A Lagrangian analysis of ice-supersaturated air over the North Atlantic

Abstract

Understanding the nature of air parcels that exhibit ice supersaturation is important because they are the regions of potential formation of both cirrus and aircraft contrails, which affect the radiation balance. Ice-supersaturated air parcels in the upper troposphere and lower stratosphere over the North Atlantic are investigated using Lagrangian trajectories. The trajectory calculations use European Centre for Medium-Range Weather Forecasts Interim reanalysis data for three winter and three summer seasons, resulting in approximately 200,000 trajectories with ice supersaturation for each season. For both summer and winter, the median duration of ice supersaturation along a trajectory is less than 6 h. Five percent of air which becomes ice supersaturated in the troposphere and 23% of air which becomes ice supersaturated in the stratosphere will remain ice supersaturated for at least 24 h. Weighting the ice-supersaturation duration with the observed frequency indicates the likely overall importance of the longer duration ice-supersaturated trajectories. Ice-supersaturated air parcels typically experience a decrease in moisture content while ice supersaturated, suggesting that cirrus clouds eventually form in the majority of such air. A comparison is made between short-lived (less than 24 h) and long-lived (greater than 24 h) ice-supersaturated air flows. For both air flows, ice supersaturation occurs around the northernmost part of the trajectory. Short-lived ice-supersaturated air flows show no significant differences in speed or direction of movement to subsaturated air parcels. However, long-lived ice-supersaturated air occurs in slower-moving air flows, which implies that they are not associated with the fastest moving air through a jet stream.