Influence of free tropospheric aerosols on the microphysical properties of a coupled low-level cloud in the central Arctic: a case study from the ARTofMELT expedition.

Low-level mixed-phase clouds (LLMPCs) containing both ice and supercooled liquid water are ubiquitous in the Arctic and play a crucial role for radiative fluxes. Mixed-phase clouds are inherently instable due to the competition over water between ice crystals and liquid droplets. The persistence of LLMPCs in the Arctic is challenging to reproduce in models, which affects predictions of the energy budget. These challenges stem from current uncertainties related to the complex web of interactions between aerosol particles, cloud microphysics, atmospheric (thermo-)dynamics and surface processes. These uncertainties can be partly attributed to the scarcity of detailed observations of aerosol characteristics and cloud microphysical properties as well as their interactions within clouds.

LLMPCs highly depend on the presence of aerosol particles acting as cloud condensation nuclei (CCN) or ice nucleating particles (INPs). In the Arctic, where aerosol concentrations can be very low, small changes in aerosol properties (e.g., size, chemical composition, hygroscopicity) can significantly impact the radiative properties and lifetime of LLMPCs. Accurate knowledge of aerosol characteristics at cloud level and their influence on cloud properties is crucial for improving the representation of clouds and their radiative behavior.

To address the need for more observations of aerosols and cloud properties at cloud level, a tethered-balloon equipped with the Modular Multiplatform Compatible Air Measurements System (MoMuCAMS) was deployed from the Swedish icebreaker Oden during the Atmospheric River and onset of sea ice melt (ARTofMELT) expedition. The expedition took place in the Fram Strait during the transition from spring to summer (May – June) in 2023. In total, 23 flights up to 645 m above mean sea level were performed, collecting unique and detailed measurements of aerosol and cloud droplet size distributions from 8 nm to 50 µm, below, inside and above LLMPCs. A key aspect addressed with the measurements is how boundary layer and free tropospheric aerosols contribute to the formation of clouds.

Results from of a case study examining the data collected from three consecutive flights through a single cloud layer located between roughly 150 and 350 meters above the surface will be presented. A combination of in situ vertical and surface-based measurements with remote sensing data and modeling studies is used to understand to what extent aerosols from below and above the cloud contribute to the formation of cloud droplets. Results indicate that the cloud is coupled to the surface and profiles of particle number size distributions show a homogenous distribution between the surface and the cloud. A comparison between estimated CCN concentrations below and above the cloud and observed cloud droplet number concentrations suggests however, that entrainment of aerosol from the free troposphere is needed to produce the amount of droplets observed.

These observations suggest that also when clouds are coupled to the surface, a different and significant source of CCN and INPs can feed the cloud from above, which is then not observable from surface-based measurements. This can have important implications for the representation of cloud microphysical and radiative properties based solely on surface-based observations.