Geography Reference
In-Depth Information
12.7
TRACE CONSTITUENT TRANSPORT
The study of global transport involves the motion of atmospheric tracers, defined
as chemical or dynamical quantities that label fluid parcels. Chemical tracers con-
sist of minor atmospheric species that have significant spatial variability in the
atmosphere. Dynamical tracers (potential temperature and potential vorticity) are
properties of the flow field that are conserved following the motion under certain
conditions. These also can be useful for interpretation of transport.
12.7.1 Dynamical Tracers
Potential temperature, defined in (2.44), can be thought of as a label for the verti-
cal position of an air parcel. Because the atmosphere is stably stratified, potential
temperature increases monotonically with height (slowly in the troposphere and
rapidly in the stratosphere as shown in Fig. 12.17) and thus can be used as an inde-
pendent vertical coordinate, the isentropic coordinates introduced in Section 1.2.7.
A parcel moving adiabatically remains on a surface of constant potential temper-
ature and can be “tagged” by its value of potential temperature. Thus the motion
of such a parcel is two dimensional when viewed in isentropic coordinates.
Potential temperature surfaces are quasi-horizontal, but they do move up and
down in physical space with adiabatic changes in temperature. Thus, it is useful
to reference trace constituent data to potential temperature, rather than pressure or
altitude, as reversible variations at local altitudes or pressures caused by adiabatic
vertical displacements associated with transient motions (e.g., gravity waves) are
then accounted for.
The other commonly used dynamical tracer, potential vorticity (PV), is con-
served for adiabatic, frictionless flow. As defined in (4.12), PV normally has a
positive gradient in the meridional direction on an isentropic surface; PV is neg-
ative in the Southern Hemisphere, zero at (or near) the equator, and positive in
the Northern Hemisphere. PV has strong gradients in both height and latitude.
Because a parcel moving in adiabatic frictionless flow conserves PV as well as
potential temperature, its motion must lie parallel to isocontours of PV on isentropic
surfaces. Rapid meridional transport implies production of strong PV anomalies
as high polar values of PV are advected equatorward or low equatorial values are
advected poleward. As discussed earlier, the meridional gradient of the background
PV field resists meridional displacements through the production of Rossby waves.
Thus, regions of strong PV gradients on isentropic surfaces can act as semiperme-
able “barriers” to transport. This PV barrier effect is one of the reasons that eddy
diffusion is often not a good model for meridional transport.
Because PV depends only on the distribution of horizontal winds and tempera-
tures on isentropic surfaces, its distribution can be determined from conventional
meteorological observations. Thus, the evolution of PV on an isentropic surface
