Showing papers on "Scintillometer published in 2017"

An End-Member-Based Two-Source Approach for Estimating Land Surface Evapotranspiration From Remote Sensing Data

Abstract: Evapotranspiration (ET) is one of the key variables in the water and energy exchange between land surface and atmosphere. This paper develops an end-member-based two-source approach for estimating land surface ET (i.e., the ESVEP model) from remote sensing data, considering the differing responses of soil water content at the upper surface layer to soil evaporation and at the deeper root zone layer to vegetation transpiration. The ESVEP model first diverges the soil-vegetation system net radiation into soil and vegetation components by considering the transmission of direct and diffuse shortwave radiation separately from the transmission of longwave radiation through the canopy, then calculates the four dry/wet soil/vegetation end-members with the diverged soil and vegetation net radiations, and last separates soil evaporation from vegetation transpiration based on the two-phase ET dynamics and the four end-member temperatures. The model can overall produce reasonably good surface energy fluxes and is no more sensitive to meteorology, vegetation, and remote sensing inputs than other two-source energy balance models and surface temperature versus vegetation index ( $T_{R}$ –VI) trapezoid models. A reasonable agreement could be found with a small bias of ±8 W/ $\text{m}^{2}$ and a root-mean-square error within 60 W/ $\text{m}^{2}$ (comparable to accuracies published in other studies) when both model-estimated sensible heat flux and latent heat flux from MODIS remote sensing data are validated with ground-based large aperture scintillometer measurements.

Evaluation of sensible heat flux and evapotranspiration estimates using a surface layer scintillometer and a large weighing lysimeter.

Abstract: Accurate estimates of actual crop evapotranspiration (ET) are important for optimal irrigation water management, especially in arid and semi-arid regions. Common ET sensing methods include Bowen Ratio, Eddy Covariance (EC), and scintillometers. Large weighing lysimeters are considered the ultimate standard for measurement of ET, however, they are expensive to install and maintain. Although EC and scintillometers are less costly and relatively portable, EC has known energy balance closure discrepancies. Previous scintillometer studies used EC for ground-truthing, but no studies considered weighing lysimeters. In this study, a Surface Layer Scintillometer (SLS) was evaluated for accuracy in determining ET as well as sensible and latent heat fluxes, as compared to a large weighing lysimeter in Bushland, TX. The SLS was installed over irrigated grain sorghum (Sorghum bicolor (L.) Moench) for the period 29 July-17 August 2015 and over grain corn (Zea mays L.) for the period 23 June-2 October 2016. Results showed poor correlation for sensible heat flux, but much better correlation with ET, with r² values of 0.83 and 0.87 for hourly and daily ET, respectively. The accuracy of the SLS was comparable to other ET sensing instruments with an RMSE of 0.13 mm·h-1 (31%) for hourly ET; however, summing hourly values to a daily time step reduced the ET error to 14% (0.75 mm·d-1). This level of accuracy indicates that potential exists for the SLS to be used in some water management applications. As few studies have been conducted to evaluate the SLS for ET estimation, or in combination with lysimetric data, further evaluations would be beneficial to investigate the applicability of the SLS in water resources management.

Assessment of actual evapotranspiration over a semiarid heterogeneous land surface by means of coupled low-resolution remote sensing data with an energy balance model: comparison to extra-large aperture scintillometer measurements

Abstract: In semiarid areas, agricultural production is restricted by water availability; hence, efficient agricultural water management is a major issue. The design of tools providing regional estimates of evapotranspiration (ET), one of the most relevant water balance fluxes, may help the sustainable management of water resources. Remote sensing provides periodic data about actual vegetation temporal dynamics (through the normalized difference vegetation index, NDVI) and water availability under water stress (through the surface temperature T surf), which are crucial factors controlling ET. In this study, spatially distributed estimates of ET (or its energy equivalent, the latent heat flux LE) in the Kairouan plain (central Tunisia) were computed by applying the Soil Plant Atmosphere and Remote Sensing Evapotranspira-tion (SPARSE) model fed by low-resolution remote sensing data (Terra and Aqua MODIS). The work's goal was to assess the operational use of the SPARSE model and the accuracy of the modeled (i) sensible heat flux (H) and (ii) daily ET over a heterogeneous semiarid landscape with complex land cover (i.e., trees, winter cereals, summer vegetables). SPARSE was run to compute instantaneous estimates of H and LE fluxes at the satellite overpass times. The good correspondence (R 2 = 0.60 and 0.63 and RMSE = 57.89 and 53.85 W m −2 for Terra and Aqua, respectively) between instantaneous H estimates and large aperture scintillome-ter (XLAS) H measurements along a path length of 4 km over the study area showed that the SPARSE model presents satisfactory accuracy. Results showed that, despite the fairly large scatter, the instantaneous LE can be suitably estimated at large scales (RMSE = 47.20 and 43.20 W m −2 for Terra and Aqua, respectively, and R 2 = 0.55 for both satellites). Additionally, water stress was investigated by comparing modeled (SPARSE) and observed (XLAS) water stress values ; we found that most points were located within a 0.2 confidence interval, thus the general tendencies are well reproduced. Even though extrapolation of instantaneous latent heat flux values to daily totals was less obvious, daily ET estimates are deemed acceptable.

Scintillation index of optical spherical wave propagating through biological tissue

Abstract: Effects of the tissue turbulence on the propagation of an optical spherical wave are analysed. For this purpose, scintillation index of an optical spherical wave which is propagating in a soft tissue is formulated and evaluated in weakly turbulent soft tissue. Scintillation index of the optical spherical wave is examined against the changes in the tissue parameters which are the tissue length between the optical spherical wave source and the detector, random variations in the refractive index of the tissue and the outer scale of the tissue turbulence. According to our graphical outputs, it is observed that increase in the random variations of the refractive index of the tissue results in an increase in the scintillation index at a certain realization of the turbulence spectrum. On the other hand, larger outer scales and longer tissue lengths yield larger scintillations. The variation of the scintillation index of the optical spherical wave versus the wavelength is also investigated. It is found th...

Comparison of Sensible Heat Fluxes Measured by a Large Aperture Scintillometer and Eddy Covariance System over a Heterogeneous Farmland in East China

Abstract: The sensible heat is an important component in surface energy partitioning over the land surface. This paper compared the sensible heat fluxes measured by a large aperture scintillometer system (LAS) and an eddy covariance system (EC) over a rice paddy with a patch of mulberry seedlings in the east China coastal region during the period from 13 September–11 October 2015. During the observation period, easterlies and northerlies prevailed, and 96% easterlies and northerlies had a speed of 0–6 m s−1. The sensible heat fluxes measured by the two systems reflected that the value of HLAS generally was inclined to be larger than HEC with the average difference of 20.30 W m−2, and the uncertainty for two instruments was less than 17 W m−2. Analysis of the average footprint resulted that the mulberry seedling field always had a higher contribution to LAS than that to EC, which could be the reason that HLAS was always larger than HEC. During the days when the contributions of the mulberry seedling field to the two systems were close to each other, the sensible heat flux measurements of the two instruments were similar. The case analysis on typical sunny days showed that there would be larger sensible heat fluxes over the mulberry seedling field than in the rice paddy field especially under larger net radiation conditions.

Towards accurate estimation of crop water requirement without the crop coefficient Kc: new approach using modern technologies

Abstract: Modern technologies to measure actual evapotranspiration, ETa, were implemented at an experimental farm near Bologna, Italy. Large-aperture scintillometer and eddy covariance instruments were installed. The results showed significant differences between actual evapotranspiration measured by eddy covariance and scintillometer when compared with the potential reference evapotranspiration, ET0, calculated from meteorological data using the Penman–Monteith equation and the crop potential evapotranspiration, ETc, which is based on the ET0 and the crop coefficient, Kc. The ETc and ET0 showed higher values than those of ETa obtained by eddy covariance and scintillometer. On average the actual evapotranspiration measured by eddy covariance and scintillometer for the cropping seasons 2014 and 2015 represented 45 and 35% of the ET0 or the ETc, respectively. The ET0, or the ETc, represent the atmospheric water demand while, fundamentally, the crop water requirement should be based on crop water demand better represented by the actual evapotranspiration. At present, the results indicate that the actual crop water requirement based on modern technologies could save at least 50% of irrigation water for this region. Another benefit is that these modern technologies do not need the crop coefficient Kc, which for many irrigation practitioners is difficult to obtain.

Influence of Mean Rooftop-Level Estimation Method on Sensible Heat Flux Retrieved from a Large-Aperture Scintillometer Over a City Centre

Abstract: The sensible heat flux (H) is determined using large-aperture scintillometer (LAS) measurements over a city centre for eight different computation scenarios. The scenarios are based on different approaches of the mean rooftop-level $$(z_{H})$$ estimation for the LAS path. Here, $$z_{H}$$ is determined separately for wind directions perpendicular (two zones) and parallel (one zone) to the optical beam to reflect the variation in topography and building height on both sides of the LAS path. Two methods of $$z_{H}$$ estimation are analyzed: (1) average building profiles; (2) weighted-average building height within a 250 m radius from points located every 50 m along the optical beam, or the centre of a certain zone (in the case of a wind direction perpendicular to the path). The sensible heat flux is computed separately using the friction velocity determined with the eddy-covariance method and the iterative procedure. The sensitivity of the sensible heat flux and the extent of the scintillometer source area to different computation scenarios are analyzed. Differences reaching up to 7% between heat fluxes computed with different scenarios were found. The mean rooftop-level estimation method has a smaller influence on the sensible heat flux (−4 to 5%) than the area used for the $$z_{H}$$ computation (−5 to 7%). For the source-area extent, the discrepancies between respective scenarios reached a similar magnitude. The results demonstrate the value of the approach in which $$z_{H}$$ is estimated separately for wind directions parallel and perpendicular to the LAS optical beam.

Path Profiles of Cn2 Derived from Radiometer Temperature Measurements and Geometrical Ray Tracing

Abstract: Atmospheric turbulence has significant impairments on the operation of Free-Space Optical (FSO) communication systems, in particular temporal and spatial intensity fluctuations at the receiving aperture resulting in power surges and fades, changes in angle of arrival, spatial coherence degradation, etc. The refractive index structure parameter C 2 n is a statistical measure of the strength of turbulence in the atmosphere and is highly dependent upon vertical height. Therefore to understand atmospheric turbulence effects on vertical FSO communication links such as space-to-ground links, it is necessary to specify C 2 n profiles along the atmospheric propagation path. To avoid the limitations on the applicability of classical approaches, propagation simulation through geometrical ray tracing is applied. This is achieved by considering the atmosphere along the optical propagation path as a spatial distribution of spherical bubbles with varying relative refractive index deviations representing turbulent eddies. The relative deviations of the refractive index are statistically determined from altitude-dependent and time varying temperature fluctuations, as measured by a microwave profiling radiometer. For each representative atmosphere ray paths are analyzed using geometrical optics, which is particularly advantageous in situations of strong turbulence where there is severe wavefront distortion and discontinuity. The refractive index structure parameter is then determined as a function of height and time.

Inhomogeneity of optical turbulence over False Bay (South Africa)

Abstract: Atmospheric turbulence impacts on the propagation of electro-optical radiation. Typical manifestations of optical turbulence are scintillation (intensity fluctuations), beam wander and (for laser systems) reduction of beam quality. For longer propagation channels, it is important to characterize the vertical and horizontal distribution (inhomogeneity) of the optical turbulence. In the framework of the First European South African Transmission ExpeRiment (FESTER) optical turbulence was measured between June 2015 and February 2016 on a 2 km over-water link over False Bay. The link ran from the Institute of Maritime Technology (IMT) in Simons Town to the lighthouse at Roman Rock Island. Three Boundary layer scintillometers (BLS900) allowed assessing the vertical distribution of optical turbulence at three different heights between 5 and 12 m above the water surface. The expected decrease of Cn 2 with height is not always found. These results are analyzed in terms of the meteorological scenarios, and a comparison is made with a fourth optical link providing optical turbulence data over a 8.7 km path from IMT to Kalk Bay, roughly 36° to the north of the three 2 km paths. The results are related to the inhomogeneous meteorological conditions over the Bay as assessed with the numerical weather prediction tool, the Weather Forecast and Research model WRF. © Copyright 2017 SPIE. The Society of Photo-Optical Instrumentation Engineers (SPIE)

Passive method to measure strength of turbulence

Abstract: Disclosed is a method to passively measure and calculate the strength of turbulence via the index of refraction structure constant Cn2 from video imagery gathered by an imaging device, such as a video camera. Processing may occur with any type computing device utilizing a processor executing machine executable code stored on memory. This method significantly simplifies instrumentation requirements, reduces cost, and provides rapid data output. This method combines an angle of arrival methodology, which provides scale factors, with a new spatial/temporal frequency domain method. As part of the development process, video imagery from high speed cameras was collected and analyzed. The data was decimated to video rates such that statistics could be computed and used to confirm that this passive method accurately characterizes the atmospheric turbulence. Cn2 accuracy from this method compared well with scintillometer data through two full orders of magnitude and more capability is expected beyond this verification.

Evapotranspiration estimation using Landsat-8 data with a two-layer framework

Abstract: Evapotranspiration (ET) plays an important role in hydrological cycle by linking land surface and atmosphere through water and energy transfers. Based on the data from the Landsat-8 satellite for typical days with clear sky condition from 2013 to 2016, a two-layer daily ET remote sensing framework was built, which includes four compartments: surface feature parameter estimation, evaporative fraction estimation, daily net radiation estimation, and daily ET extension. Based on the model, evaporation, transpiration, and daily ET in Shahe River Basin were estimated. The estimated daily ET showed a mean absolute percentage error of 8.7% in the plain areas, and 12.1% in the mountainous areas, compared to observations using large aperture scintillometer and eddy covariance system. The method gave higher accuracy than other remote sensing models applied in the same area previously, including the surface energy balance system and the ETWatch. By analyzing the relationship between land use types and surface water/heat fluxes, it was found that the surface energy balance components in the basin have prominent spatial-temporal features, and the soil component’s features are more obvious. It indicated that the proposed two-layer approach is superior to others in terms of simulation accuracy, and applicable to daily scale ET estimations on complex terrains.

Atmospheric characterization on the Kennedy Space Center Shuttle Landing Facility

Abstract: Large temperature gradients are a known source of strong atmospheric turbulence conditions. Often times these areas of strong turbulence conditions are also accompanied by conditions that make it difficult to conduct long term optical atmospheric tests. The Shuttle Landing Facility (SLF) at the Kennedy Space Center (KSC) provides a prime testing environment that is capable of generating strong atmospheric turbulence yet is also easily accessible for well instrumented testing. The Shuttle Landing Facility features a 5000 m long and 91 m wide concrete runway that provides ample space for measurements of atmospheric turbulence as well as the opportunity for large temperature gradients to form as the sun heats the surface. We present the results of a large aperture LED scintillometer, a triple aperture laser scintillometer, and a thermal probe system that were used to calculate a path averaged and a point calculation of Cn2. In addition, we present the results of the Plenoptic Sensor that was used to calculate a path averaged Cn2 value. These measurements were conducted over a multi-day continuous test with supporting atmospheric and weather data provided by the University of Central Florida.

Prediction of atmospheric turbulence on the basis of weather conditions

Abstract: The goal of the paper is in calulation of refractive index structure parameter from weather condition in selected area. Sadot - Kopeika model was chosen for determination of atmospheric turbulence during the whole day. Information about temperature, relative humidity, wind speed, time of sunrise and sunset is necessary for calculations in case of usage of the model. The main advantage of Sadot - Kopeika model is in possibility of prediction of refractive index structure parameter. We need meteorological data, so we compared the predictions for one and two days in advance from three online meteorological services. After that we have compared predicted data with data from meteorological station. Our work on the paper rely on the refractive index structure parameter prediction and comparison with calculated values from meteorological stations.

Measurement of the spatial distribution of atmospheric turbulence with SCINDAR on a mosaic of urban surfaces

Abstract: Two experiments of urban scintillometry were performed recently. Their objective was to study the SCINDAR Cn² profiler performance on a composite urbanforest ground. The SCINDAR provides horizontal Cn² profiles with a few hundred meter profile resolution. Several improvements in data processing are reported: the choice of the spatial resolution of the profile and the hyper-parameters adjustment for Cn² regularization. The distributed Cn² values along the optical path are estimated every minute with small error bars. Their non-uniformity is shown to be consistent with the differences of the line of sight to ground and the coverage of the terrain. The SCINDAR data are also in the same order of magnitude with the three scintillometer data that were simultaneously recorded.

Aerosol optical properties inferred from in-situ and path-averaged measurements

Abstract: This paper compares in-situ and path-averaged measurements of the electro-optical transmission, with emphasis on aerosol effects. The in-situ sensors consisted of optical particle counters (OPC) and a visibility meter, the path-averaged data was provided by a 7-wavelength transmissometer (MSRT) and a scintillometer (BLS). Data was collected at a test site in Northern Germany. A retrieval algorithm was developed to infer characteristics of the aerosol size distribution (Junge approximation) from the MSRT data. A comparison of the various sensors suggests that the optical particle counters are over-optimistic in their estimate of the transmission. © Copyright 2017 SPIE. The Society of Photo-Optical Instrumentation Engineers (SPIE)

Assessment of large aperture scintillometry for large-area surface energy fluxes over an irrigated cropland in north India

Abstract: Amount of available net energy and its partitioning into sensible, latent and soil heat fluxes over an agricultural landscape are critical to improve estimation of evapotranspiration and modelling parse (ecosystem modelling, hydrological and meteorological modelling). Scintillometry is a peculiar and robust methodology to provide structure parameter of refractive index and energy balance. Scintillometer has proven for assessment of sensible and latent heat flux, which is based on the principle of Monin–Obukhov similarity theory. Scintillometer has been installed in the agricultural experimental farm of ICAR-Indian Agricultural Research Institute, New Delhi, with a spatial covering path length of 990 m of irrigated and cultivable agricultural landscape. This paper discusses the patterns of energy flux as diurnal and seasonal basis at scintillometer path which was mainly covered by maize in Kharif and wheat in Rabi season during a crop growing seasons of 2014–2015. The biophysical parameters (leaf area, soil moisture, crop height) were recorded at a temporal resolution of fortnight basis along the path length at usual sampling distance. The Bowen ratio value for both Kharif and Rabi season was 0.76 and 0.88, respectively by scintillometer. Leaf area index had a significantly positive correlation with latent heat flux ( $$R^{2} =0.80$$ ) while a significantly negative correlation with sensible heat flux ( $$R^{2}{=}-0.79$$ ). Soil moisture had a significant negative correlation with sensible heat flux ( $$R^{2}{=}-0.68$$ ). The average evapotranspiration from crop land was $$1.58 ~\hbox {mm d}^{-1}$$ and total evapotranspiration was 543 mm over the 12 months study period. This study defines that large aperture scintillometer is robust instrument which can evaluate energy flux over a large area with a long term series time domain. Moreover, further studied should be conducted to use in crop simulation modelling, developing of new model with calibration and validation of remote sensing energy balance algorithm, etc.

Local optical turbulence at and in the vicinity of the GREGOR solar telescope (Conference Presentation)

Abstract: The image quality of ground-based solar telescopes depends on the amount of turbulence in the Earth’s atmosphere, which is strongest in layers close to the ground during daytime. Local optical turbulence affects the performance of adaptive optical systems and reduces the spatial resolution of solar observations. Increased turbulence which is caused by solar irradiation of the infrastructure close to the telescope and obstructions to a free airflow are major concerns, but difficult to detect and to monitor. We have conducted measurements of optical turbulence at the GREGOR solar telescope (Teide Observatory, Tenerife, Canary Islands) to assess quantitatively the influence of the infrastructure on the image quality. The strength of optical turbulence is determined by the structure function parameter of the refractive index Cn2. We have measured the temporal behavior of the free flow optical turbulence about 30m above ground using a laser scintillometer between the towers of the Vacuum Tower Telescope and of GREOR. Local measurements of Cn2 were taken on the observing platform of GREGOR using three ultrasonic anemometers. Two anemometers are located at the north and south ends of the telescope building, a third one was be placed close to the telescope main mirror cell. Air temperature, pressure, humidity as well as wind speed and direction were measured along with refractive index measurements. An image quality indicator based on an estimate of the Fried parameter r0 was recorded whenever the adaptive optics system GAOS at the GREGOR telescope was operating. Recordings of the net solar radiation were obtained from the GONG experiment which is located a few 100 m from GREGOR. All data were taken between May 2015 and March 2016. We investigate the relation between optical turbulence, solar irradiance and meteorological parameters. Under almost all conditions, optical turbulence nearby the telescope is stronger than free flow turbulence. We note significant dependencies of the strength and the horizontal gradient of Cn2 on wind direction. The moderate influence on image quality indicates that there is only a thin layer with strong turbulence just above the telescope.

Investigation of the height dependency of optical turbulence in the surface layer over False Bay (South Africa)

Abstract: Atmospheric turbulence impacts on the propagation of electro-optical radiation. Typical manifestations of optical turbulence are scintillation (intensity fluctuations), beam wander and (for laser systems) reduction of beam quality. For longer propagation channels, it is important to characterize the vertical and horizontal distribution (inhomogeneity) of the optical turbulence. In the framework of the First European South African Transmission ExpeRiment (FESTER) optical turbulence was measured between June 2015 and February 2016 over a 1.8 km over-water link over False Bay. The link ran from the Institute of Maritime Technology (IMT) at Simons Town to the lighthouse at Roman Rock Island. Three Boundary layer scintillometers (BLS900) allowed assessing the vertical distribution of optical turbulence at three different heights between 5 and 12 m above the water surface. The expected decrease with Cn 2 with height is not always found. These results are analyzed in terms of the meteorological scenario, and a comparison is made with a fourth optical link providing optical turbulence data over a 8.69 km path from IMT to St. James, roughly perpendicular to the three 1.8 km paths. © 2017 SPIE. The Society of Photo-Optical Instrumentation Engineers (SPIE)

Effects of fluctuation in refractive index of atmosphere on synthetic aperture radar images

Abstract: We quantitatively analyze the artifacts in SAR imagery due to fluctuations in the refractive index of the atmosphere. SAR image reconstruction algorithms ignore atmospheric absorption and assume unity for the refractive index of the atmosphere. We assume a complex-valued refractive index for the atmosphere and analyze the errors in reconstructed images due to absorption and fluctuations in the speed of electromagnetic (EM) waves. Absorption leads to amplitude errors which can be viewed as a filtering error. This error can be characterized as systematic variations in the ground reflectivity function. Fluctuations in the speed of EM waves lead to phase errors which manifest as positioning errors in SAR imagery.

Electro-optical propagation measurements during the MINOTAUROS experiment in the Cretan Sea

Abstract: We report on propagation measurements performed during the MINOTAUROS (Maritime INvestigations On Targets and Atmosphere Under Reduction of Optical Signatures) experiment on Crete, Greece, in late summer of 2016 The field trial has been organized by NATO STO Task Group SET-211 on Naval Platform Protection in the EO/IR Domain with strong support of the Hellenic Navy Besides meteorological measurements, the experiment included measurements of turbulence using a boundary layer scintillometer on a slant path (d = 8 km) across the entry of Souda Bay (Crete) These are compared to values obtained by a 3D sonic anemometer, which was deployed at one end of the propagation path Refraction effects have been measured using a 175 km path from Drapanos to Gerani Two meteorological buoys along the path were used to gather information about the atmospheric conditions An overview and a first analysis of the results are presented The refraction measurements are compared to simulations using MORTICIA (Model of Range and Transmission in Coastal Inland Atmospheres), a new software tool currently under development in a collaboration of Fraunhofer IOSB and TNO

Uso de datos satelitales MODIS y balance de energía para estimar la evapotranspiración

Abstract: Evapotranspiration (ET) is an important factor for the development and production of agricultural crops, its value at local and regional level is determinant for planning the management of water resources; some techniques such as vortex covariance measure evapotranspiration in a space-determined manner, while others, such as those based on remote sensing and scintilometry, do it on a regional scale. The objective was to estimate ET using MODIS sensor data and an energy balance, and to compare it against data obtained with scintillometer and vortex covariance system. The study was implemented in Culiacan Valley, Mexico, on a surface of Bell pepper crop, during 81 days of crop development. Normalized difference index (NDVI) and surface temperature data from the MODIS sensor, a BLS450 scintillometer to estimate sensible heat flux, and an IRGA EC-150 vortex covariance system to measure latent heat flux, were used. The total ET obtained was 255.4, 275.2 and 262 mm for MODIS, scintiloter and covariance of vortices, respectively; (RMSE), BIAS, Willmottʼs concordance coefficient (d), and correlation coefficient (R) were 0.44 mm d-1, -0.245 mm d-1, 0.8 y 0.75, respectively. The use of satellite data combined with the energy balance on the land surface allows a reliable estimation of evapotranspiration at a regional scale.

The AFGL thermosonde instrument, developed by Brown et al3, measures the difference in temperature of the ambient atmosphere between two probes spaced 0.5 meters apart. The two

Abstract: Turbulence parameters are determined from measurements along a 183 m line-of-sight at Palm Bay, Florida and a 300 m path at Sudbury, Massachusetts. A horizontal scintillometer is used to obtain o 2, the variance of log amplitude of the irradiance, and the path- averaged C 2, the atmospheric refractive index structure constant. Thermosondes are also used to obtain C 2 at several locations along the optical path. The mean C 2 is determined from three thermosondes and compared to that from the scintillometer. Statistics obtained from processing the 2 sec sampled data over 10 minute periods are presented. Excellent agreement is obtained between the two systems when averaged over 10 minutes. Statistics ob­ tained over shorter periods show a considerable variation in the C 2 estimates within the 10 minute period. The range of variation within 10 minute intervals can exceed 50%. Volume averaging, wind speed and direction fluctuations are discussed as possible causes of short time variations in the C 2 estimates. Atmospheric seeing conditions are an important limiting effect on modern optical systems. The refractive index structure parameter, C 2, is the fundamental quantity used to characterize atmospheric optical turbulence and its effects on optical systems. One of the most widely known turbulence effects is that of scintillation, or fluctuations in the received intensity. The parameter used to describe scintillation is o 2 , the variance of log amplitude of the irradiance. This paper presents results from a cc&iparison of measure­ ments of o 2 and Cn2. The data derive from both optical and atmospheric measurements. The optical instruments include a NOAA scintillometer and a modified AFGL transmissometer. Both these instruments respond to changes or fluctuations in intensity at the receiver aperture. However, the measurement of these fluctuations is performed differently in the two instru­ ments. Results of the comparison of these two instruments will be presented for a 300 m path at Sudbury, Massachusetts. Further comparisons will also be made between the scintil­ lometer and several AFGL thermosondes located along the scintillometer optical path. The latter comparison is from data obtained over a 183 m path at Palm Bay, Florida. A descrip­ tion of the experimental systems used follows. The horizontal scintillometer was developed at the NOAA Wave Propagation Laboratory by Ochs et al . The transmitter is a collimated, incoherent LED light source oscillating at 7 kHz with a wavelength of 940 nm. At the receiver, the log of the intensity is band-passed from 1.2 to 1000 Hz and processed through an RMS module with a 4 sec averaging time to pro­ vide an estimate of o 2. The operation of the instrument is based on the application of first order scattering^theory the relationship of C 2 to o 2. This instrument is design­ ed to avoid saturation effects and to also maintain the' path-weighting function and calibration throughout its range. The path averaged C 2 is given by the relationship 2

Estimation of turbulence parameters fromtime-lapse imagery

Abstract: A time-lapse imaging method to estimate turbulence parameters such as path-weighted Cn2 and Fried’s coherence length r0 from the differential motion of features on a distant target is demonstrated here. The time-lapse measurements can even mimic the results of a scintillometer.