WMOD00400

• Contract/Permit #: 14-06-D-6801
• Title: Structure and Seedability of San Juan Storms
• Date: 11/1/1976
• Weather Modification - Doc Type: Report

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36

elements often showed them to cont3in ImoJer concentrations of ice than those

shown in Fig. 3. 4b. This may be explained if the (~oJarmer) cloud base tempera-

ture is important in detennining the ice content, because the convective

elements frequently have' bases that are significantly ~oJarmer than the obser-

vation point.

(3) All-liquid cloud tops. 1ve often observed the tops of convective and

layer clouds to be composed entirely of supercooled water, and to contain

negligible concentrations of ice crystals. (See V.G.3.b., V.H.2, V.C.2.c,

and other examples in the case studies.) These observations can be explained

if it is assumed that most of the crystals have a common origin at cloud base.

Because the crystals have higher fall speeds than the cloud droplets, as the

air rises the crystals will fall relative to the liquid cloud in which they

formed. The result will be that the top of the cloud will consist of a

region depleted of ice. Only a continuous formation of ice would produce

ice at the tops of the clouds.

(4) Ice crystal size spectrum. The ice crystal size spectrum \\'as often

found to be relatively uniform, ~oJith few small crystals existing deep in the

cloud even in cases ~vhere a liquid cloud extended for some dis tance. An

.example is presented and analyzed in V.G.3.b. This uniformity in size in-

dicates a common origin for the crystals.

(5) Similar observations have been obtained on Elk Mountain in Wyoming,

as part of another program, and they also indicate an ice origin near the

upwind edge of the clouds (Cooper and Vali, 1976). Average crystal cone en-

tratj.ons appear to be lower in those cases, but only by about a factor of 2-5.

All of these observations point to a nucleation process associated \oJi th

the initial condensation process as the dominant ice-forming mecl1unism in these

clouds. There was no indi.cation that ice multiplication was occurring. The