Effects of Non-Uniform Crosswind Fields on Scintillometry Measurements

Ward, Helen C.; Evans, Jonathan; Grimmond, Sue

doi:10.1007/s10546-011-9626-0

Cited by 11 publications

(8 citation statements)

References 23 publications

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“…The reason for this is unknown, but two possible explanations are considered here. Firstly, Taylor's frozen turbulence hypothesis is assumed in order to relate intensity fluctuations to C 2 n (Clifford, 1971;Wang et al, 1981). When crosswind speeds are low, this assumption is less justified and eddies decay as they are slowly blown through the beam.…”

Section: Temperature-humidity Correlationmentioning

confidence: 99%

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

Ward¹,

Evans²,

Grimmond³

et al. 2014

Preprint

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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 unique 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 so-called "bichromatic-correlation" method 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 techniques under different conditions is discussed. Similar behaviour is seen between the two datasets 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 copanion paper.

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Section: Temperature-humidity Correlationmentioning

confidence: 99%

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

Ward¹,

Evans²,

Grimmond³

et al. 2014

Preprint

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“…An explanation for this large spread can be the effect of the non-uniform crosswind and wind direction on the PSD of the LAS. Ward et al (2011) compared the theoretical and experimental behaviour of microwave scintillometer spectra under various wind conditions. They showed that, in the case of variable crosswind parallel to the path, or for a non-uniform wind direction, the experimental PSD (accounting for variations in crosswind and wind direction) is located at higher frequencies than the theoretical one (considering a mean crosswind).…”

Section: Fig 12mentioning

confidence: 99%

“…13b, d, and show that both crosswind and high values of the humidity spectral density at low wavenumbers are responsible for the increase in absorption contribution. Differences between theoretical and experimental spectra are mainly due to the uncertainties in the crosswind used in the theoretical calculations (Ward et al 2011).…”

Section: Fig 12mentioning

confidence: 99%

Attenuating the Absorption Contribution on $${C_{n^{2}}}$$ Estimates with a Large-Aperture Scintillometer

Solignac

Brut

Selves

et al. 2012

Boundary-Layer Meteorol

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Large-aperture scintillometers (LAS) are often used to characterize atmospheric turbulence by measuring the structure parameter of the refractive index C n 2 . However, absorption phenomena can lead to an overestimation of C n 2 . By applying an accurate numerical filtering technique called the Gabor transform to the signal output of a LAS, we improved our knowledge of the accuracy of the measured C n 2 by determining and attenuating the contribution of absorption. Two studies are presented on a 12-day dataset using either fixed band pass or adaptive filtering. The first consists of evaluating the best-fit filter for which the resulting C n 2 is independent of meteorological conditions, especially crosswind, and the second consists in accurately reconstructing the signal to remove absorption, without losing information on C n 2 . A reference C n 2 (hereafter 'reconstructed C n 2 ') is created by accurately removing absorption from the scintillation spectrum, and is used to evaluate each filter. By comparing the 'reconstructed C n 2 ' with a raw C n 2 measured with a scintillometer, in the presence of absorption, we found that the average relative contribution of absorption to the measurement of C n 2 is approximately 9%. However, the absorption phenomenon is highly variable; occasionally, in the worst cases, we estimate that the absorption phenomenon could represent 81% of the value of C n 2 . Some explanations for this high variability are proposed with respect to theoretical considerations. Amongst the fixed band-pass filtering used, we conclude on the preferential use of a band-pass filter [0.2-400 Hz] for C n 2 , as its performance is only slightly affected by the crosswind, and that the mean absorption contribution is reduced to 5.6%, with a maximum value of 60%. Using an adaptive filter on the 12-day dataset really improves the filtering accuracy for both points discussed-the independence of meteorological conditions and the quality of signal reconstruction.

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“…Furthermore, there is also a need for scintillometer U ⊥ in more complex areas, such as mountain environments (Poggio et al, 2000) and urban environments (above the River Thames in London in Wood et al, 2013c). Ward et al (2011) studied the influence of a variable U ⊥ field along the path (U ⊥ (x), where x is the location on the scintillometer path) on the scintillometer signalhowever, their focus was on scintillation spectra and structure…”

Section: Introductionmentioning

confidence: 99%

Observing crosswind over urban terrain using scintillometer and Doppler lidar

et al. 2015

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Abstract. In this study, the crosswind (wind component perpendicular to a path, U ⊥ ) is measured by a scintillometer and estimated with Doppler lidar above the urban environment of Helsinki, Finland, for 15 days. The scintillometer allows acquisition of a path-averaged value of U ⊥ (U ⊥ ), while the lidar allows acquisition of path-resolved U ⊥ (U ⊥ (x), where x is the position along the path). The goal of this study is to evaluate the performance of scintillometer U ⊥ estimates for conditions under which U ⊥ (x) is variable. Two methods are applied to estimate U ⊥ from the scintillometer signal: the cumulative-spectrum method (relies on scintillation spectra) and the look-up-table method (relies on time-lagged correlation functions). The values of U ⊥ of both methods compare well with the lidar estimates, with root-mean-square deviations of 0.71 and 0.73 m s −1 . This indicates that, given the data treatment applied in this study, both measurement technologies are able to obtain estimates of U ⊥ in the complex urban environment. The detailed investigation of four cases indicates that the cumulative-spectrum method is less susceptible to a variable U ⊥ (x) than the look-up-table method. However, the look-up-table method can be adjusted to improve its capabilities for estimating U ⊥ under conditions under for which U ⊥ (x) is variable.

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Effects of Non-Uniform Crosswind Fields on Scintillometry Measurements

Cited by 11 publications

References 23 publications

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

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

Attenuating the Absorption Contribution on $${C_{n^{2}}}$$ Estimates with a Large-Aperture Scintillometer

Observing crosswind over urban terrain using scintillometer and Doppler lidar

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