Abstract. Upper-level fronts are often associated with the rapid transport of
stratospheric air along tilted isentropes to the middle or lower troposphere,
where this air leads to significantly enhanced ozone concentrations. These
plumes of originally stratospheric air can only occasionally be observed at
the surface because (i) stable boundary layers prevent an efficient vertical
transport down to the surface, and (ii) even if boundary layer turbulence
were strong enough to enable this transport, the originally stratospheric air
mass can be diluted by mixing, such that only a weak stratospheric signal can
be recorded at the surface. Most documented examples of stratospheric air
reaching the surface occurred in mountainous regions. This study investigates
two such events, using a passive stratospheric air mass tracer in a mesoscale
model 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-June
2006 on the Tibetan Plateau. In both cases, a tropopause fold associated with
an upper-level front enabled stratospheric air to enter the troposphere. In
our model simulation of the North American case, the strong frontal zone
reaches down to 700 hPa and leads to a fairly direct vertical transport of
the stratospheric tracer along the tilted isentropes to the surface. In the
Tibetan Plateau case, however, no near-surface front exists and a reservoir
of high stratospheric tracer concentrations initially forms at 300–400 hPa,
without further isentropic descent. However, entrainment at the top of the
very deep boundary layer (reaching to 300 hPa over the Tibetan Plateau) and
turbulence within the boundary layer allows for downward transport of
stratospheric air to the surface. Despite the strongly differing dynamical
processes, stratospheric tracer concentrations at the surface reach peak
values of 10 %–20 % of the imposed stratospheric value in both
cases, corroborating the potential of deep stratosphere-to-troposphere
transport events to significantly influence surface ozone concentrations in
these regions.
Sub-seasonal Variability in the Boundary Layer Sources for Transport into
the Tropopause Layer in the Asian Monsoon Region