Isolating aerosol impacts on cloud properties and the surface radiative budget during an extreme Arctic warm air mass intrusion

Arctic warm air mass intrusions, events characterised by the transport of strong heat and moisture anomalies from the mid-latitudes into the Arctic, have received increasing attention in recent years because of their pronounced impacts on the Arctic regional climate. However, the influence of anthropogenic aerosols transported within these intrusions on cloud properties and the Arctic radiative budget remains poorly constrained. In this study, we investigate a well-characterised warm air mass intrusion with exceptionally high aerosol loading observed during the MOSAiC expedition in spring 2020. Using the WRF-Chem-Polar model, we simulate the April 2020 event both with and without the observed anthropogenic aerosol transport, in order to isolate and quantify the aerosol impacts relative to those of the warm air mass itself.

 

We analyse the effects of the anthropogenic aerosols on simulated cloud microphysical and macrophysical properties, including cloud fraction, cloud droplet number concentration, droplet size, and cloud liquid water content, as well as the resulting cloud radiative effects at the surface. The presence of anthropogenic aerosols leads to enhanced cloud droplet formation within the plume, smaller droplet sizes, and suppressed precipitation. These changes produce a net surface cooling effect, most pronounced over dark, open ocean surfaces where shortwave radiative impacts dominate. Over ice-covered regions, however, the radiative response is substantially weaker, reflecting the high surface albedo and reduced sensitivity of the surface energy budget. We find that aerosol-driven longwave warming is small and generally offset by shortwave cooling. Furthermore, the aerosol-driven shortwave cooling associated with enhanced droplet numbers is spatially heterogeneous and confined to limited regions. The broader implications of these findings for the role of aerosol-cloud interactions in Arctic regional climate are discussed.