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<br />00249{~ <br /> <br />Different seeding techniques are applicable to orographic and <br />convective situations. An important indicator of orographic or <br />convective cloud structure is the trajectory of the air mass prior <br />to its arrival in the Sierra Nevada or, alternately, the mean <br />surface layer flow as exemplified by the 70Q-mb flow, according <br />to the MBA analysis. <br /> <br />MBA recommended an operational procedure to be used by the project <br />meteorologist to determine the seeding technique to be used during <br />seeding opportunities. The procedure is being studied by the <br />Skywater staff, which will develop the final operational procedure. <br /> <br />The main requirements for natural storm seedability are (1) the <br />absence of large numbers of natural ice nuclei so that there is a <br />potential for introducing artificial nuclei and modifying the <br />system, and (2) the availability of moisture and supercooled liquid <br />water for ice crystal growth by diffusion and accretion. The first <br />condition depends upon the temperature at the top of the cloud. <br />MBA cites Fletcher's statement that cloud tops colder than about <br />-250 to -280C will have sufficient ice nuclei to utilize available <br />cloud water for maintenance of a steady population of condensate9. <br />Additional nuclei may deplete the available water and perhaps diminish <br />precipitation. <br /> <br />, <br />v <br />r <br />I <br />! <br />i <br /> <br />The second condition depends on the relationship of the dewpoint <br />to ambient temperature and the elevation of 00 and -50C isotherms. <br />The isotherms are important in defining regions in which the ice <br />crystal modification mechanisms are valid. Consequently, the <br />forecast should provide an adequate estimate of the probability <br />that the cloud top temperature will be below -250C and an estimate <br />of the altitude of the melting level. MBA said that the general <br />idea of a probability in forecasting is not strictly defined; however, <br />20 percent and 80 percent were used to indicate three regions of low, <br />medium, and high confidence of the forecaster in the future occurrence <br />of a situation. <br /> <br />i <br /> <br />! <br /> <br />An additional concern in the operational forecasting is the condition <br />of the snowpack. Snowpack will absorb a certain amount of liquid <br />water, increasing the density of the snow. However, if too much <br />liquid water is added, some will permeate the snow and possibly cause <br />melting, so that water is actually added to the short-term runoff <br />instead of being stored for the spring snowmelt period. An estimate <br />of the ability of the snowpack to absorb water or melting snow can <br />be provided by the Central Sierra Snow Laboratory for that zone. <br /> <br />9. Fletcher, N. H., (1962). The Physics of Rainclouds , Cambridge <br />University Press, 386 pp. <br /> <br />17 <br />