WMOD00399

• Title: WMO Training Workshop on Weather Modification for Meteorologists - Lecture Notes
• Date: 12/1/1979
• Weather Modification - Doc Type: Report

50

N.-2m-5

~

~
\2
...
:I:

1.0

o

LO

2.0
DIAMETER (em)
lWC (gm/m3)

3.0

Figufte 12. The gftowth 06 V = 0.5 c.m
hailJ.>ton~ in an upd.twM
V2 = 22 mJ.>-l, c.haftaeteftized by
p Vz = c.oM:tanL p if.> the aift
deMUy which vafti~ wdh height
like in thundeftJ.,toftm~ in Coloftado.
The depletion 06 the liquid wateft
c.ontent LWC if.> non-exif.>tent 60ft
negligible hailJ.>:to ne c.onc.en:tfta:tio 11
whefte haUJ.>tonM fteac.h J.>iz~ 06 > 2
em at a balanc.e level 06 'V 3.6 kIn
above the OOC level. An oftiginal
~tone c.onc.entJLa:t-<-on 06 No = 2 m-3
ftemoveJ.> aU the cloud wateft by the
time the hydftometeoft fteach the 4.7 kIn
level. Above that heigM: no appfteci-
able 6uft:theft gftOwth tak.eJ.> plac.e. 16
No = 10 m-3 the hadJ.>tonM would be
limited to Vmax = 1.2 c.m.

-. 27 -

sparse and nature does not resort to the
multiplication mechanism.

Seeding can be used for producing
more rain, less rain, more hail and less hail,
etc., because any interference in the precip-
itation process can, in principle, be bene-
ficial or detrimental to the purpose it was
applied. To demonstrate this point we will
look at hail prevention. The two most
promising approaches to interfere with the
hail process are either through the competi-
tion or the glaciation concept. The competi-
tion concept is based on the assumption that
if more particles compete for the same amount
of available water substance their growth is
limited and they may even fall as rain after
melting. Figure 12 makes that point in a I-D
cloud model with a cylindrical updraft and
pVz = constant, where p is the air density
and Vz the updraft speed whereby the sounding
is characteristic for thunderstorms in
Colorado. Small particle concentration ('V 0)
leave the liquid water content unperturbed,
larger ones deplete it. If the number of
small hail particles is increased, but not
sufficiently to guarantee adequate competi-
tion then more hail and more damage is called
for. It is only when a certain number con-
centration is surpassed that a reduction of
hail size and number and thus damage might be
encountered.

The glaciation concepts calls for a
transformation of all cloud droplets into ice,
such that embryos of hailstones could not
grow into large hydrometeors. Again, nearly
all droplets need to be frozen, otherwise
increased hail formation can occur. This is
closely coupled to dynamic/thermodynamic
effects.

Any precipitation process depends
on water vapor influx into a cloud or cloud
system and the efficiency or degree by which
it is transformed into precipitation. This
influx in turn depends on the dynamic and
thermodynamic situation. For physical
reasons it is clear that any condensation or
increase in condensation will lead to
additional cloud buoyancy due to the release
of latent heat of condensation. This is the most important driving force for the updraft
and will inforce it when increased. Thus, the influx of water vapor will increase and more
water can be transformed into precipitation of any kind. This effect occurs also when
cloud water is transformed into ice and latent heat of .fusion is released. This lift may