Variation in PSC Occurrence Observed with ILAS-II over the Antarctic in 2003

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. IntroductionPolar stratospheric clouds (PSCs) consist mainly of nitric acid and water/ice and form in winter and early spring in the lower stratosphere over both polar regions (e.g., Solomon 1999). PSCs play a crucial role in ozone depletion processes; they provide surfaces for heterogeneous reactions that convert inactive chlorine into active chorine (e.g., Solomon 1999) and cause denitrification (e.g., Fahey et al. 1990) and nitrification (e.g., H ubler et al. 1990). Activated chlorine causes severe ozone depletion, especially in the Southern Hemisphere (SH), which is known as the "ozone hole". The climatology of PSC frequency in the SH was examined using aerosol data from the Stratospheric Aerosol Measurement (SAM) II from 1978to 1989(Poole and Pitts 1994, data from the Polar Ozone and Aerosol Measurement (POAM) II from 1994 to 1996 (Fromm et al. 1997), and data from multiple solar occultation sensors from 1978 to 2000 .Poole and Pitts (1994) examined the relationship between PSC frequency and temperature and showed that PSC formation in the SH is less likely later in PSC season than earlier. Other studies (e.g., Watterson and Tuck 1989;Hervig et al. 1997;Mergenthaler et al. 1997) have suggested that PSC formation late in PSC season in the SH is strongly affected by preceding denitrification and dehydration. However, there have been few studies on the variability of the relationship between PSC frequency and temperature over the course of a PSC season that are based on extensive observations of aerosols, nitric acid, and water vapor in the SH.Against this background, we used data obtained by the PSC identification and TNAT calculationPSCs were identified from ILAS-II AEC data over the SH using methods similar to those for ILAS data in the NH (Hayashida et al. 2000). First, averages and standard deviations for ILAS-II AEC data at te...

Can gravity waves significantly impact PSC occurrence in the Antarctic?

“…This study examines whether the enhanced PSC occurrence observed can be explained by gravity wave motions. It should be noted that previous work by Saitoh et al (2006) has identified a similar enhanced PSC occurrence in June 2003 compared to that identified by temperature thresholds using ILAS-II data. An enhanced level of PSC occurrence in June 2003 was also observed and modelled by Höpfner et al (2006).…”

“…This is a strong validation of the results presented in this study since Saitoh et al (2006) uses the mean plus five standard deviations from the ILAS-II aerosol extinction data to identify PSC, an extremely conservative threshold.…”

“…3 and 5. Saitoh et al (2006) also suggests that PSC frequency depends on the degree of denitrification later in the season. Their study also indicates that T NAT is not a good measure of PSC occurrence especially above 20 km in late winter.…”

“…In order to examine the importance of temperature and the impact of gravity wave temperature perturbations we determine the probability that a region is below various threshold temperatures relative to the frequency of PSC observations made with POAM III. This method has previously been used by many authors, including Poole and Pitts (1994) and Saitoh et al (2006).…”

Dehydration in the tropical tropopause layer estimated from the water vapor match

“…In the troposphere and stratosphere, it plays an important role in the radiation balance of the earth (e.g., Forster and Shine, 1999;Solomon et al, 2010), and it influences atmospheric chemistry via its product, the hydroxyl radical (OH) (Stenke and Grewe, 2005), and via polar stratospheric clouds (PSCs) (Saitoh et al, 2006).…”