WMOD00378

• Title: Physical Investigations of Winter Orographic Clouds in Utah
• Date: 3/1/1984
• State: UT
• Weather Modification - Doc Type: Report

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ABSTRACT

Physical investigations have been made of winter orographic clouds in

Utah as part of an evaluation of the Utah operational weather modification

program. _~an.rof the c1ouds_ i~_th_e )"ushar Mtns study _area are shallow an<:l

long-lived. Remote sensing techniques were therefore employed to collect

much of the required cloud data. Key questions addressed concern the spa-

tial and temporal distributions of supercooled liquid wate~ in the study

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clouds and the delivery of seeding materials to these clouds.

Median

radiometrically observed liquid water depths integrated over cloud depth

were 0.29 mm in deep, stratiform clouds of broad extent, 0.175 mm in

clouds of cellular form, and 0.26 mm in shallow clouds blanketin~l the

Tushar Mtns.

I n cellular clouds the liquid water is distributed over a

large range of heights but in the other clouds the water is concentrated

from 1.0 to 1.5 km above the surface near cloud bases.

Liquid water

important for precipitation development is supercooled by 5C or more.

Liquid water depths increase by a factor of 1.3 to 2.0 over a distance of

10-20 km from upwind of the mountains to over the mountains. Superco'oled

liquid water was present in the clouds about two-thirds of the time

according to lidar observations.

Radiometric liquid water depth was

g,!':eater than 0.13 mm one-half of the time or about 4.5 hr in an aV1erage

STORM. (The STORM was the research study unit and was a per.igdof signif-
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'icant cloud cover and weather typically 18-24 hr long.) During a STORM

liquid water develops first over the mountains, and it remains there

toward the end of a STORM after it has disappeared from other directions.

A quasi-periodic variation in liquid water amounts appears in at least

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