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Volker Ebert - One of the best experts on this subject based on the ideXlab platform.
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hai a new airborne absolute twin dual channel multi phase tdlas hygrometer background design setup and first flight data
Atmospheric Measurement Techniques, 2017Co-Authors: Bernhard Buchholz, Armin Afchine, Alexander Klein, C Schiller, Martina Kramer, Volker EbertAbstract:Abstract. The novel Hygrometer for Atmospheric Investigation (HAI) realizes a unique concept for simultaneous gas-phase and total (gas-phase + evaporated cloud particles) water measurements. It has been developed and successfully deployed for the first time on the German HALO research aircraft. This new instrument combines direct tunable diode laser absorption spectroscopy (dTDLAS) with a first-principle evaluation method to allow absolute water vapor measurements without any initial or repetitive sensor calibration using a reference gas or a reference humidity generator. HAI contains two completely independent dual-channel (Closed-Path, open-Path) spectrometers, one at 1.4 and one at 2.6 µm, which together allow us to cover the entire atmospheric H2O range from 1 to 40 000 ppmv with a single instrument. Both spectrometers each comprise a separate, wavelength-individual extractive, Closed-Path cell for total water (ice and gas-phase) measurements. Additionally, both spectrometers couple light into a common open-Path cell outside of the aircraft fuselage for a direct, sampling-free, and contactless determination of the gas-phase water content. This novel twin dual-channel setup allows for the first time multiple self-validation functions, in particular a reliable, direct, in-flight validation of the open-Path channels. During the first field campaigns, the in-flight deviations between the independent and calibration-free channels (i.e., Closed-Path to Closed-Path and open-Path to Closed-Path) were on average in the 2 % range. Further, the fully autonomous HAI hygrometer allows measurements up to 240 Hz with a minimal integration time of 1.4 ms. The best precision is achieved by the 1.4 µm Closed-Path cell at 3.8 Hz (0.18 ppmv) and by the 2.6 µm Closed-Path cell at 13 Hz (0.055 ppmv). The requirements, design, operation principle, and first in-flight performance of the hygrometer are described and discussed in this work.
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HAI – A new, airborne, absolute, twin dual-channel, multi-phase TDLAS-hygrometer
2016Co-Authors: Bernhard Buchholz, Armin Afchine, Alexander Klein, C Schiller, Martina Kramer, Volker EbertAbstract:Abstract. The novel Hygrometer for Atmospheric Investigations (HAI) realizes a unique concept for simultaneous gas-phase and total (gas-phase + evaporated cloud particles) water measurement. It has been developed and suc-cessfully employed for the first time on the German HALO research aircraft. This new instrument combines direct Tunable Diode Laser Absorption Spectroscopy (dTDLAS) with a first-principle evaluation method to allow absolute water vapor measurements without any initial or repetitive sensor calibration using a refer-ence gas or a reference humidity generator. HAI contains two completely independent dual-channel (Closed-Path, open-Path) spectrometers, one at 1.4 µm and one at 2.6 µm, which allow together to cover the entire at-mospheric H2O range from 1 to 40 000 ppmv with a single instrument. Both spectrometers comprise each a separate, wavelength-individual extractive, Closed-Path cell for total water (ice and gas-phase) measurements. Additionally, both spectrometers couple light into a common, open-Path-cell outside of the aircraft fuselage for a direct, sampling-free and contactless determination of the gas-phase water content. This novel twin du-al-channel setup allows for the first time multiple self-validation functions i.e. in particular a reliable, direct, in-flight validation of the open-Path channels. During the first field campaigns, the in-flight deviations be-tween the independent and calibration-free channels (i.e. Closed-Path to Closed-Path and open-Path to Closed-Path) were on average in the 2 % range. Further, the fully autonomous HAI hygrometer allows measurements up to 240 Hz with a minimal integration time of 1.4 ms. The best precision is achieved by the 1.4 µm Closed-Path cell at 3.8 Hz (0.18 ppmv) and by the 2.6 µm Closed-Path cell at 13 Hz (0.055 ppmv). The requirements, design, operation principle and in-flight performance of the hygrometer are described in this work.
Bernhard Buchholz - One of the best experts on this subject based on the ideXlab platform.
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hai a new airborne absolute twin dual channel multi phase tdlas hygrometer background design setup and first flight data
Atmospheric Measurement Techniques, 2017Co-Authors: Bernhard Buchholz, Armin Afchine, Alexander Klein, C Schiller, Martina Kramer, Volker EbertAbstract:Abstract. The novel Hygrometer for Atmospheric Investigation (HAI) realizes a unique concept for simultaneous gas-phase and total (gas-phase + evaporated cloud particles) water measurements. It has been developed and successfully deployed for the first time on the German HALO research aircraft. This new instrument combines direct tunable diode laser absorption spectroscopy (dTDLAS) with a first-principle evaluation method to allow absolute water vapor measurements without any initial or repetitive sensor calibration using a reference gas or a reference humidity generator. HAI contains two completely independent dual-channel (Closed-Path, open-Path) spectrometers, one at 1.4 and one at 2.6 µm, which together allow us to cover the entire atmospheric H2O range from 1 to 40 000 ppmv with a single instrument. Both spectrometers each comprise a separate, wavelength-individual extractive, Closed-Path cell for total water (ice and gas-phase) measurements. Additionally, both spectrometers couple light into a common open-Path cell outside of the aircraft fuselage for a direct, sampling-free, and contactless determination of the gas-phase water content. This novel twin dual-channel setup allows for the first time multiple self-validation functions, in particular a reliable, direct, in-flight validation of the open-Path channels. During the first field campaigns, the in-flight deviations between the independent and calibration-free channels (i.e., Closed-Path to Closed-Path and open-Path to Closed-Path) were on average in the 2 % range. Further, the fully autonomous HAI hygrometer allows measurements up to 240 Hz with a minimal integration time of 1.4 ms. The best precision is achieved by the 1.4 µm Closed-Path cell at 3.8 Hz (0.18 ppmv) and by the 2.6 µm Closed-Path cell at 13 Hz (0.055 ppmv). The requirements, design, operation principle, and first in-flight performance of the hygrometer are described and discussed in this work.
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HAI – A new, airborne, absolute, twin dual-channel, multi-phase TDLAS-hygrometer
2016Co-Authors: Bernhard Buchholz, Armin Afchine, Alexander Klein, C Schiller, Martina Kramer, Volker EbertAbstract:Abstract. The novel Hygrometer for Atmospheric Investigations (HAI) realizes a unique concept for simultaneous gas-phase and total (gas-phase + evaporated cloud particles) water measurement. It has been developed and suc-cessfully employed for the first time on the German HALO research aircraft. This new instrument combines direct Tunable Diode Laser Absorption Spectroscopy (dTDLAS) with a first-principle evaluation method to allow absolute water vapor measurements without any initial or repetitive sensor calibration using a refer-ence gas or a reference humidity generator. HAI contains two completely independent dual-channel (Closed-Path, open-Path) spectrometers, one at 1.4 µm and one at 2.6 µm, which allow together to cover the entire at-mospheric H2O range from 1 to 40 000 ppmv with a single instrument. Both spectrometers comprise each a separate, wavelength-individual extractive, Closed-Path cell for total water (ice and gas-phase) measurements. Additionally, both spectrometers couple light into a common, open-Path-cell outside of the aircraft fuselage for a direct, sampling-free and contactless determination of the gas-phase water content. This novel twin du-al-channel setup allows for the first time multiple self-validation functions i.e. in particular a reliable, direct, in-flight validation of the open-Path channels. During the first field campaigns, the in-flight deviations be-tween the independent and calibration-free channels (i.e. Closed-Path to Closed-Path and open-Path to Closed-Path) were on average in the 2 % range. Further, the fully autonomous HAI hygrometer allows measurements up to 240 Hz with a minimal integration time of 1.4 ms. The best precision is achieved by the 1.4 µm Closed-Path cell at 3.8 Hz (0.18 ppmv) and by the 2.6 µm Closed-Path cell at 13 Hz (0.055 ppmv). The requirements, design, operation principle and in-flight performance of the hygrometer are described in this work.
Kim Pilegaard - One of the best experts on this subject based on the ideXlab platform.
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strong low pass filtering effects on water vapour flux measurements with Closed Path eddy correlation systems
Agricultural and Forest Meteorology, 2007Co-Authors: Andreas Ibrom, Ebba Dellwik, Henrik Flyvbjerg, N O Jensen, Kim PilegaardAbstract:Turbulent water vapour fluxes measured with Closed-Path eddy correlation (EC) systems are unintentionally low-pass filtered by the system in a manner that varies with environmental conditions. Why and how is described here. So is the practical method that systematically corrects long-term flux datasets for this substantial measurement error. In contrast to earlier studies, a large number of spectra and raw data have been used in the analysis to define the low-pass filtering characteristic of the EC system. This revealed that the cut-off frequency of the Closed-Path EC system for water vapour concentration measurements decreases exponentially with increasing relative humidity. After correction for this unintended filtering, the fluxes are consistent with CO2 and H2O fluxes that were measured with an open-Path sensor at the same time. The correction of water vapour flux measurements over a Beech forest in Soro, Zealand, Denmark, amounted on average to 42% of the measured flux, while it was only 4% for the CO2 flux, which was measured with the same EC system. We recommend using the described method to correct water vapour fluxes measured in any Closed-Path EC system for unintended low-pass filtering effects. Other than for CO2 is the magnitude of the correction for water vapour flux measurements unsatisfactorily high, i.e. the EC system needs to be technically improved. Our results suggest that such high correction can be avoided by keeping relative humidity in the entire gas transport system of the EC system lower than 30%, e.g. by heating intake filters and tubes.
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on the use of the webb pearman leuning theory for Closed Path eddy correlation measurements
Tellus B, 2007Co-Authors: Andreas Ibrom, Ebba Dellwik, Søren Ejling Larsen, Kim PilegaardAbstract:We consider an imperfection of real Closed-Path eddy correlation systems—the decoupling of the water vapour and CO 2 concentrations—with respect to the application of the Webb–Pearman–Leuning (WPL) theory. It is described why and how the current application of the WPL theory needs to be adapted to the processes in Closed-Path sensors. We show the quantitative effects of applying the WPL theory in different ways using CO 2 flux measurements taken above the Danish Beech forest CarboEurope site near Soro, Zealand. Using the WPL theory in Closed-Path sensors without taking amplitude damping and decoupling into account, overcorrected the annual flux by 21%, or 31 g m -2 yr -1 , to which the decoupling effect contributed with 7%. We suggest either converting the raw data point-by-point to mixing ratios or using the uncorrected covariances of water vapour mole fractions with the vertical wind velocity that were calculated with the same time lag as for the scalar concentration when correcting the dilution effect. We showed that the two approaches yielded equivalent flux results. Correct ways of applying spectral corrections to CO 2 fluxes calculated in either way are also shown. The findings reported here do not apply to open-Path sensors. DOI: 10.1111/j.1600-0889.2007.00311.x
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On the use of the Webb–Pearman–Leuning theory for Closed-Path eddy correlation measurements
Tellus B: Chemical and Physical Meteorology, 2007Co-Authors: Andreas Ibrom, Ebba Dellwik, Søren Ejling Larsen, Kim PilegaardAbstract:We consider an imperfection of real Closed-Path eddy correlation systems—the decoupling of the water vapour and CO 2 concentrations—with respect to the application of the Webb–Pearman–Leuning (WPL) theory. It is described why and how the current application of the WPL theory needs to be adapted to the processes in Closed-Path sensors. We show the quantitative effects of applying the WPL theory in different ways using CO 2 flux measurements taken above the Danish Beech forest CarboEurope site near Soro, Zealand. Using the WPL theory in Closed-Path sensors without taking amplitude damping and decoupling into account, overcorrected the annual flux by 21%, or 31 g m -2 yr -1 , to which the decoupling effect contributed with 7%. We suggest either converting the raw data point-by-point to mixing ratios or using the uncorrected covariances of water vapour mole fractions with the vertical wind velocity that were calculated with the same time lag as for the scalar concentration when correcting the dilution effect. We showed that the two approaches yielded equivalent flux results. Correct ways of applying spectral corrections to CO 2 fluxes calculated in either way are also shown. The findings reported here do not apply to open-Path sensors. DOI: 10.1111/j.1600-0889.2007.00311.x
Andreas Ibrom - One of the best experts on this subject based on the ideXlab platform.
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Revising the high frequency response correction of scalar fluxes measured by Closed-Path eddy covariance systems
2021Co-Authors: Ivan Mammarella, Andreas Ibrom, Olli Peltola, Toprak Aslan, Eiko Nemitz, Ullar RannikAbstract:<p>Eddy covariance (EC) scalar flux loss at high frequency is due to the incapability of the measurement system to detect small-scale variation of atmospheric turbulent signals. This systematic bias is particularly important for Closed-Path systems, and it is mainly related to inadequate sensor frequency response, sensor separation and the air sampling trough tubes and filters. Here, we investigate the limitations of current approaches, based on measured power spectra (PSA) or cospectra (CSA), to empirically estimated the spectral transfer function of the EC system needed for the frequency response correction of measured fluxes. We performed a systematic analysis by using EC data from a wetland and forest site for a wide range of attenuation levels and signal-to-noise ratio. We proposed a novel approach for PSA that uses simultaneously the noise and the turbulent signals present in the power spectrum, providing robust estimates of spectral transfer function for all conditions. We further theoretically derived a new transfer function to be used in the CSA approach which specifically accounts for the interaction between the low-pass filtering induced phase shift and the high frequency attenuation. We show that current CSA approaches neglect such effect, giving a non-negligible systematic bias to the estimated scalar fluxes from the studied sites. Based on these findings, we recommend that spectral correction methods, implemented in EC data processing algorithms, are revised accordingly.</p>
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relative humidity effects on water vapour fluxes measured with Closed Path eddy covariance systems with short sampling lines
Agricultural and Forest Meteorology, 2012Co-Authors: Gerardo Fratini, Andreas Ibrom, Nicola Arriga, George Burba, Dario PapaleAbstract:Abstract It has been formerly recognised that increasing relative humidity in the sampling line of Closed-Path eddy-covariance systems leads to increasing attenuation of water vapour turbulent fluctuations, resulting in strong latent heat flux losses. This occurrence has been analyzed for very long (50 m) and long (7 m) sampling lines. To date, only a few analytical or in situ analyses have been proposed to quantify and correct such effects, among which the comprehensive method by Ibrom et al. (2007) was proved effective for the very long sampling line of a forest eddy-covariance setup. Here we analyze data from eddy-covariance systems featuring short (4 m) and very short (1 m) sampling lines running at the same clover field and show that relative humidity effects persist also for these setups, and should not be neglected. Starting from the work of Ibrom and co-workers, we propose a mixed method, a composite of two existing approaches, for correcting eddy-covariance fluxes. By means of a comparison with parallel open-Path measurements, we show that the mixed method leads to an improved estimation of latent heat fluxes, with respect to the method described by Ibrom et al. (2007) . The quantification and correction method proposed here is deemed applicable to Closed-Path systems featuring a broad range of sampling lines, and indeed applicable also to passive gases as a special case. The methods described in this paper are incorporated, as processing options, in the free and open-source eddy-covariance software packages ECO2S and EddyPro.
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strong low pass filtering effects on water vapour flux measurements with Closed Path eddy correlation systems
Agricultural and Forest Meteorology, 2007Co-Authors: Andreas Ibrom, Ebba Dellwik, Henrik Flyvbjerg, N O Jensen, Kim PilegaardAbstract:Turbulent water vapour fluxes measured with Closed-Path eddy correlation (EC) systems are unintentionally low-pass filtered by the system in a manner that varies with environmental conditions. Why and how is described here. So is the practical method that systematically corrects long-term flux datasets for this substantial measurement error. In contrast to earlier studies, a large number of spectra and raw data have been used in the analysis to define the low-pass filtering characteristic of the EC system. This revealed that the cut-off frequency of the Closed-Path EC system for water vapour concentration measurements decreases exponentially with increasing relative humidity. After correction for this unintended filtering, the fluxes are consistent with CO2 and H2O fluxes that were measured with an open-Path sensor at the same time. The correction of water vapour flux measurements over a Beech forest in Soro, Zealand, Denmark, amounted on average to 42% of the measured flux, while it was only 4% for the CO2 flux, which was measured with the same EC system. We recommend using the described method to correct water vapour fluxes measured in any Closed-Path EC system for unintended low-pass filtering effects. Other than for CO2 is the magnitude of the correction for water vapour flux measurements unsatisfactorily high, i.e. the EC system needs to be technically improved. Our results suggest that such high correction can be avoided by keeping relative humidity in the entire gas transport system of the EC system lower than 30%, e.g. by heating intake filters and tubes.
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on the use of the webb pearman leuning theory for Closed Path eddy correlation measurements
Tellus B, 2007Co-Authors: Andreas Ibrom, Ebba Dellwik, Søren Ejling Larsen, Kim PilegaardAbstract:We consider an imperfection of real Closed-Path eddy correlation systems—the decoupling of the water vapour and CO 2 concentrations—with respect to the application of the Webb–Pearman–Leuning (WPL) theory. It is described why and how the current application of the WPL theory needs to be adapted to the processes in Closed-Path sensors. We show the quantitative effects of applying the WPL theory in different ways using CO 2 flux measurements taken above the Danish Beech forest CarboEurope site near Soro, Zealand. Using the WPL theory in Closed-Path sensors without taking amplitude damping and decoupling into account, overcorrected the annual flux by 21%, or 31 g m -2 yr -1 , to which the decoupling effect contributed with 7%. We suggest either converting the raw data point-by-point to mixing ratios or using the uncorrected covariances of water vapour mole fractions with the vertical wind velocity that were calculated with the same time lag as for the scalar concentration when correcting the dilution effect. We showed that the two approaches yielded equivalent flux results. Correct ways of applying spectral corrections to CO 2 fluxes calculated in either way are also shown. The findings reported here do not apply to open-Path sensors. DOI: 10.1111/j.1600-0889.2007.00311.x
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On the use of the Webb–Pearman–Leuning theory for Closed-Path eddy correlation measurements
Tellus B: Chemical and Physical Meteorology, 2007Co-Authors: Andreas Ibrom, Ebba Dellwik, Søren Ejling Larsen, Kim PilegaardAbstract:We consider an imperfection of real Closed-Path eddy correlation systems—the decoupling of the water vapour and CO 2 concentrations—with respect to the application of the Webb–Pearman–Leuning (WPL) theory. It is described why and how the current application of the WPL theory needs to be adapted to the processes in Closed-Path sensors. We show the quantitative effects of applying the WPL theory in different ways using CO 2 flux measurements taken above the Danish Beech forest CarboEurope site near Soro, Zealand. Using the WPL theory in Closed-Path sensors without taking amplitude damping and decoupling into account, overcorrected the annual flux by 21%, or 31 g m -2 yr -1 , to which the decoupling effect contributed with 7%. We suggest either converting the raw data point-by-point to mixing ratios or using the uncorrected covariances of water vapour mole fractions with the vertical wind velocity that were calculated with the same time lag as for the scalar concentration when correcting the dilution effect. We showed that the two approaches yielded equivalent flux results. Correct ways of applying spectral corrections to CO 2 fluxes calculated in either way are also shown. The findings reported here do not apply to open-Path sensors. DOI: 10.1111/j.1600-0889.2007.00311.x
Etienne Brasselet - One of the best experts on this subject based on the ideXlab platform.
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Optical vortices from Closed-loop subwavelength slits
Optics Letters, 2013Co-Authors: Etienne BrasseletAbstract:We report on the singular shaping of light using Closed-loop subwavelength slits whose shape is homeomorphic to the circle. Various sets of optical phase singularities can be generated depending on the given Closed Path whose geometry tailors the spin-orbit interaction for the light that passes through the curved slit. Here three families of Closed-loop curves are considered--polygons, hypocycloids, and epicycloids.
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Topological Shaping of Light by Closed-Path Nanoslits
Physical Review Letters, 2013Co-Authors: Etienne Brasselet, Gediminas Gervinskas, Gediminas Seniutinas, Saulius JuodkazisAbstract:We propose a discrete set of continuous deformation of a circular nanoslit to generate and control optical vortices at the microscopic scale. The process relies on the interplay between the spin and orbital angular momentum degrees of freedom of light mediated by appropriate Closed-Path nanoslits milled on a thin gold film. Topological shaping of light is experimentally demonstrated in the visible domain. Moreover, all experimental observations are quantitatively validated by a simple model that takes into account the transverse manipulation of the optical phase via the space-variant form birefringence of subwavelength slits.