Widespread polar stratospheric ice clouds in the 2015–2016 Arctic winter – implications for ice nucleation

Christiane Voigt,Robert Baumann,Hermann Oelhaf,Benedikt Ehard,Silke M Groß,Martin Wirth,Michael C Pitts,Andreas Dörnbrack,Lamont R Poole,Björn-Martin Sinnhuber,Wolfgang Woiwode

doi:10.5194/acp-18-15623-2018

Abstract

Abstract. Low planetary wave activity led to a stable vortex with exceptionally cold temperatures in the 2015–2016 Arctic winter. Extended areas with temperatures below the ice frost point temperature Tice persisted over weeks in the Arctic stratosphere as derived from the 36-year temperature climatology of the ERA-Interim reanalysis data set of the European Centre for Medium-Range Weather Forecasts (ECMWF). These extreme conditions promoted the formation of widespread polar stratospheric ice clouds (ice PSCs). The space-borne Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument on board the CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation) satellite continuously measured ice PSCs for about a month with maximum extensions of up to 2×106 km2 in the stratosphere. On 22 January 2016, the WALES (Water Vapor Lidar Experiment in Space – airborne demonstrator) lidar on board the High Altitude and Long Range Research Aircraft HALO detected an ice PSC with a horizontal length of more than 1400 km. The ice PSC extended between 18 and 24 km altitude and was surrounded by nitric acid trihydrate (NAT) particles, supercooled ternary solution (STS) droplets and particle mixtures. The ice PSC occurrence histogram in the backscatter ratio to particle depolarization ratio optical space exhibits two ice modes with high or low particle depolarization ratios. Domain-filling 8-day back-trajectories starting in the high particle depolarization (high-depol) ice mode are continuously below the NAT equilibrium temperature TNAT and decrease below Tice∼10 h prior to the observation. Their matches with CALIPSO PSC curtain plots demonstrate the presence of NAT PSCs prior to high-depol ice, suggesting that the ice had nucleated on NAT. Vice versa, STS or no PSCs were detected by CALIPSO prior to the ice mode with low particle depolarization ratio. In addition to ice nucleation in STS potentially having meteoric inclusions, we find evidence for ice nucleation on NAT in the Arctic winter 2015–2016. The observation of widespread Arctic ice PSCs with high or low particle depolarization ratios advances our understanding of ice nucleation in polar latitudes. It further provides a new observational database for the parameterization of ice nucleation schemes in atmospheric models.

Highlights

While synoptic-scale polar stratospheric ice clouds commonly occur in the Antarctic winter stratosphere (Solomon et al, 1986), widespread ice PSCs extending over several thousand square kilometres have rarely been observed in the Arctic
First we investigate the formation of the ice, nitric acid trihydrate (NAT) and supercooled ternary solution (STS) layers of the PSC observed on 22 January 2016 using 8-day back-trajectories starting in the different PSC layers at 21.5 km altitude every 2 min
Low temperatures existed in the Arctic stratospheric winter 2015–2016 because low planetary wave activity resulted in a stable vortex

Summary

Introduction

While synoptic-scale polar stratospheric ice clouds (ice PSCs) commonly occur in the Antarctic winter stratosphere (Solomon et al, 1986), widespread ice PSCs extending over several thousand square kilometres have rarely been observed in the Arctic. Sporadic evidence for synoptic-scale ice PSCs in the Arctic is derived from in-. Arctic stratospheric temperatures are above the ice frost point temperature Tice (Murphy and Koop, 2005) on synoptic scales and limit the formation of extended ice PSCs. The reason for the warmer temperatures in the Arctic compared to the Antarctic is a more alternated land–ocean contrast in the Northern Hemisphere, supporting the generation of planetary waves, which disturb and weaken the Arctic polar vortex due to in-mixing of warmer mid-latitude air (Solomon, 2004). Radiative heating of the displaced and elongated vortex contributes to warmer Arctic vortex temperatures (Peter, 1997)

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Discussion

Conclusion