Geography Reference
In-Depth Information
tracer mass is contained in the troposphere and lower stratosphere, and the lifetime
is determined mainly by the (slow) flux of the tracer to altitudes where the chemical
timescales become comparable to or shorter than dynamical timescales.
12.7.3
Transport in the Stratosphere
Transport processes are conveniently divided between those processes that
involve mean motions of the atmosphere, or
advection,
and those processes that
may be characterized as turbulent, or diffusive in nature. In the case of point sources,
such as volcanic eruptions, the distinction is quite clear; advection moves the cen-
ter of mass of the plume along the direction of the average wind, whereas turbulent
diffusion disperses the plume in the plane orthogonal to the average wind. On a
global scale, however, the distinction between advective and diffusive processes
is not always clear. Because the atmosphere is characterized by spatially and tem-
porally varying motions with a wide range of scales, there is no obvious physical
separation between “mean” and “turbulent” motions. In practice, those transport
processes that are explicitly resolved by the particular observational network or
transport model being utilized are often regarded as the advective motions, whereas
the remaining unresolved motions are assumed to be diffusive. The effects of the
unresolved motions must then be parameterized in terms of the mean motions
in some fashion. Usually this involves assuming that tracer fluxes by the unre-
solved motions are proportional to the gradient of the resolved tracer distribution.
However, this approach is not always physically justified. A major problem in the
modeling of global transport is accurate representation of the contribution of the
unresolved eddy motions to the total transport.
As discussed in Section 12.2, the global-scale residual meridional circulation
in the middle atmosphere is driven by wave-induced zonal forces associated with
Rossby waves and gravity waves. Not surprisingly, the residual circulation plays an
essential role in the meridional and vertical transport of trace chemical constituents
in the middle atmosphere. Additionally, the waves responsible for the zonal force
that drives the residual circulation are also responsible for the quasi-isentropic
stirring and mixing that is associated with wavebreaking. Thus, understanding of
transport involves both eddy and mean-flow transport effects.
In dynamical studies it is usual to characterize a chemical constituent by the
volume mixing ratio (or mole fraction), defined as χ
n
T
/n
A
, where n
T
and n
A
designate the number densities (molecules m
−
3
) for the trace constituent and air,
respectively. The mixing ratio is conserved following the motion in the absence of
sources and sinks and hence satisfies the simple tracer continuity equation
≡
Dχ
Dt
=
S
(12.46)
where S designates the sum of all chemical sources and sinks.
