524
SAUCIER, HALL, AND NELSON
the residence half-time of the stratosphere for aerosol-size particles
is measured in years, depending on particle size and varying with
altitude, season, and geographic position. In comparison, the correSponding residence half-time in the troposphere, where the medium is
subjected to greater mixing, is of the order of just a £éw weeks. Con-
sequently one is led to surmise that, several months or ‘yearstter in-
jection of the debris into the atmosphere, the vertical distribudogrot
radioactivity should be characterized by relatively low #oncentrations
of radioactive particles and rela
agp:small vertical’gradientsoftheir
concentration 4h the tropospkere™ and By-g rapid upwiied increase in
concentration thyeugh the tr¥popaust
sé pe -lower- _Stratosphere. In a
broad sense such a. distributign
diediy exists, ag*hasbeen veri-
fied by long periods of simpiing at varios levels of the atmo
In initiating this work,weplaced faith.in.this view, beeving
j
he e.
could separate$hose storms which- onentirelytroposphericftrom
the more convective ones which penetrated_irito-the stratosp
major part of the- problem in this.eatenchwail
tei;DEE:i
height can be determined by means of ground-bagéd radar, by
vations from aircraft, and indirectly with less precision from ‘roubigpe
meteorological data. It wouldthen be a relatively simple task to relate
storm height to tropopause height, to radioactive concentrations if#’fhe
precipitation samples collected, and to atmospheric radioactivity being
determined by other agencies to assess the mechanism of thunderstorm
collection and washout of radioactive particles.
However, things are not so simple. A factor to be taken into ac-
count is that the air from which most of the water is condensed comes
from the lower troposphere. Low-level air converges horizontally and
rises; the air at the top diverges horizontally as the air from below
rises into those levels. The storm is so dominated by the low-level
air that the washout mechanism applies largely to particles that en-
tered the storm in the lower troposphere. There will be particles
aloft which enter into the precipitate because of entrainmentof air
into the vertical storm circulation in middle and high levels, injection
of water masses upward into the air above the storm during its process of upward development, and some horizontal motion of air through
the upper part of the storm where those winds are significantly greater
than the horizontal displacement of the storm.
Perhaps an even greater difficulty arises from the large varia-
tions in radioactive concentration in the atmosphere, the troposphere,
the lower stratosphere, and the tropopause region, as found by others
in their careful sampling and analyzing of content. Beyond molecularand eddy-diffusion processes, there are organized differential movements of air that produce sharp vertical and horizontal gradients in
atmospheric composition, Also, the tropopause cannot be treated as a
substantial air surface in this scale. The tropopause may displace it-