WMOD00427 - Laserfiche WebLink

Home Browse Search

hypothesis (Smith et aI., 1984). The experiment failed to demonstrate statistically all hypothesized steps leading to rain on the ground. However, the reasons for the failure were apparent in the physical data set, which was a very important result (Cooper and Lawson, 1984). Yet another inconclusive statistical experiment might have resulted without the physical observations from every case, 1.3 Factors Affecting The Success Of Physical Experiments It is worth trying to understand why marginal results were obtained from some past physical experiments, and impressive results were obtained from some others. Examples of the latter include the winter orographic seeding experiments conducted in the Cascade Project, the Bridger Range, and the Grand Mesa. 1.3.1 Influence of Precipitation Intensity. - A number of lessons became apparent in reviewing the various physical experiments cited. First, and not surprising, it is much simpler to document seeding effects leading to snowfall on the surface from nonprecipitating clouds than from those where nature is already somewhat efficient. The seeding signal can be unambiguous from clouds with a low IPC, as illustrated during the first three Grand Mesa seeding experiments of March 18, 1986 (Super and Boe, 1988). The only snow that fell throughout the period corresponded to passage of the seedlines, Also, their ground seeding experiment 2 days later produced the only snowfall anywhere on the Grand Mesa. Nonprecipitating or lightly precipitating periods with SL Ware common in Arizona winter storms, Radar, SLW, and precipitation rate observations over the Mogollon Rim indicated that the beginning and ending storm phases often had several hours with little natural snowfall and abundant SLW, and similar instances occurred during the middle of some storms (Super and Holroyd, 1989). It is, of course, important to demonstrate whether seeding can produce physical evidence of enhanced snowfall when some natural snowfall is occurring but excess SL W still exists. Hobbs (1975a) showed evidence of snowfall increases during naturally light snowfall, and similar evidence at aircraft levels is given by Super and Heimbach (1988), It will likely be increasingly difficult to demonstrate that seeding enhances snowfall as natural precipitation rates increase. When nature becomes very efficient, seeding cannot increase the snowfall because all available SL W is already converted to ice, Fortunately, Arizona winter clouds tend to be either quite efficient or very inefficient (Super and Holroyd, 1989). This tendency may be common throughout the intermountain West. For example, Hobbs (1975b) indicated that clouds over the Cascades were generally in two categories, "those in which ice particle concentrations never exceeded 0.1 L'1, and those in which the maximum concentrations were no less than about 10 L'l irrespective of temperature." It seems probable that analysis of individual well-designed physical experiments will show snowfall enhancement from naturally inefficient clouds, no enhancement from very efficient clouds, and only suggestive evidence of snowfall increases from moderately efficient clouds. However, grouping of several similar experiments should enhance the power of the analyses. ~ j 1.3.2 Airborne Tracking of Seeded Volume. - A very important factor common to successful physical experiments is airborne tracking of the seeded volume between the release point or line and the target area. The complexities of the three-dimensional airflow over mountains are sufficient to render suspect any windspeed and direction estimates based on upwind soundings and/or a few local surface measurements. It is almost essential that an aircraft monitor where the seeding material is going until it passes over the target, or is shown to have missed the target. 6