Plume Formation in Thunderstorms 
Water HitscHFELD 
McGill University, Montreal, Canada 
Abstract—Radar data displayed in the form of precipitation maps at constant alti- 
tude above ground (CAPPI) portray the structure of storms and their anvils in telling 
manner. Well-developed storms, even in the presence of severe wind shear, were ob- 
served to remain essentially vertical through their active phase. Instead of being 
strongly bent by the shear, parts of the storm appeared to be carried off by the wind, 
forming extensive plume patterns or anvils which trailed down to levels as low as 
10,000 ft while evaporating. Size and shape of the plume suggested that its particles 
had fall speeds ranging from 0.75 to about 5 m sec”, and thus were of precipitation size. 
Introduction—This paper is concerned with 
the motion of thunderstorms and their erosion 
in strong wind shears, as portrayed primarily by 
CAPPI-processed radar data. As will become 
apparent, the interpretation of the patterns, 
without the CAPPI (constant-altitude plan-po- 
sition indication) accessory to a powerful AN/ 
CPS-9 radar would have been more difficult or 
impossible. (Radar wavelength was 3.2 em, peak 
power 250 kw, pulse duration 5 microseconds, 
PRF (pulse repetition frequency) 200 sec. The 
radar is located at Montreal Airport.) A few 
words will suffice to explain how these constant- 
level records were obtained. In our procedure, the 
radar was used as a plan-position indicator, but 
with its angle of elevation being raised after every 
revolution in 1-degree steps up to 12°, and in 2- 
degree steps thereafter to 20°. The complete cycle 
took 3.6 min, and resulted in 17 photographic 
frames from which CAPPI displays were synthe- 
sized by an optical-mechanical technique [ Langle- 
ben and Gaherty, 1957]. Sets of such records for 
heights 5, 10, 15,...up to 40 kft (kft = 1000 ft) 
above ground were built up every 3.6 min, thus 
providing horizontal slices at these nine heights 
through all the showers within range. (Improved 
CAPPI records are now built up directly on the 
radar screen; in the future it will be possible to 
generate these displays on an electrical or mag- 
netic store.) Figure 1 shows in vertical section 
how the 5, 15, 25, and 35-kft constant-altitude 
displays are pieced together from successive PPI 
beams. It may be noted that a CAPPI picture, 
centered at say 15 kft, really represents a layer 
which extends in the mean through 15 + 0.75 
kft at range 18 mi, and through 15 + 3 kft at 
range 75 mi. The ranges 18 and 75 mi were mini- 
mum and maximum for the records to be dis- 
cussed. 
94 
On July 1, 1956, a series of isolated well-de- 
veloped air-mass thunderstorms swept over the 
area. Figure 2 shows a small selection of the 
CAPPI record. In the evening these storms gave 
way to wide-spread heavy rain, ahead of a cold 
front which passed Montreal at about midnight 
HEIGHT (KFT) 
RANGE (miles) 
Fic. 1—Lines of constant altitude (dashed), 
corrected for Earth curvature and normal refrac- 
tion, are plotted against range; while the antenna 
scans in azimuth continuously, its elevation angle 
increases progressively; annular segments are 
selected from photographs of the PPI display at 
elevations one or two degrees apart to make up 
the constant-altitude pictures; the sketch shows 
the segments of which CAPPI pictures at 5, 15, 
25, and 35 kft are composed, in vertical section 
(five-fold vertical exaggeration) 
EST. The first of the showers, already well de- 
veloped, was detected to the NW at 14h45m and 
decayed while still in view at 16h30m. Five sub- 
sequent showers were picked up at earlier stages 
of their development, and each could be followed 
for more than two hours before decay. Their 
life histories all followed the same pattern: 
within about 40 min of detection the echoes 
reached their greatest vertical extent (in excess 
of 40 kft), then gradually subsided, and during 
