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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. 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. 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

On 20 March 2015, a total solar eclipse occurred over Ny-Ålesund (78.9°?N, 11.9°?E), Svalbard, under ideal conditions with clear sky. The cycle of the radiation fluxes is comparable with other experiments during eclipses, with even the upward longwave radiation showing significant changes, with a delay to the shortwave radiation and a slowly linear increase after the totality. Also, under polar conditions, an increase of the wind velocity before and a decrease after the totality was found, which is an indicator of the generation of an “Eclipse cyclone”. This change of the wind direction generated a local wind system with a near-surface-layer katabatic flow. During the eclipse, a remarkably large sensible heat flux was observed. The turbulent fluxes were analysed using a wavelet technique with 1-minute time resolution, which is the ideal method for investigating these highly non-steady conditions. No influences on the boundary layer structure as measured with radiosondes were found, with the exception of a wind direction change during the eclipse cyclone below the shallow inversion layer.

Winter time atmospheric observations from the 2015 Norwegian young sea‐ICE campaign (N‐ICE2015) are compared with data from the 1997–1998 Surface Heat Budget of the Arctic (SHEBA) campaign. Both data sets have a bimodal distribution of the net longwave radiative flux for January–February, with modal values of −40 W m−2and 0 W m−2. These values correspond to the radiatively clear and opaquely cloudy states, respectively, and are likely to be representative of the wider Arctic. The new N‐ICE2015 observations demonstrate that the two winter states operate in the Atlantic sector of the Arctic and regions of thin sea ice. We compare the N‐ICE2015 and SHEBA data with ERA‐Interim and output from the coupled Arctic regional climate model HIRHAM‐NAOSIM. ERA‐Interim simulates two Arctic winter states well and captures the timing of transitions from one state to the other, despite underestimating the cloud liquid water path. HIRHAM‐NAOSIM has more cloud liquid water compared with ERA‐Interim but simulates the two states poorly. Our results demonstrate that models must simulate realistic synoptic forcing and temperature profiles to accurately capture the two Arctic winter states, and not only the presence of mixed‐phase clouds. Using ERA‐Interim, we find a positive trend in the number of opaquely cloudy days in the western Atlantic sector of the Arctic, and a strong correlation with the mean winter temperature over much of the Arctic Basin. Hence, the two Arctic winter states are important for understanding interannual variability in the Arctic. The N‐ICE2015 data set will help improve our understanding of these relationships. Two Arctic winter states are observed during the N‐ICE2015 and SHEBA campaigns with similar net longwave fluxesERA‐Interim captures the transitions from radiatively clear to opaquely cloudy states during N‐ICE2015 and SHEBAStrong correlation between number of opaquely cloudy days and mean winter temperature over large areas of the Arctic

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 expedition (N‐ICE2015) 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. Additional Arctic radiosonde observations during winter improved forecast skill of cold extremes at midlatitudesThe trajectory of high potential vorticity is important for understanding the origin of the large uncertainties in the upper troposphereThe uncertainty originated from the denial of extra observations over the Arctic reached midlatitude within a week

Multitudes of measurements are needed to understand the environment and its evolution. The Arctic region is a fundamental observation area for climate change evaluation: climate change comes first and comes faster in the arctic.The higher accuracy required to quickly capture trends; the extreme range and conditions of sensors exposure; a robust comparability asked by the different measurement networks; the need of dedicated calibration procedures, together with the logistical problems associated with such remote location, motivate the proposal for a joint effort to address metrology experience and activities for Arctic research applications. The Ny-Ålesund international research base and community offers a unique infrastructure to directly link metrological traceability to on site polar measurements. The contribution reports a study on the implementation of specific calibration procedures, metrological validation of measurements and instrument tests, uncertainties evaluations including quantities of influence, and the feasibility of a metrology laboratory in Ny-Ålesund.

Fram Strait in the northern North Atlantic is a key region for marine cold air outbreaks (MCAOs), southward discharges of polar air under northerly air flow, which have a strong impact on air‐sea heat fluxes, boundary layer processes and severe weather. This study investigates climatologies and decadal trends of Fram Strait MCAOs of different intensity classes based on the ERA5 reanalysis product for 1979–2020. Among striking interannual variability, it is shown that the main MCAO season is December through March, when MCAOs occur around 2/3 of the time. We report on significant decadal MCAO decreases in December and January, and a significant increase in March. While the mid‐winter decrease is mainly related to the different paces of warming between the surface and the lower atmosphere, the increase in March can be related to changes in synoptic circulation patterns. As an explanation for the latter, a possible feedback between retreating Barents Sea sea ice, enhanced cyclonic activity and Fram Strait MCAOs is postulated. Exemplifying the trend toward stronger MCAOs during March, the study details the recordbreaking MCAO season in early 2020, and an observational case study of an extreme MCAO event in March 2020 is conducted. Thereby, radiosonde observations are combined with kinematic air back‐trajectories to provide rare observational evidence for the diabatic cooling and drying during the MCAO preconditioning phase. 1979–2020 climatology of marine cold air outbreaks (MCAOs) in Fram StraitSignificant MCAO increase in March is dynamically driven; significant decrease in mid‐winter due to vertical distribution of warmingAnomalous circulation in February/March 2020 yields extreme MCAO season with observed air mass preconditioning 1979–2020 climatology of marine cold air outbreaks (MCAOs) in Fram Strait Significant MCAO increase in March is dynamically driven; significant decrease in mid‐winter due to vertical distribution of warming Anomalous circulation in February/March 2020 yields extreme MCAO season with observed air mass preconditioning

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. 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 the atmospheric boundary layer characteristics. During winter, we find the strongest impact of synoptic cyclones, which transport warm and moist air into the cold and dry Arctic atmosphere. In spring, incoming solar radiation warms the surface. This leads to very different thermodynamic conditions and higher moisture content, which reduces the contrast between stormy and calm periods. Further, we compare the N‐ICE2015 measurements 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. The comparisons highlight the value of the N‐ICE2015 observation and show the importance of winter time observations in the Arctic North Atlantic sector. New data set of radiosonde observations over first year sea ice in the Arctic North Atlantic sector, from January to June 2015Rapid increases in column‐integrated water vapor correspond to synoptic activity and track lower latitude air massesTropospheric static stability is similar for N‐ICE2015 and SHEBA, despite different location, sea ice thickness, and snow cover