Understanding the Observed Bimodality of Arctic Winter Boundary Layer States

The Arctic is experiencing rapid warming and sea ice loss. Climate models disagree on the magnitude of projected Arctic warming by the end of the 21st century, ranging from 5.3 to 11.2℃. Among the many factors that contribute to this uncertainty, Arctic clouds remain a key player: climate models disagree on the sign of the net Arctic cloud feedback due to uncertainties in parameterized cloud-scale processes. In winter, Arctic clouds provide a positive radiative effect on the surface energy budget. The observed Arctic winter boundary layer is characterized by a bimodal distribution of the surface net longwave radiation and low-level static stability. A cloudy state is associated with near-zero net longwave radiation and lower static stability, while a clear state with net longwave radiation lower than 40 W/m2 and higher static stability. We use observational and reanalysis data to investigate the vertical structure of these distinct states in order to shed light on the coupling between sea ice, Arctic cloud radiative effects, and boundary layer turbulence.