Antarctic clouds, supercooled liquid water and mixed-phase investigated with DARDAR: geographical and seasonal variations

Antarctic tropospheric clouds are investigated using the radar-lidar DARDAR (raDAR/liDAR)-MASK products. The cloud fraction is divided into the supercooled liquid water (SLW)-containing clouds and the all-ice clouds. The low-level SLW fraction varies according to temperature and sea ice fraction. It is the largest over water. In East Antarctica, the SLW fraction decreases sharply polewards. It is twice to three times higher in West Antarctica. The all-ice cloud geographical distribution is shaped by the interaction of the main low-pressure systems surrounding the continent and the orography, with little links with sea ice fraction. We demonstrate the largest impact of sea ice on SLW (mostly mixed-phase clouds, MPC) in autumn and winter, while it is almost null in summer and intermediate in spring. Monthly variability of MPC shows a maximum fraction at the end of summer and minimum in winter. Conversely, the unglaciated (pure) SLW (USLW) fraction has a maximum at the beginning of summer. Monthly evolutions of MPC and USLW fractions do not differ on the continent. This demonstrates a seasonality in the glaciation process in marine liquid-bearing clouds. From the literature, we identify the pattern of the monthly evolution of the MPC fraction as being similar to the one of the aerosols, which is related to marine biological activity. Marine bioaerosols are known to be efficient Ice Nucleating Particles (INPs). The emission of these INPs into the atmosphere from open waters would come on top of the temperature and sea ice fraction seasonalities as factors explaining the mixed-phase clouds monthly evolution.