Abstract This study examines thermodynamic and microphysical relationships between in‐cloud updrafts and downdrafts within tropical deep convective anvil clouds, using aircraft in situ measurements from the Aerosol, Cloud, Precipitation, and Radiation Interactions and Dynamics of Convective Cloud Systems–Cloud Processes of the Main Precipitation Systems in Brazil (ACRIDICON–CHUVA) field campaign. Our results extend current knowledge of in‐cloud drafts to higher altitudes and add detail regarding their microphysical characteristics. Key findings include observational evidence of supersaturated downdrafts and higher cloud particle number concentrations in downdrafts than in updrafts within optically thick anvil clouds. Mean draft diameters exhibit a broadening trend with altitude, while mean air‐mass fluxes decrease due to decreasing vertical velocity intensity and atmospheric density. Cloud water content is comparable in up‐ and downdrafts and increases with vertical velocity at all levels, without a clear trend with altitude. At upper levels (10–14 km), large negative vertical velocities are observed in supersaturated regions ; therefore, upper‐level downdrafts do not appear to be driven by latent cooling. Optically thick cloud regions exhibit higher median RH and ice water content (IWC) compared with optically thin clouds, with downdrafts () showing higher median and IWC than updrafts. Particle size distributions are similar for up‐ and downdrafts of comparable strength. With altitude, maximum particle size increases, and the number concentration of larger particles () increases more strongly in stronger drafts than in weaker drafts. Stronger drafts also exhibit higher concentrations of particles smaller than 100 m at upper levels. Optically thick regions exhibit higher particle number concentrations across all sizes than optically thin regions. Notably, within the optically thick regions, downdrafts show higher particle number concentrations than updrafts for all sizes. Overall, the characteristics of updrafts and downdrafts in anvil clouds exhibit strong similarities, suggesting the influence of processes, such as updraft–downdraft mixing and inertia‐driven downdrafts originating from older updraft cores, shaping microphysical and dynamical properties of anvil clouds.

Tropospheric and stratospheric airmasses are separated by the tropopause. Here we investigate the lapse rate tropopause and the cold point tropopause in the Asian summer monsoon anticyclone (ASMA) based on high-altitude airborne measurements in summer 2017. We find that the lapse rate tropopause, and not the cold point, constitutes a good estimate of the upper boundary of the well mixed tropospheric air for many species. There is slow, diabatic, upward transport in the vicinity of the lapse rate tropopause and above. The cold point is located on average about 1 km above the lapse rate tropopause and is about 3 K colder (pressure lower by about 12 hPa). The cold point is in particular important for water vapour. Above the cold point in the ASMA molar water vapour mixing ratios (including hydration patches) range between similar to 3 and 10 ppm. In the observations, no indication of substantial dehydration above the cold point was found. Ozone mixing ratios increase substantially with altitude; between the lapse rate and the cold point tropopause molar ozone mixing ratios are in the range of 50-200 ppb. For strong convection (flight on 10 August 2017) there is substantial dehydration at the cold point tropopause (indicated by high values of total water, ice particle occurrence, and strong supersaturation). Above the cold point, under such conditions, neither ice particle occurrence, nor enhanced molar mixing ratios of water vapour (above about 6 ppm) are observed.

Abstract. Dusty cirrus clouds are optically thick, convectively organized ice clouds that occur during intense mineral dust outbreaks. Previous modeling studies have found a link between mineral dust and convectively generated cirrus, but have been unable to explicitly resolve the associated convective dynamics due to limited spatial resolution. Here, we investigate the evolution and persistence of dusty cirrus using large-eddy simulations constrained by in-situ aircraft observations from the ML-CIRRUS 2014 campaign and complementary remote sensing data.The simulations indicate that sustained convective cirrus requires mineral dust concentrations exceeding climatological values by approximately one to two orders of magnitude, corresponding to number concentrations on the order of Nd ∼ 1 cm-3. Under these conditions, heterogeneous freezing on mineral dust dominates ice formation and maintains sufficiently high ice crystal number concentrations to sustain strong longwave cooling at cloud top and preserve convective overturning under shortwave radiative warming. In contrast, homogeneous freezing remains largely suppressed across a wide range of simulated conditions. The direct radiative effect of mineral dust is comparatively weak relative to cloud radiative feedbacks and does not significantly influence cirrus evolution.The simulated cirrus is also highly sensitive to the choice of deposition ice nucleation parameterization, highlighting a major source of uncertainty in representing dust–ice interactions. Overall, the results identify mineral dust availability as the primary control on dusty cirrus persistence and emphasize the need for improved representation of heterogeneous ice nucleation in atmospheric models.

Abstract. Cirrus clouds in the Arctic play a central role in the regional radiation budget, yet their formation pathways and properties remain poorly constrained, especially during Warm Air Intrusions (WAI). In this study, we analyse and contrast cirrus observed during WAI events (WAI cirrus) with those formed under undisturbed Arctic Conditions (AC cirrus) using remote sensing measurements from the HALO–(AC)³ campaign, combined with backward trajectory modelling with CLaMS-Ice and LAGRANTO. This allows us to connect the optical, macro- and microphysical properties measured via lidar and radar to the cloud origin and dynamics during their evolution. We find that WAI cirrus encountered stronger vertical transport along their trajectories compared to AC cirrus. At the time of detection, WAI cirrus were geometrically and optically thicker, exhibited higher depolarization ratios and ice supersaturation, and contained more numerous but smaller ice crystals, consistent with homogeneous ice nucleation at colder temperatures. AC cirrus, in contrast, showed larger ice crystals and a comparably greater contribution from heterogeneous ice nucleation. As WAI events are projected to become more frequent and long-lasting, a resulting increase in the formation of WAI cirrus may enhance their influence on the Arctic radiation budget leading to cooling. Our results provide observational constraints on cirrus variability in the Arctic and emphasize the need for their improved representation in weather and climate models.

Abstract. Linking cirrus cloud microphysical properties, their formation history, and their origin – whether liquid or in situ – to global satellite observations will represent a major step toward understanding these clouds, which are climatically important but not yet fully understood. This link is established by integrating Lagrangian microphysical modelling along air parcel trajectories ending at the points of satellite observations. In Part 1 of this study, detailed microphysical Lagrangian cirrus model CLaMS-Ice was coupled with DARDAR-Nice satellite retrievals to establish the DC-Ice (DARDAR → CLaMS-Ice) framework, enabling the derivation of origin-based metrics associated with satellite observations. Here, we evaluate the performance of the DC-Ice for revisited case studies and examine its sensitivity to key model parameters. Simulated ice crystal number concentration (Ni) and ice water content (IWC) are statistically compared with satellite retrievals and in situ observations. DC-Ice shows good agreement with observations for Ni and IWC, except for orographic cirrus, where the IWC from DC-Ice is lower than that from DARDAR-Nice. Sensitivity experiments indicate that variations in the parameterised sedimentation affect the simulated microphysical properties, especially IWC, while temperature fluctuations influence Ni. Nevertheless, it appears from the sensitivity analysis that origin-based metrics proved relatively robust except under most configurations. These findings highlight the viability of incorporating origin-based metrics into satellite observations, paving the way for improved global understanding of cirrus origins and their impact on the climate system. Future development includes the implementation of a three-dimensional sedimentation scheme in CLaMS-Ice and the extension of the framework to tropical regions.

Abstract Previous studies reporting the spatial inhomogeneity and heterogeneity of cirrus cloud properties have primarily been case studies derived from relatively small data sets. This study evaluates the spatial heterogeneity and inhomogeneity of cirrus bulk microphysical properties using ∼60 hr of in situ measurements from eight field campaigns. The spatial heterogeneity of ice concentration (ice water content) increases with decreasing ice concentration (ice water content), revealing more tenuous cirrus are more spatially heterogenous. Cirrus clouds often contain heavily skewed distributions of ice concentrations, which also possess high spatial heterogeneity. This is primarily due to the absence of small ice crystals (diameter < ∼50 μm) within localized regions of the clouds. These regions contain lower ozone concentrations and broader distributions of relative humidity with respect to ice. The greater variation of relative humidity suggests enhanced temperature perturbations resulting in greater frequencies of subsaturated samples, which preferentially sublimate small ice (e.g., via the kelvin effect).

Abstract. Cirrus clouds pose a challenge due to their complex microphysics and processes involved in their formation and growth. Satellite observations capture the instantaneous state of cirrus, offering limited insight into their formation history. Here, we introduce DC-Ice (DARDAR → CLaMS-Ice), a novel framework that defines origin-based metrics to characterize cirrus origin and evolution from satellite observations with Lagrangian microphysical modeling. Air parcel back trajectories are computed using Chemical Lagrangian Model of the Stratosphere (CLaMS), starting from DARDAR-Nice satellite observation point. Along these trajectories, the CLaMS-Ice microphysical box model simulates cirrus formation and evolution. Key origin-based metrics are derived to be associated with satellite observations, including ice formation pathways (homogeneous vs heterogeneous), ice crystal origin (liquid-phase or in situ), and their age (time since ice formation). DC-Ice is applied to three case studies representative of typical meteorological conditions in midlatitudes. The analysis shows that cirrus properties evolve continuously, with small-scale temperature fluctuations regulating supersaturation and ice nucleation, thereby influencing subsequent microphysical processes. Reconstructed vertical cloud profiles reveal that liquid-origin cirrus is more prevalent at lower altitudes, while in situ-origin cirrus dominates at higher altitudes, with nucleation pathways varying with cloud age and environmental conditions. A comprehensive evaluation and sensitivity analysis will be presented in Part 2 to quantify uncertainties. These studies form the basis for linking microphysical properties and the history of cirrus clouds to global satellite observations using the DC-Ice approach. The future aim is to gain broad geographical and seasonal information on cirrus clouds, improving their representation in global models.

Understanding the formation mechanisms of ice clouds is challenging due to their complex composition and diverse growth processes. Observational constraints have historically been limited, resulting in significant gaps in our understanding and representation of ice clouds. Satellite measurements are particularly limited by the absence of critical environmental context information needed to identify cloud formation mechanisms and evolution. These observations provide only a snapshot of cloud states and their microphysical properties at a single moment. This study seeks to overcome these limitations by incorporating additional metrics on ice cloud history and origin alongside operational satellite products.We introduce a novel framework that combines satellite observations with Lagrangian transport and ice microphysical modelling to provide insights into the history and origin of air parcels contributing to ice cloud formation. The Chemical LAgrangian Model of the Stratosphere (CLaMS) is employed to trace air parcel trajectories along the DARDAR-Nice track. Additionally, the CLaMS-Ice model is used to simulate cirrus clouds along these trajectories, offering metrics on cirrus age, origin (in situ vs. liquid-origin), and ice formation pathways (heterogeneous vs. homogeneous nucleation) that can be associated with satellite observations.To illustrate this approach, we present three case studies representative of distinct mid-latitude synoptic conditions: fast updraft, slow updraft, and orographically driven ice clouds. These cases demonstrate an in-depth analysis of air parcel evolution since cirrus formation, followed by a statistical examination of the relationship between microphysical properties and the origin-based metrics. Furthermore, the method is evaluated by comparing modeled microphysics with satellite retrievals. A sensitivity analysis is conducted to assess the impact of input parameters in CLaMS-Ice, including small-scale temperature fluctuations, environmental ice-nucleating particle (INP) concentrations, and sedimentation parameterizations. Overall, this comprehensive approach enhances our understanding of ice cloud processes, provides valuable context for interpreting satellite observations, and contributes to refining the representation of cirrus clouds in atmospheric models.

Water vapor (H2O) is a key trace gas in the upper troposphere (UT) and lowermost stratosphere (LMS) because it plays a crucial role in the Earth’s climate. However, accurate knowledge of the amount of H2O in this region is still insufficient due to the difficulty and lack of in-situ and space-borne measurements. This study presents a new methodology to compile H2O climatologies for the LMS from simple, extensive measurements aboard passenger aircraft between 1994 and now within the IAGOS infrastructure covering in the extratropical UT/LMS. To this end, a statistical comparison of mean H2O in sampling bins of air relative to the tropopause is performed between a dataset from ≈60.000 flights applying the IAGOS-MOZAIC and -CORE simple sensor and a dataset of only ≈500 flights using the more sophisticated IAGOS-CARIBIC instrument. We find good agreement in the UT, but a systematic positive bias in the simple measurements in the LMS. To account for this bias, mean water vapor values of the simple sensor are adjusted to the sophisticated observations based on a new statistical approach. After applying this new method, the LMS water vapor measurements are in good agreement. The extensive H2O dataset from the simple IAGOS sensor can now be used to produce highly resolved water vapor climatologies for the climatically sensitive LMS region. With the adjusted IAGOS H2O data, water vapor transport processes and (de-)hydration of air masses in the extratropical UT/LMS are analysed through backward trajectories and microphysical CLaMS-ICE simulations.

Understanding the formation mechanisms of ice clouds has been hindered by the complexity of their composition and the diversity of their growth processes. Previously, observational constraints have been limited, leading to substantial gaps in our comprehension and representation of ice clouds. Satellite measurements face a significant challenge due to the lack of essential environmental context information, that is necessary to identify and understand the cloud's formation mechanism and evolution. Indeed, these representations only capture a snapshot of the state of a cloud and its microphysical properties at a given time. This study addresses this limitation by providing additional metrics on ice cloud history and origin along with operational satellite products. Here, we present a novel framework that combines satellite observations with Lagrangian transport and ice microphysical models, to obtain information on the history and origin of air parcels that contributed to their formation. The air mass transport model CLaMS (Chemical LAgrangian Model of the Stratosphere) was employed to track the trajectory of air parcels along the DARDAR-Nice track. CLaMS-Ice model is jointly used to simulate cirrus clouds along trajectories derived by CLaMS. This approach provides information on the cloud regime as well as the ice formation (in-situ vs liquid origin) pathway. Our findings, derived from case studies involving multiple cloud types, present a realistic representation of these complex processes. We explore the sensitivity of our methodology to initial conditions and thresholds. Additionally, a statistical analysis examines how satellite cloud microphysics are sensitive to CLaMS-Ice metrics. This comprehensive approach advances our understanding of ice cloud processes and helps to refine satellite-based representations of these atmospheric phenomena.

Abstract. This study investigates the thermodynamical and microphysical links of in-cloud downdrafts of tropical deep convective clouds using aircraft measurements from ACRIDICON-CHUVA field campaign focusing on the upper-levels (10–14 km). The Cloud Water Content (CWC) does not show a discernible trend with altitude or vertical velocity. This opposes the concept of condensate loading, enhancing the downdraft strength. Furthermore, the CWC in up- and downdrafts is found to be similar. The mean draft diameters exhibit a broadening trend with altitude in updrafts and downdrafts, while the mean air mass flux decreases with altitude. In the upper-levels, strong negative vertical velocities (w < -2 m s-1) are observed in the supersaturated region (RHice > 110 %), contradicting the general idea that downdrafts are driven/maintained by latent cooling. The spread in cloud particle number concentration was found to be similar in downdraft and updraft regions with a weak linear trend for |w| > 1 m s-1 in transition (90 ≤ RHice ≤ 110 %) and supersaturated regions. The mean particle size distributions (PSDs) indicate an increase in maximum particle size with altitude. Higher particle concentrations are observed in stronger drafts for particles with Dp< 100 m. Furthermore, the number concentration of larger particles (Dp > 100 m) increases faster in stronger drafts as altitude increases. Particle number concentrations in downdrafts are comparable to those in updrafts of similar strength at the same altitude. We speculate that large eddies that allow mixing between updrafts and downdrafts have an influence on the modulation of PSDs.

Non-CO2 emissions contribute to climate effects from aviation in the same order of magnitude as carbon dioxide (CO2) emissions. However, the non-CO2 effects, comprising e.g., ozone and methane induced from NOx emissions, together with contrails, or the indirect aerosol effects, are associated with much larger uncertainties. The EU Aeronautics project ACACIA (Advancing the SCience for Aviation and ClimAte) explored the climate impacts of non-CO2 effects which show a strong dependence on atmospheric conditions and synoptic situation. While CO2 and non-CO2 effects in general introduce a warming effect for climate change, some indirect effects might result in a relatively large cooling.ACACIA investigated indirect aerosol effects comprising formation and properties of clouds. Atmospheric conditions for the formation of long-lived contrails have been investigated, with the aim to improve their predictability. Indirect effects of nitrogen oxide emissions on atmospheric ozone and methane have been estimated, using a set of global chemistry-climate models. These different effects have been brought to a common scale by various physical climate metrics. A dedicated study on prevailing uncertainties has been performed, with the goal to provide robust recommendations considering uncertainties of individual estimates. Together with the numerical studies a dedicated analysis of existing measurement data has been completed in order to identify needs and requirements for atmospheric observations.To this end, ACACIA brought together research across scales, from plume to global scale, from laboratory experiments to global models resulting in a series of scientific publications, and it proceeds from fundamental physics and chemistry to the provision of recommendations for policy, regulatory bodies, and other stakeholders in the aviation business. Acknowledgements This project ACACIA (Advancing the scienCe for Aviation and ClImAte) receives funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 875036. High performance supercomputing resources were used from the German DKRZ Deutsches Klimarechenzentrum Hamburg.

Cloud particle size distributions (PSDs) are crucial in determining the clouds physical and optical properties and hence their radiative feedback to the climate. Here we present unprecedented occurrence patterns of cloud particles derived from 270 h of cloud measurements (≈ 975 000 PSDs). The focus of the analysis is on cirrus clouds, but liquid and mixed-phase clouds are also shown. In particular, cirrus PSDs for cold to warm cirrus temperatures and microphysically thin to thick cirrus clouds are provided in a novel presentation as heat maps. The observations are accompanied by simulations of ice crystal growth in in situ-origin cirrus, showing that the maximum size to which the cirrus ice crystals can grow increases from approx. 60 µm@T < 200 K to 230 µm@T > 220 K. Crystals larger than this size are most likely of liquid-origin. The combined evaluation of observations and simulations allows the attribution of processes shaping the PSDs. Important results are that, with increasing temperature and cirrus thickness, the most frequent ice particles change from smaller and fewer crystals of in situ-origin to larger and more crystals of both in situ and liquid-origin i.e. the cirrus type changes from in situ- to liquid-origin. In addition, three characteristic ice crystal size ranges are identified. The nucleation/evaporation size interval (∼ 3–20 µm), most frequent in the coldest, thinnest in situ-origin cirrus; the – most common – overlap size interval (∼ 20–230 µm), where both in situ-origin liquid-origin cirrus occur and the uplift/sedimenation size interval (>∼ 230 µm), which consists mostly of liquid-origin ice crystals.

Tropical cirrus clouds form through in situ ice nucleation below the homogeneous freezing temperature of water or through detrainment from deep convection. Despite their importance, limited understanding of their evolution and formation pathways contributes to large uncertainty in climate projections. To address these challenges, we implement novel passive tracers in the System for Atmospheric Modeling (SAM) cloud-resolving model to track the three-dimensional development of cirrus clouds. One tracer tracks air parcels exiting convective updrafts, revealing a rapid decline in ice crystal size and number as anvils age. Another tracer focuses on in situ cirrus, capturing their formation in the cold upper atmosphere and the subsequent reduction in their ice crystal number over time. We find that in situ cirrus dominate at colder temperatures and lower ice water contents, while anvil cirrus prevail at temperatures > −60 °C. Despite the frequent occurrence of in situ cirrus within the tropical tropopause layer, they account for only 6 %–7 % of the total tropical cirrus cloud top-of-the-atmosphere radiative effect. These findings improve our ability to assess the distinct roles of convective and in situ cirrus in shaping tropical cirrus properties and their impacts on climate. We also improve the model's representation of tropical cirrus through simple, computationally inexpensive microphysics modifications, improving agreement with tropical aircraft observations. We show that updrafts critical for tropical cirrus formation are only resolved in our simulations at a horizontal grid spacing of 250 m – much finer than those used in global storm-resolving models.

Convective overshoot can result in irreversible mixing of air from the troposphere into the stratosphere, thereby influencing the radiation balance of this climate-sensitive region by altering greenhouse gas concentrations, particularly water vapor, and inducing ice and aerosol particles into the stratosphere. This study examines the cloud microphysical properties and trace gas signatures associated with a convective overshoot event observed during the TPex (TropoPause composition gradients and mixing Experiment) campaign in June 2024 over southern Sweden. While recent investigations have predominantly focused on convective overshoots related to air masses of (sub)tropical origin, this particular event took place during a cold air outbreak characterized by low tropopause altitudes (9 km; with temperatures around -55°C).For the study, microphysical data collected in-situ during Tpex aboard a Learjet by NIXE-CAPS (New Ice eXpEriment - Cloud and Aerosol Particle Spectrometer) and trace gas measurements, including water vapor and ozone, were utilized. The findings reveal that ice particles were transported into the lower stratosphere, up to 1.5 km above the tropopause. At this altitude, a pronounced stratospheric ozone concentration of approximately 800 ppbv and a notable tropospheric water vapor concentration (~40 ppmv) were recorded, the latter being twice as high as background levels at the same height. This substantial injection of tropospheric air was linked to gravity wave breaking, and subsequently irreversible mixing near the overshooting top.To gain deeper insight into the development of the overshoot, a forward trajectory analysis was conducted, and the evolution of cloud ice microphysical properties along the trajectories was simulated using the CLaMS (Chemical Lagrangian Model of the Stratosphere) model.

Accurately determining and reducing the climate impact of aviation and its uncertainties is one of the pressing challenges of our times. Contrail cirrus are estimated to contribute more than half of the total effective radiative forcing from aviation, yet the uncertainties in their optical and radiative properties are large. In contrast to midlatitude cirrus, high-latitude cirrus are less anthropogenically influenced; thus, they are more pristine. However, little is known about Arctic cirrus properties and their role in the amplified warming of this region. The Cirrus in High Latitudes (CIRRUS-HL) mission using the High Altitude and Long Range Research Aircraft (HALO) provides measurements in mid-and high latitudes during summer (June/July) 2021, exploiting HALO's capabilities and a comprehensive cloud-aerosol-trace gas and radiation instrumentation. The results of 24 HALO flights provide new insights into both natural cirrus and contrail cirrus properties in high (60 degrees-76 degrees N) and midlatitudes (38 degrees-60 degrees N). In particular, we find lower ice water content (-42%) and lower number concentrations (-88%) of cirrus particles with larger mean diameters (+22%) in high latitudes. Ice supersaturated regions were frequently observed in mid-and high latitudes, with median in-cloud relative humidity over ice between 105% and 122%. Mean aerosol number concentrations in the midlatitudes were reduced by up to 80% compared to pre-COVID-19 times. Less air traffic during the COVID-19 lockdowns, reduced contrail cirrus coverage, and lower ice nucleating particle concentrations in high latitudes help to explain the observed differences in cirrus properties. The extensive dataset will be used to improve weather and climate models. SIGNIFICANCE STATEMENT: In contrast to Arctic cirrus, midlatitude cirrus are more often modi-fied by human activities, of which air traffic is a significant contributor through the formation of contrails and contrail cirrus. These man-made cirrus warm Earth, but to constrain their effects on climate, in situ and remote sensing measurements were conducted with the German research aircraft High Altitude and Long Range Research Aircraft (HALO). During 24 flights, we used HALO's exceptional altitude and distance range to sample and contrast different cirrus types from the dense air traffic regions to the remote Arctic regions. We find that microphysical properties of high-and midlatitude cirrus differ substantially, related to their formation pathway, the abundance of air traffic, and the availability of ice nucleating particles. The measurements will help to validate contrail cirrus and climate models.

Contrail-cirrus is considered the most important component of aviation-induced climate impact. However, a reliable assessment requires better understanding of their radiative effects. Analysis of seven years of humidity observations by instrumented passenger aircraft shows that conditions promoting long-lived contrails are fulfilled most often in regions already covered by subvisible or visible cirrus: ~90% over the Northern midlatitudes and almost 100% in the Southeast Asian subtropics, approximately equally distributed among visible and subvisible cirrus clouds. A conceptual analysis shows that subvisible cirrus and clear-sky cover ~10% of the cruise altitude over Northern midlatitudes ( < 2% in the subtropics) and contrails within these regions are expected to cause additional warming. However, most contrails in the thicker, visible cirrus, only slightly enhance the cirrus warming effect or possibly reverse it to cooling. Our results suggest that potential flight rerouting concepts for contrail avoidance need to consider cirrus cloud coverage in addition to ice-supersaturation.