Microphysics of Arctic Stratiform Boundary-layer Clouds during ARCSIX 

Abstract. Clouds have a major impact on rapidly decreasing sea ice in the Arctic, yet much is still unknown how cloud microphysics influences cloud development. In situ and remote data were collected by the NASA P-3 and SPEC Inc. Learjet research aircraft in Arctic stratiform boundary-layer clouds over the oceans and sea ice bordering northern Greenland between 25 May and 15 August 2024 during the ARCSIX project. Both aircraft carried a suite of nearly identical state-of-the-art microphysical sensors. Additionally, the P-3 was equipped with aerosol and remote-sensing instrumentation and the Learjet was equipped with a zenith/nadir Ka-band radar. The total length of clouds examined remotely and in-situ by the two aircraft totaled 12,417 km, with 6,266 km of in-situ measurements. Mixed-phase clouds were sampled during 60.5 % of time in cloud, and all-liquid clouds were measured 39.5 % of the time. Cloud- top temperatures were ≥ - 9 °C during 90 % of the stratiform boundary-layer cloud investigations. Single-layer mixedphase clouds sampled with cloud-top temperatures ≥ - 4 °C often contained concentrations of ice particles more than five orders of magnitude higher than measured concentrations of ice-nucleating particles. Despite the high ice concentrations, microphysical conditions supporting secondary ice production were not always present. In contrast, in some clouds where environmental conditions met commonly accepted criteria for secondary ice production, ice particle concentrations were closer to what is expected from primary nucleation. The quality of measurements was unprecedented, but results from our preliminary analyzes raise more questions about primary and secondary nucleation mechanisms than they answer.