WMOD00399

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

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The evolution with time 06 dftoplet J.>pec~ftUm, - o~ginallq'
c.entefted at ft6 = 12 ~m - bu J.>toc.haJ.>:t-<-c. c.o~ion/c.oal~c.enc.e.
9 <x> if.> a maJ.>J.> denJ.>ity 6unc.:tion; !It. if.> the mean volume
ftadi!lJ.> and ft ~he pftedominant ftadiuJ.> p~opoft:tional ~o ~he '
ftadaft fte61ec.hvity cUvided by ~he fA-QUid utt~eft c.ontent.
ObJ.>e}we :the gfWwth 06 ~aftge dftopJ.> wuh diame,teJ:6 up to 0.8 mm
wUhin 30 minu,t~, at ~he expenJ.>e 06 ~he J.>malleft dftoplw.
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Cold Rain (Figure 2b) is also formed in clouds consisting mainly of cloud drop-
lets. In the parts of the clouds above the OOC level, these liquid particles are super-
cooled. There is also possibility - like in Warm Rain - that droplets grow by condensation,
coalescence and breakup. However, if some of the aerosol particles act as ice forming
nuclei, then ice crystals may form and grow at the expense of the water droplet~ which
exhibit a higher vapor pressure (by ~ 12.5% at -120C). Their relatively fast growth in
mass leads to higher fall speeds and sampling cross-sections. If the cloud droplets are
small (< 10 ~m) their capture is unlikely, but an aggregation of ice crystals m~y well lead
to a formation of snow. Snow can fall to the ground directly or melt on its way down if
the OOC level is above ground. Snowflakes normally breakup when hitting a surf~ce and very
often grains of agglomerates of frozen droplets can be found among remenants of I ice
crystals (Figure 4).

Ugufte 3.

In convective clouds where the liquid water contents are normally higher than in
layer clouds, the growth of ice crystals mostly occurs by accretion of large c19ud droplets
with diameters ~ 20 ~m (Figure 5). This r1ill1ng process leads to more compact, iumpy
particles of often conical shape, namely graupel (Figure 6). They consist of a:loose ice
framework with many air-filled capillaries. Faster icing or fall into less cold cloud
regions leads to a densification, a particle category called small hail (Figure: 7) .
Falling towards the ground could lead to melting and thus, rain. Further growth, however,
would lead to hail, which is defined as lumpy ice particles with sizes> 5 mm. :Figure 8
shows some 4 cm hailstones with shapes like triaxial ellipsoids with indentations along the
minor axis.

Experiments (List, 1977) have shown that not all the water accreted ori a hail-
stone needs to stay there. Depending on the state of rotation already accreted 'water may
be shed. For gyrating hailstones (spinning and precessing like a top) the water loss
occurs through spin-off of large drops with D ~ 2 mm (Figure 9). Thus, the accreted small

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