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Executive Summary The State of Environmental Science in Svalbard (SESS) report 2021 together with its predecessors contributes to the documentation of the state of the Arctic environment in and around Svalbard, and highlights research conducted within the Svalbard Integrated Arctic Earth Observing System (SIOS). Climate change is a global problem, but many of its impacts are being felt most strongly in the Arctic. Given its remote but accessible location, Svalbard constitutes an ideal place to study the Arctic environment in general, including, more specifically, the causes and consequences of climate change. The Arctic Climate Change Update (2021) emphasised the severity of global climate change for ecosystems across the Arctic. They are undergoing radical changes regarding their structure and functioning, affecting flora, fauna and livelihoods of Arctic communities. Oceanic ecosystems and food webs are directly and indirectly altered by the warming and freshening of the Arctic Ocean. A prolonged open water period and the expansion of open water areas caused by declining sea ice affect under-ice productivity and diversity. These changes have cascading effects through ecosystems and impact the distribution, abundance and seasonality of a variety of marine species. Svalbard is located at one of the key oceanic gateways to the Arctic. This land–ice–ocean transition zone is a system particularly vulnerable to environmental changes. Svalbard’s environment is influenced by maritime processes; thus extensive observation of the ocean system is nowadays necessary. The chapter on the iMOP project reports seawater temperature and salinity variability over the last decades and indicates changes of Svalbard fjord seawater properties. The chapter highlights the role of a collaborative and supportive network of observatory operators and encourages joint planning and maintenance of future marine observatories. Arctic vegetation plays a key role in land–atmosphere interactions. Alt

Executive Summary The State of Environmental Science in Svalbard (SESS) report 2021 together with its predecessors contributes to the documentation of the state of the Arctic environment in and around Svalbard, and highlights research conducted within the Svalbard Integrated Arctic Earth Observing System (SIOS). Climate change is a global problem, but many of its impacts are being felt most strongly in the Arctic. Given its remote but accessible location, Svalbard constitutes an ideal place to study the Arctic environment in general, including, more specifically, the causes and consequences of climate change. The Arctic Climate Change Update (2021) emphasised the severity of global climate change for ecosystems across the Arctic. They are undergoing radical changes regarding their structure and functioning, affecting flora, fauna and livelihoods of Arctic communities. Oceanic ecosystems and food webs are directly and indirectly altered by the warming and freshening of the Arctic Ocean. A prolonged open water period and the expansion of open water areas caused by declining sea ice affect under-ice productivity and diversity. These changes have cascading effects through ecosystems and impact the distribution, abundance and seasonality of a variety of marine species. Svalbard is located at one of the key oceanic gateways to the Arctic. This land–ice–ocean transition zone is a system particularly vulnerable to environmental changes. Svalbard’s environment is influenced by maritime processes; thus extensive observation of the ocean system is nowadays necessary. The chapter on the iMOP project reports seawater temperature and salinity variability over the last decades and indicates changes of Svalbard fjord seawater properties. The chapter highlights the role of a collaborative and supportive network of observatory operators and encourages joint planning and maintenance of future marine observatories. Arctic vegetation plays a key role in land–atmosphere interactions. Alt

Svalbard Integrated Arctic Earth Observing System (SIOS) is an international consortium for developing and maintaining a regional observing system in Svalbard and the associated waters. SIOS brings together the existing infrastructure and data from its members into a multidisciplinary network dedicated to answering Earth System Science (ESS) questions related to global change. The Observing System is built around “SIOS Core Data” – long-term data series collected by SIOS partners. SIOS Core Data (SCD) are data that fulfil the following defined criteria: to be relevant for answering key Earth System Science questions (scientific requirements), to be available according to the FAIR principles (data availability), to be secured by members for more than 5 years (member commitment). The first set of SIOS core data variables has been identified by The Science Optimisation Advisory Group (SOAG) in cooperation with the Research Infrastructure Coordination Committee (RICC) and scientific experts. Many (but not all) SIOS core data variables are selected or derived from the Essential Climate Variables (ECVs) defined by The Global Climate Observing System (GCOS), and are described by WMO standards and the Global Change Master Directory (GCMD) Keywords. SIOS core data variables are critical for characterising the climate system and its changes in the Arctic, and answering the ESS science questions outlined in the SIOS Infrastructure Optimization Report. SIOS activities related to SCD are in line with SAON's Roadmap for Arctic Observing and Data Systems process as well as the new EC ArcticPASSION project and the idea of a set of Arctic Shared Variables.

Svalbard Integrated Arctic Earth Observing System (SIOS) is an international consortium for developing and maintaining a regional observing system in Svalbard and the associated waters. SIOS brings together the existing infrastructure and data from its members into a multidisciplinary network dedicated to answering Earth System Science (ESS) questions related to global change. The Observing System is built around “SIOS Core Data” – long-term data series collected by SIOS partners. SIOS Core Data (SCD) are data that fulfil the following defined criteria: to be relevant for answering key Earth System Science questions (scientific requirements), to be available according to the FAIR principles (data availability), to be secured by members for more than 5 years (member commitment). The first set of SIOS core data variables has been identified by The Science Optimisation Advisory Group (SOAG) in cooperation with the Research Infrastructure Coordination Committee (RICC) and scientific experts. Many (but not all) SIOS core data variables are selected or derived from the Essential Climate Variables (ECVs) defined by The Global Climate Observing System (GCOS), and are described by WMO standards and the Global Change Master Directory (GCMD) Keywords. SIOS core data variables are critical for characterising the climate system and its changes in the Arctic, and answering the ESS science questions outlined in the SIOS Infrastructure Optimization Report. SIOS activities related to SCD are in line with SAON's Roadmap for Arctic Observing and Data Systems process as well as the new EC ArcticPASSION project and the idea of a set of Arctic Shared Variables.

Biomass burning related aerosol episodes are becoming a serious threat to the radiative balance of the Arctic region. Since early July 2017 intense wildfires were recorded between August and September in Canada and Greenland, covering an area up to 4674 km2 in size. This paper describes the impact of these biomass burning (BB) events measured over Svalbard, using an ensemble of ground-based, columnar, and vertically-resolved techniques. BB influenced the aerosol chemistry via nitrates and oxalates, which exhibited an increase in their concentrations in all of size fractions, indicating the BB origin of particles. The absorption coefficient data (530 nm) at ground reached values up to 0.6 Mm-1, highlighting the impact of these BB events when compared to average Arctic background values, which do not exceed 0.05 Mm-1. The absorption behavior is fundamental as implies a subsequent atmospheric heating. At the same time, the AERONET Aerosol Optical Depth (AOD) data showed high values at stations located close to or in Canada (AOD over 2.0). Similarly, increased values of AODs were then observed in Svalbard, e.g., in Hornsund (daily average AODs exceeded 0.14 and reached hourly values up to 0.5). Elevated values of AODs were then registered in Sodankyla and Andenes (daily average AODs exceeding 0.150) a few days after the Svalbard observation of the event highlighting the BB columnar magnitude, which is crucial for the radiative impact. All the reported data suggest to rank the summer 2017 plume of aerosols as one of the biggest atmosphere related environmental problems over Svalbard region in last 10 years.

The impact of aerosol spatio-temporal variability on the Arctic radiative budget is not fully constrained. This case study focuses on the intra-Arctic modification of long-range transported aerosol and its direct aerosol radiative effect (ARE). Different types of air-borne and ground-based remote sensing observations (from Lidar and sun-photometer) revealed a high tropospheric aerosol transport episode over two parts of the European Arctic in April 2018. By incorporating the derived aerosol optical and microphysical properties into a radiative transfer model, we assessed the ARE over the two locations. Our study displayed that even in neighboring Arctic upper tropospheric levels, aged aerosol was transformed due to the interplay of removal processes (nucleation scavenging and dry deposition) and alteration of the aerosol source regions (northeast Asia and north Europe). Along the intra-Arctic transport, the coarse aerosol mode was depleted and the visible wavelength Lidar ratio (LR) increased significantly (from 15 to 64–82 sr). However, the aerosol modifications were not reflected on the ARE. More specifically, the short-wave (SW) atmospheric column ARE amounted to +4.4 - +4.9 W m−2 over the ice-covered Fram Strait and +4.5 W m−2 over the snow-covered Ny-Ålesund. Over both locations, top-of-atmosphere (TOA) warming was accompanied by surface cooling. These similarities can be attributed to the predominant accumulation mode, which drives the SW radiative budget, as well as to the similar layer altitude, solar geometry, and surface albedo conditions over both locations. However, in the context of retreating sea ice, the ARE may change even along individual transport episodes due to the ice albedo feedback.

Biomass burning related aerosol episodes are becoming a serious threat to the radiative balance of the Arctic region. Since early July 2017 intense wildfires were recorded between August and September in Canada and Greenland, covering an area up to 4674 km2 in size. This paper describes the impact of these biomass burning (BB) events measured over Svalbard, using an ensemble of ground-based, columnar, and vertically-resolved techniques. BB influenced the aerosol chemistry via nitrates and oxalates, which exhibited an increase in their concentrations in all of size fractions, indicating the BB origin of particles. The absorption coefficient data (530 nm) at ground reached values up to 0.6 Mm–1, highlighting the impact of these BB events when compared to average Arctic background values, which do not exceed 0.05 Mm–1. The absorption behavior is fundamental as implies a subsequent atmospheric heating. At the same time, the AERONET Aerosol Optical Depth (AOD) data showed high values at stations located close to or in Canada (AOD over 2.0). Similarly, increased values of AODs were then observed in Svalbard, e.g., in Hornsund (daily average AODs exceeded 0.14 and reached hourly values up to 0.5). Elevated values of AODs were then registered in Sodankylä and Andenes (daily average AODs exceeding 0.150) a few days after the Svalbard observation of the event highlighting the BB columnar magnitude, which is crucial for the radiative impact. All the reported data suggest to rank the summer 2017 plume of aerosols as one of the biggest atmosphere related environmental problems over Svalbard region in last 10 years

The impact of aerosol spatio-temporal variability on the Arctic radiative budget is not fully constrained. This case study focuses on the intra-Arctic modification of long-range transported aerosol and its direct aerosol radiative effect (ARE). Different types of air-borne and ground-based remote sensing observations (from Lidar and sun-photometer) revealed a high tropospheric aerosol transport episode over two parts of the European Arctic in April 2018. By incorporating the derived aerosol optical and microphysical properties into a radiative transfer model, we assessed the ARE over the two locations. Our study displayed that even in neighboring Arctic upper tropospheric levels, aged aerosol was transformed due to the interplay of removal processes (nucleation scavenging and dry deposition) and alteration of the aerosol source regions (northeast Asia and north Europe). Along the intra-Arctic transport, the coarse aerosol mode was depleted and the visible wavelength Lidar ratio (LR) increased significantly (from 15 to 64–82 sr). However, the aerosol modifications were not reflected on the ARE. More specifically, the short-wave (SW) atmospheric column ARE amounted to +4.4 - +4.9 W m−2 over the ice-covered Fram Strait and +4.5 W m−2 over the snow-covered Ny-Ålesund. Over both locations, top-of-atmosphere (TOA) warming was accompanied by surface cooling. These similarities can be attributed to the predominant accumulation mode, which drives the SW radiative budget, as well as to the similar layer altitude, solar geometry, and surface albedo conditions over both locations. However, in the context of retreating sea ice, the ARE may change even along individual transport episodes due to the ice albedo feedback.

Biomass burning related aerosol episodes are becoming a serious threat to the radiative balance of the Arctic region. Since early July 2017 intense wildfires were recorded between August and September in Canada and Greenland, covering an area up to 4674 km2 in size. This paper describes the impact of these biomass burning (BB) events measured over Svalbard, using an ensemble of ground-based, columnar, and vertically-resolved techniques. BB influenced the aerosol chemistry via nitrates and oxalates, which exhibited an increase in their concentrations in all of size fractions, indicating the BB origin of particles. The absorption coefficient data (530 nm) at ground reached values up to 0.6 Mm–1, highlighting the impact of these BB events when compared to average Arctic background values, which do not exceed 0.05 Mm–1. The absorption behavior is fundamental as implies a subsequent atmospheric heating. At the same time, the AERONET Aerosol Optical Depth (AOD) data showed high values at stations located close to or in Canada (AOD over 2.0). Similarly, increased values of AODs were then observed in Svalbard, e.g., in Hornsund (daily average AODs exceeded 0.14 and reached hourly values up to 0.5). Elevated values of AODs were then registered in Sodankylä and Andenes (daily average AODs exceeding 0.150) a few days after the Svalbard observation of the event highlighting the BB columnar magnitude, which is crucial for the radiative impact. All the reported data suggest to rank the summer 2017 plume of aerosols as one of the biggest atmosphere related environmental problems over Svalbard region in last 10 years

Biomass burning related aerosol episodes are becoming a serious threat to the radiative balance of the Arctic region. Since early July 2017 intense wildfires were recorded between August and September in Canada and Greenland, covering an area up to 4674 km2 in size. This paper describes the impact of these biomass burning (BB) events measured over Svalbard, using an ensemble of ground-based, columnar, and vertically-resolved techniques. BB influenced the aerosol chemistry via nitrates and oxalates, which exhibited an increase in their concentrations in all of size fractions, indicating the BB origin of particles. The absorption coefficient data (530 nm) at ground reached values up to 0.6 Mm-1, highlighting the impact of these BB events when compared to average Arctic background values, which do not exceed 0.05 Mm-1. The absorption behavior is fundamental as implies a subsequent atmospheric heating. At the same time, the AERONET Aerosol Optical Depth (AOD) data showed high values at stations located close to or in Canada (AOD over 2.0). Similarly, increased values of AODs were then observed in Svalbard, e.g., in Hornsund (daily average AODs exceeded 0.14 and reached hourly values up to 0.5). Elevated values of AODs were then registered in Sodankyla and Andenes (daily average AODs exceeding 0.150) a few days after the Svalbard observation of the event highlighting the BB columnar magnitude, which is crucial for the radiative impact. All the reported data suggest to rank the summer 2017 plume of aerosols as one of the biggest atmosphere related environmental problems over Svalbard region in last 10 years.

In this work, optical and microphysical properties of Arctic aerosol as well as their radiative impact are investigated. Air-borne Lidar observations along with ground-based measurements are evaluated for the Arctic Haze season of 2018. Aerosol abundance as inferred from particle backscatter was typical for this period of the year, with nearly spherical particles. However, the inversion of microphysical properties yielded high Refractive Index (RI) together with low Single-Scattering Albedo (SSA), suggesting absorbing particles, which are not typical for the Arctic Haze period. A fitted lognormal volume distribution revealed a fine mode with effective radius (reff) of 0.23 μm and a coarse mode with reff=0.75 μm. The total aerosol forcing at ground level was negative according to observations and radiative transfer simulations.

The synergy between airborne lidar, radar, passive microwave, and passive imaging spectrometer measurements was used to characterize the vertical and small-scale (down to 10 m) horizontal distribution of the cloud thermodynamic phase. Two case studies of low-level Arctic clouds in a cold air outbreak and a warm air advection observed during the Arctic CLoud Observations Using airborne measurements during polar Day (ACLOUD) were investigated. Both clouds exhibited the typical vertical mixed-phase structure with mostly liquid water droplets at cloud top and ice crystals in lower layers. The cloud top horizontal small-scale variability observed during the cold air outbreak is dominated by the liquid water close to the cloud top and shows no indication of ice in lower cloud layers. Contrastingly, the cloud top variability of the case observed during a warm air advection showed some ice in areas of low reflectivity or cloud holes. Radiative transfer simulations considering homogeneous mixtures of liquid water droplets and ice crystals were able to reproduce the horizontal variability of this warm air advection. To account for more realistic vertical distributions of the thermodynamic phase, large eddy simulations (LES) were performed to reconstruct the observed cloud properties and were used as input for radiative transfer simulations. The simulations of the cloud observed during the cold air outbreak, with mostly liquid water at cloud top, realistically reproduced the observations. For the warm air advection case, the simulated cloud field underestimated the ice water content (IWC). Nevertheless, it revealed the presence of ice particles close to the cloud top and confirmed the observed horizontal variability of the cloud field. It is concluded that the cloud top small-scale horizontal variability reacts to changes in the vertical distribution of the cloud thermodynamic phase. Passive satellite-borne imaging spectrometer observations with pixel sizes larger than 100 m m

The Arctic CLoud Observations Using airborne measurements during polar Day (ACLOUD) cam- paign 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 us- ing 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).

Clouds play an important role in Arctic amplification. This term represents the recently observed enhanced warming of the Arctic relative to the global increase of near-surface air temperature. However, there are still important knowledge gaps regarding the interplay between Arctic clouds and aerosol particles, and surface properties, as well as turbulent and radiative fluxes that inhibit accurate model simulations of clouds in the Arctic climate system. In an attempt to resolve this so-called Arctic cloud puzzle, two comprehensive and closely coordinated field studies were conducted: the Arctic Cloud Observations Using Airborne Measurements during Polar Day (ACLOUD) aircraft campaign and the Physical Feedbacks of Arctic Boundary Layer, Sea Ice, Cloud and Aerosol (PASCAL) ice breaker expedition. Both observational studies were performed in the framework of the German Arctic Amplification: Climate Relevant Atmospheric and Surface Processes, and Feedback Mechanisms (AC)3 project. They took place in the vicinity of Svalbard, Norway, in May and June 2017. ACLOUD and PASCAL explored four pieces of the Arctic cloud puzzle: cloud properties, aerosol impact on clouds, atmospheric radiation, and turbulent dynamical processes. The two instrumented Polar 5 and Polar 6 aircraft; the icebreaker Research Vessel (R/V) Polarstern; an ice floe camp including an instrumented tethered balloon; and the permanent ground-based measurement station at Ny-Ålesund, Svalbard, were employed to observe Arctic low- and mid-level mixed-phase clouds and to investigate related atmospheric and surface processes. The Polar 5 aircraft served as a remote sensing observatory examining the clouds from above by downward-looking sensors; the Polar 6 aircraft operated as a flying in situ measurement laboratory sampling inside and below the clouds. Most of the collocated Polar 5/6 flights were conducted either above the R/V Polarstern or over the Ny-Ålesund station, both of which monitored the clouds from be

A closure experiment was conducted over Svalbard by comparing Lidar measurements and optical aerosol properties calculated from aerosol vertical profiles measured using a tethered balloon. Arctic Haze was present together with Icelandic dust. Chemical analysis of filter samples, aerosol size distribution and a full set of meteorological parameters were determined at ground. Moreover, scanning electron microscopy coupled with energy-dispersive X-ray (SEM-EDS) data were at disposal showing the presence of several mineralogical phases (i.e., sheet silicates, gypsum, quartz, rutile, hematite). The closure experiment was set up by calculating the backscattering coefficients from tethered balloon data and comparing them with the corresponding lidar profiles. This was preformed in three subsequent steps aimed at determining the importance of a complete aerosol speciation: (i) a simple, columnar refractive index was obtained by the closest Aerosol Robotic Network (AERONET) station, (ii) the role of water-soluble components, elemental carbon and organic matter (EC/OM) was addressed, (iii) the dust composition was included. When considering the AERONET data, or only the ionic water-soluble components and the EC/OM fraction, results showed an underestimation of the backscattering lidar signal up to 76, 53 and 45% (355, 532 and 1064 nm). Instead, when the dust contribution was included, the underestimation disappeared and the vertically-averaged, backscattering coefficients (1.45±0.30, 0.69±0.15 and 0.34±0.08 Mm-1 sr-1, at 355, 532 and 1064 nm) were found in keeping with the lidar ones (1.60±0.22, 0.75±0.16 and 0.31±0.08 Mm-1 sr-1). Final results were characterized by low RMSE (0.36, 0.08 and 0.04 Mm-1 sr-1) and a high linear correlation (R2 of 0.992, 0.992 and 0.994) with slopes close to one (1.368, 0.931 and 0.977, respectively). This work highlighted the importance of all the aerosol components and of the synergy between single particle and bulk chemical analysis for the opt

Aerosol strongly affect the radiation balance, especially in the Arctic where climate change is significantly faster compared to lower latitudes. The interaction between aerosol and radiation can be either direct (scattering and absorption) or indirect (aerosol serving as cloud condensation nuclei and ice nucleating particles). Aerosol optical properties can be provided by Lidar (Light Detection and Ranging) systems with high spatial and temporal resolution. In this study, we utilize data from a ground-based Lidar system located in Ny-Ålesund, Spitsbergen and an air-borne system installed onboard the research aircraft Polar5. Our focus is on a rare event of elevated aerosol layers, which persistently appeared over two different parts of the European Arctic during PAMARCMiP (Polar Air-borne Measurements and Arctic Regional Climate Model Simulation Project) campaign in spring 2018. Results show that the detected layers exhibit similar optical properties, namely aerosol backscatter coefficient, which is indicative of aerosol abundance and aerosol depolarization ratio, which is an indicator of the aerosol shape. The main hypothesis is that although the existence of those layers is rare, they impact on the radiation budget of the Arctic. In the next steps of our research, we will investigate the occurrence of similar aerosol layers in the springtime of previous years using long-term measurements from the Lidar system located in Ny-Ålesund. Our goal is to assess the effect of different aerosol layers on the surface radiation budget and gain a better understanding of their role in the amplified Arctic climate change, utilizing radiation measurements from the Ny-Ålesund BSRN (Baseline Surface Radiation Network) station.

The Arctic region is considered to be most sensitive to climate change, with warming in the Arctic occurring considerably faster than the global average due to several positive feedback mechanisms contributing to the “Arctic amplification”. Also the maritime and mountainous climate of Svalbard has undergone changes during the last decades. Here, the focus is set on the current atmospheric boundary conditions for the marine ecosystem in the Kongsfjorden area, discussed in the frame of long-term climatic observations in the larger regional and hemispheric context. During the last century, a general warming is found with temperature increases and precipitation changes varying in strength. During the last decades, a strong seasonality of the warming is observed in the Kongsfjorden area, with the strongest temperature increase occurring during the winter season. The winter warming is related to observed changes in the net longwave radiation. Moreover, changes in the net shortwave are observed during the summer period, attributed to the decrease in reflected radiation caused by the retreating snow cover. Another related aspect of radiation is the intensity of solar ultra-violet radiation that is closely coupled to the abundance of ozone in the column of air overhead. The long term evolution of ozone losses in the Arctic and their connection to climate change are discussed.

The synergy between airborne lidar, radar, passive microwave, and passive imaging spectrometer measurements was used to characterize the vertical and small-scale (down to 10 m) horizontal distribution of the cloud thermodynamic phase. Two case studies of low-level Arctic clouds in a cold air outbreak and a warm air advection observed during the Arctic CLoud Observations Using airborne measurements during polar Day (ACLOUD) were investigated. Both clouds exhibited the typical vertical mixed-phase structure with mostly liquid water droplets at cloud top and ice crystals in lower layers. The cloud top horizontal small-scale variability observed during the cold air outbreak is dominated by the liquid water close to the cloud top and shows no indication of ice in lower cloud layers. Contrastingly, the cloud top variability of the case observed during a warm air advection showed some ice in areas of low reflectivity or cloud holes. Radiative transfer simulations considering homogeneous mixtures of liquid water droplets and ice crystals were able to reproduce the horizontal variability of this warm air advection. To account for more realistic vertical distributions of the thermodynamic phase, large eddy simulations (LES) were performed to reconstruct the observed cloud properties and were used as input for radiative transfer simulations. The simulations of the cloud observed during the cold air outbreak, with mostly liquid water at cloud top, realistically reproduced the observations. For the warm air advection case, the simulated cloud field underestimated the ice water content (IWC). Nevertheless, it revealed the presence of ice particles close to the cloud top and confirmed the observed horizontal variability of the cloud field. It is concluded that the cloud top small-scale horizontal variability reacts to changes in the vertical distribution of the cloud thermodynamic phase. Passive satellite-borne imaging spectrometer observations with pixel sizes larger than 100 m m

The Arctic CLoud Observations Using airborne measurements during polar Day (ACLOUD) cam- paign 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 us- ing 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).

Clouds play an important role in Arctic amplification. This term represents the recently observed enhanced warming of the Arctic relative to the global increase of near-surface air temperature. However, there are still important knowledge gaps regarding the interplay between Arctic clouds and aerosol particles, and surface properties, as well as turbulent and radiative fluxes that inhibit accurate model simulations of clouds in the Arctic climate system. In an attempt to resolve this so-called Arctic cloud puzzle, two comprehensive and closely coordinated field studies were conducted: the Arctic Cloud Observations Using Airborne Measurements during Polar Day (ACLOUD) aircraft campaign and the Physical Feedbacks of Arctic Boundary Layer, Sea Ice, Cloud and Aerosol (PASCAL) ice breaker expedition. Both observational studies were performed in the framework of the German Arctic Amplification: Climate Relevant Atmospheric and Surface Processes, and Feedback Mechanisms (AC)3 project. They took place in the vicinity of Svalbard, Norway, in May and June 2017. ACLOUD and PASCAL explored four pieces of the Arctic cloud puzzle: cloud properties, aerosol impact on clouds, atmospheric radiation, and turbulent dynamical processes. The two instrumented Polar 5 and Polar 6 aircraft; the icebreaker Research Vessel (R/V) Polarstern; an ice floe camp including an instrumented tethered balloon; and the permanent ground-based measurement station at Ny-Ålesund, Svalbard, were employed to observe Arctic low- and mid-level mixed-phase clouds and to investigate related atmospheric and surface processes. The Polar 5 aircraft served as a remote sensing observatory examining the clouds from above by downward-looking sensors; the Polar 6 aircraft operated as a flying in situ measurement laboratory sampling inside and below the clouds. Most of the collocated Polar 5/6 flights were conducted either above the R/V Polarstern or over the Ny-Ålesund station, both of which monitored the clouds from be

A closure experiment was conducted over Svalbard by comparing Lidar measurements and optical aerosol properties calculated from aerosol vertical profiles measured using a tethered balloon. Arctic Haze was present together with Icelandic dust. Chemical analysis of filter samples, aerosol size distribution and a full set of meteorological parameters were determined at ground. Moreover, scanning electron microscopy coupled with energy-dispersive X-ray (SEM-EDS) data were at disposal showing the presence of several mineralogical phases (i.e., sheet silicates, gypsum, quartz, rutile, hematite). The closure experiment was set up by calculating the backscattering coefficients from tethered balloon data and comparing them with the corresponding lidar profiles. This was preformed in three subsequent steps aimed at determining the importance of a complete aerosol speciation: (i) a simple, columnar refractive index was obtained by the closest Aerosol Robotic Network (AERONET) station, (ii) the role of water-soluble components, elemental carbon and organic matter (EC/OM) was addressed, (iii) the dust composition was included. When considering the AERONET data, or only the ionic water-soluble components and the EC/OM fraction, results showed an underestimation of the backscattering lidar signal up to 76, 53 and 45% (355, 532 and 1064 nm). Instead, when the dust contribution was included, the underestimation disappeared and the vertically-averaged, backscattering coefficients (1.45±0.30, 0.69±0.15 and 0.34±0.08 Mm-1 sr-1, at 355, 532 and 1064 nm) were found in keeping with the lidar ones (1.60±0.22, 0.75±0.16 and 0.31±0.08 Mm-1 sr-1). Final results were characterized by low RMSE (0.36, 0.08 and 0.04 Mm-1 sr-1) and a high linear correlation (R2 of 0.992, 0.992 and 0.994) with slopes close to one (1.368, 0.931 and 0.977, respectively). This work highlighted the importance of all the aerosol components and of the synergy between single particle and bulk chemical analysis for the opt

Aerosol strongly affect the radiation balance, especially in the Arctic where climate change is significantly faster compared to lower latitudes. The interaction between aerosol and radiation can be either direct (scattering and absorption) or indirect (aerosol serving as cloud condensation nuclei and ice nucleating particles). Aerosol optical properties can be provided by Lidar (Light Detection and Ranging) systems with high spatial and temporal resolution. In this study, we utilize data from a ground-based Lidar system located in Ny-Ålesund, Spitsbergen and an air-borne system installed onboard the research aircraft Polar5. Our focus is on a rare event of elevated aerosol layers, which persistently appeared over two different parts of the European Arctic during PAMARCMiP (Polar Air-borne Measurements and Arctic Regional Climate Model Simulation Project) campaign in spring 2018. Results show that the detected layers exhibit similar optical properties, namely aerosol backscatter coefficient, which is indicative of aerosol abundance and aerosol depolarization ratio, which is an indicator of the aerosol shape. The main hypothesis is that although the existence of those layers is rare, they impact on the radiation budget of the Arctic. In the next steps of our research, we will investigate the occurrence of similar aerosol layers in the springtime of previous years using long-term measurements from the Lidar system located in Ny-Ålesund. Our goal is to assess the effect of different aerosol layers on the surface radiation budget and gain a better understanding of their role in the amplified Arctic climate change, utilizing radiation measurements from the Ny-Ålesund BSRN (Baseline Surface Radiation Network) station.

The Arctic region is considered to be most sensitive to climate change, with warming in the Arctic occurring considerably faster than the global average due to several positive feedback mechanisms contributing to the “Arctic amplification”. Also the maritime and mountainous climate of Svalbard has undergone changes during the last decades. Here, the focus is set on the current atmospheric boundary conditions for the marine ecosystem in the Kongsfjorden area, discussed in the frame of long-term climatic observations in the larger regional and hemispheric context. During the last century, a general warming is found with temperature increases and precipitation changes varying in strength. During the last decades, a strong seasonality of the warming is observed in the Kongsfjorden area, with the strongest temperature increase occurring during the winter season. The winter warming is related to observed changes in the net longwave radiation. Moreover, changes in the net shortwave are observed during the summer period, attributed to the decrease in reflected radiation caused by the retreating snow cover. Another related aspect of radiation is the intensity of solar ultra-violet radiation that is closely coupled to the abundance of ozone in the column of air overhead. The long term evolution of ozone losses in the Arctic and their connection to climate change are discussed.

Aerosol strongly affect the radiation balance, especially in the Arctic where climate change is significantly faster compared to lower latitudes, a phenomenon known as Arctic Amplification. The interaction between aerosol and radiation can be either direct (scattering and absorption) or indirect (aerosol serving as cloud condensation nuclei and ice nucleating particles).Aerosol concentration in the accumulation mode exhibits an annual maximum in the Arctic in springtime, forming the Arctic Haze. In this work, elevated layers from the European Arctic are analyzed in terms of their optical and hygroscopic properties.

Aerosol strongly affect the radiation balance, especially in the Arctic where climate change is significantly faster compared to lower latitudes, a phenomenon known as Arctic Amplification. The interaction between aerosol and radiation can be either direct (scattering and absorption) or indirect (aerosol serving as cloud condensation nuclei and ice nucleating particles).Aerosol concentration in the accumulation mode exhibits an annual maximum in the Arctic in springtime, forming the Arctic Haze. In this work, elevated layers from the European Arctic are analyzed in terms of their optical and hygroscopic properties.

The Atmosphere Flagship programme addresses various topics, including analyses of long term meteorological observations, the interaction between the surface and the atmospheric boundary layer, the composition of the atmosphere with respect to climate active trace substances like aerosols or green house gases, as well as pollutants, the role of clouds in the Arctic atmosphere, interactions of the atmosphere with the snow covered ground, land-fast and drifting sea ice, surface UV spectral fluxes and its dependency on specific conditions in the stratosphere (ozone) as well as troposphere (clouds). Recently, the role of biogenic aerosols and the observation of black carbon in snow and atmosphere received particular attention. Within the Ny-Aalesund Atmosphere Flagship, we particularly highlight the connections to observations at Barentsburg, Pyramiden, and Hornsund, as well as the pan Svalbard meteorological network.

The Atmosphere Flagship programme addresses various topics, including analyses of long term meteorological observations, the interaction between the surface and the atmospheric boundary layer, the composition of the atmosphere with respect to climate active trace substances like aerosols or green house gases, as well as pollutants, the role of clouds in the Arctic atmosphere, interactions of the atmosphere with the snow covered ground, land-fast and drifting sea ice, surface UV spectral fluxes and its dependency on specific conditions in the stratosphere (ozone) as well as troposphere (clouds). Recently, the role of biogenic aerosols and the observation of black carbon in snow and atmosphere received particular attention. Within the Ny-Aalesund Atmosphere Flagship, we particularly highlight the connections to observations at Barentsburg, Pyramiden, and Hornsund, as well as the pan Svalbard meteorological network.

This paper presents the results of measurements of aerosol physical and chemical properties during iAREA2014 campaign that took place on Svalbard between 15th of Mar and 4th of May 2014. With respect to field area, the experiment consisted of two sites: NyeÅlesund (78�550N, 11�560E) and Longyearbyen (78�130N, 15�330E) with further integration of Aerosol Robotic Network (AERONET) station in Hornsund (77�000N, 15�330E). The subject of this study is to investigate the inesitu, passive and active remote sensing observations as well as numerical simulations to describe the temporal variability of aerosol singleescattering properties during spring season on Spitsbergen. The retrieval of the data indicates several event days with enhanced singleescattering properties due to the existence of sulphate and additional seaesalt load in the atmosphere which is possibly caused by relatively high wind speed. Optical results were confirmed by numerical simulations made by the GEMeAQ model and by chemical observations that indicated up to 45% contribution of the seaesalt to a PM10 total aerosol mass concentration. An agreement between the in-situ optical and microphysical properties was found, namely: the positive correlation between aerosol scattering coefficient measured by the nephelometer and effective radius obtained from laser aerosol spectrometer as well as negative correlation between aerosol scattering coefficient and the Ångstrom exponent indicated that slightly larger particles dominated during special events. The inesitu surface observations do not show any significant enhancement of the absorption coefficient as well as the black carbon concentration which might occur during spring. All of extensive singleescattering properties indicate a diurnal cycle in Longyearbyen, where 21:00e5:00 data stays at the background level, however increasing during the day by the factor of 3e4. It is considered to be highly connected with local emissions originating in combustion, traffic

The International workshop on Atmospheric Studies in the Arctic was held at the Institute of Oceanology, Polish Academy of Sciences in Sopot, Poland on 28. and 29. January 2016. The workshop is a joint effort of the iAREA team, the Alfred Wegener Institute and the University of Florence in cooperation with the Institute of Oceanology, Polish Academy of Sciences and the Center for Polar Studies. The workshop is a contribution to the Ny-Aalesund Atmosphere Flagship Programme. This book of abstract includes the contributions building on the iAREA campaigns in 2014 and 2015 held in Ny-Aalesund, Spitsbergen.

The International workshop on Atmospheric Studies in the Arctic was held at the Institute of Oceanology, Polish Academy of Sciences in Sopot, Poland on 28. and 29. January 2016. The workshop is a joint effort of the iAREA team, the Alfred Wegener Institute and the University of Florence in cooperation with the Institute of Oceanology, Polish Academy of Sciences and the Center for Polar Studies. The workshop is a contribution to the Ny-Aalesund Atmosphere Flagship Programme. This book of abstract includes the contributions building on the iAREA campaigns in 2014 and 2015 held in Ny-Aalesund, Spitsbergen.

During ice-free periods, the Northern Sea Route (NSR) could be an attractive shipping route. The decline in Arctic sea-ice extent, however, could be associated with an increase in the frequency of the causes of severe weather phenomena, and high wind-driven waves and the advection of sea ice could make ship navigation along the NSR difficult. Accurate forecasts of weather and sea ice are desirable for safe navigation, but large uncertainties exist in current forecasts, partly owing to the sparse observational network over the Arctic Ocean. Here, we show that the incorporation of additional Arctic observations improves the initial analysis and enhances the skill of weather and sea-ice forecasts, the application of which has socioeconomic benefits. Comparison of 63-member ensemble atmospheric forecasts, using different initial data sets, revealed that additional Arctic radiosonde observations were useful for predicting a persistent strong wind event. The sea-ice forecast, initialised by the wind fields that included the effects of the observations, skilfully predicted rapid wind-driven sea-ice advection along the NSR.

This book contributes to the current discussion on global environmental changes by discussing modifications in marine ecosystems related to global climate changes. In marine ecosystems, rising atmospheric CO2 and climate changes are associated with shifts in temperature, circulation, stratification, nutrient input, oxygen concentration and ocean acidification, which have significant biological effects on a regional and global scale. Knowing how these changes affect the distribution and abundance of plankton in the ocean currents is crucial to our understanding of how climate change impacts the marine environment. Ocean temperatures, weather and climatic changes greatly influence the amount and location of nutrients in the water column. If temperatures and currents change, the plankton production cycle may not coincide with the reproduction cycle of fish. The above changes are closely related to the changes in radiative forcing, which initiate feedback mechanisms like changes in surface temperature, circulation, and atmospheric chemistry.

During ice-free periods, the Northern Sea Route (NSR) could be an attractive shipping route. The decline in Arctic sea-ice extent, however, could be associated with an increase in the frequency of the causes of severe weather phenomena, and high wind-driven waves and the advection of sea ice could make ship navigation along the NSR difficult. Accurate forecasts of weather and sea ice are desirable for safe navigation, but large uncertainties exist in current forecasts, partly owing to the sparse observational network over the Arctic Ocean. Here, we show that the incorporation of additional Arctic observations improves the initial analysis and enhances the skill of weather and sea-ice forecasts, the application of which has socioeconomic benefits. Comparison of 63-member ensemble atmospheric forecasts, using different initial data sets, revealed that additional Arctic radiosonde observations were useful for predicting a persistent strong wind event. The sea-ice forecast, initialised by the wind fields that included the effects of the observations, skilfully predicted rapid wind-driven sea-ice advection along the NSR.

This book contributes to the current discussion on global environmental changes by discussing modifications in marine ecosystems related to global climate changes. In marine ecosystems, rising atmospheric CO2 and climate changes are associated with shifts in temperature, circulation, stratification, nutrient input, oxygen concentration and ocean acidification, which have significant biological effects on a regional and global scale. Knowing how these changes affect the distribution and abundance of plankton in the ocean currents is crucial to our understanding of how climate change impacts the marine environment. Ocean temperatures, weather and climatic changes greatly influence the amount and location of nutrients in the water column. If temperatures and currents change, the plankton production cycle may not coincide with the reproduction cycle of fish. The above changes are closely related to the changes in radiative forcing, which initiate feedback mechanisms like changes in surface temperature, circulation, and atmospheric chemistry.

Aircraft borne optical in situ size distribution measurements were performed within Arctic boundary layer clouds with a special emphasis on the cloud top layer during the VERtical Distribution of Ice in Arctic clouds (VERDI) campaign in April and May 2012. An instrumented Basler BT-67 research aircraft operated out of Inuvik over the Mackenzie River delta and the Beaufort Sea in the Northwest Territories of Canada. Besides the cloud particle and hydrometeor size spectrometers the aircraft was equipped with instrumentation for aerosol, radiation and other parameters. Inside the cloud, droplet size distributions with monomodal shapes were observed for predominantly liquid-phase Arctic stratocumulus. With increasing altitude inside the cloud the droplet mean diameters grew from 10 to 20 μm. In the upper transition zone (i.e., adjacent to the cloud-free air aloft) changes from monomodal to bimodal droplet size distributions (Mode 1 with 20 μm and Mode 2 with 10 μm diameter) were observed. It is shown that droplets of both modes co-exist in the same (small) air volume and the bimodal shape of the measured size distributions cannot be explained as an observational artifact caused by accumulating data point populations from different air volumes. The formation of the second size mode can be explained by (a) entrainment and activation/condensation of fresh aerosol particles, or (b) by differential evaporation processes occurring with cloud droplets engulfed in different eddies. Activation of entrained particles seemed a viable possibility as a layer of dry Arctic enhanced background aerosol (which was detected directly above the stratus cloud) might form a second mode of small cloud droplets. However, theoretical considerations and model calculations (adopting direct numerical simulation, DNS) revealed that, instead, turbulent mixing and evaporation of larger droplets are the most likely reasons for the formation of the second droplet size mode in the uppermost region of th

Atmospheric measurements on the drifting Arctic sea ice station "North Pole-35" crossing the Eastern part of the Arctic Ocean during winter 2007/2008 have been compared with regional atmospheric HIRHAM model simulations. The observed near-surface temperature, mean sea level pressure and the vertical temperature, wind and humidity profiles are satisfactorily reproduced by the model. The strongest temperature differences between observations and the simulations occur near the surface due to an overestimated vertical mixing of heat in the stable Arctic boundary layer (ABL). The observations show very strong temperature inversions near the surface, whereas the simulated inversions occur frequently between the surface and 415 m at too high levels. The simulations are not able to reproduce the observed inversion strength. The regional model underestimates the wind speeds and the sharp vertical wind gradients. The strength of internal atmospheric dynamics on the temporal development of atmospheric surface variables and vertical profiles of temperature, wind and relative humidity has been examined. Although the HIRHAM model systematically overestimates relative humidity and produces too high long-wave downward radiation during winter, two different atmospheric circulation states, which are connected to higher or lower pressure systems over the Eastern part of the Arctic Ocean, are simulated in agreement with the NP-35 observations. Sensitivity studies with reduced vertical mixing of heat in the stable ABL have been carried out. A slower increase in the stability functions with decreasing Richardson number under stable stratification has an impact on the horizontal and vertical atmospheric structure. Changes in synoptical cyclones on time scales from 1–3 days over the North Atlantic cyclone path are generated, which influences the atmospheric baroclinic and planetary waves on time scales up to 20 days over the Arctic Ocean basin. The use of increased vertical stability in th