Polar stratospheric cloud observations from CALIPSO

Polar stratospheric clouds (PSCs) act as the primary mechanism by which inert chlorine and bromine compounds are converted into active states that catalytically destroy ozone in the late winter and early spring polar stratosphere [Solomon, 1999]. The microphysical characteristics of these clouds influence the extent and duration of polar ozone depletion based on the heterogeneous reaction rates of the halogen compounds, the temperatures at which various clouds can form, and the propensity of the clouds for altering the local gas-phase nitrogen chemistry [Solomon et al., 1986; Toon et al., 1986]. PSCs traditionally have been classified into three primary types based on particle composition [Poole and McCormick, 1988; Browell et al., 1990; Toon et al. 1990], including solid nitric acid trihydrate crystals (NAT, type 1a), supercooled liquid ternary solution (STS) droplets (H2SO4-HNO3-H2O, type 1b), and water ice (type 2). Others have used mixtures of these types in varying proportions to describe PSCs. Backscatter data from the 532 nm channel of the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP), one of three instruments employed for the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) mission, are used to identify PSCs. After normalizing the lidar backscatter profiles and performing a 30-profile (~10 km) horizontal smoothing, PSC observations are identified in the average profiles as three consecutive vertical points between 15-25 km in altitude with scattering ratios > 1.2. Polarization information from the 532 nm lidar channel is then used in combination with the scattering ratio to discriminate different PSC types by composition. Average cloud temperatures are also computed using ancillary temperature data supplied with the CALIPSO data products. The occurrence frequency, horizontal distribution, and average temperatures of the PSCs for the 2006-2008 Antarctic winter seasons are investigated. Generally, the results indicate that PSCs are observed by CALIPSO more frequently in the colder temperature polar regions, that nitric acid PSCs are observed more frequently than water ice clouds (especially during the early winter season), and that the horizontal cloud area and distribution are consistent with both temperature and the polar vortex structure.