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In situ profiles and fixed-altitude time series of all four components of net radiation were obtained at Ny-Ålesund, Svalbard (78.9° N, 11.9° E), in the period May 04–21, 2015. Measurements were performed using adapted high-quality instrumentation classified as “secondary standard” carried by a tethered balloon system. Balloon-lifted measurements of albedo under clear-sky conditions demonstrate the local dependence on altitude and on the surface inhomogeneity of this parameter over coastal terrain of Ny-Ålesund. Depending on the surface composition within the sensor’s footprint near the coastline, the albedo over predominantly snow-covered surfaces was found to decrease to 0.548 and 0.452 at 494 m and 881 m altitude compared with 0.731 and 0.788 measured with near-surface references, respectively. Albedo profiles show an all-sky maximum at 150 m above surface level due to local surface inhomogeneity, and an averaged vertical change rate of − 0.040/100 up to 750 m aboveground level (clear sky) and − 0.034/100 m (overcast). Profiling of arctic low-level clouds reveals distinct vertical gradients in all radiative fluxes but longwave upward at the cloud top. Observed radiative cooling at the top of a partly dissolving stratus cloud with heating rates of − 40.4 to − 62.1 Kd−1 in subsequent observations is exemplified., Bundesrepublik Deutschland, Deutscher Wetterdienst, vertreten durch den Vorstand, Deutsche Meteorologische Bibliothek (4242)

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 and large particles. The inversion of microphysical properties yielded high Refractive Index (RI) together with low Single-Scattering Albedo (SSA), suggesting absorbing particles. A fitted lognormal volume distribution revealed a fine mode with effective radius (reff) of μm and a coarse mode with reff=0.75 μm. The total radiative balance on ground was positive (12 Wm-2).

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.

This sixth World Ocean Review (WOR) focuses on the Arctic and the Antarctic – two regions which are, in a very real sense, polar opposites, with some of the world’s most extreme conditions. Besides presenting a wealth of facts and figures about the history and exploration of the polar regions, WOR 6 builds a deeper awareness of their key role for life on our planet. It highlights the changes that can be observed in their flora and fauna and analyses the already dramatic impacts of global warming on these extremely fragile regions.

Ny-Ålesund, Svalbard, is located in the North Atlantic part of the Arctic where warming trends are much larger than in other regions of the globe. Under polar night conditions, the increase of surface temperature during the recent two decades exceeds 3 K per decade. Although the warming is bottom-amplified, it extends throughout the entire troposphere. Here, the according increase in winter longwave radiation since 1992 is highlighted. Indications for Arctic climate change are also observed during the sunlit period: the Ny-Ålesund shortwave radiation measurements provide information on the distinct albedo drop between the snow cover surface and the dark tundra ground after snow melt. The long-term changes in the observed shortwave radiation quantities indicate that the snow free season tends to start earlier, with implications for the Svalbard ecosystem. Furthermore, an overview of BSRN-related auxiliary data from Ny-Ålesund is provided.

Bayelva is a high-arctic research site on Spitsbergen Island in the Svalbard archipelago (78.551N; 11.571E) where climate, soil and snow components are recorded since 1998 by the Alfred-Wegener-Institute. This study site is underlain by permafrost with average temperatures around -2 °C and seasonally snow-covered from October to May. The thermal insulation by the snow-pack decouples the soil temperatures from air temperature. To gain better understanding of the thermal relationships, analysis of long-term measurements of air temperature, radiation, soil temperature, and snow characteristics were examined in this study. Mean annual air temperature has increased by 0.14 °C per year for the period from 1998 to 2016 which is higher than the global average temperature change in the same period. Radiation balance trend analyses are characterized by an increase in long-wave radiation during winter months. Results of our snow characteristic studies show that the last day in the year with snow cover has been moving to earlier dates with 0.5 days per year (from 1998 to 2016), extending the snow-free season, thus resulting in more time for soil warming. This corresponds well with warming trends of all soil temperature sensors as well as the thickening of the active layer (ALT), which is estimated to have reached a depth of 2 m in 2016 for the first time. Since it is difficult to measure the exact ALT, we used the Stefan model for an estimate. To account for different magnitudes of insulation, an effective snow depth (Sdep,eff) was calculated for each year. High Sdep,eff values represent high thermo-insulation, due to early snowfall and long snow depths over 0.4 m, and can be linked to years in which re-freezing of the whole active layer took until early March of the next year. Our results support the importance of snow physical characteristics for the permafrost thermal regime, as also detailed in the Arctic Monitoring and Assessment Programme - an Arctic Council Working Group.

Permafrost around the Arctic is warming and thawing. We report data from the high arctic research site Bayelva (78.551N; 11.571E) located close to Ny-Alesund. Data on meteorology, energy balance components and subsur- face observations have been made for the last 20 years (1998-2017; Boike et al. 2017). This study site is underlain by permafrost with current mean permafrost temperature of -2.8◦C and is seasonally snow-covered from October to May. Mean annual, summer and winter soil temperature data at all depths have beenrising over the period of record with a warming trend of 0.18±0.07◦C/year in active layer and top of permafrost. However, interannual to sub-decadal variability is evident in the data and results mostly from differences of theclimate during the winter months. The modeled active layer thickness using the Stefan equation has increasedcontinuously from about 1m in 1998 and is estimated to have surpassed 2 m in 2016. The data show that snow ablation has started earlier, thus extending the snow free season, potentially re-sulting in more time for soil warming and deepening of active layer. The snow cover onset and ablation, aswell as the thermo insulation properties of the snow cover, will be investigated together with active layer and permafrost variables (temperature, volumetric water content) for further understanding of the observed warming and deepening. Boike, J., Juszak, I., Lange, S., Chadburn, S., Burke, E., Overduin, P. P., Roth, K., Ippisch, O., Bornemann, N., Stern, L., Gouttevin, I., Hauber, E., and Westermann, S.: A 20-year record (1998–2017) of permafrost, active layer and meteorological conditions at a high Arctic permafrost research site (Bayelva, Spitsbergen), Earth Syst. Sci. Data, 10, 355-390, https://doi.org/10.5194/essd-10-355-2018, 2018.

Most permafrost is located in the Arctic, and in total it contains around 600Gt of frozen organic carbon. This represents several times the remaining budget for anthropogenic emissions if we are to limit global warming below 2ºC, and this carbon could have a significant impact on global climate if it is released to the atmosphere following permafrost thaw. At present, the Arctic climate is changing much more rapidly than the rest of the globe, and yet observational data density in the region is low. The positive feedback between climate warming and permafrost carbon emissions depends on changing land-atmosphere energy and mass exchanges. There is thus a great need to understand links between the energy balance, which can vary rapidly over hourly to annual time scales, and permafrost, which changes slowly over long time periods. This understanding mandates long-term observational data sets. There is also a need to realistically incorporate permafrost into global modelling frameworks such as Earth System Models. Evaluating and parameterising these process-based models especially requires simultaneous measurements of interacting variables. Here we present an example of such a long-term data set, from the Bayelva Site at Ny-Ålesund, Svalbard, where meteorology, energy balance components and subsurface observations have been made for the last 20 years. Additional data include a high resolution digital elevation model and a panchromatic image. This paper presents the data set produced so far, explains instrumentation, calibration, processing and data quality control, as well as the sources for various resulting data sets. The resulting data set is unique in the Arctic and serves a baseline for future studies. Since the data provide observations of temporally variable parameters that mitigate energy fluxes between permafrost and atmosphere, such as snow depth and soil moisture content, they are suitable for use in integrating, calibrating and testing permafrost as a component in Earth System Models. The data set also includes a high resolution digital elevation model that can be used together with the snow physical information for snow pack modeling. The data show that mean annual, summer and winter soil temperature data from shallow to deeper depths have been warming over the period of record, indicating the degradation of permafrost at this site. This site is one of only a handful of permafrost sites where long-term automatic monitoring has taken place and data are accessible. There is a great need for continuous monitoring at more sites, to span the full range of permafrost conditions. Monitoring is often limited by scientific project funding, typically lasting only 3 or 4 years. This will continue to present a challenge unless there is investment by governments to operationalise these networks.

Over the recent decades, temperature increase in the Arctic has been almost twice as large as the global average. This amplification of global warming is attributed to various feedback mechanisms present in the Arctic environment. Some processes are locally confined to the diminishing sea ice cover of the Arctic ocean, particularly the sea ice – albedo effect during polar day. Other amplifying processes related to the increasing open water surface of the Arctic ocean include e.g. the increasing heat flux from the ocean to the atmosphere. The resulting latent heat flux and augmenting evaporation contribute to an increasing atmospheric moisture content, which affects the longwave downward radiation directly or via changing cloud microphysics. Furthermore, atmospheric moisture and heat are more frequently advected from lower latitudes into the Arctic in relation to changes in the atmospheric circulation. During the dark period of polar night, the Arctic warming trend is not homogenously distributed over the polar cap, but occurs strongest at the surface in the Barents / Kara Seas and in the free troposphere of the Arctic North Atlantic sector. Thus, Svalbard is located in a key region of climate change. Observations from Ny-Ålesund at the west coast of the Svalbard archipelago show an annual temperature increase of 1.4 K per decade since the 1990s, with an average temperature that by now exceeds those observed during the early Arctic warming period in the 1920 to 1940s. The recent winter warming is even twice as high, and is accompanied by an increase in atmospheric moisture. Surface radiation observations in winter further indicate a change in cloudiness along with an increase in net longwave radiation. Although the winter warming is bottom-amplified, radiosonde observations show that the increasing temperature signal occurs over the entire troposphere. Indeed, part of the Svalbard winter warming is associated with enhanced warm and moist air advection in the free troposphere caused by increased cyclonic activity related to changes in atmospheric circulation patterns. The various processes contributing to Arctic amplification of climate warming link the local observations from Ny-Ålesund with processes occurring both in the Arctic and in the northern hemispheric mid-latitudes.

The Norwegian young sea ICE (N-ICE2015) expedition was designed to investigate the atmosphere-snow-ice-ocean interactions in the young and thin sea ice regime north of Svalbard. Radiosondes were launched twice daily during the expedition from January to June 2015. Here we use these upper air measurements to study the multiple cyclonic events observed during N-ICE2015 with respect to changes in the vertical thermodynamic structure, moisture content, and boundary layer characteristics. We provide statistics of temperature inversion characteristics, static stability, and boundary layer extent. During winter, when radiative cooling is most effective, we find the strongest impact of synoptic cyclones. Changes to thermodynamic characteristics of the boundary layer are associated with transitions between the radiatively “clear” and “opaque” atmospheric states. In spring, radiative fluxes warm the surface leading to lifted temperature inversions and a statically unstable boundary layer. Further, we compare the N-ICE2015 static stability distributions to corresponding profiles from ERA-Interim reanalysis, from the closest land station in the Arctic North Atlantic sector, Ny-Ålesund, and to soundings from the SHEBA expedition (1997/1998). We find similar stability characteristics for N-ICE2015 and SHEBA throughout the troposphere, despite differences in location, sea ice thickness, and snow cover. For Ny-Ålesund, we observe similar characteristics above 1000 m, while the topography and ice-free fjord surrounding Ny-Ålesund generate great differences below. The long-term radiosonde record (1993–2014) from Ny-Ålesund indicates that during the N-ICE2015 spring period, temperatures were close to the climatological mean, while the lowest 3000 m were 1–3∘C warmer than the climatology during winter.

Recent cold winter extremes over Eurasia and North America have been considered to be a consequence of a warming Arctic. More accurate weather forecasts are required to reduce human and socioeconomic damages associated with severe winters. However, the sparse observing network over the Arctic brings errors in initializing a weather prediction model, which might impact accuracy of prediction results at midlatitudes. Here we show that additional Arctic radiosonde observations from the Norwegian young sea ICE cruise project 2015 drifting ice camps and existing land stations during winter improved forecast skill and reduced uncertainties of weather extremes at midlatitudes of the Northern Hemisphere. For two winter storms over East Asia and North America in February 2015, ensemble forecast experiments were performed with initial conditions taken from an ensemble atmospheric reanalysis in which the observation data were assimilated. The observations reduced errors in initial conditions in the upper troposphere over the Arctic region, yielding more precise prediction of the locations and strengths of upper troughs and surface synoptic disturbances. Errors and uncertainties of predicted upper troughs at midlatitudes would be brought with upper level high potential vorticity (PV) intruding southward from the observed Arctic region. This is because the PV contained a ‘‘signal’’ of the additional Arctic observations as it moved along an isentropic surface. This suggests that a coordinated sustainable Arctic observing network would be effective not only for regional weather services but also for reducing weather risks in locations distant from the Arctic.

Ground-based measurements of the hemispherical-directional reflectance factor (HDRF) of windblown snowcovered Arctic tundra were measured at large solar zenith angles (79◦–85◦) for six sites near the international research base in Ny-Ålesund, Svalbard. Measurements were made with the Gonio RAdiometric Spectrometer System over the viewing angles 0◦–50◦ and the azimuth angles 0◦–360◦, for the wavelength range 400–1700 nm. The HDRF measurements showed good consistency between sites for near-nadir and backward viewing angles, with a relative standard deviation of less than 10% between sites where the snowpack was smooth and the snow depth was greater than 40 cm. The averaged HDRF showed good symmetry with respect to the solar principal plane and exhibited a forward scattering peak that was strongly wavelength dependent, with greater than a factor of 2 increase in the ratio of maximum to minimum HDRF values for all viewing angles over the wavelength range 400– 1300 nm. The angular effects on the HDRF had minimal influence for viewing angles less than 15◦ in the backward viewing direction for the averaged sites and agreed well with another study of snow HDRF for infrared wavelengths, but showed differences of up to 0.24 in the HDRF for visible wavelengths owing to light-absorbing impurities measured in the snowpack. The site that had the largest roughness elements showed the strongest anisotropy in the HDRF, a large reduction in forward scattering, and a strong asymmetry with respect to the solar principal plane.

The impact of Arctic radiosonde observations on the forecasting of the 2012 early August Arctic cyclone AC12—the “strongest” since records began—has been investigated using an observing system experiment (OSE). An atmospheric ensemble reanalysis (ALERA2) was used as the control experiment (CTL) to reproduce the development of the Arctic cyclone and surrounding large-scale atmospheric fields. The OSE applies the same reanalysis as the CTL except for the exclusion of radiosonde observations from the German icebreaker Polarstern, which cruised near Svalbard during mid-July to early August 2012. Comparison of the two reanalyses revealed a difference in the upper tropospheric circulation over northern mid-Eurasia, just before the Arctic cyclone developed, in the form of a stronger tropopause polar vortex in the CTL. This indicated that the upper tropospheric field in the CTL had greater potential for baroclinic instability over mid-Eurasia. Ensemble predictions were then conducted using the two reanalyses as initial values at which the tropopause polar vortex approached northern mid-Eurasia. The CTL prediction reproduced the formation of the Arctic cyclone, but the OSE shows a significantly weaker one. These results indicate that the improved reproduction of upper tropospheric circulation in the Arctic region due to additional radiosonde observations from a mobile platform was indispensable for the prediction of AC12. In particular, observations being acquired far from the Arctic cyclone affect the prediction of the cyclone via the upper tropospheric circulation in the atmospheric west wind drift.

Between 2009 and 2013 aerosols, sea ice properties and meteorological variables were measured during several airborne campaigns covering a wide range of the western arctic ocean. The campaigns were carried out with the aircraft Polar 5 of the German Alfred-Wegener-Institute (AWI) during spring and summer periods. Optical properties of accumulation mode aerosol and clouds were measured with the nadir looking AMALi aerosol lidar covering the atmospheric boundary layer and the free troposphere up to 3000m, while dropsondes provided coincident vertical profiles of meteorological quantities. Based on these data we discuss the vertical distribution of aerosol backscatter in and above the atmospheric boundary layer and its dependence on relative humidity, dynamics and underlying sea ice properties.We analyze vertical profiles of lidar and coincident dropsonde measurements from various locations in the European and Canadian Arctic from spring and summer campaigns. Sea ice cover is derived from modis satellite and aircraft onboard camera images. The aerosol load in the arctic atmospheric boundary layer shows a high variability. Various meteorological parameters and in particular boundary layer properties are discussed with their respective influence on aerosol features. To investigate the effect of the frequency and size of open water patches on aerosol properties, we relate the profiles to the sea ice properties influencing the atmosphere in the upwind region.

In Ny-Alesund, Svalbard (78.9°N, 11.9°E), the German Alfred Wegener Institute for Polar and Marine Research (AWI) and the French Polar Institute Paul Emile Victor (IPEV) operate the joint French-German Arctic Research Base AWIPEV. Research at the base covers a broad field of modern polar research, including atmospheric research, marine and terrestrial biology, and cryosphere research, supported by many different measurements, excursions, as well as field expeditions. Prominent examples are atmospheric long term measurements, scientific diving, glacier expeditions, installation of measuring systems in the permafrost and launches of research balloons. We present the AWIPEV longterm observatories contributing to international networks.

At the Franco-German AWIPEV Arctic research base in Ny-Ålesund (79°N, 12°E), Svalbard, surface radiation measurements have been operated since 1992 in the frame of the Baseline Surface Radiation Network (BSRN). The data are, complemented with surface meteorology measurements since 1993, contributing to the study of climate change in the Arctic with focus on alpine/marine landscape. Over the last 20 years, changes both in surface radiation and temperature have been observed in Ny-Ålesund. An increase of the annual mean temperature of +1.3 ± 0.7 K per decade indicate a substantial warming of the Ny-Ålesund environment, accompanied by an increase of the annual mean radiation budget of +4.9 ± 2.9 Wm-2 per decade. Separating the annual radiation budget to its different contributions it turns out that by far the largest changes are observed in the increasing long-wave radiation during the winter season. In fact, also the temperature increase is largest during the winter season, with +3.1 ± 2.4 K per decade. In the recent warmer winters, precipitation has often occurred in the form of rain rather than snow. Furthermore, changes in reflective solar radiation indicate an earlier onset of the snow melt by 1 week compared to 20 years ago.

Dropsondes are launched from research aircraft to measure vertical profiles of temperature, humidity, pressure and wind in the atmosphere while falling to the ground. Onboard the aircraft Polar 5 of the Alfred Wegener Institute for Polar and Marine Research (AWI), they are deployed on arctic and antarctic campaigns. Here we compare temperature and humidity data from Vaisala RD94 dropsondes with data from the permanently installed sensors of the aircraft. Dropsonde profiles are combined with profiles measured during ascent or descent by the aircraft nearby. The aerosol lidar AMALi is used to identify the presence and altitude of clouds. Furthermore the time constants of the dropsonde temperature and humidity sensors are deduced from atmospheric data. The overall agreement between aircraft and dropsonde data is good, but for high humidities as in clouds the dropsonde shows a systematic dry bias of almost 10 %. Data suggest a temperature dependency of this bias. The combination of dry bias and time lag of the sensors can make cloud detection from dropsondes alone difficult in cold environments.

The Arctic research base AWIPEV is located at the western coast of Spitsbergen, on the rim of the Arctic Ocean. It is in operation year-round. The mild climate here is due to the warm West-Spitsbergen current and is in contrast to the harsh climate on Greenland or the ice covered ocean of the same latitude. The dedicated Atmospheric Observatory collects continuously meteorological parameters, and incoming and outgoing radiation at the surface. Vertical profiles of pressure, temperature, humidity, and wind are measured by balloon-borne sondes on a daily basis up to stratospheric heights of 30 km. This includes weekly ozone measurements (see Rex et al. this issue). Remote sensing techniques are employed to observe aerosols and trace gases using spectrometers, microwave-radiometers, and lidar. This paper presents the main findings of 20 years of observations.

The vertical structure of the atmospheric boundary layer (ABL), simulated with the mesoscale modelWeather Research and Forecasting (WRF) as well as with its polar optimized version Polar WRF, was compared to tethered balloon soundings and mast observations taken in March and April 2009 from two Arctic fjords in Svalbard. From twelve short (48 h) simulations, the Quasi-Normal Scale Elimination scheme for the ABL and the NOAH land surface scheme for the surface were found to perform best and were selected for one long (16 day) simulation. The differences in performance of the standard WRF and Polar WRF were marginal. A warm bias, especially near the surface, was found in the modelled temperature profiles related to underestimated temperature inversion strength and depth. The modelled humidity inversions were generally deeper but weaker than the observed, and often occurred independently of temperature inversions. The largest errors in temperature and humidity occurred under high pressure conditions. Multiple temperature and humidity inversions were usually not captured byWRF. Compared to the compact sea ice east of Svalbard, the modelled temperature and humidity inversions were weaker and less frequent over the fjords. The biases in modelled wind speed profiles were closely related to low-level jets (LLJs); the modelled LLJs were stronger and deeper, and typically located at higher altitudes than the observed LLJs. Errors in the near-surface variables were notably reduced by applying post-processing equations based on othermodelled variables.