Contribution of free tropospheric aerosols to Arctic low-level cloud droplets formation and longwave radiative forcing

Abstract. Aerosol-cloud-radiation interactions are a major source of uncertainty in the Arctic climate, particularly for low-level clouds (LLC) that dominate cloud cover. This study presents in situ measurements of aerosols and cloud droplets collected with a tethered-balloon between May 16 and June 10, 2023, during the Atmospheric Rivers and the onseT of sea ice MELT campaign above sea ice in the Fram Strait. The objective was to quantify the contributions of boundary-layer and free-tropospheric sources to the cloud condensation nuclei (CCN) budget of LLCs. Above- and below-cloud observations of five LLCs showed enhanced aerosol concentrations above cloud top in four cases.

The analysis of a case study, in which the cloud was coupled to the surface, revealed a complex layered structure of aerosol properties, including multiple distinct size distributions. Aerosol concentrations above the cloud were up to four times higher than below, and measurements at the cloud-top interface indicated mixing between in-cloud and free-tropospheric air masses. Simulations of cloud droplet concentrations based on measured particle size distributions showed that including aerosols from above cloud, rather than only below, was required to achieve closure with observations. Our highly detailed observations around the cloud allowed us to demonstrate the significance of free-tropospheric CCN sources, which influence Arctic cloud microphysical and radiative properties. Not accounting for free tropospheric CCN would have resulted in a low bias in the longwave radiative forcing of 1.3 W m^-2. These findings highlight the need for systematic vertical aerosol observations and improved model representation of elevated aerosol layers.