Showing papers on "Scintillometer published in 2007"

Comparison of Large Aperture Scintillometer and Eddy Covariance Measurements: Can Thermal Infrared Data Be Used to Capture Footprint-Induced Differences?

Abstract: 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.

Hydrometeorological application of a microwave link: 1. Evaporation

Abstract: A method to estimate areal evaporation using a microwave link (radio wave scintillometer) in combination with an energy budget constraint is proposed. This radio wave scintillometry-energy budget method (RWS-EBM) is evaluated for its applicability in different meteorological conditions and for its sensitivity to various variables (the structure parameter of the refractive index of air C n 2, the total available energy R n - G, the wind velocity u, the effective average vegetation height h 0, and the correlation coefficient between the temperature and humidity fluctuations r TQ ). The method is shown to be best suited for use in wet to moderately dry conditions, where the latent heat flux is at least a third of the total available energy (i.e., Bowen ratio =2). It is important to accurately measure the total available energy and the wind velocity as the RWS-EBM is most sensitive to these variables. The Flevoland field experiment has provided the data, obtained with a 27-GHz radio wave scintillometer (over 2.2 km), a large-aperture scintillometer (also 2.2 km), and four eddy covariance systems, which are used to test the RWS-EBM. Comparing 92 daytime measurements (30-min intervals) of the evaporation estimated using the RWS-EBM to that determined in alternative manners (eddy covariance and two-wavelength scintillometry) leads to the conclusion that the method provides consistent estimates (coefficient of determination r 2 = 0.85 in both cases) under relatively wet conditions.

Mechanics and refractive power optimization of tunable acoustic gradient lenses

Abstract: Tunable acoustic gradient index (TAG) lenses create tunable multiscale Bessel beams. These lenses are fluid-filled cylindrical cavities within which an acoustic radial standing wave is excited. This standing wave modulates the density, and thereby the refractive index within the lens. Spatial gradients in the refractive index can be used for lensing. A predictive model for the steady-state fluid mechanics behind TAG lenses driven with a sinusoidal voltage signal is presented here. The model covers inviscid and viscous regimes in both the resonant and off-resonant cases. The density fluctuations from the fluidic model are related to refractive index fluctuations. The entire model is then analyzed to determine the optimal values of lens design parameters for greatest lens refractive power. These design parameters include lens length, radius, static refractive index, fluid viscosity, sound speed, and driving frequency and amplitude. It is found that long lenses filled with a fluid of high refractive index an...

Statistical analysis of optical turbulence intensity over a 2.33 km propagation path.

Abstract: Refractive index and microclimate fluctuations can significantly affect free-space laser communications. To better understand these physics relationships, optical scintillometer data were collected over a near-horizontal propagation path along with in-situ rooftop measurements of temperature variance. Regression analysis of time-averaged data revealed that fairly high correlation values (i.e., R ≥ 0.80) occurred in 8 of 21 cases studied. Analysis suggests that point sensors can provide valuable information on optical turbulence for extended paths. Additional research is recommended to further explore point measurements and their relation to integrated values of optical turbulence over inhomogeneous paths.

Scintillometer networks for calibration and validation of energy balance and soil moisture remote sensing algorithms

Abstract: Accurate estimation of sensible and latent heat fluxes as well as soil moisture from remotely sensed satellite images poses a great challenge. Yet, it is critical to face this challenge since the estimation of spatial and temporal distributions of these parameters over large areas is impossible using only ground measurements. A major difficulty for the calibration and validation of operational remote sensing methods such as SEBAL, METRIC, and ALEXI is the ground measurement of sensible heat fluxes at a scale similar to the spatial resolution of the remote sensing image. While the spatial length scale of remote sensing images covers a range from 30 m (LandSat) to 1000 m (MODIS) direct methods to measure sensible heat fluxes such as eddy covariance (EC) only provide point measurements at a scale that may be considerably smaller than the estimate obtained from a remote sensing method. The Large Aperture scintillometer (LAS) flux footprint area is larger (up to 5000 m long) and its spatial extent better constraint than that of EC systems. Therefore, scintillometers offer the unique possibility of measuring the vertical flux of sensible heat averaged over areas comparable with several pixels of a satellite image (up to about 40 Landsat thermal pixels or about 5 MODIS thermal pixels). The objective of this paper is to present our experiences with an existing network of seven scintillometers in New Mexico and a planned network of three scintillometers in the humid tropics of Panama and Colombia.

Characterization of atmospheric turbulence during the NATO RTG-40 land field trials

Abstract: The NATO RTG-40 Active Imaging Land Field Trials were conducted at the High Energy Laser System Test Facility at White Sands Missile Range, NM, during November of 2005. This experiment intercompared six active imager systems operating in the visible, near-infrared, and short-wave infrared sensing bands. To characterize the atmospheric turbulence structure present during the optical measurements eight scintillometers were arranged along or near the atmospheric path to characterize the vertical and temporal structure of scintillation, and inner and outer scales of turbulence. A met mast, two 32-m met towers, and an 8-m tower complemented the scintillometer data. This report focuses on analysis of data from four 3-D sonic anemometers positioned at midrange on the 8-m tower and on four of the scintillometers arranged along the 2-km propagation path. First and second order statistics from the sonic sensors are illustrated, along with an analysis of the turbulence spectrum measured by the sonic temperature sensors. The analysis of this data should support both estimating turbulence strength using sonic anemometers as well as outer scale. The data acquired throughout the 10-day measurement period and have proved useful in characterization of the overall weather conditions present during testing and in prediction of various surface layer characteristics.

Measuring optical turbulence parameters with a three-aperture receiver

Abstract: Intensity fluctuations from a 532nm CW laser source were collected over an outdoor 1km path, 2m above the ground, with three different receiving apertures. The scintillation index was found for each receiving aperture and recently developed theory for all regimes of optical turbulence was used to infer three atmospheric parameters, C n 2 , l 0 , and L 0 . Parallel to the three-aperture data collection was a commercial scintillometer unit which reported C n 2 and crosswind speed. There was also a weather station positioned at the receiver side which provided point measurements for temperature and wind speed. The C n 2 measurement obtained from the commercial scintillometer was used to infer l 0 , L 0 , and the scintillation index. Those values were then compared to the inferred atmospheric parameters from the experimental data. Finally, the optimal aperture sizes for data collection with the three-aperture receiver were determined.

Estimating the refractive index structure parameter within atmospheric surface layer

Abstract: A model of atmospheric boundary layer,which has adopted Bulk method,is used to estimate the refractive index structure parameter in southeast seashore and Hefei.A high resolution three dimensional sonic anemometers was used instead of micro-thermal probes to compute the refractive index structure parameter.This avoids sea salt contaminating metal wire.The main input parameters are air temperature,surface temperature,humidity,wind speed and the measurement height of these variables.Analysis shows that applying Bulk method model to estimate the refractive index structure parameter is feasible.Meanwhile,applying Frederickson's formula or Thiermann's formula for empirically determining Monin-Obukhov similarity functions have little difference.It is difficult to get the refractive index structure parameter data directly over sea,however,which can be estimated after inputting sea surface temperature,roughness parameter and proper similarity functions into our model.

A Flux Footprint Model for Large Aperture Scintillometer

Abstract: The large aperture scintillometer(LAS)is a versatile instrument to measure area-average heat fluxes over heterogeneous surface.In order to analyze the source area of LAS data,a new Eulerian analytical footprint model is developed.The parameters of the LAS footprint are discussed in detail and some outcome of the model application over a heterogeneous field experiment are shown.The good agreement of the results demonstrates that this footprint model is an effective and practical tool for complex surface study.

Anisotropic refractive index fluctuations spectrum in the stratosphere sensed from balloon-borne observations of stellar scintillation

Abstract: Scintillation effects caused by the fluctuations of the refractive index of air are not negligible in the stratosphere. Recent experiments highlight the composite nature of optical turbulence in the stratosphere. We present an analytical model of scintillation based on a 3-D model of anisotropic and isotropic refractive index fluctuations spectrum that predicts scintillation rates inside the Rytov regime. This model uses a multi-layer decomposition of the turbulence profile. The effect of anisotropy leads to significant scintillation rates when a source is observed with a horizontal line of sight. Astronomical observations consisting in stellar scintillation made from balloon-borne spectrometer AMON-RA allow us to remotely probe statistical characteristics in the stratosphere, to validate the model of scintillation developed herein and to obtain refined values of its parameters. Data reduction from these observations brings out values of the inner scale of the anisotropic spectrum. We retrieve metric values of the inner scale that are compatible with space-based measurements. We find a major contribution of the anisotropic spectrum relatively to the isotropic contribution. This effect is particularly noticeable when the sight line plunges into the atmosphere, leading to strong scintillation as well as coupled chromatic refraction effects. This makes clear the presence of gravity waves in the stratosphere.

Evaluation of the Kipp and Zonen Large Aperture Scintillometer for estimation of sensible heat flux over irrigated and non-irrigated fields in southeastern Colorado

Abstract: .................................................................................................................................... ii Acknowledgements ................................................................................................................... iv List of Tables ........................................................................................................................... vii List of Figures ......................................................................................................................... vii

Turbulence time series data hole filling using Karhunen-Lòeve and ARIMA methods

Abstract: Measurements of optical turbulence time series data using unattended instruments over long time intervals inevitably lead to data drop-outs or degraded signals. We present a comparison of methods using both Principal Component Analysis, which is also known as the Karhunen-Loeve decomposition, and ARIMA that seek to correct for these event-induced and mechanically-induced signal drop-outs and degradations. We report on the quality of the correction by examining the Intrinsic Mode Functions generated by Empirical Mode Decomposition. The data studied are optical turbulence parameter time series from a commercial long path length optical anemometer/scintillometer, measured over several hundred metres in outdoor environments.

Turbulence Time Series Data Hole Filling using Karhunen-Loeve and ARIMA methods

Abstract: Measurements of optical turbulence time series data using unattended instruments over long time intervals inevitably lead to data drop-outs or degraded signals. We present a comparison of methods using both Principal Component Analysis, which is also known as the Karhunen--Loeve decomposition, and ARIMA that seek to correct for these event-induced and mechanically-induced signal drop-outs and degradations. We report on the quality of the correction by examining the Intrinsic Mode Functions generated by Empirical Mode Decomposition. The data studied are optical turbulence parameter time series from a commercial long path length optical anemometer/scintillometer, measured over several hundred metres in outdoor environments.

Turbulence inner scale sensor for arbitrary atmospheric paths

Abstract: Turbulence inner scale affects scintillation in laser projectio n and laser communication systems especially in strong scintillation regime. Analytical and numerical models are used for performance analysis and design of these systems. Turbulence inner scale is critically important to anchor theoretical predictions to an experiment. However, the inner scale is usually not measured in the experiments along extended atmospheric paths. Commercial scintillometer commonly operates over the range of a few hundreds meters and requires an optical transmitter and recei ver at different ends of the propagation path. We introduced a concept for turbulence inner scale sensor, which is based on phase related phenomenon and can operate along arbitrary atmospheric paths including the strong scintillation regime both during daytime and nighttime. We evaluated the feasibility of this approach. We developed an analytical model for a tilt-corrected point spread functio n (PSF) of a distant source that enables tur bulence inner scale sensor determination from optical measurements, evaluated the PSF sensitivity to the inner scale variations for ground-to-ground and space-to-ground engagement scenarios, designed and built a sensor br eadboard prototype Finally, fo r the first time we performed turbulence inner scale measurements along space-to-ground propagation paths by imaging stars. We found that the turbulence inner scale on space-to-ground paths is in the range from 1 cm to 3 cm, whereas it is in the range from 0.2 cm to 1.2 cm near the ground. Thus, initial inner scale measurements by imagin g stars revealed that turbulence inner scale on extended elevated paths exceeds that value near the ground. Key words: Adaptive optics; Tu rbulence; Inner scale; Atmospheric measur ements; Differential image motion sensor

Designing and Making a Scintillometer. Atmospheric Measures and Signal Processing

Abstract: Energetic balance between the ground and the atmosphere is a powerful tool leading to a better understanding of global warming. From all the techniques, scintillometry is the most suitable one to give access to sensible heat flux along distances of several kilometers. In association with an adaptive process, such as the Gabor transform, this device can give a new approach of atmospheric phenomena.