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REITER AND MAHLMAN
reduces to the simple form
Q=-—DQ
(2)
The divergence is easily estimated from streamline and isotach analyses on an isentropic surface. Equation 2 indicates that a decrease of
the vorticity of an air parcel will be accompanied by divergence.
Since the flow in the vicinity of a jet maximum is characterized by
a pronounced vorticity pattern, it is easily seen that in this region we
should expect relatively strong fields of divergence and convergence
which give rise to large-scale vertical motions. This has been recognized from cross sections (classical examples of which have been
given by Palmén and Newton), as well as from isobaric analyses.
(For literature references see Ref. 10.)
Jet-stream analyses on isobaric charts, say, of the 300-millibar
(mb) surface, indicate a strong vorticity gradient, almost of a zero-
order discontinuity, in the jet axis. If absolute vorticity were considered a conservative quantity, it would be difficult to visualize air
parcels crossing the jet axis in isobaric motion. The model proposed
by Reiter’: and shown in Fig. 2 resolves this difficulty, The model
shows air parcels moving along isentropic surfaces; thus they may
descend underneath the jet axis, satisfying in good approximation
Eq. 2 as well as the theorem of conservation of potential vorticity
P=Q8+QS=0
(3)
where P is the potential vorticity and S is the stability, a@/ap. The
symbol @ is the potential temperature, and p is the pressure. While
moving from the cyclonic to the anticyclonic side of the jet stream (de-
fined with respect to the position of the jet core), the air parcels apparently do not even appreciably change their speed.
Since air masses in the vicinity of a jet maximum do not move
isobarically, isentropic analyses seem more appropriate than isobaric
ones for study of the jet-stream structure. Figure 3 shows winds at
the 300°K isentropic surface for Nov. 22, 1962. Figure 2 showsthat on
the average stronger anticyclonic than cyclonic shears prevail. This is
in line with Arakawa’s instability criteria
au _
u
ay ft Rtane
au_
ff
du
ay 2 Ro
au
(> 0)
au
(<0)
(4)
(5)
where f is the Coriolis parameter, R is the earth’s radius, and ¢ is the
geographic latitude. These criteria give the limiting shear for anti-