Alistair Bell

The 2022 Hunga Tonga (HT) eruption injected an unprecedented amount of water vapour (WV) into the stratosphere and mesosphere, emerging as a potential multi-year driver of variability in those layers and associated climate feedbacks. Using satellite and reanalysis datasets, and ensemble simulations with the SOCOLv4 model, with and without the HT forcing, we diagnose the chain of processes linking the eruption to the exceptionally strong Northern Hemisphere stratospheric polar vortex (SPV) observed in winter 2024/2025. Satellite data show “tongues” of enhanced WV descending from the mesosphere into the polar stratosphere, collocated with ozone reductions and persistent cold anomalies, alongside a record-strong SPV. Our model with the HT forcing can reproduce the main structure of the descending plume, its effects on chemistry, and the SPV strengthening, albeit with underestimated magnitudes and an earlier onset than observed. Offline radiative transfer calculations indicate that the WV and ozone anomalies drive net radiative cooling over the polar stratosphere, which steepens meridional temperature gradients and thereby intensifies the Arctic polar vortex. The observed 2024/2025 winter thus represents a plausible manifestation of HT-induced vortex variability. Our simulations also predicted the observed shift towards more favorable conditions for sudden stratospheric warmings in the winter 2025/2026 as the WV forcing declines.

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.

For climate change research, it is important to have multiple independent measurement techniques. The ground-based microwave radiometer at Bern has been operated since 1994 and allows the independent derivation of the linear trend of the integrated water vapour column or integrated water vapour (IWV). According to the Clausius-Clapeyron equation, the water vapour saturation increases with increase of temperature. There is also a water vapour feedback since water vapour is a natural greenhouse gas and amplifies man-made global warming. In Switzerland, climate change is stronger than at many other places in the world. We analyse observations of the tropospheric water radiometer (TROWARA) which monitored IWV above Bern from 1995 to 2025. The relative IWV increase is 5.1 %/decade. Evaluation of coincident IWV data from ERA5 (reanalysis of European Centre for Medium-Range Weather Forecasts) gives a trend of 3.7 %/decade. The ERA5 surface air temperature in Bern increased by 0.47K/decade from 1995 to 2025. Thus, we get 10.9 % more IWV for a 1 K increase in case of TROWARA and 7.8 % more IWV for a 1K increase in case of ERA5. Though the IWV trends of TROWARA and ERA5 slightly differ, both datasets agree in the fact that water vapour above the Swiss Plateau significantly increased by 11 % or 16 % from 1995 to 2025. This strong increase of water vapour certainly has an impact on weather, climate, and hydrology.

For climate change research, it is important to have multiple independent measurement techniques. The ground-based microwave radiometer at Bern has been operated since 1994 and allows the independent derivation of the linear trend of the integrated water vapour column or integrated water vapour (IWV). According to the Clausius-Clapeyron equation, the water vapour saturation increases with increase of temperature. There is also a water vapour feedback since water vapour is a natural greenhouse gas and amplifies man-made global warming. In Switzerland, climate change is stronger than at many other places in the world. We analyse observations of the tropospheric water radiometer (TROWARA) which monitored IWV above Bern from 1995 to 2025. The relative IWV increase is 5.1 %/decade. Evaluation of coincident IWV data from ERA5 (reanalysis of European Centre for Medium-Range Weather Forecasts) gives a trend of 3.7 %/decade. The ERA5 surface air temperature in Bern increased by 0.47K/decade from 1995 to 2025. Thus, we get 10.9 % more IWV for a 1 K increase in case of TROWARA and 7.8 % more IWV for a 1K increase in case of ERA5. Though the IWV trends of TROWARA and ERA5 slightly differ, both datasets agree in the fact that water vapour above the Swiss Plateau significantly increased by 11 % or 16 % from 1995 to 2025. This strong increase of water vapour certainly has an impact on weather, climate, and hydrology.

For climate change research, it is important to have multiple independent measurement techniques. The ground-based microwave radiometer at Bern has been operated since 1994 and allows the independent derivation of the linear trend of the integrated water vapour column or integrated water vapour (IWV). According to the Clausius-Clapeyron equation, the water vapour saturation increases with increase of temperature. There is also a water vapour feedback since water vapour is a natural greenhouse gas and amplifies man-made global warming. In Switzerland, climate change is stronger than at many other places in the world. We analyse observations of the tropospheric water radiometer (TROWARA) which monitored IWV above Bern from 1995 to 2025. The relative IWV increase is 5.1 %/decade. Evaluation of coincident IWV data from ERA5 (reanalysis of European Centre for Medium-Range Weather Forecasts) gives a trend of 3.7 %/decade. The ERA5 surface air temperature in Bern increased by 0.47K/decade from 1995 to 2025. Thus, we get 10.9 % more IWV for a 1 K increase in case of TROWARA and 7.8 % more IWV for a 1K increase in case of ERA5. Though the IWV trends of TROWARA and ERA5 slightly differ, both datasets agree in the fact that water vapour above the Swiss Plateau significantly increased by 11 % or 16 % from 1995 to 2025. This strong increase of water vapour certainly has an impact on weather, climate, and hydrology.

For climate change research, it is important to have multiple independent measurement techniques. The ground-based microwave radiometer at Bern has been operated since 1994 and allows the independent derivation of the linear trend of the integrated water vapour column or integrated water vapour (IWV). According to the Clausius-Clapeyron equation, the water vapour saturation increases with increase of temperature. There is also a water vapour feedback since water vapour is a natural greenhouse gas and amplifies man-made global warming. In Switzerland, climate change is stronger than at many other places in the world. We analyse observations of the tropospheric water radiometer (TROWARA) which monitored IWV above Bern from 1995 to 2025. The relative IWV increase is 5.1 %/decade. Evaluation of coincident IWV data from ERA5 (reanalysis of European Centre for Medium-Range Weather Forecasts) gives a trend of 3.7 %/decade. The ERA5 surface air temperature in Bern increased by 0.47K/decade from 1995 to 2025. Thus, we get 10.9 % more IWV for a 1 K increase in case of TROWARA and 7.8 % more IWV for a 1K increase in case of ERA5. Though the IWV trends of TROWARA and ERA5 slightly differ, both datasets agree in the fact that water vapour above the Swiss Plateau significantly increased by 11 % or 16 % from 1995 to 2025. This strong increase of water vapour certainly has an impact on weather, climate, and hydrology.