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The rapid changes occurring in the polar regions require an improved understanding of the processes that are driving the changes. At the same time increased human activities, such as marine navigation, resource exploitation, aviation, commercial fishing, and tourism, require reliable and relevant information. One of the primary goals of the World Meteorological Organization's Year of Polar Prediction (YOPP) Project is to improve the accuracy of numerical weather prediction (NWP) at high latitudes. During YOPP, two Canadian observatories were commissioned and equipped with new ground-based instruments for enhanced meteorological and system process observations that are considered to be "supersites" for addressing YOPP objectives, while other pre-existing supersites in Canada, the United States, Norway, Finland and Russia provided data from ongoing long-term observing programs. Data from these seven supersites were amalgamated and are being used to evaluate NWP systems from several international forecast centers and to perform meteorological process studies with the aim of improving NWP performance in the Polar Regions. In order to increase data useability and station interoperability, novel Merged Observatory Data Files (MODFs) have been created for these seven international supersites over two Special Observing Periods (February to March 2018 and July to September 2018). All observations collected at the seven supersites were compiled into this new standardized NetCDF MODF format, simplifying the process of conducting pan-Arctic NWP verification and process evaluation studies. This paper describes the seven Arctic YOPP supersites, data collection and processing methods, and the novel MODF format and output files, which together comprise the observational contribution to the associated model intercomparison effort, termed YOPP supersite Model Intercomparison Project (YOPPsiteMIP). All YOPPsiteMIP MODFs are publicly accessible via the YOPP Data Portal (Whitehorse: https://doi.org/10.21343/a33e-j150, Iqaluit: https://doi.org/10.21343/yrnf-ck57, Sodankylä: https://doi.org/10.21343/m16p-pq17, Utqiaġvik: https://doi.org/10.21343/a2dx-nq55, Tiksi: https://doi.org/10.21343/5bwn w881, Ny-Ålesund: https://doi.org/10.21343/y89m-6393, Eureka: https://doi.org/10.21343/r85j-tc61), hosted by MET Norway, with corresponding output from NWP models. [ABSTRACT FROM AUTHOR]

A large and ever-growing body of geophysical information is measured on campaigns and at specialized observatories as a part of scientific expeditions/experiments. These collections of observed data include many essential climate variables (as defined by the World Meteorological Organization), but are often distinguished by a wide range of additional non-routine measurements that are designed to not only document the state of the environment, but also the drivers that contribute to that state. These field data are not only used to further understand the environmental processes through observation-based studies, but also to provide baseline data to test model performance and to codify understanding to improve predictive capabilities. To address the considerable barriers and difficulty in utilizing these diverse and complex data for observation-model research, the Merged Observatory Data File (MODF) concept has been developed. The MODF combines measurements from multiple instruments into a single file that complies with well-established data format and metadata practices and has been designed to parallel development of corresponding Merged Model Data Files (MMDFs). Using MODF and MMDF protocols will facilitate the evolution of Model Intercomparison Projects into Model Intercomparison and Improvement Projects by putting observation and model data 'on the same page' in a timely manner. The MODF concept was developed especially for weather forecast model studies in the Arctic. The surprisingly complex process of implementing MODFs in that context refined the concept itself. Thus this article explains the concept of MODFs by providing details on the issues that were revealed and resolved during this first specific implementation. Detailed instructions are provided on how to make MODFs, and this article can thus be considered a MODF creation manual. [ABSTRACT FROM AUTHOR]

The MOSAiC (Multidisciplinary drifting Observatory for the Study of Arctic Climate) expedition was the largest Arctic field campaign ever conducted. MOSAiC offered the unique opportunity to monitor and characterize aerosols and clouds with high vertical resolution up to 30 km height at latitudes from 80 to 90 ∘ N over an entire year (October 2019 to September 2020). Without a clear knowledge of the complex aerosol layering, vertical structures, and dominant aerosol types and their impact on cloud formation, a full understanding of the meteorological processes in the Arctic, and thus advanced climate change research, is impossible. Widespread ground-based in situ observations in the Arctic are insufficient to provide these required aerosol and cloud data. In this article, a summary of our MOSAiC observations of tropospheric aerosol profiles with a state-of-the-art multiwavelength polarization Raman lidar aboard the icebreaker Polarstern is presented. Particle optical properties, i.e., light-extinction profiles and aerosol optical thickness (AOT), and estimates of cloud-relevant aerosol properties such as the number concentration of cloud condensation nuclei (CCN) and ice-nucleating particles (INPs) are discussed, separately for the lowest part of the troposphere (atmospheric boundary layer, ABL), within the lower free troposphere (around 2000 m height), and at the cirrus level close to the tropopause. In situ observations of the particle number concentration and INPs aboard Polarstern are included in the study. A strong decrease in the aerosol amount with height in winter and moderate vertical variations in summer were observed in terms of the particle extinction coefficient. The 532 nm light-extinction values dropped from >50 Mm -1 close to the surface to <5 Mm -1 at 4–6 km height in the winter months. Lofted, aged wildfire smoke layers caused a re-increase in the aerosol concentration towards the tropopause. In summer (June to August 2020), much lower particle extinction coefficients, frequently as low as 1–5 Mm -1 , were observed in the ABL. Aerosol removal, controlled by in-cloud and below-cloud scavenging processes (widely suppressed in winter and very efficient in summer) in the lowermost 1–2 km of the atmosphere, seems to be the main reason for the strong differences between winter and summer aerosol conditions. A complete annual cycle of the AOT in the central Arctic could be measured. This is a valuable addition to the summertime observations with the sun photometers of the Arctic Aerosol Robotic Network (AERONET). In line with the pronounced annual cycle in the aerosol optical properties, typical CCN number concentrations (0.2 % supersaturation level) ranged from 50–500 cm -3 in winter to 10–100 cm -3 in summer in the ABL. In the lower free troposphere (at 2000 m), however, the CCN level was roughly constant throughout the year, with values mostly from 30 to 100 cm -3. A strong contrast between winter and summer was also given in terms of ABL INPs which control ice production in low-level clouds. While soil dust (from surrounding continents) is probably the main INP type during the autumn, winter, and spring months, local sea spray aerosol (with a biogenic aerosol component) seems to dominate the ice nucleation in the ABL during the summer months (June–August). The strong winter vs. summer contrast in the INP number concentration by roughly 2–3 orders of magnitude in the lower troposphere is, however, mainly caused by the strong cloud temperature contrast. A unique event of the MOSAiC expedition was the occurrence of a long-lasting wildfire smoke layer in the upper troposphere and lower stratosphere. Our observations suggest that the smoke particles frequently triggered cirrus formation close to the tropopause from October 2019 to May 2020. [ABSTRACT FROM AUTHOR]

This article presents the results of observations of selected fluxes of the radiation balance in north-western Spitsbergen in the years from 2010 to 2014. Measurements were taken in Ny-Ålesund and in the area of Kaffiøyra, on different surface types occurring in the Polar zone: moraine, tundra, snow and ice. Substantial differences in the radiation balance among the various types of surface were observed. The observations carried out in the summer seasons of 2010–2014 in the area of Kaffiøyra demonstrated that the considerable reflection of solar radiation on the Waldemar Glacier (albedo 55%) resulted in a smaller solar energy net income. During the polar day, a diurnal course of the components of the radiation balance was apparently related to the solar elevation angle. When the sun was low over the horizon, the radiation balance became negative, especially on the glacier. Diurnal, annual and multi-annual variations in the radiation balance have a significant influence on the functioning of the environment in polar conditions.

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).

The airborne field campaign HALO–(AC)³ took place from 07 March to 12 April 2022 and was designed to observe the transformation of air masses during their meridional transport in the North Atlantic sector of the Arctic. We evaluate the meteorological and sea ice conditions during the campaign based on the European Centre for Medium–Range Weather Forecasts (ECMWF) Reanalysis v5 (ERA5), satellite data, and atmospheric soundings with respect to climatology and describe special synoptic events. HALO–(AC)³ started with a warm period (11–20 March) where strong southerly winds prevailed that caused moist and warm air intrusions (MWAIs). Two MWAIs were detected as Atmospheric Rivers (ARs). Compared to the ERA5 climatology (1979–2022), record breaking vertically integrated poleward heat and moisture fluxes averaged over 75.0–81.5° N were found. The related warm and moist air masses reached the central Arctic, causing the highest rainfall rates over the sea ice northwest of Svalbard recorded since the beginning of the ERA5 climatology. Subsequently, the cold period of HALO–(AC)³ started after the passage of a Shapiro–Keyser cyclone on 21 March when the wind regime turned to northerlies, advecting colder air into the Fram Strait. Until 08 April, marine cold air outbreaks (MCAOs) prevailed, including two strong MCAO events on 21–26 March and 01–02 April. In between, aged subpolar warm air was advected to the Fram Strait with northeasterly and easterly winds. On average, the campaign period was warmer than the climatology, especially due to the exceptionally strong ARs during the warm period. In the Fram Strait, the sea ice concentration (SIC) was within the 10–90th percentiles of the climatology over the entire campaign duration. During the warm period, SIC was strongly reduced and an untypically large polynya for this season opened north of Svalbard. During the cold period, the polynya was closed again and above average SICs were found. We describe the environmental conditions of a Polar Low and far north reaching cirrus in detail, which will be subjects of research using HALO’s remote sensing suite in the future. We also present the perspective on the HALO–(AC)³ weather conditions from the research site Ny–Ålesund, where orographic effects caused by the Svalbard archipelago and temporal shifts of atmospheric signals due to the propagation of synoptic systems must be taken into account. Overall, our study may serve as basis for future analyses of the data collected during HALO–(AC)³ and to compare synoptic conditions with other field campaigns. [ABSTRACT FROM AUTHOR]