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GATZ AND DINGLE
that the debris enters from the top of the cloud as it penetrates to the
high troposphere or the stratosphere. It has been suggested® that the
peak concentration of radionuclides in precipitation will occur as the”
cloud tops reach a maximum height. Ifthe rainfall-rate peak is assumed
to be directly associated with the cloud-top maximum, the rate peak
would be expected to contain high concentrations of debris. The concentration of radioactivity at the rate peak is expected to be decreased
somewhat by dilution of the particulate matter in high concentrations
of liquid water. Nevertheless, moderate concentrations at the rate peak
would be expected rather than minimum concentrations, as were observed in the three well-organized rains.
These considerations and the fact that both classes of contaminants
decrease rapidly at the same time suggest that the low-level input is
the source of both the radioactivity and the pollens found at the beginning
of the heavy rain.
In the diffuse rains of September 30 and May 19, peculiar trends
of contaminant concentrations were noted. The absence of a consistent
temporal pattern of concentration changes in these cases might be
expected from a consideration of the likely circulations of weak convective systems. As Weickmann’ has pointed out, heavy storms with
high rainfall rates are associated with circulations of a high degree of
organization and persistent updrafts. Weak storms lack such organized
circulation; their updrafts are more transient, It is consistent with
these observations that an organized pattern of concentration changes,
such as that observed during the early portions of the heavy rains,
should be associated with an organized and persistent updraft whereas
the absence of an organized pattern should be associated with weak and
disorganized convection.
In view of the high concentration of contaminants found in thefirst
rain arriving at the station, it is clear that this rain must be associated
with a large value of the product of the volume of the air columnit
Sweeps out and the mass (or activity) of contaminants in that volume.
This criterion may be met by various combinations of drop trajectory,
distribution of contaminants, and storm-circulation patterns, The pres-
ent data donot permit adefinite specification of any of these parameters.
CONCLUSIONS
The
observation of simultaneous rapid temporal decreases in
concentrations of both radioactivity and plant pollens in rain from the
early portions of three well-organized convective rain systems is
interpreted as evidence that both classes of contaminants were removed from the same air. Because plant pollens are released into the
atmosphere at the earth’s surface, it is concluded that both radio-
activity and pollens were removed from an updraft of warm, moist
air originating at low levels of the atmosphere.