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can be used as a surrogate for the study of trace constituent transport on isentropic
surfaces in circumstances in which there are inadequate observations of relevant
chemical tracers. In addition, for dynamical studies, PV has the important property,
not shared by other tracers, that it not only is advected by the flow, but actually
determines the flow field. Thus, the distribution of PV on isentropic surfaces can be
“inverted” to yield the wind and temperature fields. Changes in the PV distribution
are then said to “induce” changes in the wind and temperature fields. In the quasi-
geostrophic case the induced changes are required to maintain geostrophic and
hydrostatic balance. More generally, such changes preserve a higher order balance
among the dynamical fields.
12.7.2
Chemical Tracers
Much can be learned about the nature of large-scale transport by considering the cli-
matological distribution of quasi-conservative chemical tracers in the atmosphere.
The distribution of such trace substances is dependent on the competition between
dynamical and chemical processes. This competition can be approximately mea-
sured by comparing the characteristic dynamical and chemical time scales. By
chemical timescale
is meant the average time for replacement or removal of a
tracer due to chemical sources and sinks. By
dynamical timescale
is meant the
average time for advective and diffusive processes to transport the tracer from
equator to pole or across a scale height in the vertical. The role of transport in
determining tracer climatology depends on the nature and distribution of tracer
sources and sinks, and on the relative magnitudes of the timescales for dynamical
and chemical processes.
If the chemical timescale is much shorter than the dynamical timescale, the
tracer will be in photochemical equilibrium and transport does not directly influ-
ence its distribution. Transport can, however, play an important indirect role by
partly determining the concentrations of other species that participate in the pho-
tochemical production or loss of the tracer in question.
If the chemical timescale is much longer than the dynamical timescale, the tracer
will be passively advected by the flow field. In the absence of localized sources
and sinks, such a tracer would eventually become well mixed due to the dispersive
effects of transport. It is for this reason that species such as nitrous oxide (N
2
O)
tend to have uniform concentrations in the troposphere and are thus not useful for
tropospheric transport studies.
When the chemical and dynamical timescales are comparable, the observed
species concentration depends on the net effects of chemical sources and sinks
and of transport. In many cases of interest, the ratio of chemical and dynamical
timescales changes drastically between the troposphere and the stratosphere, and
with altitude in the stratosphere. Tracers that have long lifetimes in the troposphere
and the lower stratosphere are referred to as
long-lived tracers
, as the bulk of the
