the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 28 Apr 2025
The AquaVIT-4 intercomparison of atmospheric hygrometers
Abstract. The AquaVIT-4 intercomparison of atmospheric hygrometers was conducted at the AIDA climate simulation chamber of the Karlsruhe Institute of Technology (KIT), Germany, in March–April 2022, within the framework of the HEMERA H2020 EU project. The objectives were to document the performance of existing hygrometers and to support the development of novel methods for water vapor (H2O) measurements in the upper atmosphere. The AquaVIT-4 intercomparison involved seven hygrometers, based on either infrared laser absorption spectroscopy or frostpoint hygrometry techniques: four deployed on aircraft or stratospheric balloon platforms, and three reference instruments. The simulated conditions in the AIDA chamber reproduced the characteristic atmospheric conditions of the upper troposphere-lower stratosphere (UTLS, altitude range ~5–28 km) in the tropics and mid-latitudes, spanning between 20–600 hPa pressure, 190–245 K temperature, and 0.5–530 ppm H2O mixing ratio. The campaign was divided into two phases, each consisting of four measurement days: an “open intercomparison”, where the simulated conditions were known to the participants, and a “blind intercomparison”, where the conditions were coordinated by independent referees and unknown to the participating teams. Here we present a statistical analysis of the entire dataset, which allows to assess the accuracy and limitations of each instrument. For the accuracy evaluation, two sets of reference measurements were defined: one for in situ instruments, located inside the AIDA vessel, and one for extractive instruments, sampling the chamber gas through a heated inlet. This distinction accounts for H2O desorption effects, which are most prominent at low pressures and low H2O concentrations. All instruments showed a good agreement with the reference values in the range of H2O > 2 ppm, with mean deviations within ±7 % for H2O > 10 ppm, and ±8 % between 2–10 ppm H2O. The largest differences were found for H2O < 2 ppm, a rarely observed range in the atmosphere, though most of the instruments still achieved average deviations within ±10 %. Overall, the results of AquaVIT-4 demonstrate the high accuracy and reliability of the four involved sensors for upper atmospheric monitoring and research applications.
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Status: closed
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RC1: 'Comment on egusphere-2025-1029', Anonymous Referee #1, 22 May 2025
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AC1: 'Reply on RC1', Simone Brunamonti, 16 Jul 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1029/egusphere-2025-1029-AC1-supplement.pdf
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AC1: 'Reply on RC1', Simone Brunamonti, 16 Jul 2025
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RC2: 'Comment on egusphere-2025-1029', Anonymous Referee #2, 18 Jun 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1029/egusphere-2025-1029-RC2-supplement.pdf
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AC2: 'Reply on RC2', Simone Brunamonti, 16 Jul 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1029/egusphere-2025-1029-AC2-supplement.pdf
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AC2: 'Reply on RC2', Simone Brunamonti, 16 Jul 2025
Status: closed
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RC1: 'Comment on egusphere-2025-1029', Anonymous Referee #1, 22 May 2025
Brunamonti et al. report a synthesis of results from the AquaVIT-4 project, providing a comparison of state-of-the-art atmospheric hygrometers for use in conditions found in the upper troposphere and lower stratosphere. Accuracy of hygrometers in these dry conditions is fundamental to assessment of ice microphysics, which controls the formation of cirrus and limits the transport of water vapor into the stratosphere. Substantial differences in the measurements reported in this region by various hygrometers has historically motivated a continued set of campaigns at AIDA and in the atmosphere, to assess the skill of research hygrometers.
This article is excellent. The experiment and analysis are well done, the figures are high quality and text is very well written. I essentially only have minor editorial comments for the authors to address, in addition to a couple of potential small changes that they could consider for the content and analysis.
Page7 Line5: Suggest to mention what kind of data acquisition mode is used for ALBATROSS, e.g. scanning with DC or 2f modulation, etc?
Page 8 Line 6: Suggest instead of saying “25+ years” state the first year that it was operated.
Page 9 lines 18 – 27: Is this paragraph really needed? As I understand it, this is summarizing results that are already reported in Ghysels et al 2024. The large differences stated of “+/- 23.6%” and “about 30%” I assume are differences in the air that was measured by the instruments on those experiments and not due to accuracy problems with the instruments, but this isn’t fully explained here. I would either provide more detail or remove that part.
Page 13 Line 15: need to delete word “of”
Figure 2: Recommend using a discrete colorbar with ~5 degree intervals.
Page 14 Line 7: “instrument” -> “instruments”
Page 26 Line 8: “Evaporates” should be changed to “sublimates”. However, I’m having trouble understanding that this is really the mechanism that results in a loss of ice. Is it rather that the ice crystals sediment to the bottom of the chamber? It doesn’t make sense that the ice cloud would sublimate while it is supersaturated.
Page 26: Does the result in Figure 9 imply a positive bias in APicT and is it worth making a comment about how that might impact the comparisons shown previously where that hygrometer was the reference? Or is it rather the case that the uncertainty range shown for 100% RHi in Figure 9 is considered uniform such that the correct value could be anywhere in that +/-5% range with equal probability?
One other comment I have is on the lack of substantial discussion or analysis about instrument precision. Precision is discussed a bit towards the end of the manuscript, but is not quantitatively summarized e.g. in Table 1 or assessed elsewhere. This paper would be a useful venue for comparison of the precision of the hygrometers as well as accuracy so I suggest that the authors consider addressing this somewhere. One suggestion if the authors do this is that some markers indicating the observed precision could be added in e.g. Figure 8, or a separate figure could be generated comparing the precision of the different instruments in this relevant P, H2O space.
Citation: https://doi.org/10.5194/egusphere-2025-1029-RC1 -
AC1: 'Reply on RC1', Simone Brunamonti, 16 Jul 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1029/egusphere-2025-1029-AC1-supplement.pdf
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AC1: 'Reply on RC1', Simone Brunamonti, 16 Jul 2025
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RC2: 'Comment on egusphere-2025-1029', Anonymous Referee #2, 18 Jun 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1029/egusphere-2025-1029-RC2-supplement.pdf
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AC2: 'Reply on RC2', Simone Brunamonti, 16 Jul 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1029/egusphere-2025-1029-AC2-supplement.pdf
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AC2: 'Reply on RC2', Simone Brunamonti, 16 Jul 2025
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Brunamonti et al. report a synthesis of results from the AquaVIT-4 project, providing a comparison of state-of-the-art atmospheric hygrometers for use in conditions found in the upper troposphere and lower stratosphere. Accuracy of hygrometers in these dry conditions is fundamental to assessment of ice microphysics, which controls the formation of cirrus and limits the transport of water vapor into the stratosphere. Substantial differences in the measurements reported in this region by various hygrometers has historically motivated a continued set of campaigns at AIDA and in the atmosphere, to assess the skill of research hygrometers.
This article is excellent. The experiment and analysis are well done, the figures are high quality and text is very well written. I essentially only have minor editorial comments for the authors to address, in addition to a couple of potential small changes that they could consider for the content and analysis.
Page7 Line5: Suggest to mention what kind of data acquisition mode is used for ALBATROSS, e.g. scanning with DC or 2f modulation, etc?
Page 8 Line 6: Suggest instead of saying “25+ years” state the first year that it was operated.
Page 9 lines 18 – 27: Is this paragraph really needed? As I understand it, this is summarizing results that are already reported in Ghysels et al 2024. The large differences stated of “+/- 23.6%” and “about 30%” I assume are differences in the air that was measured by the instruments on those experiments and not due to accuracy problems with the instruments, but this isn’t fully explained here. I would either provide more detail or remove that part.
Page 13 Line 15: need to delete word “of”
Figure 2: Recommend using a discrete colorbar with ~5 degree intervals.
Page 14 Line 7: “instrument” -> “instruments”
Page 26 Line 8: “Evaporates” should be changed to “sublimates”. However, I’m having trouble understanding that this is really the mechanism that results in a loss of ice. Is it rather that the ice crystals sediment to the bottom of the chamber? It doesn’t make sense that the ice cloud would sublimate while it is supersaturated.
Page 26: Does the result in Figure 9 imply a positive bias in APicT and is it worth making a comment about how that might impact the comparisons shown previously where that hygrometer was the reference? Or is it rather the case that the uncertainty range shown for 100% RHi in Figure 9 is considered uniform such that the correct value could be anywhere in that +/-5% range with equal probability?
One other comment I have is on the lack of substantial discussion or analysis about instrument precision. Precision is discussed a bit towards the end of the manuscript, but is not quantitatively summarized e.g. in Table 1 or assessed elsewhere. This paper would be a useful venue for comparison of the precision of the hygrometers as well as accuracy so I suggest that the authors consider addressing this somewhere. One suggestion if the authors do this is that some markers indicating the observed precision could be added in e.g. Figure 8, or a separate figure could be generated comparing the precision of the different instruments in this relevant P, H2O space.