Transport pathways and signatures of mixing in the extratropical tropopause region derived from Lagrangian model simulations

Model simulations with the Chemical Lagrangian Model of the Stratosphere (CLaMS) driven by wind fields of the National Center for Environmental Prediction (NCEP) were performed in the midlatitude tropopause region in April 2008 to study two research flights conducted during the START08 campaign. One flight targeted a deep tropospheric intrusion and another flight targeted a deep stratospheric intrusion event, both of them in the vicinity of the subtropical and polar jet. Air masses with strong signatures of mixing between stratospheric and tropospheric air masses were identified from measured CO-O-3 correlations, and the characteristics were reproduced by CLaMS model simulations. CLaMS simulations in turn complement the observations and provide a broader view of the mixed region in physical space. Using artificial tracers of air mass origin within CLaMS yields unique information about the transport pathways and their contribution to the composition in the mixed region from different transport origins. Three different regions are examined to categorize dominant transport processes: (1) on the cyclonic side of the polar jet within tropopause folds where air from the lowermost stratosphere and the cyclonic side of the jet is transported downward into the troposphere, (2) on the anticyclonic side of the polar jet around the 2 PVU surface air masses, where signatures of mixing between the troposphere and lowermost stratosphere were found with large contributions of air masses from low latitudes, and (3) in the lower stratosphere associated with a deep tropospheric intrusion originating in the tropical tropopause layer (TTL). Moreover, the time scale of transport from the TTL into the lowermost stratosphere is in the range of weeks whereas the stratospheric intrusions occur on a time scale of days.