The isotopic composition of water vapor in the upper troposphere and lower stratosphere (UTLS) can be used to understand and constrain the budget and pathways of water transport into that region of the atmosphere. Measurements of the water isotopic composition help further understanding of the region's chemistry, radiative budget, and the sublimation and growth of polar stratospheric clouds and high-altitude cirrus, both of which are also important to stratospheric chemistry and Earth's radiation budget. Here we present the first intercomparison of water isotopic composition (delta D) using in situ measurements from the ChiWIS, Harvard ICOS, and Hoxotope instruments and satellite retrievals from ACE-FTS. The in situ data come from the AVE-WIIF, TC4, CR-AVE, StratoClim, and ACCLIP field campaigns, and satellite retrievals of isotopic composition are derived from the ACE-FTS v5.2 data set. We find that in all campaign intervals, satellite retrievals above about 14 km altitude are depleted by up to 150 parts per thousand with respect to in situ measurements. This difference persists even in transit flights through stratospheric air in high-latitude regions, which should be relatively free of observational biases present in other regions. We also use in situ measurements from the ChiWIS instrument, which has flown in both the Asian Summer Monsoon (AM) and the North American Monsoon (NAM), to confirm the isotopic enhancement in delta D observed in satellite retrievals above the NAM.

Abstract. We share with the community a concise Pythonic codebase for hands-on exploration of cloud microphysics modeling concepts. It includes an adiabatic air-parcel model simulating diffusional droplet growth – leading to condensation, activation, evaporation, and ripening; as well as a box model resolving collisional growth of particles using the Super-Droplet Method Monte-Carlo scheme. Leveraging Pythonic abstractions and programmatic unit handling, the code combines pseudo-code-level readability, auditability, and dimensional-correctness enforcement with high performance through just-in-time (JIT) compilation, which generates native machine code on the fly, avoiding bytecode interpretation overhead for compute-heavy routines. With the goal of illustrating how concise yet complete implementations support both teaching and research software engineering, the paper includes and narrates the entire code, plotting logic included. The code runs with a single click "in the cloud" – on Google Colab or other Jupyter hubs, and thus constitutes a suitable resource for self-study or for a short course in microphysics modeling. Presented technical solutions originate from the PySDM particle-based simulation package, but are applicable in a wider scope and independently of PySDM as exemplified here. The paper includes section-wise exploratory exercises designed to guide further application of the methods and concepts discussed.

Abstract. 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 µm 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.

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.

Marine cloud brightening (MCB) is the deliberate injection of aerosol particles into shallow marine clouds to increase their reflection of solar radiation and reduce the amount of energy absorbed by the climate system. From the physical science perspective, the consensus of a broad international group of scientists is that the viability of MCB will ultimately depend on whether observations and models can robustly assess the scale-up of local-to-global brightening in today’s climate and identify strategies that will ensure an equitable geographical distribution of the benefits and risks associated with projected regional changes in temperature and precipitation. To address the physical science knowledge gaps required to assess the societal implications of MCB, we propose a substantial and targeted program of research—field and laboratory experiments, monitoring, and numerical modeling across a range of scales.

de Jong et al., (2023). New developments in PySDM and PySDM-examples v2: collisional breakup, immersion freezing, dry aerosol initialization, and adaptive time-stepping. Journal of Open Source Software, 8(84), 4968, https://doi.org/10.21105/joss.04968

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.

Tipping points are abrupt, drastic, and often irreversible changes in the evolution of non-stationary and chaotic dynamical systems. For instance, increased greenhouse gas concentrations are predicted to lead to drastic decreases in low cloud cover, referred to as a climatological tipping point. In this paper, we learn the evolution of such non-stationary dynamical systems using a novel recurrent neural operator (RNO), which learns mappings between function spaces. After training RNO on only the pre-tipping dynamics, we employ it to detect future tipping points using an uncertainty-based approach. In particular, we propose a conformal prediction framework to forecast tipping points by monitoring deviations from physics constraints (such as conserved quantities and partial differential equations), enabling forecasting of these abrupt changes along with a rigorous measure of uncertainty. We illustrate our proposed methodology on non-stationary ordinary and partial differential equations, such as the Lorenz-63 and Kuramoto-Sivashinsky equations. We also apply our methods to forecast a climate tipping point in stratocumulus cloud cover. In our experiments, we demonstrate that even partial or approximate physics constraints can be used to accurately forecast future tipping points.

The purpose of this study, and the companion paper Singer and Schneider (2023), is to develop a simple model to explain mechanisms controlling stratocumulus-cumulus transitions.In this second paper, we describe the extended bulk model coupled to a slab ocean that is forced only with a prescribed CO 2 concentration.We calibrate key parameters of this model based on high-resolution simulations.The simple model, like the high-resolution simulations, shows that stratocumulus clouds break up at very high CO 2 concentrations and that the boundary layer exhibits hysteresis, remaining in a cumulus-like state until CO 2 is reduced significantly past the breakup threshold.We conclude by showing a series of mechanism-denial experiments that highlight the importance of surface temperature and water vapor feedbacks on the stratocumulus breakup.

Stratocumulus clouds occurring over eastern subtropical ocean basins dominate climatological cloudiness at low latitudes and are important for Earth's global radiative energy budget.Further west, there is an observed transition to scattered cumulus cloud cover, the socalled stratocumulus-cumulus transition (SCT), which is controlled by changing large-scale meteorological conditions.To explore the mechanisms driving the SCT, we present an extension of the traditional bulk boundary layer model (bulk model) for stratocumulus-topped boundary layers that explicitly considers cloud decoupling from the surface and subsequent cloud fraction reduction.By forcing our bulk model with boundary conditions from ERA5 reanalysis, we are able to quantitatively capture the climatological SCT.We compare the modeled SCT to satellite observations across transects of the eastern Pacific and show good agreement.The bulk model highlights the importance of surface temperature variations across the transect for controlling the SCT.We also use the bulk model to explore the sensitivity of low clouds to changing meteorological factors and show how this sensitivity is nonlinear and depends on cloud regime. 2 SIGNIFICANCE STATEMENT: The purpose of this study, and the companion paper Singer and Schneider (2023), is to develop a simple model to explain mechanisms controlling stratocumulus-cumulus transitions.In this first work, we describe the bulk model in cases with prescribed boundary conditions.We validate its relevance by showing how its predictions of stratocumulus-cumulus transitions across the subtropical eastern Pacific ocean basin compares to observations from satellites.We use the simple model to explore how boundary layer clouds are sensitive to different large-scale environmental factors and show how this sensitivity is nonlinear, depending on cloud regime.

Among models participating in the Coupled Model Intercomparison Project phase 6 (CMIP6), here we show that the magnitude of the tropical low cloud feedback, which contributes considerably to uncertainty in estimates of climate sensitivity, is intimately linked to tropical deep convection and its effects on the tropical atmospheric overturning circulation. First, a reduction in tropical ascent area and an increased frequency of heavy precipitation result in high cloud reduction and upper-tropospheric drying, which increases longwave cooling and reduces subsidence weakening, favoring low cloud reduction (Radiation-Subsidence Pathway). Second, increased longwave cooling decreases tropospheric stability, which also reduces subsidence weakening and low cloudiness (Stability-Subsidence Pathway). In summary, greater high cloud reduction and upper-tropospheric drying (negative longwave feedback) lead to a more positive cloud feedback among CMIP6 models by contributing to a greater reduction in low cloudiness (positive shortwave feedback). Varying strengths of the two pathways contribute considerably to the intermodel spread in climate sensitivity.

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.

PySDM is an open-source Python package for simulating the dynamics of particles undergoing condensational and collisional growth, interacting with a fluid flow and subject to chemical composition changes. It is intended to serve as a building block for process-level as well as computational-fluid-dynamics simulation systems involving representation of a continuous phase (air) and a dispersed phase (aerosol), with PySDM being responsible for representation of the dispersed phase. The PySDM package core is a Pythonic high-performance implementation of the Super-Droplet Method (SDM) Monte-Carlo algorithm for representing collisional growth, hence the name. PySDM has two alternative parallel number-crunching backends available: multi-threaded CPU backend based on Numba and GPU-resident backend built on top of ThrustRTC. The usage examples are built on top of four simple atmospheric cloud modelling frameworks: box, adiabatic parcel, single-column and 2D prescribed flow kinematic models. In addition, the package ships with tutorial code depicting how PySDM can be used from Julia and Matlab.

The Coronavirus Disease 2019 (COVID‐19) pandemic led to a widespread reduction in aerosol emissions. Using satellite observations and climate model simulations, we study the underlying mechanisms of the large decreases in solar clear‐sky reflection (3.8 W m−2 or 7%) and aerosol optical depth (0.16 W m−2 or 32%) observed over the East Asian Marginal Seas in March 2020. By separating the impacts from meteorology and emissions in the model simulations, we find that about one‐third of the clear‐sky anomalies can be attributed to pandemic‐related emission reductions, and the rest to weather variability and long‐term emission trends. The model is skillful at reproducing the observed interannual variations in solar all‐sky reflection, but no COVID‐19 signal is discerned. The current observational and modeling capabilities will be critical for monitoring, understanding, and predicting the radiative forcing and climate impacts of the ongoing crisis.

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.