WMOD00442

• Title: A Model of Hygroscopic Seeding in Cumulus Clouds
• Date: 12/12/1978
• Weather Modification - Doc Type: Report

DECEMBER 1978

10

G ERA R DE. K L A Z U R A AND C LE MEN T J. T 0 D D

1763

9

8

7

E
~

6
~
Cl.
~ 5

a
is 4
...J
U

3

2

I 2 3 4 5 6 7 8 9 10

15
UPDRAFT (m s-i)

~
",ooot

-150 C

-50 C

6m=23.20C
BASE=lkm
M.,R.= 16.2 g kg-I

20

I
25

I
30

FIG. 4. Variation of the cloud depth at which hygroscopic drops (5-400 ~m initial size) would first break up
.as a function of updraft speed for a warm, moist cloud. Total number of breakups (40 min limit) are overlaid on
these ~urves (model computations).

2) For higher and colder cloud bases a greater
cloud depth is also required for particles to grow
large enough to begin to fall. Higher and colder based
clouds contain less liquid wat~r for a given volume
of cloud air. Consequently, the production of large
drops by the coalescence process takes longer. A 5 /-Lm
hygroscopic seed injected into a 3 km cloud base
(moist adiabat of 23.20C) for a 10 m S-1 updraft
would require 3 km more cloud depth to grow large
enough to begin to fall through the cloud than a
similar size seed injected into a 1 km cloud base at
the same moist adiabatic temperature (see Fig. 3).
And a 5 /-Lm seed injected into a 3 km cloud base
(moist adiabat of 160C) at the same updraft speed
never would grow large enough to fall through the
cloud.
3) For a given updraft speed, larger hygroscopic
seeds will result in drop breakup lower iri the cloud.
4) Stronger updraft clouds require larger hygro-
scopic seeds to produce drop breakup. Figs. 4 and 5
show the height at which the first drop breakup
would occur with respect to updraft speed for the
various initial-size seeds and for two different cloud
base temperatures. The number of breakups that
occur (40 min limit) are overlaid on these curves.
Breakup occurs before the particle reaches its peak
height in the cloud for updraft speeds greater than
10 m S-1 and after for updraft speeds less than 10 m S-I.
In the warmer cloud base situation (Fig. 4), for an
updraft speed of 10 m s-\ a 100 J.Lm seed will grow
large enough to break up about 2.2 km lower in the

cloud than where' a 10 /-Lm seed would break up. At
20 m S-1 only hygroscopic seeds larger than 100 }.lm
would have a chance to break up before they reached
the level where they glaciated.
5) For weaker updrafts, drop breakup occurs only on
the smaller seeds. Since the smaller seed travels higher
up into the cloud it can grow large enough to break up
before it falls out the base of the cloud.
6) The vertical depth of the drop breakup region
decreases as the cloud-base temperature decreases.
Smaller cloud-water contents and decreased vertical
distance to drop freezing occur as the cloud-base
temperatures become lower. The slower coalescence
growth and earlier particle freezing combine to shrink
the drop breakup zone. This also tends to shift upward
the smallest size seed that can initiate breakup. Fig. 5
illustrates this shrinking effect for higher (and thus
colder) based clouds.
7) Hygroscopic seeding produces the greatest water
yield from clouds with the warmest bases. Fig. 6
shows the maximum sizes attained by a 5 J.Lm hygro-
scopic seed in a warm (base = 1 km, 8m= 23.20C) and
cold (base= 2 km, 8m= 160C) cloud. Not only do the
precipitation particles grow larger in the warmer clouds
at higher updraft speeds, but also a broad drop
breakup region is present (note flattening of curve
at 5 mm) indicating the production of many more
drops. This trend is shown more clearly in Figs. 4
and S. Drop breakups are far more numerous and
occur over a much broader span of updraft speeds in
the warmer cloud situation.