fo) 
for) 
Heights 
© 40kft 
O35 
o e 2 e o,@ 
25 @ s 
x15 1535 1610 1642 
05 
——— 
25 MILES 
Fic. 3—Positions of the ‘centers of gravity’ of 
horizontal sections through Storm 2 at five heights 
and four instants 
the last 30 min only weak and diffuse echo resi- 
dues were detected at heights up to 20 kft. These 
diffuse echoes, in all cases, could be traced back 
to high-level echo extensions for which we used 
the name ‘plumes, for their orientation and 
development were apparently governed (like 
that of a smoke plume from the stack of a ship) 
by the motion of the source and the ambient 
winds. 
The intense cores of the echoes, the regions in 
which the convection occurred and where the 
precipitation presumably formed, had irregular 
cross sections of maximum horizontal extents 18 
mi at 10 kft, 15 mi at 30 kft and narrowing to 
less than 6 mi at 40 kft. A striking feature was 
the uprightness of all the storms in the face of 
a very strong wind shear. Except for the plumes, 
the radar echoes did not seem to bend with the 
wind, and their sections at all heights moved 
with the same velocity, about 45 mph, due east 
(from bearings ranging from 265 to 275°). Fig- 
ure 3 shows sketches made for one of the thun- 
derstorms over a period of 90 minutes. The 
relative movement of the centers of the storm 
sections at various heights is seen to be small and 
random, and is probably not much greater than 
the margin of error of the measurements. The 
wind hodograph is shown in Figure 4. Also 
shown in this figure is the echo velocity, which 
agrees in direction with the winds blowing at 
close to 4 kft and again between 8 and 9 kft; 
the measured winds are, however, several miles 
per hour slower than the echo speed. The upper 
of the two mentioned levels is close to 700 mb, 
and is representative of a height which has been 
called the ‘steering level’ of thunderstorms echoes 
[Ligda, 1953]. 
The ability of the thunderstorm to maintain 
itself upright in the face of the severe wind shear 
WALTER HITSCHFELD 
is remarkable. The relative wind from 22 to 28 
kft was about 30 mph, and between 29 and 40 
kft ranged from 40 mph to a maximum of 110 
mph at 35 kft! The only apparent effect of the 
wind-shear was the development of the plume. 
In our records, plumes started at about 35 kft, 
and gradually worked their way downwards, 
eventually dissipating somewhere below 10 kft. 
(Only in one ease did the plume reach the 5 kft 
level.) The motion and growth of the plumes ob- 
served were consistent with the following mecha- 
nism: at heights at which the velocity of the 
wind relative to the shower is great, cloud and 
precipitation laden air was swept out of the 
storm, probably from its periphery, and carried 
off horizontally. Gradually, the particles in this 
air fell into lower layers, always adapting them- 
selves to the prevailing wind pattern. They thus 
trailed down through the atmosphere, in a man- 
ner which was characteristic of their fall speeds 
and of the wind pattern; the general motion of 
the plume was equal to the motion of the region 
where the particles were released into the wind. 
Comparison of the observed plume pattern with 
that anticipated on the basis of the wind pattern, 
allowed estimates of the particle fall speed (and 
so to some extent of their nature), and helped in 
pin-pointing the height at which the plume 
270° 
n 1 
"Sy 10O0mph 
90° 
Wind ot 30kft 
relative to echo 
JULY |, 1956 
MANIWAK| WINDS ALOFT 
O° | 100mph N 
Fra. 4—Hodograph of the winds on July 1, 1956, 
up to 40 kft; the average echo velocity (45 mph, 
from 270°) is also shown; the wind information is 
based chiefly on the 1000 EST Maniwaki sound- 
ings, but is supplemented in the light of soundings 
elsewhere 
