WMOD00454

• Title: Preliminary Analysis of the Hypotheses Used in the Utah Operational Weather Modification Program - Final Report
• Date: 11/1/1983
• Weather Modification - Doc Type: Report

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The weather modification hypothesis in use in Utah basically, but not

entirely, relates to blanket clouds forming over the barrier. In basic terms

orographic clouds include (1) stable orographic clouds, and (2) convective

type clouds for which conditional instability is released by the barrier

lifting. The blanket-type clouds for which the Utah hypothesis primarily

applies are of the stable orographic type. The North American Weather

Consultants' classification of cells and bands constitute convective

type PETS. During the 1983 field program 54 (29.1%) of the 185 hou~

with precipitation occurred within these two PETS and produced some 27% of

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the observed precipitation (Table II). The other 131 hours of precipitation

(71%) occurred with more stable type cloud systems and produced some 73% of

the precipitation. 13.5% occurred with area-wide precipitation, well less than

1% occurred with PETS classified as orographic, and 56.7% occurred with NP

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events (no C-band echo but precipitation over the Tushars).
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Clearly, the NP
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events were the major contributors to the total precipitation. It seems apparent

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that the main orographic events are included in the NP category with c~

shallow for the C-band radar to observe. This was investigated by studying the

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characteristics of the clouds with the Ku-band radar for the NP events when echos

were not observed with the C-band radar (cloud systems were too shallow or too

weak for a C-band radar echo, although precipitation was occurring in the

Tushars). Ku-band radar data is available for 60 of the 105 hours classified

as NP. The results of this analysis are presented in Table III. This shows

that 86% of ~_ precipitat!on,hours du:-ing ~he NP events occu=re~ with cloud

tops at or below 4800 meters msl (<16,000 ft msl). This is equivalent to near

3000 meters (<10,000 ft) above the Ku-band radar site. These shallow clouds

obviously made the major contribution to the total precipit~tion. The

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influence of orography on these shallow clouds is apparent from Table IV which

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shows the steady increase in the precipitation over the precipitation network

from 39" at PI (near Minersville) to 2.79" at P6 (near crest of the Tushars).