Dynamical and chemical characteristics of tropospheric intrusions observed during START08

Intrusions of tropospheric air into the extratropical stratosphere above the subtropical jet potentially have a significant impact on the composition of the lower stratosphere (the "middleworld"). We present an analysis of tropospheric intrusion events observed during the Stratosphere-Troposphere Analyses of Regional Transport 2008 (START08) experiment using kinematic and chemical diagnostics. The transport processes operating during each event are discussed using high-resolution model analyses and backward trajectory calculations. Each intrusion observed during START08 can be related to a Rossby wavebreaking event over the Pacific Ocean. Trajectory analysis shows that the intruding air masses can be traced back to the tropical upper troposphere and lowermost stratosphere. In situ chemical observations of the tropospheric intrusions are used to estimate the mixing timescales of the observed intrusions through use of a simple box model and trace species with different photochemical lifetimes. We estimate that the timescale for an intrusion to mix with the background stratospheric air is about a week. Trace gas distributions and correlations are consistent with the dynamics of the high-resolution NCEP GFS analyses, suggesting that these features are captured by the GFS assimilation and forecast system. A global analysis of intrusion events observed during the START08 time period (April-June 2008) is also given.