrometeorological measurements (including eddy-covariance measurements of the surface fluxes of sensible and latent heat) were performed during the LITFASS-2003 experiment at 13 field sites over different types of land use (forest, lake, grassland, various agricultural crops) in a 20 × 20 km2 area around the Meteorological Observatory Lindenberg (MOL) of the German Meteorological Service (Deutscher Wetterdienst, DWD). Significant differences in the energy fluxes could be found between the major land surface types (forest, farmland, water), but also between the different agricultural crops (cereals, rape, maize). Flux ratios between the different surfaces changed during the course of the experiment as a result of increased water temperature of the lake, changing soil moisture, and of the vegetation development at the farmland sites. The measurements over grass performed at the boundary-layer field site Falkenberg of the MOL were shown to be quite representative for the farmland part of the area. Measurements from the 13 sites were composed into a time series of the area-averaged surface flux by taking into account the data quality of the single flux values from the different sites and the relative occurrence of each surface type in the area. Such composite fluxes could be determined for about 80% of the whole measurement time during the LITFASS-2003 experiment. Comparison of these aggregated surface fluxes with area-averaged fluxes from long-range scintillometer measurements and from airborne measurements showed good agreement.

Scintillometer-Based Turbulent Fluxes of Sensible and Latent Heat Over a Heterogeneous Land Surface – A Contribution to Litfass-2003

Abstract: 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.

Evaporation over a heterogeneous land surface - The EVA-GRIPS project

Abstract: 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.

Methodological Considerations Regarding the Measurement of Turbulent Fluxes in the Urban Roughness Sublayer: The Role of Scintillometery

Abstract: We address some of the methodological challenges associated with the measurement of turbulence and use of scintillometers in the urban roughness sublayer (RSL). Two small-aperture scintillometers were located near the roof interface in a densely urbanized part of Basel, Switzerland, as part of the Basel Urban Boundary-Layer Experiment (BUBBLE) in the summer of 2002. Eddy correlation instruments were co-located near the mid-point of each scintillometer path for data verification purposes. The study presents the first values of the inner length scale of turbulence (l 0) and the refractive index structure parameter of air $$(C_{n}^{2})$$ for a city and demonstrates the influence of mechanical driven turbulence on dissipation. Comparison of dissipation values determined from the two approaches show large scatter that is possibly due to the spatial inhomogeneity of the turbulence statistics within the RSL. Velocity and temperature spectra display a −2/3 slope in the inertial subrange, although the spectral ratio is less than the theoretical prediction of 4/3 expected for isotropy. Conventional Monin–Obukhov equations used to calculate fluxes from the scintillometer were replaced with urban forms of the equations. The results suggest that the scintillometer may be an appropriate tool for the measurement of sensible heat flux (Q H ) above the rooftops given a suitable determination of the effective measurement height.

Saturation of the large aperture scintillometer

Abstract: 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

Surface Fluxes and Characteristics of Drying Semi-Arid Terrain in West Africa

Abstract: This study examines the seasonal cycle of the components of the surface energy balance in the Volta basin in West Africa as part of the GLOWA-Volta project. The regional climate is characterized by a strong north-south gradient of mean annual rainfall and the occurrence of pronounced dry and wet seasons within one annual cycle, causing a strong seasonal variation in the natural vegetation cover. The observations are conducted with a combined system, consisting of a Large Aperture Scintillometer (LAS) for areally aver- aged sensible heat flux, radiometers and sensors for soil heat flux. For comparisons the eddy-covariance (EC) method providing the fluxes of momentum, sensible and latent heat is utilized as well. The measurements of a seasonal cycle in 2002/2003 were gathered including the rapid wet-to-dry transition after the wet season at two locations in Ghana, one in the humid tropical southern region and one in the northern region. A direct comparison and the energy balance closure of the two methods are investigated for daytime and nighttime separately. An attempt is made to understand and explain the differences between the two methods and the closure of energy budget found for these. It is found that the two systems correspond well during daytime. During nighttime the LAS seems to perform more realis- tically than the EC system. Considering the fact that a LAS system is much easier to use in the climate conditions of the Volta basin, it is concluded that the LAS approach is very suitable in this type of climate conditions. Surface conductances are estimated by rearranging the Penman-Monteith equation and compared to a Jarvis-type model optimised for savan- nah conditions. It is found that temperature dependence should be included in the conduc- tance formulation in contrast to earlier findings. Based on the findings the gathered dataset can be used for further model studies of the climate and environment of West Africa.

Turbulence strength estimation from an arbitrary set of atmospherically degraded images.

Abstract: In remote sensing, atmospheric turbulence and aerosols usually limit the image quality. For many practical cases, turbulence is shown to be dominant, especially for horizontal close-to-earth imaging in hot environments. In a horizontal long-range imaging, it is usually impractical to calculate path-averaged refractive index structure constant C2n (which characterizes the turbulence strength) with conventional equipment. We propose a method for estimating C2n from the available atmospherically degraded video sequence by calculating temporal intensity fluctuations in spatially high variance areas. Experimental comparison with C2n measurements using a scintillometer shows reliable estimation results.

Characterizing the propagation path in moderate to strong optical turbulence.

Abstract: In February 2005 a joint atmospheric propagation experiment was conducted between the Australian Defence Science and Technology Organisation and the University of Central Florida A Gaussian beam was propagated along a horizontal 1500 m path near the ground Scintillation was measured simultaneously at three receivers of diameters 1, 5, and 13 mm Scintillation theory combined with a numerical scheme was used to infer the structure constant C2n, the inner scale l0, and the outer scale L0 from the optical measurements At the same time, C2n measurements were taken by a commercial scintillometer, set up parallel to the optical path The C2n values from the inferred scheme and the commercial scintillometer predict the same behavior, but the inferred scheme consistently gives slightly smaller C2n values

Exploring Scintillometry in the Stable Atmospheric Surface Layer

Abstract: The main objective of this thesis is to investigate observation methods of heat and momentum exchange and key variables that characterise turbulence in the atmospheric stable surface layer (SSL), a layer defined as the lower part of the stable boundary layer (SBL) where surface fluxes do not change significantly with height. The SBL is often confined to a shallow layer above the surface and is often intermittent, i.e. quiescent periods with almost laminar flow are interchanged with turbulent bursts. These conditions complicate surface flux measurements considerably, since ideally these then need to take place close to the surface and over short flux averaging intervals. Scintillometers, unlike traditional flux measurement techniques such as eddy covariance (EC), can be operated just above the surface ( Two types of scintillometers will be considered, notably the displaced-beam small-aperture scintillometer (DBSAS) and the large-aperture scintillometer (LAS) deployed in three field campaigns we contributed to as part of this thesis: RAPID in Idaho, USA, (1999), CASES-99 in Kansas, USA (1999) and BBC in Cabauw, the Netherlands (2001). In addition, an old data-set is analysed with LAS data gathered during the La Poza experiment in Sonora, Mexico (1996). The DBSAS and the LAS are optical instruments that consist of a transmitter and a receiver. The receiver records intensity fluctuations of the light beam emitted by the transmitter, which are caused by refraction of the beam upon its passage through the turbulent surface layer. These intensity fluctuations are a measure of the structure parameter of temperature, C T 2 . The DBSAS obtains also the dissipation rate of turbulent kinetic energy, e , from the correlation between the two displaced beams. In itself, these quantities are important properties of turbulence. Moreover, when the flow is turbulent they are related to the turbulent fluxes of sensible heat, H , and momentum, t , usually expressed by the velocity scale u * , by virtue of Monin-Obukhov similarity theory (MOST). The DBSAS is the most suitable scintillometer to be used in the SBL, since it gives a measure of the mechanically induced turbulence (i.e. e ), which is the only turbulence generating mechanism in stable conditions. For the LAS - that does not measure e - the mechanical turbulent transport is usually included using wind speed measurement and an estimate of the roughness length. Several detailed aspects of the application of scintillometry and EC in obtaining e , C T 2 , H and t are discussed. The most general aspects presented are the following. For CASES-99 and BBC we compared the DBSAS performance against EC in obtaining e , C T 2 , H and t over a wide range of stable conditions and conclude that the DBSAS is superior in obtaining turbulence information over short intervals with remarkably little scatter, but that the derived parameters contain systematic errors. When corrected for the systematic errors (using ad-hoc solutions) the DBAS appears to provide accurate C T 2 , e and resulting H , and t for short time intervals and close to the ground. In addition, for the BBC we also investigated the LAS and combinations of LAS and DBSAS to jointly solve e and C T 2 for both stable and unstable conditions. Furthermore, for CASES-99 we derived new MOST relations for e and C T 2 and show how these can be used to evaluate the MOST relations for dimensionless wind speed and temperature gradients. Also, alternative scaling parameters based on e and C T 2 are introduced. Last, we investigated an important practical aspect of the scintillometer application, i.e. what effective height to use to calculate H when the beam-height of the instrument varies along the path. This is done based on a data-set from the La Poza experiment in Sonora, Mexico (1996).

Turbulent fluxes from Helipod flights above quasi-homogeneous patches within the LITFASS area

Abstract: Turbulent fluxes of sensible and latent heat were measured with the helicopter-borne turbulence probe Helipod over a heterogeneous landscape around the Meteorological Observatory Lindenberg during the STINHO-2 and LITFASS-2003 field experiments. Besides the determination of area-averaged heat fluxes, the analysis focused on different aspects of the response of the turbulent structure of the convective boundary layer (CBL) on the surface heterogeneity. A special flight pattern was designed to study flux profiles both over quasi-homogeneous sub-areas of the study region (representing the major land use types—forest, farmland, water) and over a typical mixture of the different surfaces. Significant differences were found between the heat fluxes over the individual surfaces along flight legs at about 80 m above ground level, in agreement with large-aperture scintillometer measurements. This flux separation was still present during some flights at levels near the middle of the CBL. Different scales for the blending height and horizontal heterogeneity were calculated, but none of them could be identified as a reliable indicator of the mixing state of the lower CBL. With the exception of the flights over water, the latent heat flux measurements generally showed a larger statistical error when compared with the sensible heat flux. Correlation coefficients a nd integral length scales were used to characterise the interplay between the vertical transport of sensible and latent heat, which was found to vary between ‘fairly correlated’ and ‘decoupled’, also depending on the soil moisture conditions.

A new method for the determination of area-averaged turbulent surface fluxes from low-level flights using inverse models

Abstract: The low-level flight method (LLF) has been combined with linear inverse models (IM) resulting in an LLF+IM method for the determination of area-averaged turbulent surface fluxes. With this combination, the vertical divergences of the turbulent latent and sensible heat fluxes were calculated from horizontal flights. The statistical errors of the derived turbulent surface fluxes were significantly reduced. The LLF+IM method was tested both in numerical and field experiments. Large-eddy simulations (LES) were performed to compare ‘true’ flux profiles with ‘measurements’ of simulated flights in an idealised convective boundary layer. Small differences between the ‘true’ and the ‘measured’ fluxes were found, but the vertical flux divergences were correctly calculated by the LLF+IM method. The LLF+IM method was then applied to data collected during two flights with the Helipod, a turbulence probe carried by a helicopter, and with the research aircraft Do 128 in the LITFASS-98 field campaign. The derived surface fluxes were compared with results from eddy-covariance surface stations and with large-aperture scintillometer data. The comparison showed that the LLF+IM method worked well for the sensible heat flux at 77 and 200 m flight levels, and also for the latent heat flux at the lowest level. The model quality control indicated failures for the latent heat flux at the 200 m level (and higher), which were probably due to large moisture fluctuations that could not be modelled using linear assumptions. Finally the LLF+IM method was applied to more than twenty low-level flights from the LITFASS-2003 experiment. Comparison with aggregated surface flux data revealed good agreement for the sensible heat flux but larger discrepancies and a higher statistical uncertainty for the latent heat flux

On the scintillation index of a multiwavelength Gaussian beam in a turbulent free-space optical communications channel.

Abstract: The scintillation statistics of a multiwavelength Gaussian optical beam are characterized when the beam is subjected to a turbulent optical channel. It is assumed that the level of turbulence in the atmosphere ensures a weak-turbulence scenario and that fluctuations in the signal intensity are due to variations in the refractive index of the medium, which in turn are caused by regional temperature variations due to atmospheric turbulence. Furthermore, it is assumed that the propagation path is nearly horizontal and that the heights of the transmitter and receiver justify a near-ground propagation assumption. The Rytov approximation is used to arrive at the desired results. Furthermore, it is assumed that the first- as well as second-order perturbation terms are present in modeling the impact of atmosphere-induced scintillation. Numerical results are presented to shed light on the performance of multiwavelength optical radiation in weak turbulence and to underscore the benefits of the proposed approach as compared with its single-wavelength counterpart in combating the effect of turbulence. Furthermore, it is shown that if the separation of wavelengths used is sufficiently large, wavelength separation affects the scintillation index in a measurable way.

Measurements and modeling of optical turbulence in a maritime environment

Abstract: Turbulence can be a dominant factor in image and laser beam degradation for optical systems operating in the near-surface maritime environment. A long-term propagation field experiment was conducted at Zuniga Shoal (near San Diego) to study the impact of environmental conditions on low-altitude laser propagation above the ocean surface. Test periods of one month duration were conducted at various points of the year, during which scintillometer measurements were obtained along a 7.2 km over-water path and a 'flux' research buoy deployed along the propagation path collected concurrent mean meteorological, atmospheric turbulence, and wave data. We use the refractive index structure parameter (C n 2 ) as the critical parameter for quantifying the effects of atmospheric turbulence on laser system performance, including received power fluctuations, beam spread and beam wander. Bulk estimates of C n 2 were derived from the buoy mean meteorological measurements using the Navy Surface Layer Optical Turbulence (NSLOT) model. C n 2 was also determined from atmospheric turbulence measurements obtained from a sonic anemometer on the buoy. These independent C n 2 values derived from the buoy data are compared with C n 2 values computed from the infrared propagation measurements to determine how the NSLOT model performs under different environmental conditions. In addition, the optical measurements and bulk estimates of C n 2 are used to study the effects of the atmospheric turbulence on operational optical systems.

Characterization of Signal Fluctuations in Optical Communications with Intensity Modulation and Direct Detection through the Turbulent Atmospheric Channel

Abstract: Laser free-space communications are now able to compete with radio communications. However, optical communications through the atmosphere still suffer from significant drawbacks. In particular, atmospheric optical turbulence, which consists of variations of the refractive index, must be considered as random. Turbulence induces phase and intensity fluctuations in the propagating wave and can cause severe degradations of the system performance. The purpose of this thesis has been to characterize the turbulence-induced fluctuations of the received optical power, also called scintillation. These fluctuations depend on parameters such as the path length, the turbulence strength, the beam shape or the size of the receiving aperture. Theoretical results are derived from the Kolmogorov model of turbulence but different approaches leading to different results are available. Distribution and temporal spectrum of the received optical power serve as inputs to the direct-detection receiver model.

An Ocean Refractometer: Resolving millimeter-scale turbulent density fluctuations via the refractive index

Abstract: A fiberoptic sensor has been constructed to measure oceanic density fluctuations via their refractive index signature. The resolution (Δz = 1 mm, Δt = 0.2 ms) and precision (Δn < 10−8, Δρ = 3.4 × 10−5 kg m−3) of the device are far better than other methods and are sufficient to resolve the entire turbulent spectrum. Spectra show the salinity Batchelor rolloff at levels undetectable via conductivity measurements. However, the low-wavenumber portion of the spectrum occupied by the turbulent inertial subrange (≈1 m–1 cm scales) is marred by noise resulting from fiber motion in response to turbulent velocity fluctuations. The technique is described, and the first ocean measurements are reported.

Statistical analysis of cloud-cover mitigation of optical turbulence in the boundary layer

Abstract: One atmospheric phenomenon that adversely affects laser propagation is optical turbulence. From ten months of observation, the refractive index structure constant in the atmospheric boundary layer was found to be significantly reduced under widespread cloudy conditions. The refractive index structure constant (C(n) (2)) depends upon the turbulent flux of momentum, sensible and latent heat. The intensity of a propagating laser beam will not be degraded nearly as much as would be expected under clear or lightly scattered cloud conditions. New experimental data are presented that support this hypothesis. The refractive index structure constant was measured for various cloud-cover conditions.

Attempt to close the energy balance for the LITFASS-2003 experiment

Abstract: For the LITFASS-2003 experiment conducted through May and June 2003 in a heterogeneous landscape around the Meteorological Observatory Lindenberg of the German Meteorological Service, different aspects relevant to close the energy balance at the surface based on radiation, soil and turbulent flux measurements were investigated The careful correction of the eddy-covariance data (and the comparison of all instruments including net radiometers) can reduce the residual of the energy balance closure, but still a significant closure gap remains The same result was found when carefully calculating the ground heat flux including all storage terms The use of the ogive test to estimate the energy loss for long wavelengths showed an effect of up to 5 % on the residual Finally, energy balance closure could not be achieved for the surface layer measurements in LITFASS-2003 Recent findings of large-eddy simulations (LES) have shown organised turbulence structures in the lower boundary layer With the application of LES and the calculation of the horizontally averaged fluxes of sensible and latent heat closure of the energy balance was possible for one selected day, when the LES was available This result was supported by large aperture scintillometer measurements as well as by averaging of the eddy-covariance measurements over longer time periods up to ten hours It is therefore concluded, that the energy balance closure problem is a scale problem and structures of the atmospheric boundary layer, which develop over heterogeneous surfaces, must also be taken into account

Validation of fluxes of an extra large aperture scintillometer at Cabauw using Sky Arrow aircraft flux measurements

Abstract: 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 (Vila-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.

Multi-wavelength laser propagation experiments

Abstract: Atmospheric propagation experiments for active and passive EO systems were performed over a 2 and 8 km path. Single and double path propagation effects were studied using retroreflectors and hot point targets. The systems used include laser systems at 1.5 and 3.5 μm wavelength as well as imaging systems in the visible, 1-2 and 8-9 μm regions. A scintillometer operating at 0.8 μm wavelength was also used for the shorter path. Experimental data will be presented and evaluated concerning statistics. The results will be discussed mainly from a laser countermeasure point of view.

Infrared image scintillation: comparison of modeling and measurement

Abstract: Atmospheric scintillation effects on point targets and small targets are studied in the middle-wave infrared MWIR band over a 2.5-km horizontal path. Expressions are presented for both exact nu- merical calculations and for a Gaussian-aperture approximation of scin- tillation that has an accurate analytic solution. Experimental results show that measurements of Cn using a commercial NIR scintillometer allow the prediction of average scintillation levels in the MWIR when correc- tions are made for wavelength and dimensions of the source and re- ceiver. The measured log-irradiance values are approximately lognor- mal, and are about a factor of 2 higher than would have been expected from theory. Our results are relevant for analysis of detection and false- alarm probabilities over long horizontal paths in the MWIR. © 2006 Society

Development of solar scintillometer

Abstract: The index of scintillation measurement is a good parameter to compare different sites for image quality or ‘seeing’. We have developed a scintillometer, which is deployed on the high resolution SPAR telescope in the island site of Udaipur Solar Observatory, for the site characterization to specify the proposed MAST (Multi Application Solar Telescope). The scintillometer consists of a miniature telescope, termed as micro telescope (4 mm aperture, 15 mm focal length) mounted on a drive which tracks the Sun continuously, associated amplifiers and a data acquisition system. A photodiode is used as the detector. The telescope along with detector was obtained from National Solar Observatory (NSO), and is similar to the one used for Advanced Technology Solar Telescope (ATST) site survey. At USO we developed the amplifier and data acquisition system for the scintillometer. A 24-bit analog to digital converter based system was designed, assembled, tested and used as the data acquisition system (DAS). In this paper, we discuss the instrumentation and present the initial results.

Turbulence strength estimation and super-resolution from an arbitrary set of atmospherically degraded images

Abstract: In remote sensing, atmospheric turbulence and aerosols lim it the image quality. For many practical cases turbulence is shown to be dominant, especially for horizontal close-to-earth imaging in hot environments. In a horizontal long-range imaging it is usually impractical to measure path-averaged refractive index structure constant C n2 (which characterizes the turbulence strength) with conventional equipment. In this paper we propose a method for estimation of C n2 based just on the available recorded turbulence-degraded image sequence. The method exploits the turbulence-induced image “dancing”. C n2 is extracted from the estimated image shifts variance. Experimental comparison with C n2 measurements using a scintillometer shows reliable estimation results. We al so estimate image motion with sub-pixel accuracy for the purpose of obtaining a high-resolution image by applying a simple super-resolution procedure. Results of super-resolution for real im agery are presented. Keywords: atmospheric turbulence, atmospheric optics, refractive index structure constant, turbulence motion estimation, super-resolution.

Optical turbulence effects on electro-optical sensors

Abstract: Based on the experimental results of the optical turbulence and the wind speed measured with laser scintillometer and sonic anemometer,the average statistical characteristics of atmospheric turbulence under the coastal environment in summer and winter was analyzed.Then,the turbulence effects on the electro-optical(EO) sensors were evaluated when accumulative probability of refractive index structure constant was 50% and 90% using modulation transfer function(MTF) and resolution power(RP).The results show that,for the horizontal propagation,the turbulence-induced MTF and RP are worse than that of the diffraction-induced under condition of moderate turbulence.For the slant propagation,the effects became more and more evident with the zenith-angle increasing.However,the turbulence effects on EO sensors will vary not more than 45% while the zenith-angle is less than 60°.After that,the effects become quite significant and the performance can degrade 3 times of that at the angle of 60° for some specific sensors.

Turbulent surface fluxes on kilometer scale obtained with scintillometry; a review.

Abstract: A review will be presented of scintillometry work carried out by the Wageningen University in collaboration with others. A scintillometer consists of a transmitter and a receiver. The transmitter produces a parallel beam of light (here wavelengths of about 1 micron and/or 1 mm) and the receiver detects light intensity fluctuations cause by atmospheric turbulent motions. The latter are related to surface fluxes. First, the performance of a large aperture scintillometer (LAS) over different surface types, e.g. heterogeneous terrain types (Flevopolder, the Netherlands; LITTFASS, Germany), grass (Cabauw, the Netherlands), savannah (GLOWA, Ghana) and forest (LITFASS, Germany; Marrakech, Morocco), using path lengths between 1 and 10 km, will be discussed. Note that a LAS provides the sensible heat flux only. Evaporation can be estimated next from a simplified energy balance equation. The so- called saturation effect appears to limit the maximum path length of a LAS. Recently developed saturation correction procedures will be presented. Next, the performance of a LAS combined with a millimeter wave scintillometer (MWS) for estimating both the surface fluxes of sensible and latent heat over natural landscape will be dealt with. Results of the Flevopolder and LITFASS-2003 field experiments will be shown, where a LAS-MWS system was installed over a heterogeneous landscape over a path length of 2.1 and 4.7 km respectively. The scintillometer derived surface fluxes will be compared with aggregated eddy-covariance (EC) measurements. It is our aim also to discuss the question whether and how the scintillometer method can be used in hydrology and agriculture.

Refractive index of HPM at transionospheric propagation

Abstract: The characteristic of high power microwave(HPM) propagation through ionosphere is mainly determined by the diversification of the atmospheric refractive index,while the refractive index has positive correlation with the electron concentration at corresponding propagation layer.The critical frequency of ionosphere in linearity condition and the atmospheric refractive index formula are derived.The electron concentration and the atmospheric refractive index are numerically simulated based on ionospheric electron concentration distributing model.The diversification of the refractive index and the refractive index gradient relating to the altitude are also analyzed, and experiment has been taken under low power microwave condition which validates that the corrected value is closer to the practicality than the value operated from general theory.Hence the altitude,elevation and incept power of HPM propagation can be modified by the theory of this paper,which can also provide theoretic reference to practical HPM propagation.

Co2 flux measuring method for forest

Abstract: PROBLEM TO BE SOLVED: To provide a CO 2 flux measuring method for a forest for accurately evaluating an absorption amount of CO 2 in the whole forest. SOLUTION: When receiving a laser beam emitted from a transmitter of a scintillometer in the sky of the forest by a receiver of the scintillometer, fluctuation of light intensity, based on scintillation caused by a turbulence flow or temperature fluctuation in an upper side of the forest is measured to find a dissipation rate e T of the temperature fluctuation, measures further the dispersion of the fluctuation about the temperature or the square root σ T thereof, the dispersion of the fluctuation about steam or the square root σ q thereof, the dispersion of the fluctuation about the CO 2 or the square root σ C thereof, by a vortex correlation sensor in the sky over the forest, the dissipation rate e q of the steam and the dissipation rate e c of the CO 2 are calculated respectively based on the expressions (1) and (2), and the flux is found from the respective dissipation rates e T , e q , e c that are scalar quantities. COPYRIGHT: (C)2006,JPO&NCIPI

Estimating surface fluxes in irrigated areas with scintillometers

Abstract: Scintillometry has proven to be a good alternative method to obtain surface fluxes over heterogeneous areas over spatial scales of up to 10 km and in non-stationary conditions in the stable surface layer (see e.g. [1]). This study concerns agro- hydrological scintillometer applications of estimating evaporation over homogeneous irrigated areas on a scale of 50 to 500 m. Two types of scintillometers will be considered, notably the displaced beam small aperture scintillometer (DBSAS) and the large aperture scintillometer (LAS) deployed in two field campaigns in Idaho, USA in 1999 [2] and in the Yaqui Valley, Sonora, Mexico in 2000 [3]. The DBSAS and the LAS are optical instruments that consist of a transmitter and receiver. The receiver records intensity fluctuations of the light beam emitted by the transmitter, which are caused by refraction of the beam upon its passage through the turbulent surface layer. These intensity fluctuations are a measure of the structure parameter of temperature, CT2. The DBSAS obtains also the dissipation rate of turbulent kinetic energy, e, from the correlation between the two displaced beams. 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. Evapotranspiration can then be estimated from net radiation and the soil heat flux measurements. In both field campaigns the irrigated agricultural area was surrounded by a desert. In these conditions dry, warm desert air can be advected over the cool evaporating surface by which sensible heat becomes negative and the water vapor deficit is increased, both enhancing evapotranspiration. As a result the surface layer is stably stratified and wind shear is the only turbulence generating mechanism. The DBSAS directly gives information on this process, the LAS does not. We will outline the potential of scintillometers of obtaining principle turbulence parameters (CT2 and e) and fluxes of latent and sensible heat, and compare these with eddy covariance method based estimates for the two experiments. We will present evidence that scintillometers have advantages over the eddy covariance (EC) method in the often non-stationary stable surface layer, since they can be used for short flux-averaging periods as they average turbulence not only in time but also in space. Furthermore, scintillometers require less complex data processing and quality control procedures. Last, the transmitter and receiver of the instrument can be installed at the borders of the field by which the instrument does not interfere with the farmer’s activities in the field. Finally, the medium aperture scintillometer (MAS) will be discussed, which has an aperture size that includes features of both the DBSAS and the LAS. Result of MAS measurements over grass at Cabauw, Netherlands will be shown. References [1] De Bruin, H.A.R.: 2002, ‘Introduction, renaissance of scintillometry’, Boundary- Layer Meteorol. 105, 1-4. and the papers of this special issue on scintillometry. [2] 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. [3] Hoedjes, J.C.B., Zuurbier, R.M., and Watts, C.J.: 2002, ‘Large aperture scintillometer used over a homogeneous irrigated are partly affected by regional advection’, Boundary-Layer Meteorol. 105, 99-117. [4] 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.

An optical fiber interferometric system for non-contact measurement of atmospheric optical turbulence

Abstract: Optical turbulence degrades the quality of laser beam propagation and the quality of the image of optical system, limiting the spatial resolution that can be obtained. A novel single-air-gap fiber optical interferometric system useful for non-contact measurement of the fine structure of optical turbulence is presented. The main idea of this system is based on the application of a specially constructed optical fiber Mach-Zehnder interferometer to measure the phase fluctuations effected by the random fluctuations of refractive index in the turbulent atmosphere. The light source is a long coherence length infrared laser operating at the wavelength of 1.31μm and the optical path exposed to the atmosphere can be adjusted to a most suitable value according to the operational environment. Theoretical estimation illustrates that the system can measure the minimal atmospheric refractive index fluctuation up to 10 -10 during a 2cm propagation path. It is easy to have a calibration of the system and the result shows that the voltage refractive index sensitivity is about 2.1x10 -6 V -1 . The system is integrated and well fixed in a burly airproof box with only the sensing arm exposed to the air. It follows that the system is suitable for the measurement of atmospheric turbulence over land and ocean surfaces.

Multi-wavelength laser system propagation experiments

Abstract: Atmospheric propagation experiments for active and passive EO systems were performed over a 2 and 8 km path. Single and double path propagation effects were studied using retroreflectors and hot point targets. The systems used include laser systems at 1.5 and 3.5 μm wavelength as well as imaging systems in the visible, 1-2 and 8-9 μm regions. A scintillometer operating at 0.8 μm wavelength was also used for the shorter path. Experimental data will be presented and evaluated concerning statistics. The results will be discussed mainly from a laser countermeasure point of view.