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The occurrence of events with increased and decreased integrated water vapor (IWV) at the Arctic site Ny-Ålesund, their relation to cloud properties, and the surface cloud radiative effect (CRE) is investigated. For this study, we used almost 2.5 years (from June 2016 to October 2018) of ground-based cloud observations processed with the Cloudnet algorithm, IWV from a microwave radiometer (MWR), long-term radiosonde observations, and backward trajectories FLEXTRA. Moist and dry anomalies were found to be associated with North Atlantic flows and air transport within the Arctic region, respectively. The amount of water vapor is often correlated to cloud occurrence, presence of cloud liquid water, and liquid water path (LWP) and ice water path (IWP). In turn, changes in the cloud properties cause differences in surface CRE. During dry anomalies, in autumn, winter, and spring, the mean net surface CRE was lower by 2–37 W m -2 with respect to normal conditions, while in summer the cloud-related surface cooling was reduced by 49 W m -2. In contrast, under moist conditions in summer the mean net surface CRE becomes more negative by 25 W m -2 , while in other seasons the mean net surface CRE was increased by 5–37 W m -2. Trends in the occurrence of dry and moist anomalies were analyzed based on a 25-year radiosonde database. Dry anomalies have become less frequent, with rates for different seasons ranging from -12.8 % per decade to -4 % per decade, while the occurrence of moist events has increased at rates from 2.8 % per decade to 6.4 % per decade. [ABSTRACT FROM AUTHOR]

Here we evaluate five atmospheric reanalyses in an Arctic gateway during late summer. The reanalyses include ERA5, ERA‐Interim, Japanese 55 year Re‐Analysis (JRA‐55), Climate Forecasting System Reanalysis‐version 2 (CFSv2), and Modern Era Retrospective analysis for Research and Applications‐version 2 (MERRA‐2). We use observations from 50 radiosondes launched in the Fram Strait around 79‐80°N, between 25 August and 11 September 2017. Crucially, data from 27 radiosondes were not transmitted to the Global Telecommunications System and therefore not assimilated into any reanalysis. In most reanalyses, the magnitude of wind speed and humidity errors is similar for profiles with and without data assimilation. In cases without data assimilation, correlation coefficients (R) exceed 0.88 for temperature, wind speed, and specific humidity, in all reanalyses. Overall, the newly released ERA5 has higher correlation coefficients than any other reanalyses as well as smaller biases and root‐mean‐square errors, for all three variables. The largest improvements identified in ERA5 are in its representation of the wind field, and temperature profiles over warm water. Plain Language Summary: The Arctic is undergoing rapid and ongoing changes. However, due to the harsh environment, there are relatively few observations from this region. To understand the drivers of these changes, we rely heavily on atmospheric reanalyses. Reanalyses are our best guess at the state of the atmosphere at a given time. Reanalyses are generated by assimilating all available atmospheric observations into a weather forecast model. A key question within the scientific community is how accurate reanalyses are in the Arctic. One problem with answering this question is that most observations used to test the performance of reanalyses were ingested in to the model and are therefore not an independent data set. Here we present a new set of balloon‐borne atmospheric observations from the Fram Strait, between Svalbard and Greenland. Many of these data were not assimilated in to any reanalyses, providing a rare opportunity to evaluate their performance in this important Arctic gateway. We test five products, including the newly released ERA5 from the European Centre for Medium Ranged Weather Forecasting. All products simulate the temperature, humidity, and wind fields well, even without data assimilation. Overall, the newly released ERA5 performs best, with the largest improvements in the wind and temperature fields. Key Points: In situ Arctic observations: 27 atmospheric profiles from radiosondes in Fram Strait (August–Septmber 2017) were not transmitted to GTSERA5 simulates observed atmospheric profiles more accurately than ERA‐Interim, JRA‐55, CFSv2, and MERRA‐2Largest improvements are found in ERA5 for wind and temperature profiles over warmer eastern Fram Strait [ABSTRACT FROM AUTHOR]

Multilayer clouds (MLCs) occur more often in the Arctic than globally. In this study we present the results of a detection algorithm applied to radiosonde and radar data from an 1-year time period in Ny-Ålesund, Svalbard. Multilayer cloud occurrence is found on 29 % of the investigated days. These multilayer cloud cases are further analysed regarding the possibility of ice crystal seeding, meaning that an ice crystal can survive sublimation in a subsaturated layer between two cloud layers when falling through this layer. For this we analyse profiles of relative humidity with respect to ice to identify super- and subsaturated air layers. Then the sublimation of an ice crystal of an assumed initial size of r=400 µ m on its way through the subsaturated layer is calculated. If the ice crystal still exists when reaching a lower supersaturated layer, ice crystal seeding can potentially take place. Seeding cases are found often, in 23 % of the investigated days (100 % includes all days, as well as non-cloudy days). The identification of seeding cases is limited by the radar signal inside the subsaturated layer. Clearly separated multilayer clouds, defined by a clear interstice in the radar image, do not interact through seeding (9 % of the investigated days). There are various deviations between the relative humidity profiles and the radar images, e.g. due to the lack of ice-nucleating particles (INPs) and cloud condensation nuclei (CCN). Additionally, horizontal wind drift of the radiosonde and time restriction when comparing radiosonde and radar data cause further deviations. In order to account for some of these deviations, an evaluation by manual visual inspection is done for the non-seeding cases. [ABSTRACT FROM AUTHOR]

Radiosonde measurements obtained at the Arctic site Ny-Ålesund (78.9°N, 11.9°E), Svalbard, from 1993 to 2014 have been homogenized accounting for instrumentation discontinuities by correcting known errors in the manufacturer provided profiles. The resulting homogenized radiosonde record is provided as supplementary material at . From the homogenized data record, the first Ny-Ålesund upper-air climatology of wind, temperature and humidity is presented, forming the background for the analysis of changes during the 22-year period. Particularly during the winter season, a strong increase in atmospheric temperature and humidity is observed, with a significant warming of the free troposphere in January and February up to 3 K per decade. This winter warming is even more pronounced in the boundary layer below 1 km, presumably amplified by mesoscale processes including e.g. orographic effects or the boundary layer capping inversion. Though the largest contribution to the increasing atmospheric water vapour column in winter originates from the lowermost 2 km, no increase in the contribution by specific humidity inversions is detected. Instead, we find an increase in the humidity content of the large-scale background humidity profiles. At the same time, the tropospheric flow in winter is found to occur less frequent from northerly directions and to the same amount more frequent from the South. We conclude that changes in the atmospheric circulation lead to an enhanced advection of warm and moist air from lower latitudes to the Svalbard region in the winter season, causing the warming and moistening of the atmospheric column above Ny-Ålesund, and link the observations to changes in the Arctic Oscillation. [ABSTRACT FROM AUTHOR]

The Arctic research village of Ny-Ålesund (79 ∘ N, 12 ∘ E) on the Spitsbergen island of Svalbard archipelago, with its logistics and infrastructure, provides a unique access to the Arctic environment. Among the several international environmental and climate monitoring programmes constantly running there, the Global Climate Observing System (GCOS) Reference Upper Air Network (GRUAN) has established one of its stations at the Alfred Wegener Institute observatory. Calibration of sensors and measurement traceability are fundamental aspects of climate observations, as requested by the GRUAN measurement procedures. In the framework of the MeteoMet project, a transportable climatic chamber for the calibrations of air temperature and pressure sensors was studied and manufactured. In June 2014, a calibration campaign involved the transport and use of one of those systems in the Ny-Ålesund GRUAN station. The result of the campaign has been the complete calibration of temperature and pressure sensors for radiosonde pre-launch ground checks. The resulting calibration curves were obtained with lower uncertainties and more robust characterization of the sensors, with respect to the usual procedures adopted. Given the opportunity of the calibration device operating already in place and the presence of the metrology staff, the calibration was extended to the sensors equipping the Amundsen-Nobile Climate Change Tower. The present study reports on the ‘Arctic metrology 2014’ campaign and the plans for the establishment of a permanent laboratory for metrology in Ny-Ålesund. The aim is to address the measurement traceability needs arising from the multidisciplinary measurements made by the scientific community operating there. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Our study shows what we know (in)consistently about the vertically integrated water vapor (IWV) trends in the Arctic and what their uncertainties are. For this, we analyze the IWV trends in most modern global reanalyses (CFSR, ERAI, JRA55, MERRA2) over the period covering 1979-2016. The presented monthly spatial trends highlight important differences from month to month and from region to region. Our results indicate the overall positive IWV trend averaged over the central Arctic (>70˚N), but also highlight the heterogeneous spatiotemporal patterns of the derived trends. Although the across-reanalyses intercomparison suggests an agreement in an Arctic becoming wetter in almost all regions, IWV trends were statistically significant only over specific regions for specific months in all reanalyses. Furthermore, the reanalyses substantially disagree on the regional trend magnitudes. We quantify the differences in the monthly IWV trends and identify the regions and months of greatest difference between the reanalyses. We further discuss the interannual and decadal IWV variations, which underlie the trend. In concert with the reanalyses, we assess IWV data derived from radiosondes at 35 stations north of 65˚N and from GPS data at the Arctic station Ny-Ålesund. The reanalyses exhibit in general a remarkable agreement with the station data. We find interesting region-specific annual characteristics in the trends, which are consistent across the data sets. Importantly, our results also demonstrate the large uncertainty in the calculated trends. [ABSTRACT FROM AUTHOR]