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The Occurrence of Polar Stratospheric Cloud over the Antarctic

The Occurrence of Polar Stratospheric Cloud over the Antarctic
Fraser Dennison

Authors

Dennison F
University of Canterbury

McDonald A
University of Canterbury

Alexander S
Australian Antarctic Division

Abstract

Polar stratospheric clouds (PSC’s) are one of the factors responsible for ozone depletion. Chlorine and bromine reservoir species in the lower stratosphere undergo a heterogeneous reaction on the surface of PSC particles and are converted into products which are then photolyzed into ozone destroying radicals. Current theory has PSC’s forming whenever the temperature falls below a certain threshold, TNAT, the equilibrium temperature for the formation of nitric acid trihydrate (NAT) – one of the constituents of PSC. However this assumption leads to overestimations of PSC coverage that are often over two times greater than observations.

The aim of this research is to investigate the usefulness of the current thresholds and develop a more accurate empirical prescription for the formation of a PSC based on the temperature histories of air parcels calculated using a simple trajectory model.

It was found that different PSC types had different characteristics in their temperature histories. The mean temperature history associated with ice PSC observations cooled by around 3-4 K over the previous 24 hours to slightly below the water ice frost point at the time of observation. However, in trajectories associated with the presence of NAT the temperature has on average stayed 4-5 K below TNAT for the entire four days of the trajectory. Using the air parcel trajectory it was possible simulate the occurrence of ice and NAT PSC more accurately than by simply using temperature thresholds as formation criteria.

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Key dates

  • 11th June 2013
    Registrations close
  • 21st June 2013
    Registrations at the AAD open for staff
  • 24th June 2013
    Registrations at the venue open
  • 24th June 2013
    Conference commences
  • 26th June 2013
    Conference concludes

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This page was last modified on 2 September 2013.