Microphysical properties and thermodynamic phase of Arctic low-level clouds from in-situ aircraft measurements

Abstract

The Arctic region is experiencing the most pronounced mean temperature rise of any region on Earth, causing drastic changes in the regional and global climate. Current investigations seek to elucidate the processes responsible for the intensified anthropogenic temperature change. Clouds in particular are suspected to play a crucial role in the Arctic climate feedback mechanisms. Clouds cool or warm the surface, depending on their ambient condition, microphysical properties, and thermodynamic phase. The gap in knowledge of microphysical cloud processes is particularly pronounced for mixed-phase clouds, which contain supercooled droplets with coexisting ice crystals and are frequently encountered in the lower part of the atmosphere at high latitudes. To better assess the role of clouds in the Arctic climate system, a comprehensive in-situ cloud data set of low-level Arctic clouds was measured within the scope of this thesis, using an advanced setup of airborne in-situ cloud probes. The airborne in-situ cloud measurements were carried out over the northern Fram Strait between Greenland and Svalbard in spring 2019, summer 2020, and spring 2022. In total, 2676 min of low-level in-situ cloud observations were performed during 33 research flights above the sea ice and the open Arctic ocean with the research aircraft Polar 5 and Polar 6 of the Alfred Wegener Institute. At first, the in-situ cloud data from spring 2019 and summer 2020 are combined to investigate the distribution of particle number concentration N, effective diameter Deff, and cloud water content CWC (liquid and ice) of Arctic low-level clouds, measured at latitudes between 76 °N and 83 °N. A method is developed to quantitatively derive the occurrence probability of their thermodynamic phase from the combination of microphysical cloud probe and Polar Nephelometer data. The changes in cloud microphysics and cloud thermodynamic phase are investigated related to the ambient meteorological situation in spring and summer, and the effects of surface conditions, including sea ice or open ocean, on low-level clouds are revealed. A median N from 0.2 cm−3 to 51.7 cm−3 is found, with about two orders of magnitude higher N for mainly liquid clouds in summer compared to ice and mixed-phase cloud conditions measured in spring. A southward directed air mass flow from the sea ice in cold air outbreaks dominates cloud formation processes at temperatures below -10 °C in spring. In contrast, northward directed warm air intrusions favor the formation of liquid clouds at warmer temperatures in summer. The median CWC is higher in summer (0.16 g m−3) than in spring (0.06 g m−3), as this is dominated by the available atmospheric water content and the temperatures at cloud formation level. Significant differences in the particle sizes in spring and summer are observed, as well as an impact of the surface conditions, which modify the heat and moisture fluxes in the boundary layer. Analyses of the cloud thermodynamic phase show that the mixed-phase state is the dominant thermodynamic cloud phase in spring, with a frequency of occurrence of 61% over the sea ice and 66% over the ocean. In summer, the cloud particles are most likely in the liquid state. In a subsequent study on Arctic low-level mixed-phase cloud conditions, the microphysical properties suggest a distinction between classic mixed-phase clouds and mixed-phase haze. The microphysical composition of this mixed-phase haze is similar to that of classic mixed-phase clouds. However, the supercooled droplets are replaced with large (> 2.8 µm) wet aerosol particles, and N is reduced by more than a factor of 150 in comparison to classic mixed-phase clouds. Further results show an increase of N in the atmospheric boundary layer over the sea ice compared to the open ocean, likely due to increased particle formation processes originating from the sea ice. The results of this work enhance our understanding of the microphysical processes and thermodynamic phase composition of Arctic low-level clouds and will contribute to improve cloud parameterizations in climate and weather models. The findings will help to assess the role of low-level clouds in the Arctic radiation budget and to quantify their feedback mechanism in the region of the world with the strongest anthropogenic climate change.