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. Upper-level fronts are often associated with the rapid transport ofstratospheric air along tilted isentropes to the middle or lower troposphere,where this air leads to significantly enhanced ozone concentrations. Theseplumes of originally stratospheric air can only occasionally be observed atthe surface because (i) stable boundary layers prevent an efficient verticaltransport down to the surface, and (ii) even if boundary layer turbulencewere strong enough to enable this transport, the originally stratospheric airmass can be diluted by mixing, such that only a weak stratospheric signal canbe recorded at the surface. Most documented examples of stratospheric airreaching the surface occurred in mountainous regions. This study investigatestwo such events, using a passive stratospheric air mass tracer in a mesoscalemodel to explore the processes that enable the transport down to the surface.The events occurred in early May 2006 in the Rocky Mountains and in mid-June2006 on the Tibetan Plateau. In both cases, a tropopause fold associated withan upper-level front enabled stratospheric air to enter the troposphere. Inour model simulation of the North American case, the strong frontal zonereaches down to 700 hPa and leads to a fairly direct vertical transport ofthe stratospheric tracer along the tilted isentropes to the surface. In theTibetan Plateau case, however, no near-surface front exists and a reservoirof high stratospheric tracer concentrations initially forms at 300–400 hPa,without further isentropic descent. However, entrainment at the top of thevery deep boundary layer (reaching to 300 hPa over the Tibetan Plateau) andturbulence within the boundary layer allows for downward transport ofstratospheric air to the surface. Despite the strongly differing dynamicalprocesses, stratospheric tracer concentrations at the surface reach peakvalues of 10 %–20 % of the imposed stratospheric value in bothcases, corroborating the potential of deep stratosphere-to-tropospheretransport events to significantly influence surface ozone concentrations inthese regions.

A numerical process study on the rapid transport of stratospheric air down to the surface over western North America and the Tibetan Plateau