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Mechanisms behind the phenomenon of Arctic amplification are widely discussed. To contribute to this debate, the (AC)3 project was established in 2016 (www.ac3-tr.de/). It comprises modeling and data analysis efforts as well as observational elements. The project has assembled a wealth of ground-based, airborne, shipborne, and satellite data of physical, chemical, and meteorological properties of the Arctic atmosphere, cryosphere, and upper ocean that are available for the Arctic climate research community. Short-term changes and indications of long-term trends in Arctic climate parameters have been detected using existing and new data. For example, a distinct atmospheric moistening, an increase of regional storm activities, an amplified winter warming in the Svalbard and North Pole regions, and a decrease of sea ice thickness in the Fram Strait and of snow depth on sea ice have been identified. A positive trend of tropospheric bromine monoxide (BrO) column densities during polar spring was verified. Local marine/biogenic sources for cloud condensation nuclei and ice nucleating particles were found. Atmospheric–ocean and radiative transfer models were advanced by applying new parameterizations of surface albedo, cloud droplet activation, convective plumes and related processes over leads, and turbulent transfer coefficients for stable surface layers. Four modes of the surface radiative energy budget were explored and reproduced by simulations. To advance the future synthesis of the results, cross-cutting activities are being developed aiming to answer key questions in four focus areas: lapse rate feedback, surface processes, Arctic mixed-phase clouds, and airmass transport and transformation.

To understand the feedbacks driving the amplified changes in the Arctic a quantification of the contribution of the involved processes is necessary. Here a detailed study of low-level Arctic clouds on the surface radiation budget is presented. These clouds frequently occur below the lowest detection range of most state-of-the-art remote-sensing instruments and were observed in summertime during 25% of the time over the marginal sea ice zone. The low altitude of these clouds poses challenges on their observation and characterization by remote-sensing techniques. Ground-based remote sensing and surface radiation flux measurements performed during the Arctic cruise PS106 in 2017 were combined with radiative transfer simulations to study low-level clouds. A multiwavelength lidar PollyXT with near-range observations capabilities down to 50m and a cloud radar with a lowest detection limit at 165m altitude were operated continuously. The liquid-water microphysical properties of clouds missed by the cloud radar were estimated using measurements of a microwave radiometer HATPRO and the lidar-detected cloud base. Thereby the surface radiative effect of these clouds was quantified. A closure between the observed and modelled radiative surface fluxes was achieved with a realistic representation of low-level liquid-containing clouds in the radiative transfer model. When omitting these low-level clouds, the cloud radiative effect at the surface was misestimated by 43Wm−2. The presented study highlights the importance of improving cloud retrievals for low-level liquid-containing clouds as they are frequently encountered in the high Arctic, together with observational capabilities, both in terms of cloud remote sensing and radiative flux observations., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

Significant uncertainties in the prediction of future warming in the Arctic arise from our lack of understanding of governing processes, including cloud radiative feedbacks.The present study compares preliminary simulations of 1D radiative fluxes based on the Cloudnet and ShupeTurner cloud retrievals for the yearlong Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition.The analysis is conducted in the context of a radiative closure study at the surface and the top-of-the atmosphere for the upward and downward broadband solar and terrestrial radiative fluxes. The consistency of our simulations and satellite-based estimates from the Clouds and the Earth’s Radiant Energy System (CERES) are analysed by considering several atmospheric and surface-type conditions. Particular focus is given to the effect of clouds on the radiation budget.Based on our simulations and CERES estimates, we find that clouds increase the net radiative fluxes at the surface by about 35.5 W/m² for the entire MOSAiC expedition period. Nevertheless, based on in-situ observations, it is argued that significant uncertainties in the solar and terrestrial affect this estimate of cloud radiative effects. Our research will also address the spatiotemporal variability of clouds and how this might impact the comparison between the point-like ground measurements with the CERES satellite footprint., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)