Analysis of PSC Formation using Parcel Temperature Histories

Abstract

Polar Stratospheric Clouds (PSCs) are composed of HSO4/HNO3/H2O aerosols called supercooled ternary solution (STS), solid nitric acid trihydrate (NAT) particles and ice particles. These clouds require the low temperatures found in the polar regions of the lower stratosphere to occur. PSCs are important due to the critical role they play in the destruction of ozone over the poles. The role of PSCs in ozone destruction has been understood since the 1980’s however the mechanisms involved in PSC formation, particularly NAT formation, is still not well understood. Typically formation of PSCs in climate models is handled by using a temperature threshold as a proxy. A commonly used threshold is the equilibrium temperature for the existence of NAT, TNAT ; however when the area with temperatures below TNAT are compared with observations of PSC it is found that this measure significantly over-estimates the abundance of PSC. This research uses back trajectories stemming from the site of PSC observations to investigate the effect that the temperature history has on PSC formation. The trajectories are calculated using a simple Lagrangian model on an isentropic surface. The observational PSC data is obtained from the CALIOP (Cloud Aerosol Lidar with Orthogonal Polarization) instrument, which is from the CALIPSO (Could Aerosol Lidar and Infrared Pathfinder Satellite Observations) mission launched in 2006. The CALIOP data used in this research is taken from the June to September period from the years 2007-10 and for latitudes between -55◦ and -82◦S, and totals in excess of 7 million individual measurement profiles, a four day back trajectory is calculated for each observation along the 450 K isentropic level. Temperature and wind data is from the MERRA (Modern Era Retrospective-analysis for Research and Applications) reanalysis and H2O and HNO3 concentrations (required for calculating equilibrium temperatures) are from Earth Observing System (EOS) Aura Microwave Limb Sounder (MLS) version 3.3. It was found that different PSC types had different characteristics in their temperature histories. The mean temperature history associated with ice observations cooled by around 3 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. Based on these findings and theories of PSC formation, a system for predicting PSC occurrence based on the temperature histories is proposed. This system provides a better approximation for the total extent of PSC occurrence than the TNAT threshold. The average area covered by PSC on the 450 K level over the period examined as observed by CALIOP was 7.6 million km2 while the area predicted by the temperature histories was 8.6 million km2 as compared to 20.4 million km2 with temperatures below TNAT. The average geographical coverage of the different PSC types is reasonably well replicated by the temperature history classification; however, the ability to discriminate between different PSC types based on individual temperature histories is somewhat limited.