Adrianos Filinis

Accurate and sustained monitoring of water vapor (H₂O) in the upper troposphere-lower stratosphere (UTLS) is essential for quantifying its role in the atmospheric radiative balance, chemistry, and climate variability. However, in situ measurements in this region remain challenging due to the extremely low H₂O concentrations, harsh environmental conditions, and strict payload limitations on meteorological balloon platforms. ALBATROSS is a compact (3.6 kg), balloon-borne mid-infrared laser absorption spectrometer developed to provide highly accurate, SI-traceable H₂O measurements with high vertical resolution in the UTLS. Following recent comprehensive laboratory characterization, we present the first in-flight validation of ALBATROSS, based on seven balloon flights conducted between 2022 and 2026 from the meteorological observatories of Payerne, Switzerland (six flights) and Lindenberg, Germany (one flight). All flights included simultaneous measurements by a cryogenic frostpoint hygrometer (CFH) and a meteorological radiosonde (Vaisala RS41), with CFH operating either in a tandem-flight configuration or on the same payload as ALBATROSS. In the troposphere (up to 12 km altitude), the ALBATROSS retrieval agrees with both reference instruments within a mean relative difference of –2 ± 7 % with respect to RS41 and –1 ± 8 % with respect to CFH over nearly three orders of magnitude in H₂O mixing ratio. Fine‑scale vertical structures and cirrus cloud layers are consistently resolved. In the stratosphere, following the implementation of an in-flight purging system to eliminate contamination from internal moisture sources, ALBATROSS achieves an agreement of 3 ± 10 % relative to CFH between 12 and 28 km altitude. An important advantage of laser absorption spectroscopy is demonstrated through observations of balloon-induced H₂O spikes detected simultaneously by ALBATROSS and CFH in the stratosphere. These measurements reveal fast dynamic response of the laser spectrometer of ~1 s, corresponding to an effective vertical resolution of 5 m, compared to 3–4 s (15–20 m) for frostpoint hygrometry. Overall, these results demonstrate the robust performance of ALBATROSS under real atmospheric conditions and establish mid-infrared laser absorption spectroscopy as a reference-quality technique for balloon‑borne observations and long‑term monitoring of UTLS H₂O.

Accurate and sustained monitoring of water vapor (H₂O) in the upper troposphere-lower stratosphere (UTLS) is essential for quantifying its role in the atmospheric radiative balance, chemistry, and climate variability. However, in situ measurements in this region remain challenging due to the extremely low H₂O concentrations, harsh environmental conditions, and strict payload limitations on meteorological balloon platforms. ALBATROSS is a compact (3.6 kg), balloon-borne mid-infrared laser absorption spectrometer developed to provide highly accurate, SI-traceable H₂O measurements with high vertical resolution in the UTLS. Following recent comprehensive laboratory characterization, we present the first in-flight validation of ALBATROSS, based on seven balloon flights conducted between 2022 and 2026 from the meteorological observatories of Payerne, Switzerland (six flights) and Lindenberg, Germany (one flight). All flights included simultaneous measurements by a cryogenic frostpoint hygrometer (CFH) and a meteorological radiosonde (Vaisala RS41), with CFH operating either in a tandem-flight configuration or on the same payload as ALBATROSS. In the troposphere (up to 12 km altitude), the ALBATROSS retrieval agrees with both reference instruments within a mean relative difference of –2 ± 7 % with respect to RS41 and –1 ± 8 % with respect to CFH over nearly three orders of magnitude in H₂O mixing ratio. Fine‑scale vertical structures and cirrus cloud layers are consistently resolved. In the stratosphere, following the implementation of an in-flight purging system to eliminate contamination from internal moisture sources, ALBATROSS achieves an agreement of 3 ± 10 % relative to CFH between 12 and 28 km altitude. An important advantage of laser absorption spectroscopy is demonstrated through observations of balloon-induced H₂O spikes detected simultaneously by ALBATROSS and CFH in the stratosphere. These measurements reveal fast dynamic response of the laser spectrometer of ~1 s, corresponding to an effective vertical resolution of 5 m, compared to 3–4 s (15–20 m) for frostpoint hygrometry. Overall, these results demonstrate the robust performance of ALBATROSS under real atmospheric conditions and establish mid-infrared laser absorption spectroscopy as a reference-quality technique for balloon‑borne observations and long‑term monitoring of UTLS H₂O.

Accurate and sustained monitoring of water vapor (H2O) in the upper troposphere-lower stratosphere (UTLS) is essential for quantifying its role in the atmospheric radiative balance, chemistry, and climate variability. However, in situ measurements in this region remain challenging due to the extremely low H2O concentrations, harsh environmental conditions, and strict payload limitations on meteorological balloon platforms. ALBATROSS is a compact (3.6 kg), balloon-borne mid-infrared laser absorption spectrometer developed to provide highly accurate, SI-traceable H2O measurements with high vertical resolution in the UTLS. Following recent comprehensive laboratory characterization, we present the first in-flight validation of ALBATROSS, based on seven balloon flights conducted between 2022 and 2026 from the meteorological observatories of Payerne, Switzerland (six flights) and Lindenberg, Germany (one flight). All flights included simultaneous measurements by a cryogenic frostpoint hygrometer (CFH) and a meteorological radiosonde (Vaisala RS41), with CFH operating either in a tandem-flight configuration or on the same payload as ALBATROSS. In the troposphere (up to 12 km altitude), the ALBATROSS retrieval agrees with both reference instruments within a mean relative difference of –2 ± 7 % with respect to RS41 and –1 ± 8 % with respect to CFH over nearly three orders of magnitude in H2O mixing ratio. Fine‑scale vertical structures and cirrus cloud layers are consistently resolved. In the stratosphere, following the implementation of an in-flight purging system to eliminate contamination from internal moisture sources, ALBATROSS achieves an agreement of 3 ± 10 % relative to CFH between 12 and 28 km altitude. An important advantage of laser absorption spectroscopy is demonstrated through observations of balloon-induced H2O spikes detected simultaneously by ALBATROSS and CFH in the stratosphere. These measurements reveal fast dynamic response of the laser spectrometer of ~1 s, corresponding to an effective vertical resolution of 5 m, compared to 3–4 s (15–20 m) for frostpoint hygrometry. Overall, these results demonstrate the robust performance of ALBATROSS under real atmospheric conditions and establish mid-infrared laser absorption spectroscopy as a reference-quality technique for balloon‑borne observations and long‑term monitoring of UTLS H2O.

High-quality water vapor measurements from the ground to space are essential for assessing Earth's radiative balance. Water vapor contributes substantially to the uncertainty of the outgoing longwave radiation, emphasizing the need for accurate and reliable measurements. The current generation of commercially available operational radiosondes exhibits rapidly degrading reliability above the tropopause, leaving a critical observational gap. The Cryogenic Frost Point Hygrometer (CFH), a widely used instrument for water vapor profile measurements relies on the R23 coolant, which was banned under the Montreal Protocol.The Swiss H2O Hub addresses this challenge using two balloon-borne instruments and a newly developed cryogenic water vapor radiometer. These observations are complemented by the MeteoSwiss RALMO Lidar system. ALBATROSS is a compact mid-IR laser spectrometer that measures the water vapor absorption spectrum in an open-path multipass cell, providing SI-traceable water vapor data. The second balloon-borne instrument is the Peltier Cooled Frost Point Hygrometer (PCFH) that replaces the R23 coolant with a Peltier cooler and measures the frost point temperature via a thermocouple calibrated at METAS (SI-traceable). Stratospheric and mesospheric water vapor is measured with two passive radiometers: the already established MIAWARA instrument at the Zimmerwald Observatory near Bern and a newly developed cryogenic instrument operated at the laboratory building. In total, we conducted 29 balloon soundings with various payload configurations and performed detailed instrument intercomparisons for the troposphere and the lower stratosphere. Both balloon instruments achieve breakthrough requirements for the stratosphere and reach threshold requirements for the troposphere.These combined observations enabled the detection of increased water vapor volume-mixing ratios in the lower stratosphere and mesosphere above Switzerland. Both the passive radiometers and the balloon soundings revealed enhanced water vapor levels in the years following the eruption.

This paper reports on the instrumental design of a new cryogenically cooled middle-atmosphere water vapor radiometer developed by the University of Bern at the Institute of Applied Physics (IAP). Here, we present the instrument design for the breadboard stage. The key innovation of this new instrument is its cryogenically cooled front-end, which is designed to keep its size compact, reducing the required cooling power compared to existing cryogenically cooled radiometers. The advantage compared to uncooled instruments is the reduced receiver noise temperature and the possibility to extend the altitude coverage of the retrieval of water vapor profiles to even higher altitudes with better temporal resolution. The new radiometer is part of the Swiss H₂O Hub and is supposed to replace the existing 22 GHz radiometer, MIAWARA, which has been in operation at the University of Bern for over 20 years at the Zimmerwald observatory. The calibration of the new instrument includes tipping curve calibration to determine tropospheric opacity, using the sky as a cold target. An ambient load serves as the hot target for the Hot-Cold calibration, and we also explore the possibility of using frequency-switch calibration to reduce the impact of non-linearities in the receiver chain, allowing for a higher integration time of the line observation compared to other calibration techniques. The combination of a cryogenic front-end and frequency switch microwave radiometers at 22 GHz has not been previously implemented in a single instrument. In addition to detailing the instrumental design and calibration techniques, we present preliminary results of atmospheric spectra obtained with the breadboard setup.

This paper reports on the instrumental design of a new cryogenically cooled middle-atmosphere water vapor radiometer developed by the University of Bern at the Institute of Applied Physics (IAP). Here, we present the instrument design for the breadboard stage. The key innovation of this new instrument is its cryogenically cooled front-end, which is designed to keep its size compact, reducing the required cooling power compared to existing cryogenically cooled radiometers. The advantage compared to uncooled instruments is the reduced receiver noise temperature and the possibility to extend the altitude coverage of the retrieval of water vapor profiles to even higher altitudes with better temporal resolution. The new radiometer is part of the Swiss H2O Hub and is supposed to replace the existing 22 GHz radiometer, MIAWARA, which has been in operation at the University of Bern for over 20 years at the Zimmerwald observatory. The calibration of the new instrument includes tipping curve calibration to determine tropospheric opacity, using the sky as a cold target. An ambient load serves as the hot target for the Hot-Cold calibration, and we also explore the possibility of using frequency-switch calibration to reduce the impact of non-linearities in the receiver chain, allowing for a higher integration time of the line observation compared to other calibration techniques. The combination of a cryogenic front-end and frequency switch microwave radiometers at 22 GHz has not been previously implemented in a single instrument. In addition to detailing the instrumental design and calibration techniques, we present preliminary results of atmospheric spectra obtained with the breadboard setup.

This paper reports on the instrumental design of a new cryogenically cooled middle-atmosphere water vapor radiometer developed by the University of Bern at the Institute of Applied Physics (IAP). Here, we present the instrument design for the breadboard stage. The key innovation of this new instrument is its cryogenically cooled front-end, which is designed to keep its size compact, reducing the required cooling power compared to existing cryogenically cooled radiometers. The advantage compared to uncooled instruments is the reduced receiver noise temperature and the possibility to extend the altitude coverage of the retrieval of water vapor profiles to even higher altitudes with better temporal resolution. The new radiometer is part of the Swiss H2O Hub and is supposed to replace the existing 22 GHz radiometer, MIAWARA, which has been in operation at the University of Bern for over 20 years at the Zimmerwald observatory. The calibration of the new instrument includes tipping curve calibration to determine tropospheric opacity, using the sky as a cold target. An ambient load serves as the hot target for the Hot-Cold calibration, and we also explore the possibility of using frequency-switch calibration to reduce the impact of non-linearities in the receiver chain, allowing for a higher integration time of the line observation compared to other calibration techniques. The combination of a cryogenic front-end and frequency switch microwave radiometers at 22 GHz has not been previously implemented in a single instrument. In addition to detailing the instrumental design and calibration techniques, we present preliminary results of atmospheric spectra obtained with the breadboard setup.

This paper reports on the instrumental design of a new cryogenically cooled middle-atmosphere water vapor radiometer developed by the University of Bern at the Institute of Applied Physics (IAP). Here, we present the instrument design for the breadboard stage. The key innovation of this new instrument is its cryogenically cooled front-end, which is designed to keep its size compact, reducing the required cooling power compared to existing cryogenically cooled radiometers. The advantage compared to uncooled instruments is the reduced receiver noise temperature and the possibility to extend the altitude coverage of the retrieval of water vapor profiles to even higher altitudes with better temporal resolution. The new radiometer is part of the Swiss H2O Hub and is supposed to replace the existing 22 GHz radiometer, MIAWARA, which has been in operation at the University of Bern for over 20 years at the Zimmerwald observatory. The calibration of the new instrument includes tipping curve calibration to determine tropospheric opacity, using the sky as a cold target. An ambient load serves as the hot target for the Hot-Cold calibration, and we also explore the possibility of using frequency-switch calibration to reduce the impact of non-linearities in the receiver chain, allowing for a higher integration time of the line observation compared to other calibration techniques. The combination of a cryogenic front-end and frequency switch microwave radiometers at 22 GHz has not been previously implemented in a single instrument. In addition to detailing the instrumental design and calibration techniques, we present preliminary results of atmospheric spectra obtained with the breadboard setup.

This paper reports on the instrumental design of a new cryogenically cooled middle-atmosphere water vapor radiometer developed by the University of Bern at the Institute of Applied Physics (IAP). Here, we present the instrument design for the breadboard stage. The key innovation of this new instrument is its cryogenically cooled front-end, which is designed to keep its size compact, reducing the required cooling power compared to existing cryogenically cooled radiometers. The advantage compared to uncooled instruments is the reduced receiver noise temperature and the possibility to extend the altitude coverage of the retrieval of water vapor profiles to even higher altitudes with better temporal resolution. The new radiometer is part of the Swiss H2O Hub and is supposed to replace the existing 22 GHz radiometer, MIAWARA, which has been in operation at the University of Bern for over 20 years at the Zimmerwald observatory. The calibration of the new instrument includes tipping curve calibration to determine tropospheric opacity, using the sky as a cold target. An ambient load serves as the hot target for the Hot-Cold calibration, and we also explore the possibility of using frequency-switch calibration to reduce the impact of non-linearities in the receiver chain, allowing for a higher integration time of the line observation compared to other calibration techniques. The combination of a cryogenic front-end and frequency switch microwave radiometers at 22 GHz has not been previously implemented in a single instrument. In addition to detailing the instrumental design and calibration techniques, we present preliminary results of atmospheric spectra obtained with the breadboard setup.