STABILITY PROPERTIES OF LARGE-SCALE ATMOSPHERIC DISTURBANCES 
constant. But since the lengths of the streamlines for 
v’ increase, the average intensity of | v’ | must decrease 
correspondingly. When this stabilizing influence domi- 
nates the destabilizing influence from the transverse 
transport of vorticities of the basic flow, which can be 
shown to be the case for the shortest waves [14, p. 31], 
kinetic energy must flow from the disturbances to the 
mean flow so that in accordance with what was said 
above, « will have to increase where «/R is large and 
decrease where @/F is small. 
Combined Baroclinic and Barotropic Disturbances 
While there is enough evidence for the importance 
both of baroclinic and barotropic effects, there must 
be a limit to the extent to which phenomena can be 
explained purely barotropically or baroclinically. In 
general, one must expect that barotropic and baroclinic 
effects either add together in a more or less simple 
fashion, or they may be coupled to such a degree 
that the consideration of both effects simultaneously 
may give rise to entirely new types of phenomena. 
The only studies until now on waves for which both 
potential and kinetic energy are possible sources for 
the growth of the disturbance are those on polar front 
waves. Because of the complexity in the solutions for 
these waves one does not know whether the one or the 
other of these two possible sources is the more im- 
portant, although the shearing instability has been 
interpreted as the decisive one, seemingly, however, 
without any convincing justification. The unstable baro- 
clinic waves treated in this article have accordingly 
been looked upon as physically entirely different waves. 
It is the writer’s opinion that this probably is not 
true. Further investigations on this subject can be 
carried out with relative ease when the quasi-geo- 
strophic approximation is made. Relatively simple equa- 
tions appropriate for the most simple polar front model 
have been worked out by Phillips [21]. 
It is not unlikely that the study of an atmosphere 
where typical barotropic and baroclinic effects are op- 
erating in full generality may contribute considerably 
to a further understanding of the behavior of the at- 
mosphere. For this purpose equations (23) and (25), 
or essentially similar equations, may prove useful, at 
least for theoretical investigations, because of their 
great simplicity and generality. 
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