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Continuous height-resolved observations of aerosol profiles over the central Arctic throughout a full year were performed for the first time. Such measurements covering aerosol layering features are required for an adequate modeling of Arctic climate conditions, especially with respect to a realistic consideration of cloud formation and here, in particular, of ice nucleation processes. MOSAiC (Multidisciplinary drifting Observatory for the Study of Arctic Climate) offered this favorable opportunity to monitor aerosol and clouds over the central Arctic over all four seasons, from October 2019 to September 2020. In this article, a summary of MOSAiC lidar observations aboard the icebreaker Polarstern of tropospheric aerosol products is presented. Particle optical properties, i.e., light-extinction profiles and aerosol optical thickness (AOT), and estimates of cloud-relevant aerosol properties (cloud condensation nucleus, CCN, and ice-nucleating particle concentrations, INPs) are discussed, separately for the lowest part of the troposphere (near the surface at 250 m height), within the lower free troposphere (2000 m height), and regarding INPs also near the tropopause (cirrus level, 8–10 km height). In situ observations of the particle number concentration and INPs aboard Polarstern are included in the study. Strong differences between summer and winter aerosol conditions were found. During the winter months (Arctic haze period) a strong decrease of the aerosol light extinction coefficient (532 nm) with height up to about 4–5 km height was found with values of 20–100 Mm-1 close to the surface and an order of magnitude less at 1500–2000 m height. Lofted aged wildfire smoke layers caused a re-increase of the aerosol concentration from the middle troposphere up to stratospheric heights and were continuously observable from October 2019 to May 2020. In summer (June to August 2020), much lower particle extinction coefficients, frequently as low as 1–5 Mm-1, were observed. Aerosol removal, controlled by cloud scavenging processes (widely suppressed in winter, very efficient in summer) in the lowermost 1–2 km of the atmosphere, seem to be the main reason for the strong differences between winter and summer aerosol conditions. In line with this pronounced annual cycle in the aerosol optical properties, CCN concentrations (0.2 % supersaturation level) ranged from 50–500 cm-3 in the atmospheric boundary layer (ABL) in winter and 1–40 cm-3 in summer. In the lower free troposphere, however, the CCN level was roughly constant throughout the year with values mostly from 30–100 cm-3. A strong contrast between winter to summer was also given in terms of ABL INPs which control ice production in low-level clouds. INP concentration of 0.01–0.2 L-1 prevailed in the ABL in winter at typical ice-nucleating cloud temperatures of -25 °C and assuming soil dust as the main INP type, and were roughly 2 orders of magnitude lower in the ABL in summer at typical cloud top temperatures of -10 °C. In the summer ABL, marine aerosol (biogenic components) is most probably the main INP type, continental INP contributions (e.g., soil dust INPs) are suppressed by efficient wet removal during long-range transport. A strong reduction in the INP population was also found in the lower free troposphere at 2000 m height from winter to summer (2 orders of magnitude), mostly due to the change in the prevailing ice-nucleation temperatures. Estimated INP concentration accumulated from 0.004–0.02 L-1 during the winter months. The highlight of the MOSAiC lidar studies was the detection of a persistent wildfire smoke layer in the upper troposphere and lower stratosphere from October 2019 to May 2020. The smoke particles (organic aerosol) triggered continuously cirrus formation at INP concentrations mostly from 1–20 L-1 close to the tropopause during the entire winter period. [ABSTRACT FROM AUTHOR]

The microwave radiometers HATPRO (Humidity and Temperature Profiler) and MiRAC-P (Microwave Radiometer for Arctic Clouds - Passive) continuously measured radiation emitted from the atmosphere throughout the Multidisciplinary drifting Observatory for the Study of the Arctic Climate (MOSAiC) expedition on board the research vessel Polarstern. From the measured brightness temperatures, we have retrieved atmospheric variables using statistical methods in a temporal resolution of 1 s covering October 2019 to October 2020. The integrated water vapour (IWV) is derived individually from both radiometers. In addition, we present the liquid water path (LWP), temperature and absolute humidity profiles from HATPRO. To prove the quality and to estimate uncertainty, the data sets are compared to radiosonde measurements from Polarstern. The comparison shows an extremely good agreement for IWV, with standard deviations of 0.08–0.19 kg m−2 (0.39–1.47 kg m−2) in dry (moist) situations. The derived profiles of temperature and humidity denote uncertainties of 0.7–1.8 K and 0.6–0.45 gm−3 in 0–2 km altitude. Measurement(s) radio wave radiation • far-infrared radiation Technology Type(s) microwave radiometer Factor Type(s) brightness temperature Sample Characteristic - Environment planetary atmosphere Sample Characteristic - Location Arctic Ocean [ABSTRACT FROM AUTHOR]

During the 1-year MOSAiC (Multidisciplinary drifting Observatory for the Study of Arctic Climate) expedition, the German icebreaker Polarstern drifted through Arctic Ocean ice from October 2019 to May 2020, mainly at latitudes between 85 and 88.5 ∘ N. A multiwavelength polarization Raman lidar was operated on board the research vessel and continuously monitored aerosol and cloud layers up to a height of 30 km. During our mission, we expected to observe a thin residual volcanic aerosol layer in the stratosphere, originating from the Raikoke volcanic eruption in June 2019, with an aerosol optical thickness (AOT) of 0.005–0.01 at 500 nm over the North Pole area during the winter season. However, the highlight of our measurements was the detection of a persistent, 10 km deep aerosol layer in the upper troposphere and lower stratosphere (UTLS), from about 7–8 to 17–18 km height, with clear and unambiguous wildfire smoke signatures up to 12 km and an order of magnitude higher AOT of around 0.1 in the autumn of 2019. Case studies are presented to explain the specific optical fingerprints of aged wildfire smoke in detail. The pronounced aerosol layer was present throughout the winter half-year until the strong polar vortex began to collapse in late April 2020. We hypothesize that the detected smoke originated from extraordinarily intense and long-lasting wildfires in central and eastern Siberia in July and August 2019 and may have reached the tropopause layer by the self-lifting process. In this article, we summarize the main findings of our 7-month smoke observations and characterize the aerosol in terms of geometrical, optical, and microphysical properties. The UTLS AOT at 532 nm ranged from 0.05–0.12 in October–November 2019 and 0.03–0.06 during the main winter season. The Raikoke aerosol fraction was estimated to always be lower than 15 %. We assume that the volcanic aerosol was above the smoke layer (above 13 km height). As an unambiguous sign of the dominance of smoke in the main aerosol layer from 7–13 km height, the particle extinction-to-backscatter ratio (lidar ratio) at 355 nm was found to be much lower than at 532 nm, with mean values of 55 and 85 sr, respectively. The 355–532 nm Ångström exponent of around 0.65 also clearly indicated the presence of smoke aerosol. For the first time, we show a distinct view of the aerosol layering features in the High Arctic from the surface up to 30 km height during the winter half-year. Finally, we provide a vertically resolved view on the late winter and early spring conditions regarding ozone depletion, smoke occurrence, and polar stratospheric cloud formation. The latter will largely stimulate research on a potential impact of the unexpected stratospheric aerosol perturbation on the record-breaking ozone depletion in the Arctic in spring 2020. [ABSTRACT FROM AUTHOR]

An advanced multiwavelength polarization Raman lidar was operated aboard the icebreaker Polarstern during the MOSAiC (Multidisciplinary drifting Observatory for the Study of Arctic Climate) expedition to continuously monitor aerosol and cloud layers in the central Arctic up to 30 km height. The expedition lasted from September 2019 to October 2020 and measurements were mostly taken between 85 and 88.5 ∘ N. The lidar was integrated into a complex remote-sensing infrastructure aboard the Polarstern. In this article, novel lidar techniques, innovative concepts to study aerosol–cloud interaction in the Arctic, and unique MOSAiC findings will be presented. The highlight of the lidar measurements was the detection of a 10 km deep wildfire smoke layer over the North Pole region between 7–8 km and 17–18 km height with an aerosol optical thickness (AOT) at 532 nm of around 0.1 (in October–November 2019) and 0.05 from December to March. The dual-wavelength Raman lidar technique allowed us to unambiguously identify smoke as the dominating aerosol type in the aerosol layer in the upper troposphere and lower stratosphere (UTLS). An additional contribution to the 532 nm AOT by volcanic sulfate aerosol (Raikoke eruption) was estimated to always be lower than 15 %. The optical and microphysical properties of the UTLS smoke layer are presented in an accompanying paper. This smoke event offered the unique opportunity to study the influence of organic aerosol particles (serving as ice-nucleating particles, INPs) on cirrus formation in the upper troposphere. An example of a closure study is presented to explain our concept of investigating aerosol–cloud interaction in this field. The smoke particles were obviously able to control the evolution of the cirrus system and caused low ice crystal number concentration. After the discussion of two typical Arctic haze events, we present a case study of the evolution of a long-lasting mixed-phase cloud layer embedded in Arctic haze in the free troposphere. The recently introduced dual-field-of-view polarization lidar technique was applied, for the first time, to mixed-phase cloud observations in order to determine the microphysical properties of the water droplets. The mixed-phase cloud closure experiment (based on combined lidar and radar observations) indicated that the observed aerosol levels controlled the number concentrations of nucleated droplets and ice crystals. [ABSTRACT FROM AUTHOR]

Within the transpolar drifting expedition MOSAiC (Multidisciplinary drifting Observatory for the Study of Arctic Climate), the Global Navigation Satellite System (GNSS) was used among other techniques to monitor variations in atmospheric water vapor. Based on 15 months of continuously tracked GNSS data including GPS, GLONASS and Galileo, epoch-wise coordinates and hourly zenith total delays (ZTDs) were determined using a kinematic precise point positioning (PPP) approach. The derived ZTD values agree to 1.1 ± 0.2 mm (root mean square (rms) of the differences 10.2 mm) with the numerical weather data of ECMWF's latest reanalysis, ERA5, computed for the derived ship's locations. This level of agreement is also confirmed by comparing the on-board estimates with ZTDs derived for terrestrial GNSS stations in Bremerhaven and Ny-Ålesund and for the radio telescopes observing very long baseline interferometry in Ny-Ålesund. Preliminary estimates of integrated water vapor derived from frequently launched radiosondes are used to assess the GNSS-derived integrated water vapor estimates. The overall difference of 0.08 ± 0.04 kg m -2 (rms of the differences 1.47 kg m -2) demonstrates a good agreement between GNSS and radiosonde data. Finally, the water vapor variations associated with two warm-air intrusion events in April 2020 are assessed. [ABSTRACT FROM AUTHOR]

Water vapor is an important component in the water and energy cycle of the Arctic. Especially in light of Arctic amplification, changes in water vapor are of high interest but are difficult to observe due to the data sparsity of the region. The ACLOUD/PASCAL campaigns performed in May/June 2017 in the Arctic North Atlantic sector offers the opportunity to investigate the quality of various satellite and reanalysis products. Compared to reference measurements at R/V Polarstern frozen into the ice (around 82 ∘ N, 10 ∘ E) and at Ny-Ålesund, the integrated water vapor (IWV) from Infrared Atmospheric Sounding Interferometer (IASI) L2PPFv6 shows the best performance among all satellite products. Using all radiosonde stations within the region indicates some differences that might relate to different radiosonde types used. Atmospheric river events can cause rapid IWV changes by more than a factor of 2 in the Arctic. Despite the relatively dense sampling by polar-orbiting satellites, daily means can deviate by up to 50 % due to strong spatio-temporal IWV variability. For monthly mean values, this weather-induced variability cancels out, but systematic differences dominate, which particularly appear over different surface types, e.g., ocean and sea ice. In the data-sparse central Arctic north of 84 ∘ N, strong differences of 30 % in IWV monthly means between satellite products occur in the month of June, which likely result from the difficulties in considering the complex and changing surface characteristics of the melting ice within the retrieval algorithms. There is hope that the detailed surface characterization performed as part of the recently finished Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) will foster the improvement of future retrieval algorithms. [ABSTRACT FROM AUTHOR]

Water vapor is an important component in the water and energy cycle of the Arctic. Especially in the light of Arctic amplification, changes of water vapor are of high interest but are difficult to observe due to the data sparsity of the region. The ACLOUD/PASCAL campaign performed in May/June 2017 in the Arctic North Atlantic sector offers the opportunity to investigate the quality of various satellite and reanalysis products. Compared to reference measurements at R/V Polarstern frozen into the ice (around 82° N, 10° E) and at Ny-Ålesund, the Integrated Water Vapor (IWV) from IASI shows the best performance among all satellite products. Using all radiosonde stations within the region indicates some differences that might relate to different radiosonde types used. Though the region is well sampled by polar orbiting satellites daily means can deviate by up to 50 % due to strong spatio-temporal IWV variability associated with atmospheric river events. For monthly mean values, this weather induced variability cancels out but systematic differences dominate which particularly appear over different surface types, e.g. ocean, sea ice. In the data sparse central Arctic above 84° N, strong differences of 30 % in IWV monthly means between satellite products occur in the month of June which likely results from the difficulties to consider the complex and changing surface characteristics of the melting ice within the retrieval algorithms. There is hope that the detailed surface characterization performed as part of the recently finished Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) will foster the improvement of future retrieval algorithms. [ABSTRACT FROM AUTHOR]

The Svalbard archipelago in the Arctic North Atlantic is experiencing rapid changes in the surface climate and sea ice distribution, with impacts for the coupled climate system and the local society. This study utilizes observational data of surface air temperature (SAT) from 1980–2016 across the whole Svalbard archipelago, and sea ice extent (SIE) from operational sea ice charts to conduct a systematic assessment of climatologies, long‐term changes and regional differences. The proximity to the warm water mass of the West Spitsbergen Current drives a markedly warmer climate in the western coastal regions compared to northern and eastern Svalbard. This imprints on the SIE climatology in southern and western Svalbard, where the annual maxima of 50–60% area ice coverage are substantially less than 80–90% in the northern and eastern fjords. Owing to winter‐amplified warming, the local climate is shifting towards more maritime conditions, and SIE reductions of between 5 and 20% per decade in particular regions are found, such that a number of fjords in the west have been virtually ice‐free in recent winters. The strongest decline comes along with SAT forcing and occurs over the most recent 1–2 decades in all regions; while in the 1980s and 1990s, enhanced northerly winds and sea ice drift can explain 30–50% of SIE variability around northern Svalbard, where they had correspondingly lead to a SIE increase. With an ongoing warming it is suggested that both the meteorological and cryospheric conditions in eastern Svalbard will become increasingly similar to what is already observed in the western fjords, namely suppressed typical Arctic climate conditions. [ABSTRACT FROM AUTHOR]

Clouds strongly impact the available energy at the surface and at the top of the atmosphere as well as its vertical distribution within the atmosphere by modifying the shortwave (SW) and longwave (LW) fluxes and heating rates. The so-called cloud radiative effect (CRE) and the cloud radiative forcing (CRF), i.e. the difference between the all-sky and clear-sky fluxes and heating rates, respectively, strongly depend on the cloud macrophysical (e.g. frequency of occurrence, cloud vertical distribution) and microphysical (e.g. phase, water content, hydrometeor size distribution) properties.In the Arctic, the cloud–radiative interactions are even more complex due to low temperatures, frequently occurring temperature inversions, the dryness of the atmosphere, large solar zenith angles and a high surface albedo. In particular (supercooled) liquid containing clouds, which frequently occur in the Arctic and often have very low amounts of liquid water, exhibit a strong impact on the radiative fluxes.For the first time, the impact of clouds on the shortwave (SW) and longwave (LW) fluxes and heating rates is estimated for Ny-Ålesund exploiting more than 2 years (06/2016 -10/2018) of continuous vertical cloud measurements at the French-German research station AWIPEV. To this end, cloud macro- and microphysical properties have been retrieved from ground-based remote sensing observations (cloud radar, ceilometer) and used as input for the Rapid Radiative Transfer Model RRTMG. Further input data, e.g. thermodynamic profiles, as well as aerosol and surface albedo information, have been generated from numerical weather prediction model data and from observations of further AWIPEV instrumentation. In general, the agreement between simulated and observed SW and LW downward surface fluxes is very good with only a small bias in both clear-sky and cloudy conditions. Regarding the surface CRE, low-level liquid clouds lead to a warming (~50 Wm-2) throughout the year. During polar day, clouds strongly cool the surface in the SW with a large daily variability. The net CRE at the surface is generally positive between September and May and negative from June to August which is due to the low surface albedo. The net CRF is dominated by the LW CRF with warming taking place in principal below the height of the maximum frequency of occurrence of liquid around 1 km, and cooling above. The strength of this cooling and warming is closely related to the amount of liquid.We gratefully acknowledge the funding by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – Projektnummer 268020496 – TRR 172, within the Transregional Collaborative Research Center "ArctiC Amplification: Climate Relevant Atmospheric and SurfaCe Processes, and Feedback Mechanisms (AC)³". We gratefully thank Ewan O'Connor for providing the Cloudnet algorithms. [ABSTRACT FROM AUTHOR]

Arctic warming is currently one of the most discussed topics in atmospheric research. Besides the observed significant trend in the near-surface temperature, moisture and temperature anomalies on daily to monthly time scales occur frequently in the Arctic [1]. The positive and negative anomalies in temperature and amount of water vapour are often driven by certain weather patterns which are related to the transport of warm and moist air from North Atlantic or the transport of cold and dry air from the Polar regions. These anomalous conditions will potentially affect cloud occurrence and their phase partitioning and therefore, influence the radiation budget. In this study we look at temperature and water vapour anomalies which occur at Ny-Ålesund. We identify anomalous situations using daily two-meter surface temperature and integrated water vapour to calculate frequency of occurrence of extreme events. We consider cases as extreme events when temperature or water vapour are below the 10th and larger than the 90th percentiles for negative and positive anomalies, respectively. For the analysis, we use observations of the various instrumentation from the AWIPEV (the German Alfred Wegener Institute for Polar and Marine Research and the French Polar Institute Paul Emile Victor) observatory. Two-meter temperature is obtained from long-term Baseline Surface Radiation Network (BSRN) observations. The integrated water vapour is provided by a microwave radiometer. Cloud observations at Ny-Ålesund were performed by a set of active remote sensing instruments: a ceilometer operating since 2011 was complemented with a 94 GHz FMCW cloud radar in 2016. In order to get information on vertical distribution of hydrometeor types, the CLOUDNET algorithms have been applied to the observations, including also a target classification.In this study, we present an analysis of clouds occurring under different anomaly conditions at Ny-Ålesund. We analyse the occurrence of clouds and hydrometeors, cloud type (liquid, ice, mixed-phase) as well as cloud microphysical properties. We relate different types of clouds occurring within these anomalous episodes to the weather patterns and atmospheric circulations. The analysis is also performed for different seasons.In the future we will also analyse the cloud radiative effect for these temperature and water vapour anomalies. If such anomalies will occur more often in the future, this might lead to larger impact on Arctic climate. We gratefully acknowledge the funding by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – Projektnummer 268020496 – TRR 172, within the Transregional Collaborative Research Center "ArctiC Amplification: Climate Relevant Atmospheric and SurfaCe Processes, and Feedback Mechanisms (AC)³". We acknowledge Ewan O'Connor for providing the Cloudnet algorithms.References[1] J. E. Overland and M. Wang, 2016: Recent Extreme Arctic Temperatures are due to a Split Polar Vortex, Journal of Climate, 29, 5609-5616, doi: 10.1175/JCLI-D-16-0320.1. [ABSTRACT FROM AUTHOR]

The role of atmospheric moisture within the Arctic climate system is raising increasing interest. However, with sparse surface and radiosonde measurements mainly limited to land areas, satellite observations provide the main information on moisture distribution across the Arctic. While a wealth of satellite measurements from different instruments exist, in particular for integrated water vapor (IWV), the special conditions in the Arctic hamper accurate observations. For example, methods based on solar radiation cannot be applied during polar night and the highly variable emissivity of snow and ice hinders microwave retrievals. With few measurements available, reanalysis is less constrained by observations increasing the influence of the underlying numerical model. Therefore, it is no surprise that the Arctic belongs to those regions that show strongest relative discrepancies between different long-term, global IWV data sets, i.e. satellite and reanalysis products (GEWEX water vapor assessment (G-VAP); WCRP Report 16/2017).In this contribution, we provide a systematic assessment of different reanalysis, i.e. ERA-Interim, ERA5, MERRA2, CFSR, JRA55, and satellite products in the Arctic. Satellite products include operational retrievals using among others Microwave Integrated Retrieval System (MIRS), Infrared Atmospheric Sounding Interferometer (IASI), Moderate-resolution Imaging Spectroradiometer (MODIS), and Global Ozone Monitoring Experiment (GOME-2). As reference, we make use of the comprehensive set of measurements from the Arctic CLoud Observations Using airborne measurements during polar Day (ACLOUD) and the Physical feedbacks of Arctic planetary boundary level Sea ice, Cloud and AerosoL (PASCAL) campaigns which took place near Svalbard from May 23 to June 26, 2017. Thus, we focus on the Nordic Seas including the marginal sea ice zone and the neighboring land areas for the two months May and June in 2017. In addition to campaign observations by ground-based remote sensing, aircraft and radiosondes, all operational radiosonde soundings and surface-based IWV from the GNSS network are used. We investigate the specific advantages and disadvantages of the different data sources in order to finally answer the question to which degree we can reliably describe Arctic IWV patterns on the monthly scale. [ABSTRACT FROM AUTHOR]