Showing papers on "Scintillometer published in 2009"

Estimation of urban sensible heat flux using a dense wireless network of observations

Abstract: The determination of the sensible heat flux over urban terrain is challenging due to irregular surface geometry and surface types. To address this, in 2006–07, a major field campaign (LUCE) took place at the Ecole Polytechnique Federale de Lausanne campus, a moderately occupied urban site. A distributed network of 92 wireless weather stations was combined with routine atmospheric profiling, offering high temporal and spatial resolution meteorological measurements. The objective of this study is to estimate the sensible heat flux over the built environment under convective conditions. Calculations were based on Monin–Obukhov similarity for temperature in the surface layer. The results illustrate a good agreement between the sensible heat flux inferred from the thermal roughness length approach and independent calibrated measurements from a scintillometer located inside the urban canopy. It also shows that using only one well-selected station can provide a good estimate of the sensible heat flux over the campus for convective conditions. Overall, this study illustrates how an extensive network of meteorological measurements can be a useful tool to estimate the sensible heat flux in complex urban environments.

New Mexico Scintillometer Network: Supporting Remote Sensing and Hydrologic and Meteorological Models

Abstract: In New Mexico, a first-of-its-kind network of seven large aperture scintillometer (LAS) sites was established in 2006 to measure sensible heat fluxes over irrigated fields, riparian areas, deserts, lava flows, and mountain highlands. Wireless networking infrastructure and auxiliary meteorological measurements facilitate real-time data assimilation. LAS measurements are advantageous in that they vastly exceed the footprint size of commonly used ground measurements of sensible and latent heat fluxes (~100 m2), matching the pixel size of satellite images or grid cells of hydrologic and meteorological models (~0.1–5 km2). Consequently, the LAS measurements can be used to validate, calibrate, and force hydrologic, remote sensing, and weather forecast models. Initial results are presented for 1) variability and error of sensible heat flux measurements by scintillometers over heterogeneous terrain and 2) the validation of the Surface Energy Balance Algorithm for Land (SEBAL) applied to Moderate Resolution Imagin...

Uncertainty analysis of computational methods for deriving sensible heat flux values from scintillometer measurements

Abstract: . The use of scintillometers to determine sensible heat fluxes is now common in studies of land-atmosphere interactions. The main interest in these instruments is due to their ability to quantify energy distributions at the landscape scale, as they can calculate sensible heat flux values over long distances, in contrast to Eddy Covariance systems. However, scintillometer data do not provide a direct measure of sensible heat flux, but require additional data, such as the Bowen ratio (β), to provide flux values. The Bowen ratio can either be measured using Eddy Covariance systems or derived from the energy balance closure. In this work, specific requirements for estimating energy fluxes using a scintillometer were analyzed, as well as the accuracy of two flux calculation methods. We first focused on the classical method (used in standard softwares) and we analysed the impact of the Bowen ratio on flux value and uncertainty. For instance, an averaged Bowen ratio (β) of less than 1 proved to be a significant source of measurement uncertainty. An alternative method, called the "β-closure method", for which the Bowen ratio measurement is not necessary, was also tested. In this case, it was observed that even for low β values, flux uncertainties were reduced and scintillometer data were well correlated with the Eddy Covariance results. Besides, both methods should tend to the same results, but the second one slightly underestimates H while β decreases (

Scintillometer Intercomparison Study—Continued

Abstract: An earlier study by one of the authors reported significant differences of up to 21% in linear regression slopes between six Kipp & Zonen large-aperture scintillometers. In this note, the consistency of this increasingly popular instrument for measuring sensible heat fluxes at the km scale was quantified by comparing measurements from four Scintec boundary-layer scintillometers and one large-aperture scintillometer over nearly identical transects. The Kipp & Zonen instrument’s sensible heat fluxes were more than 20% larger than those from the Scintec instruments, while the difference in regression slopes amongst the Scintec instruments was 3% or less.

Footprint issues in scintillometry over heterogeneous landscapes

Abstract: . Scintillometry is widely recognized as a potential tool for obtaining spatially aggregated sensible heat fluxes at regional scales. Although many investigations have been made over contrasting component surfaces, few aggregation schemes consider footprint contributions. In this paper, an approach is presented to infer average sensible heat flux over a very heterogeneous landscape by using a large aperture scintillometer. The methodology is demonstrated on simulated data and tested on a time series of measurements obtained during the SPARC2004 experiment in Barrax, Spain. Results show that the two-dimensional footprint approach yields more accurate results of aggregated sensible heat flux than traditional methods.

Combining a large aperture scintillometer and estimates of available energy to derive evapotranspiration over several agricultural fields in a semi-arid region

Abstract: The objective of the present study was to investigate the potential of a large aperture scintillometer (LAS) combined with a simple available energy model to estimate area-averaged latent heat flux in difficult environmental conditions. The difficulties are related to the sparseness of the vegetation, the heterogeneity of the soil characteristics, and, most importantly, the heterogeneity in terms of soil moisture induced by the “flood irrigation” method. In this context, three sites (Agdal, R3 and Sâada) in the Tensift Al Haouz plain (region of Marrakech city, central Morocco) have been equipped with a LAS and eddy covariance (EC) system (local scale measurements). Agdal and R3 are a flood-irrigated olive yard and wheat field, respectively. Sâada is a drip-irrigated orange orchard. Due to the irrigation method applied, the Agdal and R3 sites shifted from being almost homogeneous between two irrigations (dry conditions) and completely heterogeneous during the irrigation events (large variability o...

Scintillation index of optical wave propagating in turbulent atmosphere

Abstract: A concise expression of the scintillation index is proposed for a plane optical wave and a spherical optical wave both propagating in a turbulent atmosphere with a zero inner scale and a finite inner scale under an arbitrary fluctuation condition. The expression is based on both the results in the Rytov approximation under a weak fluctuation condition and the numerical results in a strong fluctuation regime. The maximum value of the scintillation index and its corresponding Rytov index are evaluated. These quantities are affected by the ratio of the turbulence inner scale to the Fresnel size.

Long-Path Scintillometry over Complex Terrain to Determine Areal-Averaged Sensible and Latent Heat Fluxes

Abstract: Landscape scale measurements (1 - 10 km2) of sensible and latent heat fluxes over heterogeneous areas are required for hydrological and meteorological modelling. Evaporation is strongly dependent on land surface properties, thus aggregating field-scale measurements has much uncertainty because of the need for detailed land-cover maps and the possible disproportionate contribution of vegetation transition zones; in any case, such a collection of field-scale instruments is a highly resource intensive approach. A potentially better alternative is reported here: the long-path (large aperture) scintillometer (LAS) has been used over topographically complex chalk downland with mixed vegetation, to measure the sensible heat fluxes at the landscape scale, using a 2.4 km pathlength. These sensible heat fluxes agreed well with aggregated eddy covariance measurements made at the field scale for different vegetation types – the contrasting range of sensible heat fluxes in the late summer over various agricultural fields in southern England is reported. The LAS in combination with a new custom-built millimetre-wave (94 GHz) scintillometer (MWS) was trialled to measure large-scale area averaged latent and sensible heat fluxes, using the two-wavelength method of scintillometry, over the same complex terrain. The LAS-MWS fluxes were found to close the energy balance well, except during periods of either high windspeed or very low cross-wind conditions. The latter conditions may lead to inappropriate filtering or reach a fundamental limitation of the method. The over-estimation of flux at high windspeeds may be due to limitations of the Monin-Obukhov similarity theory which was developed for homogeneous surfaces. The difference in the effective heights of the two scintillometers for the measurement of their respective fluxes is recognised, and the formula for these heights is derived. Application over certain complex topography shows that an increased measurement height would lead to bettermatched source areas for the LAS and MWS.

Time To Think: Reflections of a Pre-Pensioned Scintillometer Researcher

Abstract: In this essay, Henk de Bruin shares his experiences in fostering research in the field of scintillometry, the applications of the technique to various topics in meteorology, as well as details of the way these flux measurements are obtained.

Influence of the source spectrum on the optical measurements of turbulence.

Abstract: It is considered how the source spectrum influences the measurement accuracy of optical wave arrival angles, as well as the estimation of the path-averaged structure parameter of the refractive index fluctuations. Two reasons that can cause the wavelength dependence of the variance of fluctuations of wave arrival angles are analyzed. The first one is connected with the fact that phases depend on a wavelength in the approximation of smooth perturbations. The second reason is associated with the wavelength dependence of the refractive index and, consequently, its fluctuations. Strict equations are obtained to take into account the influence of the source spectrum on the measurement accuracy of the variance of arrival angle fluctuations and, indirectly, on the estimation accuracy of the path-averaged refractive index structure parameter. It can be stated that for most radiation sources (even nonmonochromatic) the influence of the source spectral composition can be neglected.

Area-averaged Evapotranspiration Fluxes Measured from Large Aperture Scintillometer in the Hai River Basin

Abstract: Compared with traditional observation techniques, Large Aperture Scintillometer (LAS) can measure area-averaged evapotranspiration (ET) at a larger scale (500m-10Km), for area-averaged evapotranspiration and water resources management, LAS has become more and more popular in recent years. Through the improvement of data processing scheme of LAS, the area-averaged ET was calculated from observation data collected from Miyun and Guantao site in Hai Basin from Jan to July, 2008. The results show the ET in different surface has obvious diurnal and seasonal variations. At the same time, the comparison between ET and net radiation, precipitation as well as the height of ground vegetation was analyzed, the results showed ET was affected by the three factors. The total amount of ET from Jan to July is 260mm and 293mm in Miyun and Guantao site respectively. In the end, from the discussion of the difference between sensible heat flux measured from LAS and Eddy Covariance System(EC), it can be concluded the difference between Hlas and Hec were caused by the landuse of the surface, the overlap degree of the LAS and EC footprint, as well as energy closure ratio of EC together.

Effects of light propagation in middle intensity atmospheric turbulence

Abstract: The purpose of this report is to present an experimental study of the effects of light propagation through atmospheric turbulence Free space optical communication is a line-of-sight technology that transmits a modulated beam of visible light through the atmosphere for broadband communication The fundamental limitations of free space optical communications arise from the environment through which it propagates However these systems are vulnerable to atmospheric turbulence, such as attenuation and scintillation Scintillation is due to the air index variation under the temperature effects These factors cause an attenuated receiver signal and lead to higher bit error rate (BER) An experiment of laser propagation was carried out to characterize the light intensity through turbulent air in the laboratory environment The experimental results agree with the calculation based on Rytov for the case of weak to intermediate turbulence Also, we show the characteristics of irradiance scintillation, intensity distribution and atmospheric turbulence strength By means of laboratory simulated turbulence, the turbulence box is constructed with the following measurements: 05 m wide, 2 m long and 05 m high The simulation box consists of three electric heaters and is well described for understanding the experimental set up The fans and heaters are used to increase the homogeneity of turbulence and to create different scintillation indices The received intensity scintillation and atmosphere turbulence strength were obtained and the variation of refractive index, with its corresponding structure parameter, is calculated from the experimental results

The Characteristics of Heat and Water Vapor Fluxes over Different Surfaces in the Heihe River Basin

Abstract: Several flux stations at difference sufaces have been established for the project "Simultaneous remote sensing and ground based experiment in the Heihe River basin".In this paper,we selected the observation data on grasslands,forests and farmland and analyzed the characteristics of water vapor,heat and carbon dioxide(CO2) fluxes.The results showed that there are obvious diurnal and seasonal variations of energy balance components(net radiation,sensible heat,latent heat and soil heat flux,etc.) and different surface in the Heihe River basin has distinct characteristics.The Eddy-Covariance(EC) flux data have been compared with Large Aperture Scintillometer(LAS) observations at Arou Station(in the upper Heihe Basin) and Linze Station(in the middle basin).Both have good correlations,however,the sensible heat fluxes observed by LAS are generally larger than that of EC.The differences are mainly from the different footprints′ or source areas of LAS and EC in the non-uniform surfaces of the specific station.Besides,LAS observation of heat flux may comprise contributions from larger scale atmospheric turbulence.By considering soil heat storage in the upper soil layers,the surface energy balance closure has also been analyzed in each station.

Analysis of rapid Cn2 fluctuations observed during a 5-km communication link experiment

Abstract: The path-integrated turbulence strength is usually thought of as a parameter that varies slowly with time. In a recent free-space communications experiment the Cn2 n value over a 5-km horizontal path was monitored almost continuously for a period of nearly a month. In addition to well defined and repeatable diurnal fluctuations, strong short-term fluctuations were observed in which the turbulence strength changed by an order of magnitude within a period of minutes. These rapid changes were independently measured by a commercial scintillometer and the high-rate output from the communications transceiver. The characteristics and probable causes for these dynamic atmospheric events and their impact on the design of free-space communication systems are discussed in this article.

Investigation of Spatial Representativeness for Surface Flux Measurements in the Heihe River Basin

Abstract: It is very important to determine the land surface fluxes,including sensible heat fluxes and latent heat fluxes,for the issues involving many aspects of climate,agriculture,and water resources management.Nowadays,the Eddy Covariance system(EC) and the Large Aperture Scintillometer(LAS) are the main instruments measuring surface fluxes.The former is point observation and can observe hundreds of meters and the latter can observe area average fluxes in the scale of hundreds of meters to ten kilometers.In order to analyze the source area,we develop Eulerian analytical footprint models for the EC and LAS.Detailed descriptions of the theory basis and methods of the two models are given,and their differences are discussed.Simultaneous remote sensing and ground-based experiment was being carried out in the Heihe River Basin,its observing major components of water cycle were in three experiment areas,i.e.,cold region,forest,and arid region hydrology experiment areas.This paper shows some outcomes of the flux footprint models application in A′Rou freeze/thaw observation station,Yingke irrigated oasis station,Dayekou Guantan forest station and Linze grassland station.The representatinve one day or one month is selected.The analysis of the source area and spatial representativeness of the EC and LAS is given.The difference of observing flux is discussed.In the end,it reveals that reasonable source area of LAS and EC can be derived from their flux footprint models and the footprint model is an effective and practical tool.

Investigation of Spatial Representativeness for Surface Flux Measurements with Eddy Covariance System and Large Aperture Scintillometer

Abstract: Based on the analysis of sensitivity of analytical footprint models of an eddy covariance system(EC) and a Large Aperture Scintillometer(LAS),as well as the observation of EC and LAS at Miyun flux station of Beijing(from August 2006 to December 2007),the flux spatial representativeness of EC and LAS were preliminarily analyzed.The results are as follows:(1)The source areas of EC and LAS are sensitive to wind direction,Monin-Obukhov length,aerodynamic roughness length and measurement height.(2)The source areas of the eddy covariance system distribute in the southwest and south area during the daytime,while in the northeast and north area during the nighttime.The source areas of the LAS are in the direction from southeast to northwest.The source areas of the eddy covariance system are different in each month,but all locate in an area of 1000 m(S_N) by 850 m(W_E) area.The source areas of the LAS are in the direction from southeast to northwest in each month,about 2060 m long and 620 m wide.(3)The main types of land use to determine the measured fluxes of the eddy covariance system are orchard(67%) and farmland(19%).The flux contribution of orchard is larger in summer and autumn,but smaller in winter and spring.And it is contrary for farmland.The primary land use types of flux contribution for LAS are orchard(49%),farmland(28%) and residential area(11%).The seasonal variation of flux contribution of orchard and farmland is the same as the eddy covariance system,but it is not obvious.

Experiment to characterize optical turbulence along a 2.33 km free-space laser path via differential image motion measurements

Abstract: In a previous experiment (Tunick, 2008: Optics Express 16, 14645-14654), values for the refractive index structure constant and the Fried parameter were calculated from measurements of signal intensity and angle-of-arrival statistics based on idealized models. Calculated turbulence parameters were evaluated in comparison to scintillometer-based measurements for several cases. It was found that the idealized models alone were insufficient to accurately describe complex, non-uniform microclimate and turbulence conditions. In addition, the signal intensity and focal spot displacement measurements were quite sensitive to platform and light source jitter. In order to compensate for adverse effects such as platform vibrations, an alternative differential image motion method is explored for optical turbulence parameter characterization. Hence, further experimental research is conducted along a 2.33 km free-space laser path to capture differential image centroid data from which Fried parameter and refractive index structure constant information can be obtained. This research is intended to provide useful information for US Army laser communications, long-range imaging and energy-on-target.

Height distribution profiles and its application of atmosphere refractive index

Abstract: When beam wave propagates through the non-homogenous atmosphere,the beams are refracted,optical path is increased and the track is curved,hence the measuring error is caused by angular position and distance of tested target.Some main experimental datum of the atmosphere parameters were obtained in desert of Xinjiang.According to the measuring data,the atmospheric refractive index characteristics at various heights were summarized,and the measuring profiles of the atmospheric refractive index were given.Exponential distributing mode and three-parameters-γ distributing mode of the atmospheric refractive index were given by mode fitting.Influence of factors such as pressure,temperature and wavelength on the atmospheric refractive index were analyzed.Based on the modeling profiles of the atmospheric refractive index,the effect of atmosphere refractive of optical propogation was analyzed in theory,the refractive angles and dispersion angles were calculated as beams propagated through the atmosphere at different zenith on typical wavelength.Calculation results provide reference for correction of the refractive effects on beams propagation.

Wandering of laser beams: a useful tool for local atmospheric investigations

Abstract: Wandering of a laser beam in the atmosphere is due to fluctuations of the refractive index of the atmosphere and, having a strong dependence on the path length, is very useful to investigate random or continuous changes of the refractive index during time. First, we describe methods we developed and applied to locally investigate the parameters of turbulence (inner scale, outer scale and structure constant) based on our previous theory of propagation of "thin" beams. Then we describe use of thin beams to investigate the evolution of the refractive index gradient and show experimental results including non stationary and non isotropic conditions.

Team 4: Problems Associated with Remotely Sensing Wind Speed

Abstract: Atmospheric turbulence combined with wind causes optical scintillations. By measuring this optical scintillation we can gain insight into our turbulent atmosphere. The main objective of this research area is to measure the average wind speed along the path of a laser beam and also to determine the refractive index structure parameter C 2 n. We propose Brownian model for the refractive index structure parameter C 2 n . Also a perturbation analysis is described for refractive index structure parameter C 2 and the velocity v(z). Next we compare the cross covariance for spherical and Gaussian waves.

Laser guide star scintillometer (LAGUSS) for profiling atmospheric turbulence

Abstract: High-resolution Cn2 profiling is essential for characterizing atmospheric turbulence, especially when correction of the laser beam propagation. This is can be done by using distributed light sources. One way of doing so can be achieved by using the Rayleigh or Mie laser guide star as a light source. In this case the time-resolved characteristics of the backward scattered pulse can be used to characterize turbulence continuously along the measured range. This report discusses the operational principles of the laser guide-star scintillometer (LAGUSS) and analyses the results of experimental data for profiling the Cn2 as a function of the range.