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Global warming is amplified in the Arctic. However, numerical models struggle to represent key processes that determine Arctic weather and climate. To collect data that help to constrain the models, the HALO–(𝒜𝒞)3 aircraft campaign was conducted over the Norwegian and Greenland seas, the Fram Strait, and the central Arctic Ocean in March and April 2022. The campaign focused on one specific challenge posed by the models, namely the reasonable representation of transformations of air masses during their meridional transport into and out of the Arctic via northward moist- and warm-air intrusions (WAIs) and southward marine cold-air outbreaks (CAOs). 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 were flown in colocated formation whenever possible. To follow the air mass transformations, a quasi-Lagrangian flight strategy using trajectory calculations was realized, enabling us to sample the same moving-air parcels twice along their trajectories. Seven distinct WAI and 12 CAO cases were probed. From the quasi-Lagrangian measurements, we have quantified the diabatic heating/cooling and moistening/drying of the transported air masses. During CAOs, maximum values of 3 K h−1 warming and 0.3 g kg−1 h−1 moistening were obtained below 1 km altitude. From the observations of WAIs, diabatic cooling rates 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 5.5 km altitude were derived. Furthermore, the development of cloud macrophysical (cloud-top height and horizontal cloud cover) and microphysical (liquid water path, precipitation, and ice index) properties along the southward pathways of the air masses were documented during CAOs, and the moisture budget during a specific WAI event was estimated. In addition, we discuss the statistical frequency of occurrence of the different thermodynamic phases of Arctic low-level clouds, the interaction of Arctic cirrus clouds with sea ice and water vapor, and the characteristics of microphysical and chemical properties of Arctic aerosol particles. Finally, we provide a proof of concept to measure mesoscale divergence and subsidence in the Arctic using data from dropsondes released during the flights.

Mixed-phase clouds are not well represented in climate and weather forecasting models, due to a lack of the key processes controlling their life cycle. Developing methods to study these clouds is therefore essential, despite the complexity of mixed-phase cloud processes and the difficulty of observing two cloud phases simultaneously. We propose in this paper a new method to retrieve the microphysical properties of mixed-phase clouds, ice clouds and supercooled water clouds using airborne or satellite radar and lidar measurements, called VarPy-mix. This new approach extends an existing variational method developed for ice cloud retrieval using lidar, radar and passive radiometers. We assume that the lidar attenuated backscatter β at 532 nm is more sensitive to particle concentration and is consequently mainly sensitive to the presence of supercooled water. In addition, radar reflectivity Z at 95 GHz is sensitive to the size of hydrometeors and hence more sensitive to the presence of ice particles. Consequently, in the mixed phase the supercooled droplets are retrieved with the lidar signal and the ice particles with the radar signal, meaning that the retrievals rely strongly on a priori and error values. This method retrieves simultaneously the visible extinction for ice αice and liquid αliq particles, the ice and liquid water contents IWC and LWC, the effective radius of ice re,ice and liquid re,liq particles, and the ice and liquid number concentrations Nice and Nliq. Moreover, total extinction αtot, total water content (TWC) and total number concentration Ntot can also be estimated. As the retrieval of ice and liquid is different, it is necessary to correctly identify each phase of the cloud. To this end, a cloud-phase classification is used as input to the algorithm and has been adapted for mixed-phase retrieval. The data used in this study are from DARDAR-MASK v2.23 products, based on the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) and Cloud Profiling Radar (CPR) observations from the CALIPSO and CloudSat satellites, respectively, belonging to the A-Train constellation launched in 2006. Airborne in situ measurements performed on 7 April 2007 during the Arctic Study of Tropospheric Aerosol, Clouds and Radiation (ASTAR) campaign and collected under the track of CloudSat–CALIPSO are compared with the retrievals of the new algorithm to validate its performance. Visible extinctions, water contents, effective radii and number concentrations derived from in situ measurements and the retrievals showed similar trends and are globally in good agreement. The mean percent error between the retrievals and in situ measurements is 39 % for αliq, 398 % for αice, 49 % for LWC and 75 % for IWC. It is also important to note that temporal and spatial collocations are not perfect, with a maximum spatial shift of 1.68 km and a maximum temporal shift of about 10 min between the two platforms. In addition, the sensitivity of remote sensing and that of in situ measurements is not the same, and in situ measurement uncertainties are between 25 % and 60 %.

Abstract There is growing evidence that marine microorganisms may influence cloud cover over the ocean through their impact on sea spray and trace gas emissions, further forming cloud droplets or ice crystals. However, evidence of a robust causal relationship based on observations is still pending. In this study, we use 4 years of multi‐instrument satellite data to segregate low‐level clouds into ice‐containing and liquid‐water clouds to obtain clear relationships between cloud types and ocean biological tracers, especially with nanophytoplankton cell abundances. Results suggest that microorganisms may be involved in compensating effects on cloud properties, increasing the frequency of occurrence of warm‐liquid clouds, and decreasing the occurrence of ice‐containing clouds in most regions during springtime. The relationships observed in most regions do not apply to the South Pacific Ocean in the 40°S–50°S latitude band. These results shed light on overlooked potential compensating effects of ocean microorganisms on cloud cover.

Airborne in situ cloud measurements were carried out over the northern Fram Strait between Greenland and Svalbard in spring 2019 and summer 2020. In total, 811 min of low-level cloud observations were performed during 20 research flights above the sea ice and the open Arctic ocean with the Polar 5 research aircraft of the Alfred Wegener Institute. Here, we combine the comprehensive in situ cloud data to investigate the distributions of particle number concentration N, effective diameter Deff, and cloud water content CWC (liquid and ice) of Arctic clouds below 500 m altitude, measured at latitudes between 76 and 83∘ N. We developed a method to quantitatively derive the occurrence probability of their thermodynamic phase from the combination of microphysical cloud probe and Polar Nephelometer data. Finally, we assess changes in cloud microphysics and cloud phase related to ambient meteorological conditions in spring and summer and address effects of the sea ice and open-ocean surface conditions. We find median N from 0.2 to 51.7 cm−3 and about 2 orders of magnitude higher N for mainly liquid clouds in summer compared to ice and mixed-phase clouds measured in spring. A southerly flow from the sea ice in cold air outbreaks dominates cloud formation processes at temperatures mostly below −10 ∘C in spring, while northerly warm air intrusions favor the formation of liquid clouds at warmer temperatures in summer. Our results show slightly higher N in clouds over the sea ice compared to the open ocean, indicating enhanced cloud formation processes over the sea ice. The median CWC is higher in summer (0.16 g m−3) than in spring (0.06 g m−3), as this is dominated by the available atmospheric water content and the temperatures at cloud formation level. We find large differences in the particle sizes in spring and summer and an impact of the surface conditions, which modifies the heat and moisture fluxes in the boundary layer. By combining microphysical cloud data with thermodynamic phase information from the Polar Nephelometer, we find mixed-phase clouds to be the dominant thermodynamic cloud phase in spring, with a frequency of occurrence of 61 % over the sea ice and 66 % over the ocean. Pure ice clouds exist almost exclusively over the open ocean in spring, and in summer the cloud particles are most likely in the liquid water state. The comprehensive low-level cloud data set will help us to better understand the role of clouds and their thermodynamic phase in the Arctic radiation budget and to assess the performance of global climate models in a region of the world with the strongest anthropogenic climate change.

Low-level (cloud tops below 2 km) mixed-phase clouds are important in amplifying warming in the Arctic region through positive feedback in cloud fraction, water content and phase. In order to understand the cloud feedbacks in the Arctic region, good knowledge of the vertical distribution of the cloud water content, particle size and phase is required. Here we investigate the vertical extent of the cloud-phase and ice-phase optical properties in six case studies measured in the European Arctic during the ACLOUD campaign. Late spring- and summertime stratiform clouds were sampled in situ over pack ice, marginal sea ice zone and open-ocean surface, with cloud top temperatures varying between −15 and −1.5 ∘C. The results show that, although the liquid phase dominates the upper parts of the clouds, the ice phase was frequently observed in the lower parts down to cloud top temperatures as warm as −3.8 ∘C. In the studied vertical cloud profiles, the maximum of average liquid phase microphysical properties, droplet number concentration, effective radius and liquid water content, varied between 23 and 152 cm−3, 19 and 26 µm, 0.09 and 0.63 g m−3, respectively. The maximum of average ice-phase microphysical properties varied between 0.1 and 57 L−1 for the ice number concentration, 40 and 70 µm for the effective radius, and 0.005 and 0.08 g m−3 for the ice water content. The elevated ice crystal number concentrations and ice water paths observed for clouds, with cloud top temperatures between −3.8 and −8.7 ∘C can be likely attributed to secondary ice production through rime splintering. Low asymmetry parameters between 0.69 and 0.76 were measured for the mixed-phase ice crystals with a mean value of 0.72. The effect of the ice-phase optical properties on the radiative transfer calculations was investigated for the four cloud cases potentially affected by secondary ice production. Generally the choice of ice-phase optical properties only has a minor effect on the cloud transmissivity and albedo, except in a case where the ice phase dominated the upper cloud layer extinction. In this case, cloud albedo at solar wavelengths was increased by 10 % when the ice phase was given its measured optical properties instead of treating it as liquid phase. The presented results highlight the importance of accurate vertical information on cloud phase for radiative transfer and provide a suitable data set for testing microphysical parameterizations in models.

Aerosol–cloud interaction is considered one of the largest sources of uncertainty in radiative forcing estimations. To better understand the role of black carbon (BC) aerosol as a cloud nucleus and the impact of clouds on its vertical distribution in the Arctic, we report airborne in situ measurements of BC particles in the European Arctic near Svalbard during the “Arctic CLoud Observations Using airborne measurements during polar Day” (ACLOUD) campaign held in the summer of 2017. BC was measured with a single-particle soot photometer aboard the Polar 6 research aircraft from the lowest atmospheric layer up to approximately 3500 m a.s.l (metres above sea level). During in-cloud flight transects, BC particles contained in liquid droplets (BC residuals) were sampled through a counterflow virtual impactor (CVI) inlet. Four flights, conducted in the presence of low-level, surface-coupled, inside-inversion, and mixed-phase clouds over sea ice, were selected to address the variability in BC above, below, and within the cloud layer. First, the increase in size and coating thickness of BC particles from the free troposphere to the cloud-dominated boundary layer confirmed that ground observations were not representative of upper atmospheric layers. Second, although only 1 % of liquid droplets contained a BC particle, the higher number concentration of BC residuals than BC particles sampled below cloud indicated that the totality of below-cloud BC was activated by nucleation scavenging but also that alternative scavenging processes such as the activation of free-tropospheric BC at the cloud top might occur. Third, the efficient exchange of aerosol particles at cloud bottom was confirmed by the similarity of the size distribution of BC residuals and BC particles sampled below cloud. Last, the increase in the BC residual number concentration (+31 %) and geometric mean diameter (+38 %) from the cloud top to the cloud bottom and the absolute enrichment in larger BC residuals compared with outside of the cloud supported the hypothesis of concomitant scavenging mechanisms but also suggested the formation of BC agglomerates caused by cloud processing. The vertical evolution of BC properties from inside the cloud and below the cloud indicated an efficient aerosol exchange at cloud bottom, which might include activation, cloud processing, and sub-cloud release of processed BC agglomerates. In the case of persistent low-level Arctic clouds, this cycle may reiterate multiple times, adding an additional degree of complexity to the understanding of cloud processing of BC particles in the Arctic.

The aim of this paper is to present the Monte Carlo code McRALI that provides simulations under multiple-scattering regimes of polarized high-spectral-resolution (HSR) lidar and Doppler radar observations for a three-dimensional (3D) cloudy atmosphere. The effects of nonuniform beam filling (NUBF) on HSR lidar and Doppler radar signals related to the EarthCARE mission are investigated with the help of an academic 3D box cloud characterized by a single isolated jump in cloud optical depth, assuming vertically constant wind velocity. Regarding Doppler radar signals, it is confirmed that NUBF induces a severe bias in velocity estimates. The correlation of the NUBF bias of Doppler velocity with the horizontal gradient of reflectivity shows a correlation coefficient value around 0.15 m s−1 (dBZ km-1)-1, close to that given in the scientific literature. Regarding HSR lidar signals, we confirm that multiple-scattering processes are not negligible. We show that NUBF effects on molecular, particulate, and total attenuated backscatter are mainly due to unresolved variability of cloud inside the receiver field of view and, to a lesser extent, to the horizontal photon transport. This finding gives some insight into the reliability of lidar signal modeling using independent column approximation (ICA).

The Arctic CLoud Observations Using airborne measurements during polar Day (ACLOUD) campaign was carried out north-west of Svalbard (Norway) between 23 May and 6 June 2017. The objective of ACLOUD was to study Arctic boundary layer and mid-level clouds and their role in Arctic amplification. Two research aircraft (Polar 5 and 6) jointly performed 22 research flights over the transition zone between open ocean and closed sea ice. Both aircraft were equipped with identical instrumentation for measurements of basic meteorological parameters, as well as for turbulent and radiative energy fluxes. In addition, on Polar 5 active and passive remote sensing instruments were installed, while Polar 6 operated in situ instruments to characterize cloud and aerosol particles as well as trace gases. A detailed overview of the specifications, data processing, and data quality is provided here. It is shown that the scientific analysis of the ACLOUD data benefits from the coordinated operation of both aircraft. By combining the cloud remote sensing techniques operated on Polar 5, the synergy of multi-instrument cloud retrieval is illustrated. The remote sensing methods were validated using truly collocated in situ and remote sensing observations. The data of identical instruments operated on both aircraft were merged to extend the spatial coverage of mean atmospheric quantities and turbulent and radiative flux measurement. Therefore, the data set of the ACLOUD campaign provides comprehensive in situ and remote sensing observations characterizing the cloudy Arctic atmosphere. All processed, calibrated, and validated data are published in the World Data Center PANGAEA as instrument-separated data subsets (Ehrlich et al., 2019b, https://doi.org/10.1594/PANGAEA.902603).

Ice crystal submicron structures have a large impact on the optical properties of cirrus clouds and consequently on their radiative effect. Although there is growing evidence that atmospheric ice crystals are rarely pristine, direct in situ observations of the degree of ice crystal complexity are largely missing. Here we show a comprehensive in situ data set of ice crystal complexity coupled with measurements of the cloud angular scattering functions collected during a number of observational airborne campaigns at diverse geographical locations. Our results demonstrate that an overwhelming fraction (between 61 % and 81 %) of atmospheric ice crystals sampled in the different regions contain mesoscopic deformations and, as a consequence, a similar flat and featureless angular scattering function is observed. A comparison between the measurements and a database of optical particle properties showed that severely roughened hexagonal aggregates optimally represent the measurements in the observed angular range. Based on this optical model, a new parameterization of the cloud bulk asymmetry factor was introduced and its effects were tested in a global climate model. The modelling results suggest that, due to ice crystal complexity, ice-containing clouds can induce an additional short-wave cooling effect of −1.12 W m2 on the top-of-the-atmosphere radiative budget that has not yet been considered.

Air traffic affects cloudiness, and thus climate, by emitting exhaust gases and particles. The study of the evolution of contrail properties is very challenging due to the complex interplay of vortex dynamics and the atmospheric environment (e.g. temperature, supersaturation). Despite substantial progress in recent years, the optical, microphysical, and macrophysical properties of contrails and ambient cirrus during contrail formation and subsequent ageing are still subject to large uncertainties due to instrumental and observational limitations and the large number of variables influencing the contrail life cycle. In this study, various contrail cases corresponding to different aircraft types and atmospheric conditions are investigated using a statistical method based on the in situ optical measurements performed during the Contrail and Cirrus Experiments (CONCERT) campaigns 2008 and 2011. The two aircraft campaigns encompass more than 17 aircraft contrail cases. A principal component analysis (PCA) of the angular scattering coefficients measured by the polar nephelometer is implemented. The goal is to classify the sampled ice cloud measurements in several clusters representative of different contrail development stages (primary wake, young contrail, aged contrail, and cirrus). Extinction and asymmetry coefficients, nitrogen oxide concentrations, and relative humidity with respect to ice and particle size distributions are analysed for each cluster to characterize the evolution of ice cloud properties during the contrail to cirrus evolution. The PCA demonstrates that contrail optical properties are well suited to identify and discriminate between the different contrail growth stages and to characterize the evolution of contrail properties.

This study aims to characterize the microphysical and optical properties of ice crystals and supercooled liquid droplets within low-level Arctic mixed-phase clouds (MPCs). We compiled and analyzed cloud in situ measurements from four airborne spring campaigns (representing 18 flights and 71 vertical profiles in MPCs) over the Greenland and Norwegian seas mainly in the vicinity of the Svalbard archipelago. Cloud phase discrimination and representative vertical profiles of the number, size, mass and shape of ice crystals and liquid droplets are established. The results show that the liquid phase dominates the upper part of the MPCs. High concentrations (120 cm−3 on average) of small droplets (mean values of 15 µm), with an averaged liquid water content (LWC) of 0.2 g m−3 are measured at cloud top. The ice phase dominates the microphysical properties in the lower part of the cloud and beneath it in the precipitation region (mean values of 100 µm, 3 L−1 and 0.025 g m−3 for diameter, particle concentration and ice water content (IWC), respectively). The analysis of the ice crystal morphology shows that the majority of ice particles are irregularly shaped or rimed particles; the prevailing regular habits found are stellars and plates. We hypothesize that riming and diffusional growth processes, including the Wegener–Bergeron–Findeisen (WBF) mechanism, are the main growth mechanisms involved in the observed MPCs. The impact of larger-scale meteorological conditions on the vertical profiles of MPC properties was also investigated. Large values of LWC and high concentration of smaller droplets are possibly linked to polluted situations and air mass origins from the south, which can lead to very low values of ice crystal size and IWC. On the contrary, clean situations with low temperatures exhibit larger values of ice crystal size and IWC. Several parameterizations relevant for remote sensing or modeling studies are also determined, such as IWC (and LWC) – extinction relationship, ice and liquid integrated water paths, ice concentration and liquid water fraction according to temperature.

Aerosols of the volcanic degassing plumes from Mt. Etna and Mt. Stromboli were probed with in situ instruments on board the Deutsches Zentrum für Luft- und Raumfahrt research aircraft Falcon during the contrail, volcano, and cirrus experiment CONCERT in September 2011. Aerosol properties were analyzed using angular-scattering intensities and particle size distributions measured simultaneously with the Polar Nephelometer and the Forward Scattering Spectrometer probes (FSSP series 100 and 300), respectively. Aerosols of degassing plumes are characterized by low values of the asymmetry parameter (between 0.6 and 0.75); the effective diameter was within the range of 1.5–2.8 µm and the maximal diameter was lower than 20 µm. A principal component analysis applied to the Polar Nephelometer data indicates that scattering features of volcanic aerosols of different crater origins are clearly distinctive from angular-scattering intensities of cirrus and contrails. Retrievals of aerosol properties revealed that the particles were "optically spherical" and the estimated values of the real part of the refractive index are within the interval from 1.35 to 1.38. The interpretation of these results leads to the conclusion that the degassing plume aerosols were porous with air voids. Our estimates suggest that aerosol particles contained about 18 to 35 % of air voids in terms of the total volume.

This study reports on the origin of small-scale ice crystal complexity and its influence on the angular light scattering properties of cirrus clouds. Cloud simulation experiments were conducted at the AIDA (Aerosol Interactions and Dynamics in the Atmosphere) cloud chamber of the Karlsruhe Institute of Technology (KIT). A new experimental procedure was applied to grow and sublimate ice particles at defined super- and subsaturated ice conditions and for temperatures in the −40 to −60 °C range. The experiments were performed for ice clouds generated via homogeneous and heterogeneous initial nucleation. Small-scale ice crystal complexity was deduced from measurements of spatially resolved single particle light scattering patterns by the latest version of the Small Ice Detector (SID-3). It was found that a high crystal complexity dominates the microphysics of the simulated clouds and the degree of this complexity is dependent on the available water vapor during the crystal growth. Indications were found that the small-scale crystal complexity is influenced by unfrozen H2SO4 / H2O residuals in the case of homogeneous initial ice nucleation. Angular light scattering functions of the simulated ice clouds were measured by the two currently available airborne polar nephelometers: the polar nephelometer (PN) probe of Laboratoire de Métérologie et Physique (LaMP) and the Particle Habit Imaging and Polar Scattering (PHIPS-HALO) probe of KIT. The measured scattering functions are featureless and flat in the side and backward scattering directions. It was found that these functions have a rather low sensitivity to the small-scale crystal complexity for ice clouds that were grown under typical atmospheric conditions. These results have implications for the microphysical properties of cirrus clouds and for the radiative transfer through these clouds.

Clouds have an important role in Earth's radiative budget. Since the late 1970s, considerable instrumental developments have been made in order to quantify cloud microphysical and optical properties, for both airborne and ground-based applications. Intercomparison studies have been carried out in the past to assess the reliability of cloud microphysical properties inferred from various measurement techniques. However, observational uncertainties still exist, especially for droplet size distribution measurements and need to be reduced. In this work, we discuss results from an intercomparison campaign, performed at the Puy de Dôme in May 2013. During this campaign, a unique set of cloud instruments was operating simultaneously in ambient air conditions and in a wind tunnel. A Particle Volume Monitor (PVM-100), a Forward Scattering Spectrometer Probe (FSSP), a Fog Monitor (FM-100), and a Present Weather Detector (PWD) were sampling on the roof of the station. Within a wind tunnel located underneath the roof, two Cloud Droplet Probes (CDPs) and a modified FSSP (SPP-100) were operating. The main objectives of this paper are (1) to study the effects of wind direction and speed on ground-based cloud observations, (2) to quantify the cloud parameters discrepancies observed by the different instruments, and (3) to develop methods to improve the quantification of the measurements. The results revealed that all instruments showed a good agreement in their sizing abilities, both in terms of amplitude and variability. However, some of them, especially the FM-100, the FSSP and the SPP, displayed large discrepancies in their capability to assess the magnitude of the total number concentration of the cloud droplets. As a result, the total liquid water content can differ by up to a factor of 5 between the probes. The use of a standardization procedure, based on data of integrating probes (PVM-100 or visibilimeter) and extinction coefficient comparison substantially enhanced the instrumental agreement. During this experiment, the total concentration agreed in variations with the visibilimeter, except for the FSSP, so a corrective factor can be applied and it ranges from 0.44 to 2.2. This intercomparison study highlights the necessity to have an instrument which provides a bulk measurement of cloud microphysical or optical properties during cloud ground-based campaigns. Moreover, the FM and FSSP orientation was modified with an angle ranging from 30 to 90° angle with wind speeds from 3 to 7 m s−1. The results show that the induced number concentration loss is between 29 and 98 % for the FSSP and between 15 and 68 % for the FM-100. In particular, FSSP experiments showed strong discrepancies when the wind speed was lower than 3 m s−1 and/or when the angle between the wind direction and the orientation of the instruments is greater than 30°. An inadequate orientation of the FSSP towards the wind direction leads to an underestimation of the measured effective diameter.

Snow consists of non-spherical grains of various shapes and sizes. Still, in many radiative transfer applications, single-scattering properties of snow have been based on the assumption of spherical grains. More recently, second-generation Koch fractals have been employed. While they produce a relatively flat phase function typical of deformed non-spherical particles, this is still a rather ad hoc choice. Here, angular scattering measurements for blowing snow conducted during the CLimate IMpacts of Short-Lived pollutants In the Polar region (CLIMSLIP) campaign at Ny Ålesund, Svalbard, are used to construct a reference phase function for snow. Based on this phase function, an optimized habit combination (OHC) consisting of severely rough (SR) droxtals, aggregates of SR plates and strongly distorted Koch fractals is selected. The single-scattering properties of snow are then computed for the OHC as a function of wavelength λ and snow grain volume-to-projected area equivalent radius rvp. Parameterization equations are developed for λ = 0.199–2.7 μm and rvp = 10–2000 μm, which express the single-scattering co-albedo β, the asymmetry parameter g and the phase function P11 as functions of the size parameter and the real and imaginary parts of the refractive index. The parameterizations are analytic and simple to use in radiative transfer models. Compared to the reference values computed for the OHC, the accuracy of the parameterization is very high for β and g. This is also true for the phase function parameterization, except for strongly absorbing cases (β > 0.3). Finally, we consider snow albedo and reflected radiances for the suggested snow optics parameterization, making comparisons to spheres and distorted Koch fractals.

The Arctic region is known to be very sensitive to climate change. Clouds and in particular mixed-phase clouds (MPCs) remain one of the greatest sources of uncertainties in the modelling of the Arctic response to climate change due to an inaccurate representation of their variability and their quantification. In this study, we present a characterisation of the vertical, spatial and seasonal variability of Arctic clouds and MPCs over the entire Arctic region based on satellite active remote sensing observations. MPC properties in the region of the Svalbard archipelago (78° N, 15° E) are also investigated. The occurrence frequency of clouds and MPCs are determined from CALIPSO/CLOUDSAT measurements processed with the DARDAR retrieval algorithm, which allow for a reliable cloud thermodynamic phase classification (warm liquid, supercooled liquid, ice, mixing of ice and supercooled liquid). Significant differences are observed between MPC properties over the entire Arctic region and over the Svalbard region. Results show that MPCs are encountered all year long, with a minimum occurrence of 30% in winter and 50% during the rest of the year on average over the entire Arctic. Over the Svalbard region, MPC occurrence is more constant with time with larger values (55%) compared to the average observed in the Arctic. MPCs are especially located at low altitudes, below 3000 m, where their frequency of occurrence reaches 90%, particularly during winter, spring and autumn. Moreover, results highlight that MPCs are statistically more frequent above open sea than land or sea ice. The temporal and spatial distribution of MPCs over the Svalbard region seems to be linked to the supply of moister air and warmer water from the North Atlantic Ocean, which contribute to the initiation of the liquid water phase. Over the whole Arctic, and particularly in western regions, the increase of MPC occurrence from spring to autumn could be connected to the sea ice melting. During this period, the open water transports some of the warm water from the North Atlantic Ocean to the rest of the Arctic region. This facilitates the vertical transfer of moisture and thus the persistence of the liquid phase. Particular attention is also paid to the measurement uncertainties and how they could affect our conclusions.

The 3DCLOUD algorithm for generating stochastic three-dimensional (3-D) cloud fields is described in this paper. The generated outputs are 3-D optical depth (τ) for stratocumulus and cumulus fields and 3-D ice water content (IWC) for cirrus clouds. This model is designed to generate cloud fields that share some statistical properties observed in real clouds such as the inhomogeneity parameter ρ (standard deviation normalized by the mean of the studied quantity), the Fourier spectral slope β close to −5/3 between the smallest scale of the simulation to the outer Lout (where the spectrum becomes flat). Firstly, 3DCLOUD assimilates meteorological profiles (humidity, pressure, temperature and wind velocity). The cloud coverage C, defined by the user, can also be assimilated, but only for stratocumulus and cumulus regime. 3DCLOUD solves drastically simplified basic atmospheric equations, in order to simulate 3-D cloud structures of liquid or ice water content. Secondly, the Fourier filtering method is used to constrain the intensity of ρ, β, Lout and the mean of τ or IWC of these 3-D cloud structures. The 3DCLOUD model was developed to run on a personal computer under Matlab environment with the Matlab statistics toolbox. It is used to study 3-D interactions between cloudy atmosphere and radiation.

Cirrus are cloud types that are recognized to have a strong impact on the Earth-atmosphere radiation balance. This impact is however still poorly understood, due to the difficulties in describing the large variability of their properties in global climate models. Consequently, numerous airborne and space-borne missions have been dedicated to their study in the last decades. The satellite constellation A-Train has for instance proven to be particularly helpful for the study of cirrus. More particularly, the Infrared Imaging Radiometer (IIR) carried onboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite shows a great sensitivity to the radiative and microphysical properties of these clouds. Our study presents a novel methodology that uses the thermal infrared measurements of IIR to retrieve the ice crystal effective size and optical thickness of cirrus. This methodology is based on an optimal estimation scheme, which possesses the advantage of attributing precise uncertainties to the retrieved parameters. Two IIR airborne validation campaigns have been chosen as case studies for illustrating the results of our retrieval method. It is observed that optical thicknesses could be accurately retrieved but that large uncertainties may occur on the effective diameters. Strong agreements have also been found between the products of our method when separately applied to the measurements of IIR and of the airborne radiometer CLIMAT-AV, which consolidates the results of previous validation studies of IIR level-1 measurements. Comparisons with in situ observations and with operational products of IIR are also discussed and appear to be coherent with our results. However, we have found that the quality of our retrievals can be strongly impacted by uncertainties related to the choice of a pristine crystal model and by poor constraints on the properties of possible liquid cloud layers underneath cirrus. Simultaneous retrievals of liquid clouds radiative and microphysical properties and/or the use of different ice crystal models should therefore be considered in order to improve the quality of the results.

In this paper, we show that in mixed phase clouds, the presence of ice crystals may induce wrong FSSP 100 measurements interpretation especially in terms of particle size and subsequent bulk parameters. The presence of ice crystals is generally revealed by a bimodal feature of the particle size distribution (PSD). The combined measurements of the FSSP-100 and the Polar Nephelometer give a coherent description of the effect of the ice crystals on the FSSP-100 response. The FSSP-100 particle size distributions are characterized by a bimodal shape with a second mode peaked between 25 and 35 μm related to ice crystals. This feature is observed with the FSSP-100 at airspeed up to 200 m s−1 and with the FSSP-300 series. In order to assess the size calibration for clouds of ice crystals the response of the FSSP-100 probe has been numerically simulated using a light scattering model of randomly oriented hexagonal ice particles and assuming both smooth and rough crystal surfaces. The results suggest that the second mode, measured between 25 μm and 35 μm, does not necessarily represent true size responses but corresponds to bigger aspherical ice particles. According to simulation results, the sizing understatement would be neglected in the rough case but would be significant with the smooth case. Qualitatively, the Polar Nephelometer phase function suggests that the rough case is the more suitable to describe real crystals. Quantitatively, however, it is difficult to conclude. A review is made to explore different hypotheses explaining the occurrence of the second mode. However, previous cloud in situ measurements suggest that the FSSP-100 secondary mode, peaked in the range 25–35 μm, is likely to be due to the shattering of large ice crystals on the probe inlet. This finding is supported by the rather good relationship between the concentration of particles larger than 20 μm (hypothesized to be ice shattered-fragments measured by the FSSP) and the concentration of (natural) ice particles (CPI data). In mixed cloud, a simple estimation of the number of ice crystals impacting the FSSP inlet shows that the ice crystal shattering effect is the main factor in observed ice production.

During the Arctic Study of Tropospheric Aerosol, Clouds and Radiation (ASTAR) campaign, which was conducted in March and April 2007, an optically thin ice cloud was observed south of Svalbard at around 3 km altitude. The microphysical and radiative properties of this particular subvisible midlevel cloud were investigated with complementary remote sensing and in situ instruments. Collocated airborne lidar remote sensing and spectral solar radiation measurements were performed at a flight altitude of 2300 m below the cloud base. Under almost stationary atmospheric conditions, the same subvisible midlevel cloud was probed with various in situ sensors roughly 30 min later. From individual ice crystal samples detected with the Cloud Particle Imager and the ensemble of particles measured with the Polar Nephelometer, microphysical properties were retrieved with a bi-modal inversion algorithm. The best agreement with the measurements was obtained for small ice spheres and deeply rough hexagonal ice crystals. Furthermore, the single-scattering albedo, the scattering phase function as well as the volume extinction coefficient and the effective diameter of the crystal population were determined. A lidar ratio of 21(±6) sr was deduced by three independent methods. These parameters in conjunction with the cloud optical thickness obtained from the lidar measurements were used to compute spectral and broadband radiances and irradiances with a radiative transfer code. The simulated results agreed with the observed spectral downwelling radiance within the range given by the measurement uncertainty. Furthermore, the broadband radiative simulations estimated a net (solar plus thermal infrared) radiative forcing of the subvisible midlevel ice cloud of −0.4 W m−2 (−3.2 W m−2 in the solar and +2.8 W m−2 in the thermal infrared wavelength range).

During the international ASTAR experiment (Arctic Study of Aerosols, Clouds and Radiation) carried out from Longyearbyen (Spitsbergen) from 10 May to 11 June 2004, the AWI (Alfred Wegener Institute) Polar 2 aircraft was equipped with a unique combination of remote and in situ instruments. The airborne AMALi lidar provided downward backscatter and Depolarisation ratio profiles at 532 nm wavelength. The in situ instrumental setup comprised a Polar Nephelometer, a Cloud Particle Imager (CPI) as well as a Nevzorov and standard PMS probes to measure cloud particle properties in terms of scattering characteristics, particle morphology and size, and in-cloud partitioning of ice/water content. The objective of the paper is to present the results of a case study related to observations with ice crystals precipitating down to supercooled boundary-layer stratocumulus. The flight pattern was predefined in a way that firstly the AMALi lidar probed the cloud tops to guide the in situ measurements into a particular cloud formation. Three kinds of clouds with different microphysical and optical properties have therefore been quasi-simultaneously observed: (i) water droplets stratiform-layer, (ii) drizzle-drops fallstreak and (iii) precipitating ice-crystals from a cirrus cloud above. The signatures of these clouds are clearly evidenced from the in situ measurements and from the lidar profiles in term of backscatter and Depolarisation ratio. Accordingly, typical lidar ratios, i.e., extinction-to-backscatter ratios, are derived from the measured scattering phase function combined with subsequent particle shapes and size distributions. The backscatter profiles can therefore be retrieved under favourable conditions of low optical density. From these profiles extinction values in different cloud types can be obtained and compared with the direct in situ measurements.

Faced with the pandemic viral circulation of SARS-CoV-2, healthcare establishments have had to maintain an effective screening strategy in order to prevent nosocomial clusters. Automated antigenic tests appear to be a reliable and complementary alternative to RT-PCR (reverse transcriptase polymerase chain reaction) in order to optimize patient care in the emergency department. We report our experience of the deployment of the LumiraDx antigen tests on the LumiraDx platform, as well as the comparison of these tests' results with the RT-PCR results on a population of patients sampled in the emergency department.

Two airborne field campaigns focusing on observations of Arctic mixed-phase clouds and boundary layer processes and their role with respect to Arctic amplification have been carried out in spring 2019 and late summer 2020 over the Fram Strait northwest of Svalbard. The latter campaign was closely connected to the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition. Comprehensive datasets of the cloudy Arctic atmosphere have been collected by operating remote sensing instruments, in-situ probes, instruments for the measurement of turbulent fluxes of energy and momentum, and dropsondes on board the AWI research aircraft Polar 5. In total, 24 flights with 111 flight hours have been performed over open ocean, the marginal sea ice zone, and sea ice. The datasets follow documented methods and quality assurance and are suited for studies on Arctic mixed-phase clouds and their transformation processes, for studies with a focus on Arctic boundary layer processes, and for satellite validation applications. All datasets are freely available via the world data center PANGAEA., (© 2022. The Author(s).)