| OCR Text |
Show very ~gh space and _time resolution in input flow and temperature data, largely negatmg the economics of the generating system. On the other hand, if the atmosphere stratification is neutral and constant, up to the desired treatment height, ground generators do provide an effective and economical means of uniformly treating a large cloud volume over an extended period of time. In the Park Range area a trapping inversion between 8, 000 and 9, 000 ft. MSL is typically present for about half of each precipitation event, although there is a wide variation, with some events characterized by no inversion, and others by an inversion that does not break during the entire period. The more convective and shorter spring storms will usually break an inversion system sooner than a purely orographic winter storm. Inversion and stable layers are· a complicating factor in treatment with ground generators in the Park Range area during approximately two thirds of the total precipitation. Aircraft delivery of nucleating materials definitely provides a more reliable delivery system for seeding of winter orographic cloud systems than ground generators, except in very special cases. However, reliability is provided at a price. If the aircraft system is to be realistically reliable in operation, redundancy in equipment must be provided; also, aircraft adequate in payload, altitude, and range must be provided. The use of light, single engine aircraft is an essentially hit and run approach, and will not provide reliable, long period treatment of a large cloud volume. The pyrotechnic devices currently available, and thos.e which will be available in the near future, have made aircraft treatment of a cloud system practical. No longer is it necessary to fly into the zone of maximum aircraft icing, or to carry large volumes of flammable materials. With the devices now available the cloud system can be reliably and predictably treated from levels well above any serious aircraft icing problems. Devices soon to be available should make the uniform treatment of large volumes over long periods of time practical. The reliability of these delivery systems is equivalent to the reliability of aircraft operations at high altitude for extended periods of time. 5. 7 Operational Considerations Precipitation periods tend to have a frequency peak somewhere between 24 and 48 hours, controlled by the upper air short wave pattern, and a second peak at around two weeks caused by major shifts in the long wave pattern. These periodic characteristics create personnel scheduling problems, since desirable test periods tend to come in groups followed by fairly long periods of no precipitation at all. The problem arises because a large n:umbe~ of skilled personnel are needed for very long hours, concentrated m periods of a week or so; then very few skilled personnel are required through ex~ tended fair weather periods. This nature of weather modification o~erations in the Colorado River Basin is one of the problems to be overcome if an opera- 313 |
