Terrestrial Influences on Summertime Polar Mixed Phase Cloud Freezing

Terrestrial aerosols such as dust can act as ice nucleating particles and have been proposed to explain why clouds at similar temperatures are substantially icier over the Arctic than over the Antarctic. However, so far, models provide no consensus on the magnitude or even the sign of dust effects on Arctic clouds. Moreover, directly attributing interpolar differences in cloud phase to aerosol-cloud interactions using observations can be challenging due to factors such as differing and co-varying meteorology. Here we aim to shed light on some of these questions using a combination of CloudSat/CALIPSO satellite data, aerosol reanalysis and model products, and aircraft data from NASA’s 2024 ARCSIX aircraft campaign in Northern Greenland. Based on the satellite data, dust is robustly associated with glaciation and cloud radiative cooling effects during summer, but the cloud radiative effects are observed at temperatures warmer than -10 C where the inorganic minerals in dust are no longer expected to be efficient ice nucleating particles. Satellite-derived statistics on the fraction of Arctic clouds that are glaciated at warmer temperatures (> -6 C) show concentrations in “hot spots” along coasts, suggesting a strong local terrestrial influence. However, the spatial locations of higher warm-cloud glaciation do not match well with CALIPSO dust concentrations. Taken together, this evidence suggests that a terrestrial aerosol source might be impacting cloud glaciation at warmer temperatures, perhaps in concert with secondary ice production across a regional scale. INP sources may include, for example, primary organics internally mixed with inorganic minerals, sourced from permafrost sediment and/or glacial dust. Lastly, we use satellite data to compare cloud glaciation differences between the poles within highly similar meteorological regimes. This analysis provides constraints on the extent to which terrestrial aerosols are impacting mixed phase cloud freezing and related cloud radiative effects in the Arctic.