In May 2024, the Earth Cloud, Aerosol and Radiation Explorer (EarthCARE) satellite was launched. For the first time a satellite payload combines two active instruments, i.e., the Atmospheric Lidar and the Cloud Profiling Radar, together with two passive instruments, a multi-spectral imager and a broad-band radiometer, on one single spacecraft platform. EarthCARE is thus the most complex satellite mission to date for collocated aerosol, cloud, radiation and precipitation measurements. To utilize the data collected by the EarthCARE mission to its full extent and to support and quantify the data quality and measurement uncertainty, careful and holistic validation activities are needed. For this purpose, we set up an airborne instrument payload on the German High Altitude and LOng-range research aircraft (HALO), which is similar to the EarthCARE instrumentation. We used this payload during an extensive measurement campaign in summer and fall 2024 in the tropic and mid- to high-latitudes to validate the EarthCARE measurements and data products early in its commissioning phase. Here we aim to give a detailed overview of the PERCUSION (Persistent Earth CARE underflight studies of the ITCZ and organized convection) mission, and to advertise the use of its data in future more detailed validation studies. We give examples of how to use PERCUSION data to approach the validation of all four instruments of EarthCARE as well as of higher level (i.e. multi-sensor) products, and give first confidence in the quality of EarthCARE data.

Abstract. This study presents a statistical analysis of the microphysical properties and ice crystal habits of low-level Arctic mixed-phase clouds (MPCs) using in situ observations from four aircraft campaigns conducted near Svalbard, Norway. The dataset comprises 45 hours of in-cloud measurements, with 731 600 ice crystal images classified using a convolutional neural network. Observations were categorized into four meteorological regimes using the Marine Cold Air Outbreak index: Marine Cold Air Outbreaks (MCAOs), Warm Air Advections (WAAs), and two transitional states. Vertical profiles of microphysical properties exhibit clear differences between regimes. MCAO conditions are associated with deeper mixed-phase layers, larger ice water contents, and higher concentrations of ice particles. In contrast, WAA cases are associated with shallower clouds dominated by liquid water with lower ice water content. The analysis indicates that temperature inversions influence cloud microphysics: weaker inversions support deeper clouds and enhanced ice development, while stronger inversions limit the extent of the liquid-containing layer. Analysis of ice crystal habits reveals distinct growth patterns based on the meteorological regime and surface type. Rimed particles appear more frequently during MCAOs, particularly over open ocean where increased turbulence likely promotes droplet–ice interactions. Over sea ice, pristine and aggregated crystals are more prevalent, implying that riming plays a lesser role while depositional growth and aggregation are more significant. This combined multi-campaign dataset offers a comprehensive statistical view on the interaction of synoptic regime, temperature inversion, and surface conditions in determining the microphysical structure and ice crystal habits of Arctic MPCs.

The Southern Ocean (SO) is one of the cloudiest regions on Earth. However, cloud radiative effects are not well represented over the SO in atmospheric models, which is mainly due to an underestimation of aerosols. To address this and other fundamental and pressing open questions on the interaction of atmospheric radiation, aerosol nucleation and growth, cloud formation and impacts over the SOI, the HALO-South aircraft mission was conducted in September and October 2025 based in Christchurch, Aotearoa New Zealand. HALO stands for High Altitude and Long Range Research Aircraft. HALO-South covered the full cycle of processes from aerosol formation, cloud evolution, and radiative interaction with a special focus on the characteristics and effects of mixed-phase clouds. The instrumental payload of HALO included a unique and comprehensive in-situ and remote sensing suite of instruments. It was designed to collect data to improve our understanding of fundamental atmospheric processes and to extrapolate and upscale the results using satellite data and global climate models in order to resolve long-standing measurement-modelling discrepancies. In addition, the ground-based stations in Tāwhaki and Invercargill with remote sensing and in-situ long term measurements will extend the data to a larger scale in time. The first analysis of the campaign shows promising insights into cloud and aerosol processes over the SO, which will be presented and discussed.Acknowledgments: This work was supported by the DFG (Deutsche Forschungsgemeinschaft, German Research Foundation) Priority Program SPP 1294, the Max Planck Society, Priority Program SPP 1294, the German Aerospace Center (DLR)

The clouds associated with Marine Cold Air Outbreaks (MCAOs) exhibit characteristic structures, initially forming as roll clouds or cloud streets parallel to the wind direction, and eventually breaking up into a cellular cloud field.Here, a novel correlation-based metric, the Correlation clOud Street Index (COSI) is introduced. It is defined as the Pearson correlation coefficient between an image and an optimally oriented and scaled Gabor kernel, providing a quantitative measure of cloud street presence and distinctness. The calculation of this index also extracts cloud street spacing (wavelength) and orientation as structural properties.Applied to satellite observations with extensive spatial and temporal coverage, we utilise the COSI to get novel insights into the spatio-temporal evolution of cloud street structures in marine cold air outbreaks. By analysing sequences of consecutive satellite images for individual events, we capture the cloud evolution for both the overall MCAO and along quasi-Lagrangian trajectories. We quantify the systematic increase in cloud street wavelength with increasing distance from the ice edge and assess the aspect ratio (wavelength divided by cloud top height) across a larger dataset. The dependence on the MCAO strength is also evaluated. The cases analysed correspond to periods with (AC)3 aircraft campaigns, allowing the aircraft observations to be placed in a broader context and providing more detailed observations of meteorological conditions along flight trajectories.This work was supported by the DFG funded Transregio-project TRR 172 “Arctic Amplification (AC)3“.

A comprehensive in-situ dataset of low-level Arctic clouds was collected in the Fram Strait during the HALO-(AC)(3) campaign in spring 2022 using the research aircraft Polar 6. The clouds observed at altitudes below 1000 m were frequently in a mixed-phase state. We demonstrate that despite comparable optical properties, classic mixed-phase clouds (MPC) and mixed-phase haze (MPH) can be distinguished on the basis of their microphysical properties, with MPH observed about 8 times more frequently than MPC. While the thermodynamic phases of the particles within the MPH are similar to those in the MPC, the supercooled droplets observed in MPC are replaced by large (> 3 mu m) wet aerosol particles in MPH. Furthermore, the particle number concentration measured in MPH is reduced by approximately 3 orders of magnitude compared to MPC. MPH is observed in subsaturated air with respect to water, suggesting that the small liquid particles are haze droplets and are in equilibrium below the activation threshold to form cloud droplets. Chemical analysis suggested that the haze particles contained significant amounts of sea salt. Additional in-situ measurements with an optical particle counter indicated that their number concentration was 2 times larger over the sea ice compared to the open ocean. Furthermore, measurements of the vertical distribution of the thermodynamic phases in low-level Arctic clouds revealed a characteristic structure, with a liquid regime frequently occurring at the top of the atmospheric boundary layer, followed by MPCs, and an MPH layer below. The findings from this study enhance our understanding of the microphysical composition of clouds in mixed-phase conditions.

We present a comprehensive balloon-borne measurement dataset collected during a dedicated Arctic observation campaign conducted from 19 March to 18 April 2024 in the transition from polar night to polar day at the Villum Research Station (VRS) at Station Nord in Greenland. The objective of the observations was to characterise the temporal evolution of the Arctic atmospheric boundary layer (ABL), focusing on key transition periods, including cloud development, low-level jet evolution, and day to night shifts. Data were collected by the Balloon-bornE moduLar Utility for profilinG the lower Atmosphere (BELUGA) tethered-balloon system performing in-situ measurements of temperature, relative humidity, wind speed, turbulence, and thermal infrared irradiance from the surface to several hundred meters altitude, with frequent profiling in high vertical resolution. Twenty-eight research flights delivered more than 300 profiles, with up to 8 profiles per hour, complemented by daily radiosonde launches. This paper specifies the BELUGA instrumentation at VRS, data processing procedures, and the publicly available Level-2 data (BELUGA and radiosonde), provided in instrument-separated data subsets listed in a data collection (10.1594/PANGAEA.986431, ). One possible major application of the data is to evaluate different model types (such as numerical weather prediction, single-column, large-eddy simulations) in representing processes controlling the Arctic ABL. To prepare such evaluations, we give an overview of the observations, environmental conditions during the campaign, and highlight specific events that are valuable for model comparison. We introduce an event in which temporal temperature changes influence the ABL inversion, radiative heating-rate profiles associated with transitions between cloudy and cloud-free conditions, and an observed Arctic low-level jet compared with reanalysis, offering insights into the Arctic ABL evolution.

Two retrieval methods for the determination of Arctic surface skin temperature and surface type based on radiance measurements from the thermal infrared (TIR) imager VELOX (Video airbornE Longwave Observations within siX channels) were developed. VELOX captured TIR radiances in terms of brightness temperatures for wavelengths from 7.7 to 12 mu m in six spectral channels. The imager was deployed on the High Altitude and LOng Range research aircraft (HALO) during the HALO-(AC)3 aircraft field campaign conducted in the framework of the Arctic Amplification: Climate Relevant Atmospheric and SurfaCe Processes and Feedback Mechanisms (AC)3 research programme. The measurements were taken over the Fram Strait and the central Arctic in March and April 2022. Radiative transfer simulations assuming cloud-free atmospheric conditions were performed showing that the influence of water vapour on the measured brightness temperature can be neglected. Therefore it was possible to apply a single-channel retrieval technique to obtain the surface skin temperature from the VELOX data. The retrieval results were compared with data from the MODerate-resolution Imaging Spectroradiometer (MODIS) showing an agreement within 2.0 K. Secondly, a pixel-by-pixel surface classification retrieval was developed using a random forest algorithm. It classifies surfaces into types of open water, sea-ice-water mixture, thin sea ice, and snow-covered sea ice. The resulting sea-ice concentrations were compared with satellite data, yielding a mean absolute difference (MAD) of 5 %. In addition, the classified pixels were aggregated into segments of the same surface type, providing different segment size distributions for all surface types. When grouped by the distance to the sea-ice edge, the segment size distribution showed a shift to fewer but larger floes in the direction of the pack ice.

In May 2024 the EarthCARE satellite mission EarthCARE was launched. For the first time, the satellite combines a high spectral resolution lidar and a cloud radar with doppler capability as key instruments on one single platform. In addition, it is equipped with a multi spectral imager and a broadband radiometer. This unique combination makes EarthCARE the most complex satellite mission to study aerosol, clouds, precipitation, and radiation. To fully use these new and advanced data for science applications, a careful validation of the measurements and data products is required. We have implemented an EarthCARE-like payload onboard the German research aircraft HALO (High Altitude and LOng range) to prepare and validate the EarthCARE data. This instrumentation was flown during PERCUSION (Persistent EarthCARE underflight studies of the ITCZ and organized convection) as a contribution to ORCESTRA (Organized Convection and EarthCARE Studies over the Tropical Atlantic).ORCESTRA is a network of different campaigns conducted to better understand the organized tropical convection at the mesoscale, e.g. including the interaction of convective organization with tropical waves and air-sea interaction, and the impact of convective organization on the Earth’s climate and radiation budget. In addition, ORCESTRA helps to validate satellite remote sensing (especially EarthCARE). To achieve these objectives, ORCESTRA combines several sub-campaigns taking place on the Cape Verde Islands and Barbados in August and September 2024.One of the campaigns within ORCESTRA is the PERCUSION campaign. PERCUSION aims to test factors hypothesized to influence the organization of deep maritime convection in the tropics and the influence of convective organization on its larger-scale environment. One focus of PERCUSION was to establish confidence in the EarthCARE measurements and products. For this purpose, we conducted one EarthCARE underpass within each research flight HALO measurements were performed during the EarthCARE commissioning phase in August 2024 out of Sal, Cape Verde, and out of Barbados in September 2024. In addition, we performed flights out of Oberpfaffenhofen, Germany in November 2024 for validation of conditions that could not be captured in the two first campaign parts. Altogether, 33 EarthCARE underpasses were carried out in different aerosol and cloud situations. Some of the flights were coordinated with in-situ measurements onboard other aircrafts (e.g. the French ATR42), with shipborne measurements onboard the German research vessel METEOR, or with ground-based radar and lidar measurements at Mindelo (Cape Verde), Barbados, and the ACTRIS stations Antikythera, Leipzig, Lindenberg and Munich. Four underpasses under NASA’s PACE mission were also performed.In our presentation we will give an overview of ORCESTRA with the main focus on PERCUSION. We will present the HALO PERCUSION measurements and will show first comparisons of HALO lidar and radar and EarthCARE lidar and radar measurements.

Marine stratocumulus clouds (MSC) strongly influence Earth's radiation budget, yet the mechanisms governing the descent of entrainment-affected (diluted) parcels, and the relative roles of cloud-top entrainment instability (CTEI) and longwave radiative cooling (RC), remain debated. Using helicopter-borne observations from the ACORES campaign that combine high-resolution in situ vertical profiling with co-located remote sensing, we examine vertical variations of microphysics, thermodynamics, and the entrainment interfacial layer (EIL). When CTEI conditions were strongly met, inhomogeneous mixing (IM) traits appeared near cloud top and transitioned to homogeneous mixing (HM) traits deeper in the cloud layer, accompanied by localized elevations of cloud base, signatures consistent with enhanced descent of diluted parcels. We argue that these apparent HM traits arise from adiabatic warming and evaporation during descent rather than true HM. When CTEI was weakly met or not met, IM traits near the top were weaker, HM traits emerged deeper in the cloud, and cloud base elevation was not observed; these differences are explained by RC-driven buoyancy contrasts modulated by turbulence and EIL thickness. Even in such cases, diluted parcels descended, but weakly. Integrating these results with prior field studies, we provide observational evidence that sufficiently strong CTEI can dominate RC and drive diluted-parcel descent, clarifying how CTEI, RC, and EIL thickness jointly shape MSC structure and offering guidance for improved representation in weather and climate models.

Arctic low-level clouds play an important, albeit uncertain, role in the Arctic climate system. Consequently, their effect on the radiative energy budget (REB) is subject to considerable uncertainty as well. To reduce this uncertainty and to assess the importance of processes driving the cloud radiative effect (CRE), it is crucial to quantitatively disentangle the impact of essential parameters that non-linearly affect the CRE. Therefore, this study uses a CRE parameterization and low-level airborne REB observations in combination with an approach similarly applied in climate dynamics to quantify the contributions of the concurrently observed solar zenith angle (SZA), cloud optical thickness, and surface albedo on the solar CRE at the surface. Based on a case study characterized by inhomogeneous cloud and surface conditions in the marginal sea ice zone, it is shown that the surface albedo contributed more than 95 % to the solar CRE difference between open ocean and sea ice. Using the same approach, the analysis is extended to observations from a series of aircraft campaigns and indicates that the variability in the non-cloud properties SZA and surface albedo between seasons and surface types, respectively, has a larger impact on the resulting difference in the solar CRE than the variability in cloud properties.

Field observations of vegetation indices (VIs) are derived from ratios of spectral reflectance data that are collected by drones and aircraft, providing higher spatial resolution than satellites. These reflectance data require periodic reference measurements over calibrated reflectance panels under cloud-free conditions. However, the reference measurements are partly performed in cloudy situations with the effect that wavelength-dependent scattering and absorption of solar radiation by clouds affects the subsequently derived VIs. This paper quantifies these effects using combined atmosphere-vegetation radiative transfer (RT) simulations. We study the general case when VIs are obtained from reflectance ratios of two wavelengths, and for the special cases of the normalized difference vegetation index (NDVI), the normalized difference water index (NDWI), and the enhanced vegetation index (EVI). For the general case of two-band VIs the lowest sensitivity to cloud changes was found for wavelength combinations below 1400 nm and outside the water vapor absorption bands. The NDVI was almost insensitive to changes in cloud conditions, while greater biases were identified for the NDWI. The EVI was most susceptible to cloud changes, with biases of 0.2 in the selected example. This lead to biases in the estimated leaf area index of 0.9. Biophysical properties derived from EVI, such as gross primary product, are also affected with variations of up to +/- 2 gCm-2d-1 in the selected cases.

This case study explores the evolution of clouds during an Arctic cold-air outbreak in the Fram Strait region observed during the HALO-(AC)3 aircraft campaign. Our research provides information about the formation, structure, microphysical and macrophysical properties, and radiative effects and investigates the role of vertical wind shear and buoyancy forces in the transition from regular cloud streets to rather isotropic cloud patterns. Our findings show that lower horizontal boundary layer wind speeds (<12 ms-1) disrupt the formation of cloud streets, leading to more isotropic cloud patterns, characterized by increasing cloud fraction (from 0.73 to 0.84) and cloud top height (from 330 to 390 m), and quantify the increase in liquid water path as well. In addition, we observe an increase in the number concentration of ice crystals in a size range between 100-1000 m and notable riming processes within organized cloud streets. Concurrent radiation measurements in our case study reveal that isotropic cloud patterns can exhibit either low or high albedo as well as low or high Fnet,TIR, suggesting that these patterns represent different developing stages.

The HALO–(𝒜𝒞)3 aircraft campaign was carried out in March and April 2022 over the Norwegian and Greenland seas, the Fram Strait, and the central Arctic Ocean. Three research aircraft – the High Altitude and Long Range Research Aircraft (HALO), Polar 5, and Polar 6 – performed 54 partly coordinated research flights on 23 flight days over areas of open ocean, the marginal sea ice zone (MIZ), and the central Arctic sea ice. The general objective of the research flights was to quantify the evolution of air mass properties during moist and warm-air intrusions (WAIs) and cold-air outbreaks (CAOs). To obtain a comprehensive data set, the three aircraft operated following different strategies. HALO was equipped with active and passive remote sensing instruments and dropsondes to cover the regional evolution of cloud and thermodynamic processes. Polar 5 carried a similar remote sensing payload to HALO, and Polar 6 was instrumented with in situ cloud, aerosol, and trace gas instruments focusing on the initial air mass transformation close to the MIZ. The processed, calibrated, and validated data are published in the World Data Center PANGAEA as instrument-separated data subsets and listed in aircraft-separated collections for HALO (Ehrlich et al., 2024a, https://doi.org/10.1594/PANGAEA.968885), Polar 5 (Mech et al., 2024a, https://doi.org/10.1594/PANGAEA.968883), and Polar 6 (Herber et al., 2024, https://doi.org/10.1594/PANGAEA.968884). A detailed overview of the available data sets is provided here. Furthermore, the campaign-specific instrument setup, the data processing, and quality are summarized. Based on measurements conducted during a specific CAO, it is shown that the scientific analysis of the HALO–(𝒜𝒞)3 data benefits from the coordinated operation of the three aircraft.

Cloud streets are a common feature of cold air outbreaks in the Arctic region. These are long, parallel bands of cumulus clouds that form perpendicular to the wind direction. They are caused by the interaction between the cold air mass and the warm ocean surface. Within the framework of (AC)³, the HALO-(AC)³ campaign was performed in spring 2022 involving several research aircraft to study cold air outbreaks and their belonging cloud streets. In this study we use a spectral imaging instrument, called AISA Hawk, to retrieve cloud microphysical properties in the very initial phase of these cloud streets and therefore focus on their development over the leads in the marginal sea ice zone.

Albedo, defined as the ratio between reflected radiation and total incoming radiation, is a key variable in the Earth radiative budget. In a fast changing climate with more frequent extreme events, such as droughts and excessive heat, vegetation is under constant stress. Such stress factors might modify the tree physiology, the reflectivity of individual leaves, and, eventually, the forest albedo as an entity. This might alter the local radiative budget and contribute to changes in the local climate, e.g., intensifying drought - a potential feedback loop. The understating of those effects might be further complicated by the occurrence of clouds. Therefore, this study presents spectral solar measurements of upward and downward irradiance that are used to determine the spectral albedo over a forest canopy. Since June 2021, ongoing measurements are performed on top of the Leipzig Canopy Crane located in the Leipzig floodplain forest. The measurements are separated for illumination geometries, i.e., the solar zenith angle, as well as for different cloud conditions. The interpretation of the measurements is aided and validated by coupled radiative transfer simulations using the library for radiative transfer model (libRadtran) and the Soil Canopy Observation of Photosynthesis and Energy fluxes (SCOPE2.0) model. Both models allow for simulations in the visible, near- and far-infrared wavelength range. By that, the impact of clouds on the spectral and broad band albedo, as well as the net radiative budget can be investigated. First simulations revealed that the presence of clouds enhance the spectral forest albedo. The magnitude of the effect is controlled by the cloud optical thickness, i.e., the ratio of direct and diffuse radiation. The enhancement is more pronounced for small solar zenith angles. However, the effect from clouds appears to be smaller than influences of variations in the surface properties. The presentation aims to outline the measurement set-up and strategy, and to discuss preliminary results. Furthermore, the new, iterative coupling of the atmosphere and soil-vegetation model is presented, which aims to improve the understating of cloud-vegetation radiation interactions.

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.

Marine cold-air outbreaks (MCAOs) strongly affect the Arctic water cycle and, thus, climate through large-scale air mass transformations. The description of air mass transformations is still challenging, partly because previous observations do not resolve fine scales, particularly for the initial development of an MCAO, and due to a lack of information about the thermodynamical evolution starting over sea ice and continuing over open ocean and associated cloud microphysical properties. Therefore, we focus on the crucial initial development within the first 200 km over open water for two case studies in April 2022 during the HALO-(AC)3 campaign (named after the High Altitude and Long Range Research Aircraft and Transregional Collaborative Research Centre ArctiC Amplification: Climate Relevant Atmospheric and SurfaCe Processes and Feedback Mechanisms (AC)3). The two events, just 3 d apart, belong to a particularly long-lasting MCAO and occurred under relatively similar thermodynamic conditions. Even though both events were stronger than the climatological 75th percentile of that period, the first event was characterized by colder air masses from the central Arctic which led to an MCAO index twice as high compared to that of the second event.The evolution and structure were assessed by flight legs crossing the Fram Strait multiple times at the same location, sampling perpendicularly to the cloud streets. Airborne remote sensing and in situ measurements were used to build statistical descriptions of the boundary layer, dynamics, clouds, and precipitation. For this purpose, we established a novel approach based solely on radar reflectivity measurements to detect roll circulation that forms cloud streets. The two cases exhibit different properties of clouds, riming, and roll circulations, though the width of the roll circulation is similar. For the stronger event, cloud tops are higher; more liquid-topped clouds exist; the liquid water path, mean radar reflectivity, precipitation rate, and precipitation occurrence have increased; and riming is active. The variability in rime mass has the same horizontal scale as the roll circulation, implying the importance of roll circulation on cloud microphysics and precipitation.Boundary layer and cloud properties evolve with distance over open water, as seen by, e.g., cloud top height rising. In general, cloud streets form after traveling 15 km over open water. After 20 km, this formation enhances cloud cover to just below 100 %. After around 30 km, precipitation forms, though for the weaker event, the development of precipitation is shifted to larger distances. Within our analysis, we developed statistical descriptions of various parameters (i) within the roll circulation and (ii) as a function of distance over open water. These detailed cloud metrics are particularly well suited for the evaluation of cloud-resolving models close to the sea ice edge to evaluate their representation of dynamics and microphysics.

Abstract. The global warming is amplified in the Arctic. To collect data that help to constrain weather and climate models, which often do not realistically represent the enhanced Arctic warming, the HALO-(AC)³ aircraft campaign was conducted in March and April 2022 over the Norwegian and Greenland Seas, the Fram Strait, and the central Arctic Ocean. Observations were made over areas of open ocean, the marginal sea ice zone, and the central Arctic sea ice. Two low-flying and one long-range, high-altitude research aircraft have been employed. Whenever possible, the three aircraft were flown in collocated formation. The campaign focused on one specific challenge posed by the models: The reasonable representation of transformations of air masses during their meridional transport into (northward by moist and warm air intrusions, WAIs) and out of (southward via marine cold air outbreaks, CAOs) the Arctic. To observe the air mass transformations, a quasi-Lagrangian flight strategy using trajectory calculations was realized enabling to sample the moving air mass parcels twice along their trajectories. Eight distinct WAI and 12 CAO cases were probed extensively. From the quasi-Lagrangian measurements, we have derived the diabatic heating and moistening of the moving air masses during CAOs and WAIs, the development of cloud macrophysical and microphysical properties along the southward pathways of the air masses during CAOs, and the moisture budget of WAIs. As an example result, we have obtained typical values of the surface-driven diabatic heating between 1–3 K h-1 and of the near-surface moistening between 0.05–0.3 g kg-1 h-1 within the lowest about 0.5 km. From the observations of WAIs, a weak diabatic cooling of up to 0.4 K h-1 and a moisture loss of up to 0.1 g kg-1 h-1 from the ground to about 5 km altitude were derived. In addition, we discuss the frequency of occurrence of the different thermodynamic phases of Arctic low-level clouds, the interaction of Arctic cirrus with sea ice, water vapor, and aerosol particles, and the characteristic microphysical and chemical properties of Arctic aerosol particles. Finally, we provide proof of a concept to measure mesoscale divergence and subsidence in the Arctic using data from dropsondes released during circular flight patterns.