RELATION OF INDEX PATTERNS TO FALLOUT PEAKS
465
that the stratosphere provides the debris source for these maximums.!,4
As a result of the higher static stabilities and lack of precipitation
‘scavenging in the stratosphere, much longer mean particle residence
times -would be expected in this region than in the troposphere. At
present, a plausible estimate of mean residence time is five years for
the tropical stratosphere and one year or less for the polar stratosphere.° It is thus reasonable to assume that the stratosphere is the
probable debris source for these seasonal oscillations in surface
fallout intensity.
With this assumption the problem of explaining the seasonal fallout
variations now becomes one of understanding the physical processes
that lead to an exchange of mass between the stratosphere and tropo-
sphere. It has been suggested by several authors®® that the transport
of mass and radioactive debris downward from the stratosphere is
associated with extratropical cyclones. Staley’ demonstrates that this
type of mass exchange occurs as a discrete intrusion and hence results
in the transport of large amounts of contaminated air into the troposphere. It has been further hypothesized that the sinking is associated
with cyclogenetic processes at tropopause level and leads to the
occurrence of individual shorter period surface fallout peaks.'° More-
recent research’! has in part verified this hypothesis and also revealed
that this sinking process is characterized by extremely strong vorticity
advection and mass convergence.
Because of the apparent dependence of individual fallout maximums
upon upper tropospheric cyclones, one may inquire whether the
fluctuations in mean seasonal fallout are thusa result of yearly changes
in cyclonic activity. A proposed way to partially examine this will be to
develop
an index parameter that describes the relative amount of
tropopause-level cyclonic activity in the middle latitudes and compare
the seasonal variations of the index with those of the mean fallout
intensity.
DEVELOPMENT OF THE CYCLONE INDEX
Some of the initial attempts toward the development of a simple
quantitative description of the state of atmospheric flow ata given
level were begun by Rossby’? and Allen ef al.'* These efforts to produce numerical indexes that would reduce the complexities of atmospheric motions resulted in the well-known zonal index. The concept
of utilizing this index to describe atmospheric motions on a global
basis has proved to be exceedingly valuable in many areas of atmo-
spheric research. However, for more specialized problems, this index
fails to provide a sufficiently reliable description of the state of atmo-
spheric motions.'4:!5 Also, if hand computation is necessary, the time
required to calculate a series of index values may be prohibitive.