Seasonal variability of middle-latitude ozone in the lowermost stratosphere derived from probability distribution functions

We present a study of the distribution of ozone in the lowermost stratosphe re with the goal of characterizing the observed variability. The air in the lowermost stratosphere is divided into two population groups based on Erte l's potential vorticity at 300 hPa. High (low) potential vorticity at 300 h Pa indicates that the tropopause is low (high), and the identification of t hese two groups is made to account for the dynamic variability. Conditional probability distribution functions are used to define the statistics of th e ozone distribution from both observations and a three-dimensional model s imulation using winds from the Goddard Earth Observing System Data Assimila tion System for transport. Ozone data sets include ozonesonde observations from northern midlatitude stations (1991-1996) and midlatitude observations made by the Halogen Occultation Experiment (HALOE) on the Upper Atmosphere Research Satellite (UARS) (1994-1998). The conditional probability distrib ution functions are calculated at a series of potential temperature surface s spanning the domain from the midlatitude tropopause to surfaces higher th an the mean tropical tropopause (similar to 380 K). The probability distrib ution functions are similar for the two data sources, despite differences i n horizontal and vertical resolution and spatial and temporal sampling. Com parisons with the model demonstrate that the model maintains a mix of air i n the lowermost stratosphere similar to the observations. The model also si mulates a realistic annual cycle. Results show that during summer, much of the observed variability is explained by the height of the tropopause, Duri ng the winter and spring, when the tropopause fluctuations are larger, less of the variability is explained by tropopause height. This suggests that m ore mixing occurs during these seasons. During all seasons, there is a tran sition zone near the tropopause that contains air characteristic of both th e troposphere and the stratosphere. The relevance of the results to the ass essment of the environmental impact of aircraft effluence is also discussed .