Scintillometer Path Research Articles

Application and Evaluation of a Two-Wavelength Scintillometry System for Operation in a Complex Shallow Boreal-Forested Valley

Two-wavelength scintillometer systems can provide much needed measurements of area-averaged sensible and latent heat fluxes. However, these devices rarely have been deployed on canopy-covered complex terrain, and never in the circumpolar boreal biome, where large-scale fluxes are essential to hydroclimate modellers. We present a comparison of fluxes measured above a boreal-forested valley with a two-wavelength scintillometer and an eddy covariance system. Instruments were deployed in late summer 2017, and 19 days of data were retained for the analysis. The scintillometer path was 1347-m long and projected across the valley between 5 and 100 m above the ground, with an effective height of $$\approx $$88 m. The limitations of deriving surface fluxes using scintillometry in complex terrain are discussed, and the effects of atmospheric conditions on the flux comparison are quantified. Fluxes are calculated with the scintillometer only, and using a number of atmospheric variables from the eddy-covariance system; impacts of these calculation methods on the correlation between instrumental systems are assessed. Despite a weak agreement of structure parameters between instruments, the comparison of scintillometer and eddy-covariance fluxes yields good correlation ($$R^2$$ up to 0.82). Scintillometry correlates best with eddy-covariance data when the atmospheric surface-layer top is above the scintillometer effective height, but $$R^2$$ only drops slightly otherwise (average decrease of 0.11). The validity of scintillometer fluxes appears dubious during night-time and stable periods. We show that area-averaged flux measurements using two-wavelength scintillometers are possible in hilly forests, but more studies are needed to pinpoint the best methodological framework.

Scaling-up of evapotranspiration measurements: the promising role of scintillometry

The assessment of evapotranspiration (ET) is of primary interest, especially for agricultural and environmental issues. Traditional micrometeorological techniques require homogenous surface and are characterized by limited footprint. Thus, they cannot be applied to assess ET over inhomogeneous and extensive surfaces, typical conditions at the catchment scale and of extensive farming systems. In this context, a suitable technique to measure turbulent fluxes is scintillometry, which can give measurements of sensible heat flux at larger scale, providing averages over heterogeneous surfaces. ET can then be estimated as residual of the energy budget. In this study, we present results from a one-week campaign held during summer 2016 in Southern Italy. We deployed a Large Aperture Scintillometer (LAS) in an extensive vineyard of 140 ha on a path length of 760 m. The site was characterized by gently slope terrain with uniform crop. In order to have reference measurements of local sensible heat flux, we deployed three sonic anemometers along the scintillometer path. The aim of the study was to test the ability of scintillometry to provide a spatially averaged flux, representative of the possibly diverse conditions in an extended footprint upwind to the measurement path. The relationship between sensible heat flux measured by EC and LAS showed to be very good for the EC station in the middle of the path, whereas off-centre areas were less represented.

Long-term Turbulent Sensible-Heat-Flux Measurements with a Large-Aperture Scintillometer in the Centre of \u0141\xf3d\u017a, Central Poland

We investigate the area-averaged sensible heat flux (Q_{H}) obtained with a scintillometer along a 3.1-km path length over the city centre of Łódź, Central Poland. The annual cycle of Q_{H} peaks in June but is lower by the middle of summer. In winter, due to a large amount of anthropogenic heat input, Q_{H} remains positive all day long, with positive night-time fluxes also found during months with frequent cold advection, e.g., June 2010. In the diurnal cycle of this flux, several features specific to urban areas are seen: the peak shifts 1–2 h after noon, the heat flux turns from positive to negative 1–2 h after sunset. In Łódź Q_{H} was observed during inflow from the north and north-west, i.e. from the city centre. As this area is mostly covered with impervious materials, most of the heat exchanged between the ground and the overlying air is in the form of sensible heat flux. Under the conditions of inflow from the east and south-east, the maximum heat flux is approximately 100,hbox {W},hbox {m}^{-2} lower than during the inflow from the city centre, since more vegetation exists to the east and south-east of the scintillometer path. Cold and warm advection are found to be a vital factor in the observed heat-flux variability in the centre of Łódź.

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

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.

Evaluation of scintillometery measurements of fluxes of momentum and sensible heat in the roughness sublayer

Scintillometer measurements of turbulent fluxes of momentum and sensible heat in the roughness sublayer over a regular array of cubes in an outdoor environment were tested with direct measurement from sonic anemometers. The dissipation rate, e, and temperature structure parameter, C T 2 , obtained from the scintillometer agreed well with those from four sonic anemometers located along the scintillometer path. The fluxes measured by the scintillometer also corresponded well to those from the line-averaged eddy covariance approach, although this agreement was greatly influenced by the choice of the zero-plane displacement length and the form of the similarity function used in the scintillometer software. A guide for choosing the appropriate similarity function for the urban roughness sublayer is proposed.

Infrared and millimetre-wave scintillometry in the suburban environment – Part 1: Structure parameters

Abstract. Scintillometry, a form of ground-based remote sensing, provides the capability to estimate surface heat fluxes over scales of a few hundred metres to kilometres. Measurements are spatial averages, making this technique particularly valuable over areas with moderate heterogeneity such as mixed agricultural or urban environments. In this study, we present the structure parameters of temperature and humidity, which can be related to the sensible and latent heat fluxes through similarity theory, for a suburban area in the UK. The fluxes are provided in the second paper of this two-part series. A millimetre-wave scintillometer was combined with an infrared scintillometer along a 5.5 km path over northern Swindon. The pairing of these two wavelengths offers sensitivity to both temperature and humidity fluctuations, and the correlation between wavelengths is also used to retrieve the path-averaged temperature–humidity correlation. Comparison is made with structure parameters calculated from an eddy covariance station located close to the centre of the scintillometer path. The performance of the measurement techniques under different conditions is discussed. Similar behaviour is seen between the two data sets at sub-daily timescales. For the two summer-to-winter periods presented here, similar evolution is displayed across the seasons. A higher vegetation fraction within the scintillometer source area is consistent with the lower Bowen ratio observed (midday Bowen ratio < 1) compared with more built-up areas around the eddy covariance station. The energy partitioning is further explored in the companion paper.

Multi-Scale Sensible Heat Fluxes in the Suburban Environment from Large-Aperture Scintillometry and Eddy Covariance

Sensible heat fluxes ( $$Q_{H}$$ ) are determined using scintillometry and eddy covariance over a suburban area. Two large-aperture scintillometers provide spatially integrated fluxes across path lengths of 2.8 and 5.5 km over Swindon, UK. The shorter scintillometer path spans newly built residential areas and has an approximate source area of 2–4 $$\text {km}^{2}$$ , whilst the long path extends from the rural outskirts to the town centre and has a source area of around 5–10 $$\text {km}^{2}$$ . These large-scale heat fluxes are compared with local-scale eddy-covariance measurements. Clear seasonal trends are revealed by the long duration of this dataset and variability in monthly $$Q_{H}$$ is related to the meteorological conditions. At shorter time scales the response of $$Q_{H}$$ to solar radiation often gives rise to close agreement between the measurements, but during times of rapidly changing cloud cover spatial differences in the net radiation ( $$Q^{*}$$ ) coincide with greater differences between heat fluxes. For clear days $$Q_{H}$$ lags $$Q^{*}$$ , thus the ratio of $$Q_{H}$$ to $$Q^{*}$$ increases throughout the day. In summer the observed energy partitioning is related to the vegetation fraction through use of a footprint model. The results demonstrate the value of scintillometry for integrating surface heterogeneity and offer improved understanding of the influence of anthropogenic materials on surface-atmosphere interactions.

Using the Time-Lag Correlation Function of Dual-Aperture Scintillometer Measurements to Obtain the Crosswind

AbstractIn this study the crosswind (U⊥) is determined from the time-lag correlation function [r12(τ)] measured by a dual large-aperture scintillometer; U⊥ is defined as the wind component perpendicular to a path—in this case, the scintillometer path. A scintillometer obtains a path-averaged U⊥, which for some applications is an advantage compared to other wind measurement devices. Four methods were used to obtain U⊥: the peak method, the Briggs method, the zero-slope method, and the lookup table method. This last method is a new method introduced in this paper, which obtains U⊥ by comparing r12(τ) of a measurement to r12(τ) of a theoretical model. The U⊥ values obtained from the scintillometer were validated with sonic anemometer measurements. The best results were obtained by the zero-slope method for U⊥ < 2 m s−1 and by the lookup table method for U⊥ > 2 m s−1. The Briggs method also showed promising results, but it is not always able to obtain U⊥. The results showed that a high parallel wind component (>2.5 m s−1) on the scintillometer path can cause an overestimation of U⊥ mainly for low U⊥ values (<2 m s−1).

Variability of the Structure Parameters of Temperature and Humidity Observed in the Atmospheric Surface Layer Under Unstable Conditions

The structure parameters of temperature and humidity are important in scintillometry as they determine the structure parameter of the refractive index of air, the primary atmospheric variable obtained with scintillometers. In this study, we investigate the variability of the logarithm of the Monin-Obukhov-scaled structure parameters (denoted as $$\log ({\widetilde{C_{s}^2}_{\mathrm {}}})$$ ) of temperature and humidity. We use observations from eddy-covariance systems operated at three heights (2.5, 50, and 90 m) within the atmospheric surface layer under unstable conditions. The variability of $$\log ({\widetilde{C_{s}^2}_{\mathrm {}}})$$ depends on instability and on the size of the averaging window over which $$\log ({\widetilde{C_{s}^2}_{\mathrm {}}})$$ is calculated. If instability increases, differences in $$\log ({\widetilde{C_{s}^2}_{\mathrm {}}})$$ between upward motions (large $$C_{s}^2$$ ) and downward motions (small $$C_{s}^2$$ ) increase. The differences are, however, not sufficiently large to result in a bimodal probability density function. If the averaging window size increases, the variances of $$\log ({\widetilde{C_{s}^2}_{\mathrm {}}})$$ decrease. A linear regression of the variances of $$\log ({\widetilde{C_{s}^2}_{\mathrm {}}})$$ versus the averaging window size for various stability classes shows an increase of both the offset and slope (in absolute sense) with increasing instability. For temperature, data from the three heights show comparable results. For humidity, in contrast, the offset and slope are larger at 50 and 90 m than at 2.5 m. In the end we discuss how these findings could be used to assess whether observed differences in $$C_{s}^2$$ along a scintillometer path or aircraft flight leg are just within the range of local variability in $$C_{s}^2$$ or could be attributed to surface heterogeneity. This is important for the interpretation of data measured above a heterogeneous surface.

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

AbstractThe 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.

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

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

Introduction: Renaissance of Scintillometry

In the 1970s impressive progress was made in research on turbulence and the use of scintillometers. Muschinski and Lenschow (2002) summarised the outcome of the workshop on ‘Future Directions for Research on Meterand Submeter-Scale Atmospheric Turbulence’ held at Boulder, Colorado, U.S.A. where the development in this field was discussed. One of their comments noted that, after a blooming period in the 1970s and consolidation in the 1980s, funding and interest in scintillometry faded rapidly in the early 1990s. This special BLM issue is a result of a renewed interest, a renaissance, in scintillometry. Of work done in the last 40 years, some of the most important contributions include the work of Tatarskii (1961), the textbook by Monin and Yaglom (1975), the collection of scintillation papers brought together by Andreas (1990) and reviews by e.g., Hill (1997). Here are some main lines. A scintillometer is an instrument that consists of a transmitter and a receiver. The receiver measures intensity fluctuations in the radiation emitted by the transmitter caused by refractive scattering of turbulent eddies in the scintillometer path. For laser sources, or small aperture scintillometers (SAS), the observed intensity fluctuations are a measure of the structure parameter of the refractive index, C2 n, and the inner scale of turbulence, 0. For large aperture scintillometers (LAS), they are a measure of C2 n only. At optical wavelengths the contribution of temperature fluctuations dominates, i.e., the structure parameter of temperature C2 T can be deduced from the C2 n measurement. For radio wavelengths (>1 mm) on the other hand, water vapour fluctuations contribute most to the scintillometer signal, i.e. the structure parameter of moisture C2 q can be deduced from the C 2 n measurement. Surface fluxes of sensible heat, latent heat and momentum can be determined from the obtained C2 T , C 2 n and 0 respectively by applying Monin–Obukhov similarity theory (MOST). A number of length scales play a role in scintillometry: • The inner and outer length scales of turbulence. The first, 0, is proportional to the Kolmogorov length (ν3/ )1/4, where ν is the molecular kinematic viscosity and the dissipation rate of turbulent kinetic energy (TKE). The outer length scale is proportional to the height above the surface, z, in the surface layer; • the wavelength of the light source of the scintillometer, λ; • the aperture, D, of the scintillometer. Note that the transmitter and receiver have the same aperture in the scintillometers used in the studies presented here;