We describe a new version of the Chicago Water Isotope Spectrometer (ChiWIS), designed for airborne measurements of vapor-phase water isotopologues in the dry upper troposphere and lower stratosphere (UTLS) aboard research aircraft. This version of the instrument is a tunable diode laser (TDL), off-axis integrated cavity output spectrometer (OA-ICOS). The instrument was designed to measure the HDO / H2O ratio in the 2017 Asian Summer Monsoon flight aboard the M-55 Geophysica during the StratoClim campaign, and so far has also flown aboard the WB-57F in the 2021 and 2022 ACCLIP campaigns. The spectrometer scans absorption lines of both H2O and HDO near 2.647 mu m wavelength in a single current sweep, and has an effective path length of 7.5 km under optimal conditions. The instrument utilizes a novel non-axially-symmetric optical component which increases the signal-to-noise ratio by a factor of 3. Ultra-polished, 4 in. (101.6 mm) diameter cavity mirrors suppress scattering losses, maximize mirror reflectivity, and yield optical fringing significantly below typical electrical noise levels. In laboratory conditions, the instrument has demonstrated a 5 s measurement precision of 3.6 ppbv and 82 pptv in H2O and HDO, respectively.

The summertime Asian Monsoon (AM) is the single most important contributor to water vapor in the UTLS and overworld stratosphere. Much of that water comes from sublimating ice, but the life cycle of the condensate lofted by overshooting convection is not well understood. We report here on insights into that life cycle derived from the first in-situ measurements of water vapor isotopic composition over the Asian Monsoon. The Chicago Water Isotope Spectrometer (ChiWIS) flew on high-altitude aircraft in the monsoon center during the StratoClim (2017) campaign out of Nepal, and in monsoon outflow during ACCLIP (2022) out of South Korea. Both campaigns sampled a broad range of convective and post-convective conditions, letting us trace how convective ice sublimates, reforms, and leaves behind characteristic isotopic signatures. We use isotopic models, along with TRACZILLA backtrajectories and convective interactions derived from cloud-top products, to follow the evolving isotopic composition along flight paths in both campaigns. Results support the wide diversity of isotopic enhancement seen in both campaigns and show how temperature cycles downstream of convective events modify environmental isotopic compositions.

Tropical cyclones (TCs) induce gravity waves which radiate outwards and upwards in concentric rings from the core, or spiral formations. These waves deposit momentum at higher altitudes and create temperature fluctuations that can control local cirrus formation. The induced cold temperature fluctuation from the cold phase of the gravity wave can induce relative humidities above the threshold needed for condensate formation. However, TC-induced gravity waves are mainly known from simulations and satellite observations, which are unable to fully capture the fine scale structure of the perturbations. An overflight of TC Hinnamnor during the 2022 ACCLIP campaign provides new, high resolution data. We find a gravity wave amplitude of 4.7 K (centerline to peak) above the TC at 87 mb (~17.6 km). ERA5 reanalysis underestimates the amplitude of the TC-induced gravity waves: amplitudes in ERA5 at 70 mb are more than three times smaller (1.3 K), and the frequency is two times larger, than in-situ measurements. The in-situ-measured amplitude over Hinnamnor is large but not unprecedented for stratospheric gravity waves. We show that in ERA5, TCs regularly produce gravity waves, albeit their amplitudes are underestimated, and that TC Hinnamnor has temperature variations representative of other TCs. Underestimates of gravity wave amplitude can result in an underestimate of cirrus formation in the cold phase of the wave under appropriate conditions.

The isotopic composition of water vapor provides a novel source of information on the convective origins and life cycle of water in the UT/LS region. The Chicago Water Isotope Instrument (ChiWIS) has now provided a deep catalog of measurements over both the North American and Asian summer monsoons, from both European and American high-altitude aircraft campaigns: StratoClim (Stratospheric and upper tropospheric processes for better climate predictions) in 2017, and ACCLIP (Asian summer monsoon Chemical and CLimate Impact Project in 2021-2022. Isotopic measurements show the importance of deep convection in carrying moisture as lofted ice, but also the strong distinction between the two monsoons, with higher convective influence and more complete sublimation over North America. Convective outflow remnants remain distinct for as long as two weeks and govern the locations of formations of in-situ ‘secondary cirrus’. We show the consistency and power of the complete dataset collected and examples highlighting the utility of isotopic measurements in understanding the life cycle of UT/LS water, including convective inputs, mixing, sedimentation, and sublimation.

The 2022 ACCLIP (Asian summer monsoon Chemical and CLimate Impact Project) high-altitude aircraft campaign has provided a sampling of the diversity of processes that affect moisture transport in the upper troposphere / lower stratosphere (UT/LS). We report here on ACCLIP observations of water vapor isotopologues, which trace the origin and microphysical history of water vapor. Measurements with the Chicago Water Isotope Instrument (ChiWIS) show isotopic variations in the UT/LS that correlate with airmass history, and 100-150 ‰ variation even at the same water content. ACCLIP flights out of Osan, South Korea sampled monsoon anticyclone outflow with CO values over 200 ppb, recent local convection, extensive in-situ cirrus, and an overflight of tropical cyclone Hinnanmor showing strong isotopic depletion. Flights out of Houston, TX sampled week-old remnants of sublimated ice from deep convection, producing enriched vapor, and possible mixing of convective overshoots with stratospheric air before sinking. We show through case studies from both Asia and North America that isotopologues provide a sensitive diagnostic of ice sublimation, and demonstrate how different meteorological contexts produce distinct isotopic signatures.

The eruption of the submarine Hunga Tonga Hunga Hapaii volcano on 15 January 2022 was associated with a powerful blast that injected sulfur and water to altitudes up to 58 km. In this study, we combine the data from various satellite instruments (MLS, OMPS-LP, CALIOP, SAGE III, Aeolus, COSMIC-2, ACE-FTS, GOES, Himawari), ground-based lidars at various locations, meteorological radiosoundings as well as model simulation using CLaMS chemistry-transport model to investigate the evolution of the stratospheric moisture and sulfate aerosol plume at a wide range of scales—from minutes and kilometres to monthly and planetary scale. We show that due to extreme altitude reach of the eruption, the volcanic plume has circumnavigated the Earth in only one week and dispersed nearly pole-to-pole in three months. The observations provide evidence for an unprecedented increase in the global stratospheric water mass by 13% as compared to climatological levels. As there are no efficient sinks of water vapour in the stratosphere, this perturbation is expected to persist several years. The eruption has also led to a 5-fold increase in the stratospheric aerosol load, the highest in the last three decades yet factor of 6 smaller than the previous major eruption of Mt Pinatubo in 1991.The unique nature and magnitude of the global stratospheric perturbation by the Hunga eruption ranks it among the most remarkable climatic events in the modern observation era. Given the expected longevity of the stratospheric humidity perturbation, the Hunga eruption can be said to have initiated a new era in stratospheric gaseous chemistry and particle microphysics with a wide range of potential long-lasting repercussions for the global stratospheric composition and dynamics. The eruption has thus provided a unique natural testbed, lending itself to studies of climate sensitivity to strong change in both stratospheric gaseous and particulate composition.Spanning more than one year, the satellite and ground-based observations available to-date enable the first accurate assessment of the annual-scale stratospheric aftermath of the Hunga Tonga eruption, uncovering its climate-altering capacity.

Abstract. Ice growth from vapor deposition is an important process for the evolution of cirrus clouds, but the physics of depositional ice growth at the low temperatures (<235 K) characteristic of the upper troposphere and lower stratosphere is not well understood. Surface attachment kinetics, generally parameterized as a deposition coefficient αD, control ice crystal habit and also may limit growth rates in certain cases, but significant discrepancies between experimental measurements have not been satisfactorily explained. Experiments on single ice crystals have previously indicated the deposition coefficient is a function of temperature and supersaturation, consistent with growth mechanisms controlled by the crystal's surface characteristics. Here we use observations from cloud chamber experiments in the Aerosol Interactions and Dynamics in the Atmosphere (AIDA) aerosol and cloud chamber to evaluate surface kinetic models in realistic cirrus conditions. These experiments have rapidly changing temperature, pressure, and ice supersaturation such that depositional ice growth may evolve from diffusion limited to surface kinetics limited over the course of a single experiment. In Part 1, we describe the adaptation of a Lagrangian parcel model with the Diffusion Surface Kinetics Ice Crystal Evolution (DiSKICE) model (Zhang and Harrington, 2014) to the AIDA chamber experiments. We compare the observed ice water content and saturation ratios to that derived under varying assumptions for ice surface growth mechanisms for experiments simulating ice clouds between 180 and 235 K and pressures between 150 and 300 hPa. We found that both heterogeneous and homogeneous nucleation experiments at higher temperatures (>205 K) could generally be modeled consistently with either a constant deposition coefficient or the DiSKICE model assuming growth on isometric crystals via abundant surface dislocations. Lower-temperature experiments showed more significant deviations from any depositional growth model, with different ice growth rates for heterogeneous and homogeneous nucleation experiments.

During the StraoClim Geophysica campaign, moist air with total water mixing ratios up to 200 ppmv was observed within the Asian Summer Monsoon anticyclone, above the local cold point tropopause (CPT). High ozone mixing ratios of up to 250 ppbv suggest substantial stratospheric moistening. We used 60-day back- and forward trajectories to classify the observations into two groups based on their distance to the Lagrangian dry point (LDP): those where the LDP has just occurred or is still expected to occur (type A, 0-3 days from LDP), and those where the LDP was passed 15-35 days before (type B). We applied a microphysical box model (CLaMS-Ice) and a simple freeze drying model (FDM) to simulate the evolution of ice mixing ratios along the trajectories. Type A air masses, with ice mixing ratios larger than 1 ppm, underwent multiple transitions between the solid and gas phase, in good agreement with CALIPSO ice and MLS water vapor observations of around 5 ppm. In contrast, type B air masses showed less agreement with CALIPSO ice and significantly overestimated MLS observations when CLaMS-Ice or FDM were applied. However, water vapor reconstructed from the LDP of the merged back- and forward trajectories agreed much better with MLS, indicating that the wet air masses of type B, observed up to 1.7 km above the CPT, are not representative of the large-scale water vapor distribution detected by MLS. Our results suggest that the full backward and forward evolution of the sampled air masses needs to be considered when inferring stratospheric moistening in the Asian monsoon region. Water vapor concentrations set by LDPs seem to be a better proxy for the stratospheric water vapor budget than rare observations of enhanced water mixing ratios above the local CPT. These observations call into question their applicability to quantify long-term stratospheric water vapor trends.

Data sets used in the publication Re-evaluating cloud chamber constraints on depositional ice growth in cirrus clouds-- Part 1: Model description and sensitivity tests K.D. Lamb, J. Harrington, B.W. Clouser, E.J. Moyer, L. Sarkozy, V. Ebert, O. Möhler, H. Saathoff. Atmospheric Chemistry and Physics (2023)

Abstract. During the StratoClim Geophysica campaign, air with total water mixing ratios up to 200 ppmv and ozone up to 250 ppbv was observed within the Asian summer monsoon anticyclone up to 1.7 km above the local cold point tropopause (CPT). To investigate the temporal evolution of enhanced water vapor being transported into the stratosphere, we conduct forward trajectory simulations using both a microphysical and an idealized freeze-drying model. The models are initialized at the measurement locations and the evolution of water vapor and ice is compared with satellite observations of MLS and CALIPSO. Our results show that these extremely high water vapor values observed above the CPT are very likely to undergo significant further freeze-drying due to experiencing extremely cold temperatures while circulating in the anticyclonic dehydration carousel. We also use the Lagrangian dry point (LDP) of the merged backward and forward trajectories to reconstruct the water vapor fields. The results show that the extremely high water vapor mixed in with the stratospheric air has a negligible impact on the overall water vapor budget. The LDPs are a better proxy for the large-scale water vapor distributions in the stratosphere during this period.

During the StratoClim Geophysica campaign, air with total water mixing ratios up to 200 ppmv and ozone up to 250 ppbv was observed within the Asian summer monsoon anticyclone up to 1.7 km above the local cold-point tropopause (CPT). To investigate the temporal evolution of enhanced water vapor being transported into the stratosphere, we conduct forward trajectory simulations using both a microphysical and an idealized freeze-drying model. The models are initialized at the measurement locations and the evolution of water vapor and ice is compared with satellite observations of MLS and CALIPSO. Our results show that these extremely high water vapor values observed above the CPT are very likely to undergo significant further freeze-drying due to experiencing extremely cold temperatures while circulating in the anticyclonic “dehydration carousel”. We also use the Lagrangian dry point (LDP) of the merged back-and-forward trajectories to reconstruct the water vapor fields. The results show that the extremely high water vapor mixed with the stratospheric air has a negligible impact on the overall water vapor budget. The LDP mixing ratios are a better proxy for the large-scale water vapor distributions in the stratosphere during this period.

The eruption of the submarine Hunga volcano in January 2022 was associated with a powerful blast that injected volcanic material to altitudes up to 58 km. From a combination of various types of satellite and ground-based observations supported by transport modeling, we show evidence for an unprecedented increase in the global stratospheric water mass by 13% relative to climatological levels, and a 5-fold increase of stratospheric aerosol load, the highest in the last three decades. Owing to the extreme injection altitude, the volcanic plume circumnavigated the Earth in only 1 week and dispersed nearly pole-to-pole in three months. The unique nature and magnitude of the global stratospheric perturbation by the Hunga eruption ranks it among the most remarkable climatic events in the modern observation era, with a range of potential long-lasting repercussions for stratospheric composition and climate.

The Asian monsoon anticyclone (AMA) represents one of the wettest regions in the lower stratosphere (LS) and is a key contributor to the global annual maximum in LS water vapour. While the AMA wet pool is linked with persistent convection in the region and horizontal confinement of the anticyclone, there remain ambiguities regarding the role of tropopause-overshooting convection in maintaining the regional LS water vapour maximum. This study tackles this issue using a unique set of observations from aboard the high-altitude M55-Geophysica aircraft deployed in Nepal in summer 2017 within the EU StratoClim project. We use a combination of airborne measurements (water vapour, ice water, water isotopes, cloud backscatter) together with ensemble trajectory modelling coupled with satellite observations to characterize the processes controlling water vapour and clouds in the confined lower stratosphere (CLS) of the AMA. Our analysis puts in evidence the dual role of overshooting convection, which may lead to hydration or dehydration depending on the synoptic-scale tropopause temperatures in the AMA. We show that all of the observed CLS water vapour enhancements are traceable to convective events within the AMA and furthermore bear an isotopic signature of the overshooting process. A surprising result is that the plumes of moist air with mixing ratios nearly twice the background level can persist for weeks whilst recirculating within the anticyclone, without being subject to irreversible dehydration through ice settling. Our findings highlight the importance of convection and recirculation within the AMA for the transport of water into the stratosphere.

Abstract. In situ measurements in the climatically important upper troposphere / lower stratosphere (UTLS) are critical for understanding controls on cloud formation, the entry of water into the stratosphere, and hydration/dehydration of the tropical tropopause layer. Accurate in situ measurement of water vapor in the UTLS however is difficult because of low water vapor concentrations (< 5 ppmv) and a challenging low temperature/pressure environment. The StratoClim campaign out of Kathmandu, Nepal in July and August 2017, which made the first high-altitude aircraft measurements in the Asian Summer Monsoon (ASM), also provided an opportunity to intercompare three in situ hygrometers mounted on the M-55 Geophysica: ChiWIS (Chicago Water Isotope Spectrometer), FISH (Fast In situ Stratospheric Hygrometer), and FLASH (Fluorescent Lyman-α Stratospheric Hygrometer). Instrument agreement was very good, suggesting no intrinsic technique-dependent biases: ChiWIS measures by mid-infrared laser absorption spectroscopy and FISH and FLASH by Lyman-α induced fluorescence. In clear-sky UTLS conditions (H2O < 10 ppmv), mean differences between ChiWIS and FLASH were only −1.42 % and those between FISH and FLASH only −1.47 %. Agreement between ChiWIS and FLASH for in-cloud conditions is even tighter, at +0.74 %. In general, ChiWIS and FLASH agreed to better than 10 % for 92 % (87 %) of clear-sky (in-cloud) datapoints. Agreement between FISH and FLASH to 10 % occurred in 78 % of clear-sky datapoints. Estimated realized instrumental precision in UTLS conditions was 0.05, 0.1, and 0.2 ppmv for ChiWIS, FISH, and FLASH, respectively. This level of accuracy and precision allows the confident detection of fine-scale spatial structures in UTLS water vapor required for understanding the role of convection and the ASM in the stratospheric water vapor budget.

Deployment of the high-altitude M55-Geophysica aircraft in Kathmandu during Summer 2017 within StratoClim campaign has yielded a wealth of unique high-resolution measurements in the Asian Monsoon Anticyclone (AMA). In a particular flight (F8, 10 August 2017) the aircraft flew at the cold-point tropopause level through active overshoots and their outflows minutes to hours old. The measurements reveal up to 2500 ppmv of ice water above 17 km in large aggregated ice crystals up to 700 µm in diameter. Smaller crystals were observed as high as 18.8 km (410 K). Tracer and thermodynamical measurements show manifestations of vigorous vertical motions and provide evidence for ongoing mixing of tropospheric and stratospheric air around the tropopause. We use an ensemble of airborne and satellite measurements inside and downwind of convective overshoots together with trajectory modeling to characterize the impact of overshooting convection on the thermodynamical structure and chemical composition of the Asian tropopause layer. The effect of cross-tropopause convective transport on the Asian lower stratospheric water vapour is discussed.

The Asian monsoon anticyclone (AMA) represents one of the wettest regions in the lower stratosphere (LS) and is a key contributor to the global annual maximum in LS water vapour. While the AMA wet pool is linked with persistent convection in the region and horizontal confinement of the anticyclone, there remain ambiguities regarding the role of tropopause-overshooting convection in maintaining the regional LS water vapour maximum. This study tackles this issue using a unique set of observations from aboard the high-altitude M55-Geophysica aircraft deployed in Nepal in summer 2017 within the EU StratoClim project. We use a combination of airborne measurements (water vapour, ice water, water isotopes, cloud backscatter) together with ensemble trajectory modelling coupled with satellite observations to characterize the processes controlling water vapour and clouds in the confined lower stratosphere (CLS) of the AMA. Our analysis puts in evidence the dual role of overshooting convection, which may lead to hydration or dehydration depending on the synoptic-scale tropopause temperatures in the AMA. We show that all of the observed CLS water vapour enhancements are traceable to convective events within the AMA and furthermore bear an isotopic signature of the overshooting process. A surprising result is that the plumes of moist air with mixing ratios nearly twice the background level can persist for weeks whilst recirculating within the anticyclone, without being subject to irreversible dehydration through ice settling. Our findings highlight the importance of convection and recirculation within the AMA for the transport of water into the stratosphere.

High-altitude cirrus clouds are climatically important: their formation freeze-dries air ascending to the stratosphere to its final value, and their radiative impact is disproportionately large. However, their formation and growth are not fully understood, and multiple in situ aircraft campaigns have observed frequent and persistent apparent water vapor supersaturations of 5 %–25 % in ultracold cirrus (T<205 K), even in the presence of ice particles. A variety of explanations for these observations have been put forth, including that ultracold cirrus are dominated by metastable ice whose vapor pressure exceeds that of hexagonal ice. The 2013 IsoCloud campaign at the Aerosol Interaction and Dynamics in the Atmosphere (AIDA) cloud and aerosol chamber allowed explicit testing of cirrus formation dynamics at these low temperatures. A series of 28 experiments allows robust estimation of the saturation vapor pressure over ice for temperatures between 189 and 235 K, with a variety of ice nucleating particles. Experiments are rapid enough (∼10 min) to allow detection of any metastable ice that may form, as the timescale for annealing to hexagonal ice is hours or longer over the whole experimental temperature range. We show that in all experiments, saturation vapor pressures are fully consistent with expected values for hexagonal ice and inconsistent with the highest values postulated for metastable ice, with no temperature-dependent deviations from expected saturation vapor pressure. If metastable ice forms in ultracold cirrus clouds, it appears to have a vapor pressure indistinguishable from that of hexagonal ice to within about 4.5 %.

We describe a new tunable diode laser (TDL) absorption instrument, the Chicago Water Isotope Spectrometer, designed for measurements of vapor-phase water isotopologues in conditions characteristic of the upper troposphere [190-235 K temperature and 2-500 parts per million volume (ppmv) water vapor]. The instrument is primarily targeted for measuring the evolving ratio of HDO/H2O during experiments in the "Aerosol Interaction and Dynamics in the Atmosphere" (AIDA) cloud chamber. The spectrometer scans absorption lines of both H2O and HDO near the 2.64 µm wavelength in a single current sweep, increasing the accuracy of isotopic ratio measurements. At AIDA, the instrument is configured with a 256-m path length White cell for in situ measurements, and effective sensitivity can be augmented by enhancing the HDO content of chamber water vapor by an order of magnitude. The instrument has participated to date in the 2012-2013 IsoCloud campaigns studying isotopic partitioning during the formation of cirrus clouds and in the AquaVIT-II instrument intercomparison campaign. Realized precisions for 1-s measurements during these campaigns were 22 ppbv for H2O and 16 ppbv for HDO, equivalent to relative precisions of less than 0.5% for each species at 8 ppmv water vapor. The 1-s precision of the [HDO]/[H2O] ratio measurement ranged from 1.6‰ to 5.6‰ over the range of experimental conditions. H2O measurements showed agreement with calculated saturation vapor pressure to within 1% in conditions of sublimating ice and agreement with other AIDA instruments (the AIDA SP-APicT reference TDL instrument and an MBW 373LX chilled mirror hygrometer) to within 2.5% and 3.8%, respectively, over conditions suitable for all instruments (temperatures from 204 K to 234 K and H2O content equivalent to 15-700 ppmv at 200 hPa).

In-situ measurements of the HDO/H2O isotopic ratio from the Chicago Water Isotope Spectrometer (ChiWIS) during the 2017 StratoClim campaign help diagnose convective processes in the Asian Monsoon. Isotopic measurements show enormous diversity in isotopic composition, likely reflecting degree of recent convective influence. Eight flights in July—August sampled a wide range of convective influence at near-tropopause altitudes, with timescales of minutes to weeks, and mean isotopic compositions from -700 per mil in recent convective outflow to -350 per mil in more aged air that is at least several days from last convective influence. Above the tropopause, we use isotopic composition to understand the fate of convective remnants. Isotopic measurements suggest much in-situ cirrus measured during StratoClim campaign is actually secondary cirrus which has reformed in an area of prior convective moistening. These flights allow detailed comparison between North American and Asian monsoons, and we compare StratoClim results to both satellite and in-situ measurements in other monsoon and tropical locations. Finally, we discuss prospects for detection and interpretation of convective remnants during the in the 2021/2022 ACCLIP campaign.