DONALDSON, CHMELA, AND SHACKFORD 
LOGEZimnax 
3 STORM CLASS 5x10 
HAIL REPEATER 
===" HAIL <20 MINUTES 
| 5x1074 
\ =i ue pe a aes oS ET Ses ees ate — I 
-150 -120 -90 -60 -30 0 30 60 90 120 150 
TIME AFTER FIRST HAIL — minutes 
Fic. 8—The trends of maximum echo intensity in 17 hailstorms, related to time before or after first 
hail; see text for explanation of log Zmax 
an assortment of hailstones, for three reasons: 
(1) the dielectric backscattering factor is lower 
in ice of unit density than in water by a factor of 
4 or 5; (2) hailstone densities may be as low as 
0.5 and, of course, are always less than the density 
of water; and (3) for 3.2-em radar practically all 
observable hailstones are outside the Rayleigh 
scattering region and begin the transition through 
the complicated Mie region toward geometrical 
scattering which is approximately proportional 
to the square instead of the sixth power of par- 
ticle diameter. 
Figure 8 shows the time variation of log Zmax 
in the 17 storms in which this was measured. The 
right-hand ordinate gives the maximum ice con- 
tent, assuming the maximum echo is scattered 
from unit density ice spheres of 1-cm diameter. 
Note the rapid growth in Storm 10, in which Zax 
measurements were made near the time of first 
echo. The maximum and median Zmax trends for 
the two classes of storms are illustrated in Figure 
9. Similar to the echo top situation, the hail re- 
peaters attain a higher maximum and at a later 
time. 
Radar echo maximum heights and maximum 
intensities appear to be conveniently observable 
indicators of the convective activity within a 
storm. Although the two parameters are loosely 
correlated, they reflect somewhat different as- 
pects of the convective situation, the heights in- 
dicating more about updraft speeds and persist- 
ence, and the intensities perhaps more about 
moisture supply and the efficiency of converting 
the available moisture to large hailstones. A con- 
vective index, the product of Hmax and log Zmax , 
was plotted for the median trends of the two 
storm groups (Fig. 10). Hmax is expressed in k 
ft and Zmax im mm‘/m*. The median convective 
index change with time was also plotted for the 
three tornado-producing storms, with each tor- 
nado storm maximum plotted separately. The 
diagram also shows the range of convective index 
calculated from the medians and quartiles of two 
populations of hailstorms and rain-thunder- 
storms (the Hmax population includes the Zmax 
population and many more cases besides). 
Figure 10 is intended to be provocative rather 
than definitive. Several general features are worth 
mentioning: (1) the more severe the storm class, 
the higher and later the peak of convective in- 
dex; (2) the close association in time (within the 
seale of time resolution in the original observa- 
tions) between the maximum convective index 
in a tornado storm and the time of the most de- 
structive tornado; and (3) the rapid rise but slow 
