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The Arctic is warming faster than anywhere else on Earth, prompting glacial melt, permafrost thaw, and sea ice decline. These severe consequences induce feedbacks that contribute to amplified warming, affecting weather and climate globally. Aerosols and clouds play a critical role in regulating radiation reaching the Arctic surface. However, the magnitude of their effects is not adequately quantified, especially in the central Arctic where they impact the energy balance over the sea ice. Specifically, aerosols called ice nucleating particles (INPs) remain understudied yet are necessary for cloud ice production and subsequent changes in cloud lifetime, radiative effects, and precipitation. Here, we report observations of INPs in the central Arctic over a full year, spanning the entire sea ice growth and decline cycle. Further, these observations are size-resolved, affording valuable information on INP sources. Our results reveal a strong seasonality of INPs, with lower concentrations in the winter and spring controlled by transport from lower latitudes, to enhanced concentrations of INPs during the summer melt, likely from marine biological production in local open waters. This comprehensive characterization of INPs will ultimately help inform cloud parameterizations in models of all scales.

The planetary boundary layer height (PBLH) is a crucial indicator reflecting the region of the atmosphere characterized by continuous turbulence. Here, we use radiosonde and surface meteorological observations (4–7 times per day, year‐round measurements) during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition to derive the PBLH (PBLHMOSAiC), and further evaluate the PBLH from the ERA5 reanalysis (PBLHERA5). Comparisons between PBLHMOSAiC and PBLHERA5 from different perspectives reveal that: (a) The overestimation of PBLHERA5 when the sea ice concentration is >90% is significant with the centered root mean squared error reaching up to 201 m; (b) The difference between the two products is notably pronounced in cold seasons, while it is comparatively diminished in warm seasons; (c) In neutral boundary layers, differences in PBLHERA5 are larger compared with stable and convective boundary layers. In addition, the analysis of error sources indicates that the bias of PBLHERA5 is sensitive to the bias of vertical thermal structure and wind speed profiles in ERA5 data sets in all conditions. Finally, we find a Random Forest model effectively reduces the bias of PBLHERA5 with the index of agreement reaching up to 0.71 in the test data set, while a multiple linear regression demonstrates comparable performance to the Random Forest model. Plain Language Summary: The planetary boundary layer (PBL) is a crucial component in the interactions between the atmosphere and the surface, especially for the shallow PBL in the Arctic. However, the scarcity of observations over the central Arctic limits our understanding of physical characteristics of the Arctic boundary layer, and current research relies heavily on atmospheric reanalysis. This study focuses on evaluating the planetary boundary layer height (PBLH) of ERA5 over the Arctic sea‐ice surface using observations collected during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition. We find the PBLHERA5 is often overestimated, especially with high sea ice concentrations and in cold seasons. Additionally, the PBLHERA5 performs comparatively worse against observations in neutral boundary layers when contrasted with stable and convective boundary layers. Analysis of error sources reveals the PBLHERA5 bias is sensitive to vertical thermal structure and wind speed profiles in ERA5 data sets under all conditions. To rectify the errors in the PBLHERA5, we employ a Random Forest model, significantly improving PBLHERA5's accuracy. Our model‐corrected PBLHERA5 shows strong agreement with observations, providing a more reliable representation of PBLH. This study highly enhances our understanding of the performance of the Arctic atmospheric boundary layer structure in ERA5 data sets. Key Points: The MOSAiC observations are employed to comprehensively assess the ERA5 planetary boundary layer height (PBLH) performance in the ArcticThe bias of the ERA5 PBLH is more sensitive to biases in temperature and wind speed profiles than to biases in specific humidity profileThe random forest model can reduce the ERA5 PBLH bias with the index of agreement reaching up to 0.71 in the test data set [ABSTRACT FROM AUTHOR]

The effects of atmospheric thermal structure on the surface energy flux are poorly understood over the Arctic sea‐ice surface. Here, we explore the mechanism of sensible heat exchange under different types of thermal structures over the Arctic sea‐ice surface by using data collected during the Multidisciplinary drifting Observatory for the Study of Arctic Climate expedition. The quadrant analysis indicates that strong surface temperature inversions below 100 m enhance non‐local effects on the positive (upward) sensible heat flux (w′θ′‾ $\overline{{w}^{\prime }{\theta }^{\prime }}$) through entrainment of large eddies from the convective boundary layer aloft. However, strong surface inversions restrict the contributions of large eddies to the negative (downward) w′θ′‾ $\overline{{w}^{\prime }{\theta }^{\prime }}$ due to intensified surface stability. By inspecting the existing parameterization schemes, we found that the European Center for Medium‐Range Weather Forecasts Integrated Forecasting System scheme fails to predict the impacts of non‐local processes on the positive w′θ′‾ $\overline{{w}^{\prime }{\theta }^{\prime }}$, and an adjustment term to correct the bias of parameterized w′θ′‾ $\overline{{w}^{\prime }{\theta }^{\prime }}$ is proposed. Plain Language Summary: Since pre‐industrial times, the increase of Arctic near‐surface air temperature has been 2–3 times larger than the global average, known as the Arctic amplification. The atmospheric physical processes occurring in the boundary layer are essential for understanding the ongoing climate changes because they modulate the surface energy budget over the Arctic. However, scarce observations limit our understanding of surface heat flux exchange in this area. Recently, the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition was conducted in the central Arctic to support the urgent need for understanding and modeling the rapidly changing Arctic atmosphere‐ice‐ocean system. Based on the MOSAiC observations, we investigate the role of non‐local effects on sensible heat flux under different types of atmospheric thermal structures over the Arctic sea‐ice surface. To the best of our knowledge, this study represents the first time that the effects of strong surface inversions on negative and positive sensible heat fluxes over the Arctic sea‐ice surface are found to be opposite. In addition, we propose a method for taking non‐local effects into account, which can be applied in the European Center for Medium‐Range Weather Forecasts Integrated Forecasting System flux parameterization or other numerical models to correct the bias in parameterized positive sensible heat flux. Key Points: The characteristics of thermal structure during the Multidisciplinary drifting Observatory for the Study of Arctic Climate expedition are presentedThe effects of strong surface inversions on the negative and positive sensible heat flux are contrastingWe propose an improved solution to predict the impacts of non‐local processes on the positive sensible heat flux [ABSTRACT FROM AUTHOR]

Clouds play an important role in controlling the radiative energy budget of the Arctic atmospheric boundary layer. To quantify the impact of clouds on the radiative heating or cooling of the lower atmosphere and of the surface, vertical profile observations of thermal-infrared irradiances were collected using a radiation measurement system carried by a tethered balloon. We present 70 profiles of thermal-infrared radiative quantities measured in summer 2020 during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition and in autumn 2021 and spring 2022 in Ny-Ålesund, Svalbard. Measurements are classified into four groups: cloudless, low-level liquid-bearing cloud, elevated liquid-bearing cloud, and elevated ice cloud. Cloudless cases display an average radiative cooling rate of about - 2 K d -1 throughout the atmospheric boundary layer. Instead, low-level liquid-bearing clouds are characterized by a radiative cooling up to - 80 K d -1 within a shallow layer at cloud top, while no temperature tendencies are identified underneath the cloud layer. Radiative transfer simulations are performed to quantify the sensitivity of radiative cooling rates to cloud microphysical properties. In particular, cloud top cooling is strongly driven by the liquid water path, especially in optically thin clouds, while for optically thick clouds the cloud droplet number concentration has an increased influence. Additional radiative transfer simulations are used to demonstrate the enhanced radiative importance of the liquid relative to ice clouds. To analyze the temporal evolution of thermal-infrared radiation profiles during the transitions from a cloudy to a cloudless atmosphere, a respective case study is investigated. [ABSTRACT FROM AUTHOR]