592
KRUGER, HOSLER, AND MILLER
time period of growth and evaporation history during descent to the
ground compared to the time period of maximum growth of the clouds
from the middle troposphere would likely obscure resolution of the two
mechanisms, The use of THI cross sections in 1963 showing the radar
reflectivity allowed more detailed observations of ground-level “Sr
concentrations with respect to individual bursts of precipitation observed at given altitudes in the cloud structure. The THI cross-
section analysis of the showers of May 10, 1963, Fig. 2, shows the
relation between the height at which the precipitation originated and
the observed “Sr concentration in ground-level rain. The Sr concentration curve from 1920 to 2025 EST follows the maximum radar-tops
curve during this period, and the sharp risefrom 60 to 249 dis/min per
liter follows the growth of the closed isopleth of 10° mm*/m’ reflec-
tivity from an altitude between 11,000 and 17,000 ft above the radarsite
to an altitude between 10,000 and 23,000 ft, with a high cell of similar
reflectivity at 29,500 ft.
A final observation of convective activity has been made with
respect to the spring peak of nuclear-debris deposition.® Itis suggested
that the greater occurrence of thunderstorms during the spring months
may be responsible in part for the observed spring peak. It is likely
that convective activity acts in concert withoneor more stratospheric—
tropospheric debris transfer processes in incorporating the nuclear
debris into precipitation that transports it to the ground or the lower
layers of the atmosphere through virga, where it can deposit through
subsequent low-level precipitation processes, This aspect was examined in two giant thunderstorm systems in Oklahoma during May
1963. Although the data are not yet ready for publication, an interesting
feature of these severe storms has been the lack of convectiveactivity Sr deposition features. The radioactivity data appear to be
more indicative of steady-state precipitation and will be the subject of
a separate report when the evaluation is completed.
EXTRATROPICAL CYCLONES
Much of the precipitation falling over the mid-latitudes of the
northern and southern hemispheres is associated with extratropical
cyclones. These large vortexes extend to great heights covering large
areas and are probably responsible for a large fraction of the nucleardebris deposition from the troposphere to the ground. The general large-
scale uplift of such cyclones may be augmented with smaller-scale
convective activity or orographic lifting or both, leading to precipita-
tion caused by one or more mechanisms acting simultaneously. Elliott
and Hovind?! have recently analyzed such complex storms entering the
West Coast from the Pacific. When the concurrent effects of convective
activity or orographic lifting are small, the time variations observed