Widespread persistent polar stratospheric ice clouds in the Arctic

Abstract. Despite a general decline in ozone depleting substances in the stratosphere due to the multi-national commitment to substantially reduce the emissions of their precursors, the magnitude of Arctic polar ozone loss has not decreased in recent years. Thus new observations at cold conditions can help to enhance our knowledge of polar stratospheric cloud (PSC) formation and life cycle which is of relevance for Arctic ozone loss. In the unique winter 2015/16, cold and persistent areas with temperatures below the ice frost point T_ice developed in the Arctic stratosphere, caused by reduced perturbations of the polar vortex through planetary waves. Due to these extreme conditions, unprecedented and widespread ice PSCs formed and persisted for more than a month in the Arctic. These ice PSCs were repeatedly detected by lidars on the CALIPSO satellite and on the high altitude long range research aircraft HALO. A new lower threshold of the backscatter ratio⁻¹ of 0.3 for ice PSCs derived from high-resolution lidar measurements at 532 nm wavelength leads to enhanced ice PSC coverage compared to previous analysis. The ice PSCs were generally surrounded by nitric acid trihydrate (NAT) and supercooled ternary solution (STS) clouds. By combining optical PSC data and trajectory analysis, we investigate ice formation pathways. In addition to ice nucleation in STSm with meteoric dust, we find that ice nucleation on pre-existing NAT may play an important role in polar winter. Persistent synoptic-scale Arctic ice PSCs have not been observed so far. Hence, ice PSCs are a sensitive marker for cold stratospheric winter temperatures modulated by natural variability and climate change.