ATMOSPHERIC TRANSPORT PROCESSES
453
eyclonic and cyclonic flow derived for zonal motion. The anticyclonic
shears shown in Fig. 3a, measured to the south of the jet maximum,
are of the order of 1.4x10‘*/sec. This suggests negative values of
absolute vorticity and dynamic instability in this area.
According. to Eq. 5 limiting shears of 0.643 x 107*/sec should pre-
vail at the geographic latitude of the Great Lakes. Thus cyclonic dy-
namic instability should be expected there.
From Figs. 2, 3a, 3b, and 4 [Fig. 4 shows a cross section through
the jet stream from Green Bay, Wis., to Oklahoma City, Okla., at 12
Greenwich civil time (GCT) on Nov. 22], we may concludethat there
is a marked difference between wind profiles on an isobaric surface
and on an isentropic surface. Isobaric profiles show strong cyclonic
and weaker anticyclonic shears. Usually the jet axis is indicated by a
sharp peak in wind speeds. Isentropic profiles, when taken along a
potential-temperature surface contained within the stable layer underneath the jet core, show a sharp anticyclonic shear, a weaker cyclonic
shear, and a “platform” of almost uniform winds inside the stable
layer. This is indicated schematically in Fig. 5. Also the locations of
jet axes on isentropic charts may be quite different from those on isobaric surfaces, as may be seen from Fig. 4.
Figures 3a and 3b show a splitting of the jet stream. One branch
moves toward the northeast with the air contained in the middle and
upper troposphere. The other moves toward the south and is characterized by strong decelerations and a sinking motion, It is this branch
which carries radioactive debris to the ground.
TROPOPAUSE
ISENTROPIC
SURFACES
STRATOSPHERE
DEBRIS
r TRAJECTORY
500 MB
FRONTAL ZONE
Fig. 2—Schematic section through a jet stream and its associated
““et-stream front.”