Abstract
The Improved Limb Atmospheric Spectrometer (ILAS)-II frequently observed polar stratospheric clouds (PSCs) in the Southern Hemisphere (SH) throughout the winter of 2003. Simultaneous observations of the aerosol extinction coefficient (AEC) at 780 nm, nitric acid, and water vapor data were analyzed to investigate the ambient thermodynamic conditions associated with observed PSCs. PSCs were first observed with ILAS-II at the end of May, and observed most frequently in August/September as temperatures cooled. At approximately 20 km late in the PSC season, however, PSCs were less likely to occur, despite cold temperatures, because of the lower concentration of nitric acid due to denitrification caused by sedimentation of previously occurring PSCs. The probability of PSC occurrence and the probability of ambient temperatures colder than nitric acid trihydrate (NAT) saturation temperature (TNAT) were well correlated below -20 km throughout the winter. In contrast, PSC frequency at -22 km from late August to early September was low even when temperatures were sufficiently colder than TNAT; this is, at least, partly because of the decrease in background aerosol particles in the atmosphere.
