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Last modified
7/28/2009 2:29:05 PM
Creation date
2/27/2007 9:01:05 AM
Metadata
Fields
Template:
Weather Modification
Applicant
Arlin B. Super and James A. Heimbach Jr.
Sponsor Name
Colorado Water Conservation Board and US Bureau of Reclamation
Project Name
Literature Review/Scientific Study
Title
Feasibility of Snowpack Enhancement from Colorado Winter Mountain Clouds: Emphasis on Supercooled Liquid Water and Seeding with Silver Iodide and Propane
Prepared For
Coloado Water Conservatoin Board
Prepared By
USBR
Date
9/30/2005
State
CO
Weather Modification - Doc Type
Scientific Study
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<br />times during a storm passage as natural precipitation processes cycle between inefficient and <br />efficient phases. Depending upon how a seedable episode with SL W present is defined (and no <br />standard definition exists), typical seedable periods last from a few to several hours. Given that <br />spatial variations occur simultaneous with temporal variations, it is not practical to predict just <br />when seedable periods will began and how long they will last. Existing models are not adequate <br />for that task. The most practical means of making seeding decisions is to measure SL W presence <br />and begin seeding as soon as it is detected. Seeding could cease an hour or two after SL W is no <br />longer detectable, with the delay necessary to deal with short-term variations. There is little point <br />in attempting to forecast seedable periods when they can be measured with available <br />instrumentation. There is simply no practical way to forecast short-term variations in winter <br />orographic cloud SL W, and those who attempt to do so with models or other approaches are <br />ignoring the numerous observations by vertically-pointed microwave radiometers which clearly <br />show rapid fluctuations in time. Attempting to make and react to forecasts of SL W presence is <br />simply an unnecessary exercise in futility when real-time continuous observations are now <br />practical. The costs of seeding materials, both AgI and propane, are a small part of total seeding <br />operations. Seeding throughout any period which has potential for production of SL W is <br />worthwhile to avoid squandering seedable opportunities. <br /> <br />8) A relatively thin layer of SL W often exists along cloud bases of shallow clouds. It can <br />extend well upwind of mountain barriers in come cases (Rauber and Grant 1986). Seeding this <br />layer could provide important additional ice crystal/snowflake growth times when such seeding is <br />practical. It is not a zone practical to seed with aircraft unless quite expensive vertical drop flares <br />are used. Achieving adequate horizontal dispersion of the seeding agent and crystals by this <br />means would be prohibitively expensive. The most practical means of seeding the cloud base <br />layer, when it exists, would be with high altitude releases where the presence of upwind terrain <br />permits such releases. <br /> <br />9) Gravity waves have been found to sometimes produce high SL W amounts downwind of <br />mountain barriers (Bruintjes et al. 1994; Heimbach et al. 1997). Reinking et al. (2000) discuss <br />how storms passing parallel ridges, which are common worldwide, will generate embedded <br />gravity waves which may form significant SL W zones and precipitating cloud. But gravity <br />waves require a stable atmosphere, and are transient, subject to rapid buildup and decline, as <br />atmospheric conditions change. Reinking et al. (ibid.) suggest that further work is needed to <br />determine, "m whether a seeding methodology can be designed to tap the wave-cloud water <br />source. II Gravity waves may aid in the vertical transport of seeding material, but this approach <br />may be too unpredictable to apply operationally (Heimbach et al. ibid.). <br /> <br />A recent feasibility study suggests that the State of Wyoming seeding program, scheduled to <br />start during the 2005/06 winter, will attempt to seed gravity wave-induced SL W over and <br />downwind of the Wind River Range with an aircraft. It is not obvious how seeded crystals <br />formed far above the range and/or downwind of it will increase the mountain snowpack. <br /> <br />It is suggested that the State of Colorado continue to monitor investigations of gravity wave <br />SL W zones. However, until a proven seeding technology is shown to target such transient zones, <br />it is strongly recommended that emphasis be given to seeding the frequently existing SL W zone <br />just above the windward slopes and crests of mountains. <br /> <br />10) A relatively thin layer of SL W often exists along cloud tops (Rauber and Grant 1986). <br />While sometimes important for natural snowfall production, seeding this layer would require <br />aircraft releases which have a number of limitations discussed below. The thin cloud top SL W <br />layer is not considered to have serious seeding potential. <br /> <br />6 <br />
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