The dependence of contrail formation on weather pattern and altitude in the North Atlantic

1. The dependence of contrail formation on weather pattern and altitude in the North Atlantic Emma Irvine, Keith Shine, Brian Hoskins Meteorology Department, University of Reading Contact: e.a.irvine@reading.ac.uk

2. Motivation • The climate impact of persistent contrails and contrail cirrus could be comparable to CO2 impact (e.g. Burkhardt and Kärcher, 2011) • Persistent contrails only form when aircraft fly through cold ice-supersaturated regions (ISSRs) in the upper-troposphere • an AVOIDABLE climate impact of aviation Lee et al (2009)

3. Motivation • Climate optimal aircraft routing: find the route which minimises the climate impact of the aircraft’s emissions on a flight-by-flight basis (REACT4C project) • Aircraft routes depend on the large-scale weather pattern • What are preferred locations and altitudes for ISSRs? New York London

4. Climatological ISSR frequency overthe North Atlantic in ERA-Interim 300 hPa, FL300 250 hPa, FL340 200 hPa, FL390 • Qualitatively reproduces satellite climatologies (Spichtinger 2003; Lamquin 2012) • Case studies show ISSRs are associated with anticyclonic flow (Kästner 1999; Immler 2008) and warm conveyor belts (Spichtinger 2005)

5. Methodology • Identify cold ice-supersaturated regions (ISSRs) in ERA-Interim as regions where temperature < 233 K, relative humidity w.r.t. ice > 100 % • Two methods: • Synoptic weather pattern analysis links contrail formation conditions to specific meteorological features (Irvine et al., 2013 Met Apps and Irvine et al., 2012 GRL) • Lagrangiantrajectories are used to investigate long-lived ice-supersaturated air which can support long-lived contrails (Irvine et al., 2013 JGR – in review)

6. Cold ISSR frequency by winterweather pattern and altitude 300 hPa 250 hPa 200 hPa W1. Zonal jet W2. Tilted jet W4. Confined jet Eastbound routes Westbound routes % • Location linked to various features: jet stream, Greenland, ridges • Altitude distribution depends on weather pattern

7. Probability of persistent contrail formation along a route Eastbound W1. Zonal jet W2. Tilted jet Westbound W4. Confined jet Eastbound routes Westbound routes • Flying higher does not always produce fewer contrails!

8. Trajectory Analysis • Trajectories released on 1°x1°grid covering north Atlantic, from 200 hPa, 250 hPa and 300 hPa • Lagrangian trajectory code (Methven, 1997) run on ERA-Interim data • Generated 249,874 trajectories with ice-supersaturation (winter) T+0 T+48 h T-48 h

9. Trajectories of air that becomes ice-supersaturated over the UK 24 h before… 24 h after… • For trajectories starting in the troposphere with ice-supersaturation duration of at least 24 h • Air that becomes ice-supersaturated over the UK comes from the south-west and moves north-eastwards.

10. Comparison of longer-lived ISS air with shorter-lived ISS air • Long-lived ISS air comes from a more southerly direction and has a slower speed than short-lived ISS air • This suggests that a greater proportion of long-lived ISS air is associated with high-pressure ridges rather than the jet stream Direction air comes from Speed of air (along trajectory) W S

11. Summary • ERA-Interim re-analysis data are used to analyse regions where persistent contrails could be formed, for the north Atlantic region. • Preferred locations for persistent contrail formation are in regions of uplift associated with the jet stream, around the northern periphery of high-pressure ridges, and over Greenland. • The formation of contrails shows a strong dependence on altitude in a given weather pattern • Air parcels which remain ice-supersaturated for long periods (and therefore can support long-lived contrails) may be associated with certain weather features, e.g. high-pressure ridges

12. Thank you! Information from: e.a.irvine@reading.ac.uk www.react4c.eu

13. Dependence of route latitude on the jet stream Eastbound: New York - London fly in the jet stream Westbound: London - New York avoid the jet stream Irvine et al., 2012, Meteorological Applications, in press

14. Probability of persistent contrail formation along a route W GC Estimates of contrail formation are very sensitive to route location! E • Flying higher forms LESS contrails (type W1, both directions) • Flying higher forms MORE contrails (types W2 and W3 eastbound)

15. Duration of ice-supersaturation ALL ISS points Tropospheric ISS Stratospheric ISS • Median duration of ice-supersaturation is less than 6 hours • % ice-supersaturated air parcels that have duration of at least 24 h: • 5% (tropospheric air) • 23% (stratospheric air)