WMOD00299

• Title: Physical Structure of Northern Colorado River Basin Cloud Systems
• Date: 9/1/1985
• State: CO
• Weather Modification - Doc Type: Report

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19

presented recently by Cooper and Vali (1981) and Politovich and Vali
(1983),

--='"-:
.e::..-

Cooper and Va1i discuss the origin of ice in mountain cap clouds.

Their stUdy indicated that ice particles originate in association with

the initial condensation process at the upwind edge of the cloud and

that ice particle production is limited to this region. They determined

that contact nucleation or the condensation freezing process must be the

mechanism of nucleation of these crystals. No evidence of secondary

multiplication processes was observed and nucleation was observed to

exhibit a clear temperature trend. Their study indicated that cloud top

was composed almost entirely of supercooled water.

Politovich and Vali (1983) discussed the structure and evolution of

the supercooled water in these cloud systems. They found that droplet
-3
concentrations were typic~J,.ly_2~O-30_0. cD!, . and that droplet spectra
__---.......-- --'._~.,,~._ _"~__'L_'~.-=--"_~.' . ">.;.

were consistently very narrow. CCN concentrations were also found to be

low. This was attributed to the lack of anthropogenic sources and

wintertime snow cover inhibiting mechanical interaction wi~h the

surface. _~!q!1!ctJI.C3.tercontents in the cloud .lif!_I"~ gene.rally i? the rang~
-3
of 0.16 ~ 0.15 gm . Cloud droplet concentrations were found to

stabilize near cloud edge and were not a function of liquid water

content indicating smooth airflow and continuous condensate produotion

throughout the cloud. Increases in liquid water content were generally

associated with a shift in the droplet spectra as the cloud advected

across the mountain peak.

(7) Utah Dhvsical eXDeriments

Measurements of supercooled cloud water, precipitation, vertiQll

air. moti.on and cloud top temperature were made by Utah State Urd.ver.ity