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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]

Clouds play an important role in controlling the radiative energy budget of the Arctic atmospheric boundary layer. To quantify their impact on diabatic heating or cooling of the atmosphere and of the surface, vertical profile observations of thermal-infrared irradiances were collected using a tethered balloon. We present 70 profiles of thermal-infrared radiative quantities measured in summer 2020 at 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 a radiative cooling rate of about -2 K day-1. Observed low-level liquid-bearing clouds are characterized by a radiative cooling up to -80 K day-1 in a shallow layer at cloud top. Radiative transfer simulations are performed to quantify the sensitivity of radiative cooling rates to cloud microphysical properties. In particular, cloud top cooling has a strong response to variation of the liquid water path, especially in optically thin clouds, while for optically thick clouds the cloud droplet number concentration has an increased relative importance. Two case studies with a changing cloud cover are presented to investigate the temporal evolution of radiation profiles during the transitions between (a) cloudy to cloudless and (b) low-level to elevated clouds. Additional radiative transfer simulations are used to reproduce the observed scenarios and to showcase the radiative impacts of elevated liquid and ice clouds, demonstrating the increased radiative significance of the liquid clouds. [ABSTRACT FROM AUTHOR]

This work investigates the characteristics of submeso motions and their influence on the dynamics of the atmospheric boundary layer in different stability conditions. Submeso motions generally refer to a complex mix of non-stationary stochastic processes of different nature on scales that separate the main turbulent eddies (few meters in stable conditions) and the smallest mesoscale motions (few kilometres). Submeso motions can take a variety of forms including gravity waves, density currents or drainage flows. During low-speed conditions submeso activities manifest themselves as horizontal meandering of the wind velocity vector. Meandering generally refers to large non-turbulent oscillations of the horizontal wind velocity components, associated to the oscillation of the wind direction and temperature. Although horizontal meandering is typically associated to stable stratifications of the boundary layer, wavelike oscillations associated to low turbulent activity have also been observed in diurnal conditions. Some insight in the understanding of the meandering dynamic has been achieved, but the causes of the meandering phenomenon and its complex interactions with turbulence are still poorly understood. While these modes may have a strong impact on the horizontal transport of pollutants in the presence of weak and intermittent turbulent mixing, their accurate parameterisation within weather and climate models remains an open question. In this work we study the role of radiative forcing on the generation of horizontal meandering in different stability conditions. To this purpose, measurements of wind velocity components, temperature, radiation, CO2 and water vapour concentrations carried out at two distinct sites (Santa Maria, Brazil; Ny-Ålesund, Svalbard, Arctic) were analysed. To detect meandering occurrences and their characteristic time‐scales Eulerian autocorrelation functions and spectral analysis of the wind components and temperature are employed. Horizontal meandering is identified by a clear negative lobe in the Eulerian autocorrelation functions corresponding to a peak located at low-frequency in the spectra of wind components and temperature. The performed analysis confirmed that horizontal modes represents a distinctive feature of the very stable boundary layer, characterised by a strong radiative cooling, low wind conditions and a weak and intermittent mechanical turbulent production. Horizontal meandering has been observed also during daytime in presence of low positive radiative forcing, often related to cloudy conditions and weak large scale flows. In all atmospheric conditions meandering appears strongly correlated to scalar oscillations, confirming its important role in the horizontal transport of pollutant in low-wind conditions. [ABSTRACT FROM AUTHOR]