SESSION IIB DISCUSSION
691
SHLEIEN: The reason I mention this is that we have found 2°*Ra
concentrations
in precipitation
which
very
likely originated from a
ground source.
GATZ: The occurrence of *“Ra in rain is very strong evidence
that resuspended particulate matter from the earth’s surface gets into
the rain. If we were able to obtain the temporal variation of 7*°Ra in
rain, we would have additional evidence pertaining to the question of
where the scavenged material enters the storm.
HUFF: Dr. Kruger, I’m surprised by your finding that there is a
direct relation between the rainout concentration and the rainfall in-
tensity. Did you have a large number of cases, and did this relation
hold for all storms ?
KRUGER: The *Sr concentration as a function of rainfall intensity is a difficult relation to establish for convective showers in
that, instead of just gross measurements, the relation should be examined in detail under the same storm conditions. On a gross basis,
for our 14 convective showers, increased "Sr concentrations occurred
with decreasing precipitation rates in about 50% of the cases. Many
of these increased concentrations occurred in the first cells passing
over the site, and the increase could be ascribed to evaporation
effects below the clouds. For later cells with more humid lower
layers, most of the §’Sr concentration curves followed the precipitationrate curve quite well. Thus in a single storm we could observean inverse relation during the initial and the final period and a direct relation in between where two or more radar tops could be identified.
SAUCIER: You presented rather great detail for the radar storm
analyses. Do you have similar detail for radioactivity measurements?
For instance, what is the time duration of your rain samples?
KRUGER: In general, we do. The time duration of rain samples
are generally taken as the minimum time to collect sufficient volume
of water such that %Sr concentrations can be determined with a pre-
cision of better than +5%. The time, therefore, varies not only with
the precipitation rate but also with the general levels of nuclear
debris in the atmosphere. In 1962 and 1963, when the nuclear-debris
levels were reasonably high, we were able to measure *’Sr in samples
of less than 0.1 liter. Thus, generally, we could limit our time span
to meteorologically interesting time periods of half hours or less for
large-scale systems and to minutes for convective showers. For
some convective showers, we were able to obtain half-minute samples
during intense periods. Our radar cross sections usually show the
Sr concentration and the precipitation-rate data sample by sample.
LIN