RADIATION STANDARDS, INCLUDING FALLOUT
59
MODELS OF STRATOSPHERIC TRANSPORT
The earliest predictions of fallout from nuclear test clouds injected into the
stratosphere were of two varieties. First, a mathematical model was, and is
still being used, in which the stratosphere is assumedto be a thoroughly mixed
reservoir. Material added anywhere in the stratosphere would be completely
mixed within it in a matter of days, weeks, or a few months and leaks slowly
into the troposphere below. Such a model permits a numerical treatment in
which the holdup by the stratosphere may be characterized by a “residence” time.
Specifically, the half-residence time is the time required for half of the material in the stratosphere to enter the troposphere. Early and preliminaryestimates for equatorial injections suggested a 5- to 10-year residence time which
was subsequently reduced to a few years. Then, with the advent of readily
distinguishable Soviet polar fallout, the residence time for the Soviet debris was
estimated to be much shorter than a few years. Finally, the half-residence
time of the U.S. high altitude fallout was estimated to be much longer than a
few years. Thus, it became evident that the half-residence time varies with the
latitude, altitude, and season of the test and further, the half-residence time
might change from year to year even for the same point of injection. The concept of half-residence time is still probably the simplest way of comparing fallout rates from different tests.
The meteorologist, on the other hand, has tried from the first to model the
stratosphere with the best possible concepts of circulations and mixing processes.
But unable to be quantitative, he has failed to provide real help to the fallout
predictor. Nevertheless, there is qualitative and semiquantitative information
which is derivable from truly meteorological models of the stratosphere and these
will be discussed below.
The earliest model brought to the attention of the fallout field was that derived
from ideas expressed by two distinguished British scientists, Brewer and
Dobson. They speculated that the dry air observed in the lower stratosphere
over England could best be accounted for by the air having previously passed
through the tropopause region at the Equator. Here the air temperature is very
cold and literally can wring the air dry. Stewart in England and then Machta
suggested models which, as seen in figure 11, had fountains of rising air in the
lower equatorial stratosphere and compensating subsiding air poleward. Some
early bomb carbon 14 measurements led Machta to extend the upward motions to
at least 80,000 feet, while Stewart was, wisely, less specific.
This modified Brewer-Dobson picture explained a number of the observed
characteristics of fallout. Though the early stratospheric injections were made
near the Equator the peak in fallout was in the temperate zone. This is understandable from the poleward motions. There was an unequal partitioning between
’ hemispheres, presumably due to the Eniwetok clouds having been caught mainly
in the Northern Hemisphere circulation arm. Brewer and Dobson on the basis
of certain meteorological considerations, also predicted a maximum exit from
the stratosphere in late winter or spring. This too is, as noted earlier, a fearure
of the fallout pattern.
The death knell for a theory of rising air to altitudes of over 80,000 feet over
the Equator resulted from the tungsten experiment. The stratospheric measurements, illustrated by figure 12, disagreed with the model. In this placard,
the horizontal axis is again latitude, the North Pole to the left, the South Pole
to the right, and the Equator in the center. The vertical axis is height increasing upwards in thousands of feet. The solid-dashed lines are the tropopause.
The observation points are indicated by the black squares. The isolines delineate
lines of equal concentration of tungsten 181, an isotupe injected in the lower
equatorial stratosphere over Eniwetokin the summer of 1958. This figure shows
the distribution of the material about 244 years later, in November 1960. If
there were an upward current as envisaged in the previous placard, the maximum concentration would have moved away from the source. But this is not
the case; the highest concentration of radiotungsten is still in the vicinity of
the source region at 11° N.
Thus, the radiotungsten tracer experiment has
proven invaluable to the meteorologist in eliminating one kind of circulation
maodel.
But Brewer and Dobson never cluimed that the fountain reached to 80 ,000
or even 70,000 feet. Two other British scientists, Brown and Goldsmith, have
tried to rescue the rising current model as shown by their picture in figure
86853 O—62-—pt. 1——5
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