Showing papers on "Scintillometer published in 2011"

A comparison of eddy-covariance and large aperture scintillometer measurements with respect to the energy balance closure problem

Abstract: . We analyzed the seasonal variations of energy balance components over three different surfaces: irrigated cropland (Yingke, YK), alpine meadow (A'rou, AR), and spruce forest (Guantan, GT). The energy balance components were measured using eddy covariance (EC) systems and a large aperture scintillometer (LAS) in the Heihe River Basin, China, in 2008 and 2009. We also determined the source areas of the EC and LAS measurements with a footprint model for each site and discussed the differences between the sensible heat fluxes measured with EC and LAS at AR. The results show that the main EC source areas were within a radius of 250 m at all of the sites. The main source area for the LAS (with a path length of 2390 m) stretched along a path line approximately 2000 m long and 700 m wide. The surface characteristics in the source areas changed with the season at each site, and there were characteristic seasonal variations in the energy balance components at all of the sites. The sensible heat flux was the main term of the energy budget during the dormant season. During the growing season, however, the latent heat flux dominated the energy budget, and an obvious "oasis effect" was observed at YK. The sensible heat fluxes measured by LAS at AR were larger than those measured by EC at the same site. This difference seems to be caused by the so-called energy imbalance phenomenon, the heterogeneity of the underlying surfaces, and the difference between the source areas of the LAS and EC measurements.

Estimation of catchment averaged sensible heat fluxes using a large aperture scintillometer

Abstract: [1] Evapotranspiration rates at the catchment scale are very difficult to quantify. One possible manner to continuously observe this variable could be the estimation of sensible heat fluxes (H) across large distances (in the order of kilometers) using a large aperture scintillometer (LAS), and inverting these observations into evapotranspiration rates, under the assumption that the LAS observations are representative for the entire catchment. The objective of this paper is to assess whether measured sensible heat fluxes from a LAS over a long distance (9.5 km) can be assumed to be valid for a 102.3 km2 heterogeneous catchment. Therefore, a fully process-based water and energy balance model with a spatial resolution of 50 m has been thoroughly calibrated and validated for the Bellebeek catchment in Belgium. A footprint analysis has been performed. In general, the sensible heat fluxes from the LAS compared well with the modeled sensible heat fluxes within the footprint. Moreover, as the modeled H within the footprint has been found to be almost equal to the modeled catchment averaged H, it can be concluded that the scintillometer measurements over a distance of 9.5 km and an effective height of 68 m are representative for the entire catchment.

Confronting the WRF and RAMS mesoscale models with innovative observations in the Netherlands: Evaluating the boundary layer heat budget

Abstract: The Weather Research and Forecasting Model (WRF) and the Regional Atmospheric Mesoscale Model System (RAMS) are frequently used for (regional) weather, climate and air quality studies. This paper covers an evaluation of these models for a windy and calm episode against Cabauw tower observations (Netherlands), with a special focus on the representation of the physical processes in the atmospheric boundary layer (ABL). In addition, area averaged sensible heat flux observations by scintillometry are utilized which enables evaluation of grid scale model fluxes and flux observations at the same horizontal scale. Also, novel ABL height observations by ceilometry and of the near surface longwave radiation divergence are utilized. It appears that WRF in its basic set-up shows satisfactory model results for nearly all atmospheric near surface variables compared to field observations, while RAMS needed refining of its ABL scheme. An important inconsistency was found regarding the ABL daytime heat budget: Both model versions are only able to correctly forecast the ABL thermodynamic structure when the modeled surface sensible heat flux is much larger than both the eddy-covariance and scintillometer observations indicate. In order to clarify this discrepancy, model results for each term of the heat budget equation is evaluated against field observations. Sensitivity studies and evaluation of radiative tendencies and entrainment reveal that possible errors in these variables cannot explain the overestimation of the sensible heat flux within the current model infrastructure.

Estimation of evaporation and sensible heat flux from open water using a large‐aperture scintillometer

Abstract: [1] The use of scintillometers to determine sensible and latent heat flux is becoming increasingly common because of their ability to quantify convective fluxes over distances of hundreds of meters to several kilometers. The majority of investigations using scintillometry have focused on processes above land surfaces, but here we propose a new methodology for obtaining sensible and latent heat fluxes from a scintillometer deployed over open water. This methodology has been tested by comparison with eddy covariance measurements and through comparison with alternative scintillometer calculation approaches that are commonly used in the literature. The methodology is based on linearization of the Bowen ratio, which is a common assumption in models such as Penman's model and its derivatives. Comparison of latent heat flux estimates from the eddy covariance system and the scintillometer showed excellent agreement across a range of weather conditions and flux rates, giving a high level of confidence in scintillometry-derived latent heat fluxes. The proposed approach produced better estimates than other scintillometry calculation methods because of the reliance of alternative methods on measurements of water temperature or water body heat storage, which are both notoriously hard to quantify. The proposed methodology requires less instrumentation than alternative scintillometer calculation approaches, and the spatial scales of required measurements are arguably more compatible. In addition to scintillometer measurements of the structure parameter of the refractive index of air, the only measurements required are atmospheric pressure, air temperature, humidity, and wind speed at one height over the water body.

Turbulent mixing theory applied to radio scattering

Abstract: Obukhoff's statistical theory of turbulent mixing is proposed as a replacement for the heuristic theories of Gallet and Villars‐Weisskopf, and is applied to the problem of the scattering of radio waves by refractive index fluctuations. In the case of ionospheric scattering, order‐of‐magnitude agreement with the observed scattered power is obtained if the refractive index fluctuations are attributed to electron density fluctuations produced by turbulent mixing in the lower edge of the E layer. In the case of tropospheric scattering, it appears that order‐of‐magnitude agreement with the observed scattered power can be obtained, except during the summer months, by attributing the refractive index fluctuations to temperature fluctuations. During the summer months and at low scattering heights, humidity and its fluctuations are expected to play a prominent role. Experimental and theoretical evidence is cited in favor of perennial fractional‐degree temperature fluctuations in the troposphere. A comparison of th...

High-precision diode-laser-based temperature measurement for air refractive index compensation

Abstract: We present a laser-based system to measure the refractive index of air over a long path length. In optical distance measurements, it is essential to know the refractive index of air with high accuracy. Commonly, the refractive index of air is calculated from the properties of the ambient air using either Ciddor or Edlen equations, where the dominant uncertainty component is in most cases the air temperature. The method developed in this work utilizes direct absorption spectroscopy of oxygen to measure the average temperature of air and of water vapor to measure relative humidity. The method allows measurement of temperature and humidity over the same beam path as in optical distance measurement, providing spatially well-matching data. Indoor and outdoor measurements demonstrate the effectiveness of the method. In particular, we demonstrate an effective compensation of the refractive index of air in an interferometric length measurement at a time-variant and spatially nonhomogeneous temperature over a long time period. Further, we were able to demonstrate 7 mK RMS noise over a 67 m path length using a 120 s sample time. To our knowledge, this is the best temperature precision reported for a spectroscopic temperature measurement.

Stability and height dependant variations of the structure function parameters in the lower atmospheric boundary layer investigated from measurements of the long-term experiment VERTURM (vertical turbulence measurements)

Abstract: Operation and design of electro-optical systems are affected by atmospheric optical turbulence quantified by the refractive index parameter Cn2 Regarding wave propagation in the visible and infrared (IR), Cn2 is a function of height, dependant on temperature, pressure, and the structure temperature function parameter Cn2 The long-term experiment VerTurM (vertical turbulence measurements) was designed to characterize the vertical variations of optical turbulence up to 250 m in the lower atmospheric boundary layer for a moderate typical central European climate Since May 2009 three independent measurement systems have been operated in a flat pasture site in north-western Germany In the atmospheric surface layer at a tall tower sonic anemometer measurements are performed on four discrete heights between 4 and 64 m providing information about atmospheric stability and turbulence Cn2 is derived From 30 to 250 m a SODAR-RASS system (Sound Detection and Ranging - Radio acoustic sounding system) yields every half an hour profiles of Cn2 Additional direct measurements of Cn2 have been performed near the ground using a scintillometer First results of the three measurement systems are presented Vertical profiles and stability dependence are analysed in respect of Monin- Obukhov-similarity theory (MOST) Differences in the measurement systems and the expected height variations are discussed

The Effective Height of a Two-Wavelength Scintillometer System

Abstract: A scaling factor, S, is derived to account for the difference in path-weighted measurement heights of a combined system consisting of a large-aperture scintillometer (LAS) and a millimetre-wave scintillometer (MWS), operating at wavelengths of 0.88 μm and about 3 mm respectively, and designed to determine the area-averaged latent heat flux. This work extends an earlier derivation of Zef, the effective height for the LAS yielding the area-averaged sensible heat flux. The LAS and MWS have different path-weighting functions, therefore, in general, it is expected that the path-weighted beam heights are different, for paths other than with the beam parallel to the land surface. Their difference will depend on the detail of the experimental set-up. The objective is to introduce a scaling factor that transforms the MWS refractive-index structure parameter measurement to the same path-weighted height of the LAS measurement. By applying S as a prelude to the calculation of the temperature and humidity structure parameters, and the sensible and latent heat fluxes, these quantities are made representative of the same measurement height, thus simplifying the application of two-wavelength scintillometry. The equations presented here enable the analysis of LAS-MWS paths such that they can be selected to optimize S towards its ideal value of unity. For this purpose we provide a new analytical approximation of the LAS path-weighting function. The importance of accounting for S is demonstrated by example applications over varying topography as well as for slanted beams.

Path-Average Rainfall Estimation from Optical Extinction Measurements Using a Large-Aperture Scintillometer

Abstract: The potential of a near-infrared large-aperture boundary layer scintillometer as path-average rain gauge is investigated. The instrument was installed over a 2.4-km path in Benin as part of the African Monsoon Multidisciplinary Analysis (AMMA) Enhanced Observation Period during 2006 and 2007. Measurements of the one-minute-average received signal intensity were collected for 6 rainfall events during the dry season and 16 events during the rainy season. Using estimates of the signal base level just before the onset of the rainfall events, the optical extinction coefficient is estimated from the path-integrated attenuation for each minute. The corresponding path-average rain rates are computed using a power-law relation between the optical extinction coefficient and rain rate obtained from measurements of raindrop size distributions with an optical spectropluviometer and a scaling-law formalism for describing raindrop size distribution variations. Comparisons of five-minute rainfall estimates with measurements from two nearby rain gauges show that the temporal dynamics are generally captured well by the scintillometer. However, the instrument has a tendency to underestimate rain rates and event total rain amounts with respect to the gauges. It is shown that this underestimation can be explained partly by systematic differences between the actual and the employed mean power-law relation between rain rate and specific attenuation, partly by unresolved spatial and temporal rainfall variations along the scintillometer path. Occasionally, the signal may even be lost completely. It is demonstrated that if these effects are properly accounted for by employing appropriate relations between rain rate and specific attenuation and by adapting the pathlength to the local rainfall climatology, scintillometer-based rainfall estimates can be within 20% of those estimated using rain gauges. These results demonstrate the potential of large-aperture scintillometers to estimate path-average rain rates at hydrologically relevant scales.

Validating remotely sensed land surface fluxes in heterogeneous terrain with large aperture scintillometry

Abstract: The Large Aperture Scintillometer (LAS) has emerged as one of the best tools for quantifying areal averaged fluxes over heterogeneous land surfaces. This is particularly useful as a validation of surface energy fluxes derived from satellite sources. We examine how changes in surface source area contributing to the scintillometer and eddy covariance measurements relate to satellite derived estimates of sensible heat flux. Field data were collected on the Konza Prairie in Northeastern Kansas, included data from two eddy covariance towers: one located on an upland, relatively flat homogeneous area, and the second located in a lowland area with generally higher biomass and moisture conditions. The large aperture scintillometer spanned both the upland and lowland areas and operated with a path length of approximately 1 km specifically to compare to Moderate Resolution Imaging Spectroradiometer (MODIS) derived estimates of surface fluxes. The upland station compares well with the LAS (correlation of 0.96), with...

Observation and analysis of aero-optic effects on the ORCA laser communication system

Abstract: In this paper we show evidence of aero-optic effects on the measured beacon beam as the gimbal angle of a nosemounted turret changes from 0 to 90 degrees and greater with respect to the line of flight. Data from the beacon beam was collected with a new technology 3-aperture scintillometer over a 82km to 104km air-to-ground downlink during field testing of the ORCA system in Nevada in May 2009. In this paper we present data analysis on the impact of an aero-optic boundary layer on a laser link between an aircraft and a ground-based stationary node. Particularly we look at the impact of an aero-optic boundary layer on the mean, variance, scintillation, probability density function (PDF), power spectral density (PSD), and fading of the received irradiance. We find that the most compelling argument for the presence of strong aero-optic effects comes from calculating the PSD of the received beacon intensity. We also find the cumulative effect of the aero-optic boundary layer differs depending on the transmitted beam parameters, i.e. collimated or divergent.

Analysis of the Systematic Errors Found in the Kipp & Zonen Large-Aperture Scintillometer

Abstract: Studies have shown a systematic error in the Kipp & Zonen large-aperture scintillometer (K&ZLAS) measurements of the sensible heat flux, H. We improved on these studies and compared four K&ZLASs with a Wageningen large-aperture scintillometer at the Chilbolton Observatory. The scintillometers were installed such that their footprints were the same and independent flux measurements were made along the measurement path. This allowed us to compare H and the direct scintillometer output, the refractive index structure parameter, \({C_{n}^{2}}\) . Furthermore, spectral analysis was performed on the raw scintillometer signal to investigate the characteristics of the error. Firstly, correlation coefficients ≥ 0.99 confirm the robustness of the scintillometer method, and secondly we discovered two systematic errors: the low-\({C_{n}^{2}}\) error and the high-\({C_{n}^{2}}\) error. The low-\({C_{n}^{2}}\) error is a non-linear error that is caused by high-frequency noise, and we suspect the error to be caused by the calibration circuit in the receiver. It varies between each K&ZLAS, is significant for H ≤ 50 W m−2, and we propose a solution to remove this error using the demodulated signal. The high-\({C_{n}^{2}}\) error identified by us is the systematic error found in previous studies. We suspect this error to be caused by poor focal alignment of the receiver detector and the transmitter light-emitting diode that causes ineffective use of the Fresnel lens in the current Kipp & Zonen design. It varies between each K&ZLAS (35% up to 240%) and can only be removed by comparing with a reference scintillometer in the field.

A New Triangulation-Like Technique for the Evaluation of the Refractive Index Structure Constant $(C_{n}^{2})$ in Free-Space Optical Links

Abstract: A light beam propagating through the atmosphere has its amplitude, phase, and propagation direction changed along the optical path due to atmospheric turbulence. This effect is caused by the randomness of the air refractive index and depends on the local temperature, pressure, and humidity. The physical parameter that quantifies the turbulence strength is well known in the literature as the refractive index structure constant (Cn2). In this paper, a simple and low-cost technique based on a triangulation-like method to measure the Cn2 is presented for the first time to the best knowledge of the authors. Through a novel device comprised of three photodetectors in a triangle-shaped array placed on the receiver plane, it is possible, after power measurements and applying a developed mathematical model, to obtain precisely the position of the beam spot. Therefore, calculates the Cn2 and evaluates the atmospheric turbulence strength. Furthermore, the technique is useful to check the suitability of a free-space optical (FSO) link in a specific geographic region by monitoring the local turbulence. Also, the device will be useful to improve the deployment and maintenance of FSO systems.

Effects of non-uniform crosswind fields on scintillometry measurements

Abstract: The effects of a non-uniform wind field along the path of a scintillometer are investigated. Theoretical spectra are calculated for a range of scenarios where the crosswind varies in space or time and compared to the ‘ideal’ spectrum based on a constant uniform crosswind. It is verified that the refractive-index structure parameter relation with the scintillometer signal remains valid and invariant for both spatially and temporally-varying crosswinds. However, the spectral shape may change significantly preventing accurate estimation of the crosswind speed from the peak of the frequency spectrum and retrieval of the structure parameter from the plateau of the power spectrum. On comparison with experimental data, non-uniform crosswind conditions could be responsible for previously unexplained features sometimes seen in observed spectra. By accounting for the distribution of crosswind, theoretical spectra can be generated that closely replicate the observations, leading to a better understanding of the measurements. Spatial variability of wind speeds should be expected for paths other than those that are parallel to the surface and over flat, homogenous areas, whilst fluctuations in time are important for all sites.

Turbulence effects in a horizontal propagation path close to ground: implications for optics detection

Abstract: Atmospheric turbulence effects close to ground may affect the performance of laser based systems severely. The variations in the refractive index along the propagation path cause effects such as beam wander, intensity fluctuations (scintillations) and beam broadening. Typical geometries of interest for optics detection include nearly horizontal propagation paths close to the ground and up to kilometre distance to the target. The scintillations and beam wander affect the performance in terms of detection probability and false alarm rate. Of interest is to study the influence of turbulence in optics detection applications. In a field trial atmospheric turbulence effects along a 1 kilometre horizontal propagation path were studied using a diode laser with a rectangular beam profile operating at 0.8 micrometer wavelength. Single-path beam characteristics were registered and analysed using photodetectors arranged in horizontal and vertical directions. The turbulence strength along the path was determined using a scintillometer and single-point ultrasonic anemometers. Strong scintillation effects were observed as a function of the turbulence strength and amplitude characteristics were fitted to model distributions. In addition to the single-path analysis double-path measurements were carried out on different targets. Experimental results are compared with existing theoretical turbulence laser beam propagation models. The results show that influence from scintillations needs to be considered when predicting performance in optics detection applications.

Zero-Plane Displacement Height in a Highly Built-Up Area of Tokyo

Abstract: The zero-plane displacement height d was evaluated in downtown Tokyo by the two independent methods of temperature variance and scintillometer heat flux. Regardless of the method, d exceeds the area-weighted average building height. This may be because d, which represents the point of effect for wind drag, is elevated by some tall buildings that jut above others. The areaweighted average building height would then be unsuitable as a geometrical index of urban canopies with large height variations. Thus, the height of the upper envelope of the canopy was proposed as the representative canyon height.

Estimating Evapotranspiration using Improved Fractional Vegetation Cover and Land Surface Temperature Space

Abstract: Vegetation index—land surface temperature (VI—Ts ) space has been widely used to estimate evapotranspiration and soil moisture. The limitation of this method is the uncertainty of the observed dry edge, which is usually fitted by scatter plots. Here, a method was used to locate true dry and wet edges based on energy balance formulation, and a simple method to estimate surface energy flux is proposed based on the improved Fractional vegetation cover—Land surface temperature (Fv —Ts ) space. Seventeen days of MODIS products were selected to estimate evapotranspiration and the estimated sensible heat flux (H) is compared with Large Aperture Scintillometer (LAS) data at a site in Zhengzhou. resulting in a RMSE of 44.06 W m-2, bias of 36.99 W m-2 and R 2 of 0.71. The H scatter plots of estimation versus observation show clearly that most points are around the 1:1 line. Overall, the located true and wet edges are more accurate than the observed true edge. Our results can also be applied to improve the...

Method and apparatus for determining gas flux

Abstract: Disclosed embodiments of the present invention provide means to obtain correct gas density and flux measurements using (i) gas analyzer (open-path, or closed-path gas analyzers with short intake tube, for example 1 m long, or any combination of the two); (ii) fast temperature or sensible heat flux measurement device (such as, fine-wire thermocouple, sonic anemometer, or any other device providing fast accurate gas temperature measurements); (iii) fast air water content or latent heat flux measurement device (such as, hygrometer, NDIR analyzer, any other device providing fast accurate gas water content measurements); (iv) vertical wind or sampling device (such as sonic anemometer, scintillometer, or fast solenoid valve, etc.) and (v) algorithms in accordance with the present invention to compute the corrected gas flux, compensated for T-P effects. In case when water factor in T-P effects is negligible, the fast air water content or latent heat flux measurement device (item iii in last paragraph) can be excluded.

Measurement of atmospheric refractive index structure constant near ground based on CAN bus

Abstract: The principle of measuring atmospheric refractive index structure constant was introduced, and the specialities of the CAN bus were described. A system based on CAN bus was developed to measure the atmospheric refractive index structure constant in the open air and transmit the multi-parameter data from micro-thermal probe and weather transmitter for over 30m far through a 4-wire cable. Continuous measurements for weeks have been taken, and daystart and sunset, the two weakest periods of the atmospheric turbulence, have been found on the experiment spot, and the regular pattern of the terbulence at different heights has been achieved. These results supply necessary information for the laser propagation experiment near earth surface and show that the system is practical and effective for the multi-parameter measurements and transmissions in the field.

Slant path 1.5 μm range gated imaging close to ground

Abstract: This paper will report experiments, analysis and simulations of slant path imaging using 1.5 μm gated imaging. The measurements were taking place at a former airfield along a 2 km path. The sensor was elevated by a lift in steps from 2-12.5 meters. Targets were resolution charts. The turbulence was measured along the path with a scintillometer. Turbulence information was also obtained at various path positions including the elevated cage using anemometers. The camera was collecting both passive and active images in the SWIR region. In the passive mode (using solar illumination) the noise due to speckles are eliminated and the influence by scintillation limited. In the active mode on the other hand these noise sources are present to a varying degree depending on stabilized frame averaging and on the sensor elevation. A trend is that the image quality is improved for elevated sensor positions. Two light sources in the camera FOV (head lights from a car) gave independent turbulence level estimates. The paper will present evaluated images for both passive and active modes obtained at different elevations and the result will be compared with theory including image simulation.

First results from optical turbulence measurements at Cerro Las Campanas in 2010

Abstract: We report preliminary results from optical turbulence measurements carried out in 2010 at Cerro Las Campanas, the future site for the Giant Magellan Telescope (GMT). The instruments involved are MooSci, a lunar scintillometer for the near-ground optical turbulence profile, Differential Image Motion Monitor (DIMM) for the whole atmosphere total seeing, and MASS Multiple Aperture Scintillation Sensor (MASS) for high-altitude optical turbulence estimation. The main purpose of these measurements is to anticipate the optical turbulence strength above the future GMT enclosure, and to provide a means to model the future adaptive optics performance. We also discuss the significance of such a combination of instruments and some hypothetical limitations.

Full range determination of 222 rn at the watershed scale by liquid

Abstract: Rn has been increasingly used to identify groundwater contribution to surface water. Particular attention has been paid to analytical protocols and counting parameters used for liquid alpha scintillation measurements over a range of activities covering river and groundwater domains. Direct measurements and Rn-extraction protocols are optimized, and scintillometer efficiency is calibrated using international standards over the 0.5-35 Bq/L range. The interval of activities was performed in surface water and groundwater from a small Canadian watershed.

Comparison of the sensible heat flux measurements on different spatial scales in an irrigated cropland

Abstract: This study examined the relationship between LAS (Large Aperture Scintillometer) and EC (Eddy Covariance technique) under different crop conditions in term of the sensible heat flux using the data collected from a typical cropland in the Weishan Irrigation District,Liaocheng city,Shandong province.The results showed that the measurements of LAS and EC are in good agreement in this cropland.But the estimated sensible heat flux by LAS is slightly higher than that by EC,which might be due to either the disagreement in their heterogeneous source areas or the underestimation of EC measurement.LAS could provide a reliable way of measuring area-average sensible heat flux

Scintillation measurements over False Bay, South Africa

Abstract: A commercial long-range scintillometer was deployed over a 2-km path in False Bay, South Africa, for a timeframe of one year. The turbulence data retrieved from the instrument are compared to turbulence parameters inferred from micrometeorological data and models, and the relation between experimental and model-data is explored.

Monitoring the flux of carbon dioxide gas with tunable diode laser absorption spectroscopy

Abstract: The greenhouse effect exacerbated by the increase of Carbon-containing gases is the more important causes of the climate change, It is very meaningful to the large-scale flux of carbon dioxide detection for the estimate the contributions of the main greenhouse gases in the atmosphere of various errestrial eco-systems. Tunable diode laser absorption spectroscopy (TDLAS) is a highly sensitive, highly selective and fast time response trace gas detection technique. In the present paper, the authors used a DFB laser was used as the light source, and by employing wavelength modulation method, and measuring the second harmonic signal of one absorption line near 1.573 microm of carbon dioxide molecule, the authors built a system for online monitoring of carbon dioxide concentration within the optical path of more than 700 meters at different heights. Combined with Alonzo Mourning -Obukhov length and characteristic velocity detected by large aperture scintillometer, the flux of carbon dioxide gas within one day calculated by the formula is within--1.5-2.5, breaking through the phenomenon of only providing the flux of trace gases near the ground at present, makking the measurement of trace gas fluxes within a large area possible.

Comparative advantages of Large Aperture Scintillometer and Eddy Covariance instrument for measuring evapotranspiration in irrigated farmlands

Abstract: Evapotranspiration(ET),a critical element of surface energy balance,plays an important role in the exchange of energy,mass and momentum in the soil-plant-atmosphere continuum.Large Aperture Scintillometer(LAS) is recent emerging instrument put on the ground to observe surface fluxes across pixel dimensions.This study validated the reliability of LAS observations with observation data from Eddy Covariance(EC) instruments.EC is the officially recognized and commonly used ET equipment in the Chinese Ecosystem Research Network(CERN).Heat flux was measured in summer maize fields via both LAS and EC at the Luancheng Agro-Ecosystem Experimental Station of Chinese Academy of Sciences in August 2010 and compared.The results showed a significant consistency in both daily and monthly changes in heat flux measurements by LAS and EC.ET obtained from LAS was almost consistent with that from EC(R2=0.800 4) at the monthly scale.However,some differences existed in the daily change probably due to the effects of underlying principles,environment conditions and measurement scale.The study suggested that surface water and heat flux data collected via LAS was reliable and applicable in verifying estimated ET via remote sensing.

Investigation of Air Refractive Index Profile over Silchar Region to Predict the Propagation of Radio Wave

Abstract: To investigate the atmospheric dynamism over Silchar is of great significance. Though it is practically important to understand and reveal the atmospheric variation of this region, yet no such study has been reported over this region till now. This study is of technical interest as the high frequency (radio wave) communications are greatly influenced due to the variation of refractive index profile. In the present investigation, we have considered “air refractive index” through-out the year, as refractive index profile plays the central role in controlling dynamism of tropospheric atmosphere. Lastly, this work highlights the radio wave propagation behaviour with variation in air refractive index.

Integrated role of the urban canopy on turbulent transfer with the roughness sub-layer from observations

Abstract: were placed within the wake of large roughness elements, which suggest that under some wind directions the measurements were made too close to the surface. Yet, scintillometer measurements in the urban environment depend on a large number of corrections. From these, the most critical is the determination of an appropriate value of zero-plane displacement height, which needs to be considered to account for flow modification by the roughness elements. The dependence on a large number of factors and limitations on the determination of appropriate values of zero-plane displacement height suggest that at this point in time scintillometer measurements made that close to the canopy should be considered with care.