### The dynamical and transport properties of the
extratropical tropopause (with
Professor P.H. Haynes)

The tropopause is among the most
fundamental structures in the atmosphere. It is the interface between the
water vapour-rich and dynamically active troposphere below and the
ozone-rich and relatively quiescent stratosphere above. Traditional
radiative-dynamical theories suggest that the position of the tropopause
is determined by radiative-convective equilibrium in tropical latitudes.
But this theory is not really consistent in the extratropics and we require
a new theory to explain the position of the tropopause here.
Dr I.Held (GFDL, Princeton)
has already suggested that the position of the extratropical tropopause
is determined by baroclinic eddies, which are the predominant form
of large-scale motion in the extratropical troposphere. Professor Haynes and
I are using numerical models, new diagnostic tools, and
analytical techniques to try to assess Held's suggestion.
I have developed a numerical model which solves the three-dimensional
quasigeostrophic equations in a channel geometry, which is the simplest
realistic model for baroclinic eddy activity in the extratropical atmosphere.
I have applied a new transport diagnostic (the effective diffusivity
of an advected-diffused tracer field, introduced recently by Nakamura)
to this model. There is a direct connection between the transport
properties of the flow and its dynamical structure because the key
dynamical quantity, potential vorticity (PV), from which all other
dynamical fields can be derived, is nearly materially conserved.
The action of the baroclinic eddies can be seen as the stirring
of PV, with the tropopause representing the upper limit of the region
of stirring. In this sense the tropopause is a barrier to transport,
separating a well-stirred region below from a less well-stirred region
above. The effective diffusivity diagnostic in our simulations reveals
a very robust barrier structure as the result of the baroclinic
activity in our model. We believe that this barrier structure is
analogous to the extratropical tropopause and that a study of
its behaviour will lead to an improved understanding of tropopause
dynamics. We have examined the dependence of the barrier structure
on model parameters, including the radiative relaxation rate,
the Ekman friction in the surface layer, the supercriticality of the flow,
and the structure of the relaxation state. Our results are
presented in a paper `Transport barriers and jets in multi-layer
baroclinically unstable flows', a preprint of which will be
available shortly.