The transport of stratospheric air deep into the troposphere via convection is
investigated numerically using the UK Met Office Unified Model. A convective system
that formed on 27 June 2004 near southeast England, in the vicinity an upper level
potential vorticity anomaly and a lowered tropopause, provides the basis for analysis.
Transport is diagnosed using a stratospheric tracer that can either be passed through or
withheld from the model’s convective parameterization scheme. Three simulations are
performed at increasingly finer resolutions, with horizontal grid lengths of 12, 4, and 1 km.
In the 12 and 4 km simulations, tracer is transported deeply into the troposphere by the
parameterized convection. In the 1 km simulation, for which the convective
parameterization is disengaged, deep transport is still accomplished but with a much
smaller magnitude. However, the 1 km simulation resolves stirring along the tropopause
that does not exist in the coarser simulations. In all three simulations, the concentration of
the deeply transported tracer is small, three orders of magnitude less than that of the
shallow transport near the tropopause, most likely because of the efficient dilution of
parcels in the lower troposphere
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