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Last modified
7/28/2009 2:32:23 PM
Creation date
4/11/2008 3:38:50 PM
Metadata
Fields
Template:
Weather Modification
Contract/Permit #
14-06-D-6467
Title
An Operational Adaptation Program for the Colorado River Basin
Prepared By
Lewis O. Grant, Chappell, Crow, Mielke Jr., Rasmussen, Shobe, Stockwell, Wykstra
Date
10/1/1969
State
CO
Country
United States
Weather Modification - Doc Type
Report
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<br />For purposes of demonstrating impor- <br />tant aspects of modification poLenLial a H1t::drl <br />orographic component may be estimated and treated <br />as a known parameter. <br /> <br />Following this approach the rate of <br />condensation production per unit volume in the doud <br />system can be expressed as <br />-9 <br />C =1.11(10 )w(q -q (9) <br />a s700 s500 <br /> <br />where w is the mean upward speed in mb per hour, <br />q is specific humidity at water saturation in gm per <br />k~m, and C is expressed in gm per sec liter. From <br />(9) it is see~ that the rate at which liquid water is <br />supplied in the cloud system is a function of the <br />upward air speed and cloud system temperatures. <br />c. Relation of ice crystals to cold cloud <br />precipitation efficiency <br />For maximum utilization of cloud <br />moisture in the cold cloud precipitation process it is <br />desirable to have the integrated growth rate of ';he <br />ice crystals per unit volume (N ) proceed at the <br />identical rate as the cloud wate~ is supplied per unit <br />volume by the condensation process. This condition <br />can be expressed by equating (8) and (9). The <br />optimum ice crystal conc entration can then be <br />expressed as <br /> <br />N = 7.7 (10-11)( w /r)[(q - q )/F(T)] (10) <br />c s700 s500 <br /> <br />where N is the number of ice crystals per liteJ~. <br />c <br /> <br />It is apparent from (10) that no unique <br />ice crystal concentration is associated with a maxi- <br />mum utilization of cloud water since the radius of the <br />ice crystal remains a variable. For a given upward <br />speed in the cloud system the ultimate crystal E:ize <br />can adapt to the concentration of growing crysta.ls for <br />best utilization of cloud moisture. <br /> <br />There are practical boundary conditions <br />that confine desired adjustments between crysta,l size <br />and concentration. The number of crystals could <br />increase until crystal size ultimately is so small that <br />the settling velocity of the crystal is radically <br />reduced. In this event, the crystal could be carried <br />over the mountain barrier without deposition. <br />Evaporation of this moisture in the lee of the m::>untain <br />would then represent a loss of precipitation below <br />that which would have fallen naturally. <br /> <br />The extreme limiting condition is <br />reached if the crystal concentration becomes sc <br />large that the ultimate crystal size attained res'~lts in <br />complete suspension of the tiny crystals. <br /> <br />The other boundary condition limiting <br />the interplay of crystal size and concentration arises <br />when required crystal sizes for maximum utilization <br />of cloud water become too large. Under these condi- <br />tions the crystal may be unable to remain in the cloud <br />system for a sufficient growth period, or the <br />accretion process maybegin to remove important <br />amounts of cloud water. <br /> <br />The possibility of targeting snowfall <br />is quite intE:rc8ting. If the crystal size adjusts to the <br />crystal concentration then some control can be <br />exerted over the crystal trajectory. This might come <br />from either affecting the crystal settling speed <br />directly, or indirectly by promoting agglomeration. <br /> <br />Solutions to (10) can be obtained as a <br />function of the 500 mb temperature for specific <br />upward speeds and representative ice crystal sizes. <br />A mean upward speed applicable to the Wolf Creek <br />Pass area is estimated to be about 50 cps. This is <br />obtained by taking the mean wind speed (15 mps) <br />observed in the 700 mb to 500 mb layer during snow- <br />fall, and multiplying by the terrain grade toward the <br />southwest (.063). This results in a value approxi- <br />mately e9ual to 1 mps. Since a mean upward motion <br />for the cloud layer is desired, and upward speeds <br />must approach zero toward the upper limit of the <br />cloud, the 1 mps is reduced by one half, or 50 cps. <br /> <br />In order to obtain a representative <br />mean upward speed for the Climax area, the ratio of <br />the average hourly snowfall at Climax to the average <br />hourly snowfall at Wolf Creek Pass is multiplied by <br />the 50 cps upward speed derived for the Wolf Creek <br />area. This ratio is about one third resulting in an <br />estimate of the mean vertical speed for the Climax <br />area of 15 cps. This indirect approach was taken <br />for the Climax area because of the very complex <br />terrain features located in that area. <br /> <br />Figure 1 shows various solutioo s to <br />(10) for upward speeds of 15 cps and 50 cps, and <br />crystal radii of 100 microns, 300 microns and 500 <br />microns. <br /> <br />Figure 1 indicates that at colder <br />temperatures the required concentrations of ice <br />crystals nearly stabilizes for a given crystal size <br />and upward speed. This stabilization reflects the <br />region where the amount of moisture supplied <br />decreases at nearly the same rate as the capacity of <br />the crystal to grow.' The optimum concentration of <br />ice crystals required increases rapidly for a given <br />crystal size and upward speed as 500 mb tempera- <br />tures become warmer than about -14C. This is due <br />to the increasing water supplied at these warmer <br />temperatures coupled with slower crystal growth <br />rates which results in an accelerating need for more <br />ice crystals to utilize the cloud water. <br /> <br />d. Temperature activation spectrum for ice <br />nuclei <br />The number of ice nuclei activating <br />in the atmosphere is highly variable in time and <br />space. In general most observations have indicated <br />a crude exponential rise in ice nuclei counts with <br />decreasing temperature. An average spectrum of <br />ice forming nuclei in the atmosphere may be <br />determined from the relation <br /> <br />NN=(10-S)e-O.6T (11) <br /> <br />where T is the temperature in degrees centigrade <br /> <br />7 <br />
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