WMOD00176

• Project Name: Annual Report
• Title: Seeding Operations & Atmospheric Research, 2005 Annual Report
• Prepared By: Duncan Axisia
• Date: 12/31/2005
• Weather Modification - Doc Type: Report

.
.
.
II
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
t
t
t
t
t
,
I

CFIR_~,_ fX2OOolOe07
o ]00 '1llI 600 ~ 'lXIll
-,

-- . .' - "-
,~ f
--. I... -~ .-'" ..
r - . . ---
.
I - - -.....
. -
.-
'000.. ..... ..
,
II .
- .
'"
I
-,
, 00' '" ,. ,.

-'

. .~- .... .....-..

i

i

"'

C1P'c_~m'~"oo....,

Figure 8: Precipitation drop diameter (red dots) and
~ecipilallon drop concentration (blue squares) as a
functioo of allltude. Data obtained from the SOM research
aircraft for on 7~ Augusl2004.

Figure 9 (image strip belOW'): large graupe! esbmated at 2
millimeters in diameter Imaged by !he SOAR researd1
aircraft on 7'"' August at an allltude of 8143m and
uncorrected lemperatureof-ll'C.

Figure 9 (image strip to the left)
shows a graupel partide
estimated at 2 millimeters in
diameter al a temperature of
about -12C. This demonstrates
that precipitation is very active at
this attitude (8.1 km), In figure 5. doud droplets
with diameters around 19 to 22 microns are
measured. Although these larger droplets are
present in large concentrations (600/cc), it is
evident that these droplets do not contribute to

Figure 8 shows a plot of the
precipitation drop sizes up to
1000 microns (1mm), It can be
observed that in the 71t1 August
severe stann, precipitation
particles above 100 microns
become more numerous above
an altitude of 4km (5:C).
Precipitation reaches a
concentration of 0.01/cc at 6.5km
(Se) and increases to 0.03/cc
above 7km (-10 C). This
indicates that ice processes
dominate the precipitation
mechanism.

the precipitation process since the droplets are
found in large concentrations. Therefore. the
microphysical properties of this cloud can be
described as continental. with very low
precipitation efficiency. Precipitation processes
become active at very cold temperatures. and
there is an abundance of 19 to 22 micron
droplets at the cloud tops. A deficiency of ice
crystals can be also observed by the low
concentration (<O.1/cc) of precipitation
hydrometeors within the vertical development
of the cloud

The 7tr' August case is just an example of a
typical west Texas convective cloud with very
low precipitation efficiency. In fact, hail up to 2
inches in diameter was observed from this
stann.

Precipitation mechanisms

Very simply. cloud precipitatkm mechanisms
can be grouped in 2 processes;

1. the supercooled cloud droplets and ice
crystal process (or the cold--cloud
mechanism).
2. and the collision-coalescence process (or
the warm rain process).

The cold--cloud mechanism postulates the
nucleation of ice particles in supercooled
clouds followed by their growth by vapor
diffusion into snow particles, Under favorable
conditions they may aggregate as snow or
rime to fann low-density graupel or snow
pellets. This process is important in clouds of
all types where temperatures are colder than
about .15"C. induding the upper parts of
cumulonimbus clouds.

The collision--coalescence process or the warm
rain process occurs in relatively warm clouds
with tops warmer than .15"C and with bases
warmer than + 15"C by the collision bet\Neen
water droplets. To produce the large amount of
collisions required to form a raindrop that would
eventually fall to the ground. some cloud
droplets must be larger than others. Larger
drops may form on larger condensation nudei,
such as salt particles, or through random
collisions of droplets. Recent studies show that
convective clouds that ingest polluted doud

16