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<br />. <br />.. <br /> <br />;, <br /> <br />number of ways of reducing bias in analysis of operational programs so that some useful information can <br />be gleaned. For example, target and control gages should be selected and made known before a project <br />starts. <br /> <br />Each of the three analyses cited above used a ~arkedly different set of control gages and target gages. <br />For example, two Arizona control gages were used in the 1981 analysis, none in the 1991 analysis and <br />four totally different Arizona gages in the 1997 analysis. Some changes in gage selection may be <br />necessary as gages are discontinued or relocated. However, such changes are unfortunate because <br />consistent results from each analysis, based on the same target and control gages, would be more <br />...... .. . <br />convmcmg. <br /> <br />Because of these and other problems with post-hoc statistical analysis of the Utah operational program, <br />the Utah Division of Water Resources decided that physical evidence was needed to better evaluate the <br />operational program. Accordingly, the NOAAlUtah AMP was heavily based on physical observations <br />and reasoning, including sophisticated numerical modeling. <br /> <br />1.4 Selection of Wasatch Plateau Experimental Area <br /> <br />Soon after the author began to serve as Principal Investigator for the NOAA/Utah AMP, late in 1989, <br />observational emphasis was shifted from the Tushar Mountains of southern Utah to the Wasatch Plateau <br />of central Utah. This shift in the experimental area occurred because of several practical considerations, <br />which significantly improved field observations. A limited winter field program was conducted during <br />early 1990 on both the Wasatch Plateau of central Utah and the Wasatch Range just east of Salt Lake <br />City (Super and Huggins 1992a, 1992b). Both are long north-south mountain barriers which should <br />minimize transport of valley-released seeding material around them. However, the Plateau offered <br />several advantages for field observational studies including less rugged terrain which permitted in-cloud <br />aircraft sampling much closer to the barrier top, and all-weather roads across and along the Plateau, <br />permitting widespread surface sampling by instrumented vehicles and access to fixed installation"s. The <br />importance of low-level instrumented aircraft sampling and instrumented vehicle sampling along the <br />Plateau's all weather highways, cannot be overemphasized. It has simply not been practical to obtain <br />such observations for other mountain regions, with a few exceptions like the Grand Mesa of western <br />Colorado (Super and Boe 1988). Aircraft sampling over the Plateau was conducted under special <br />waivers from the Federal Aviation Administration: This procedure allowed flight to within 300 m of <br />nearby highest terrain, while standard flight rules require 600 m minimum separation over mountainous <br />terrain. Moreover, lowest sampling passes were made in a terrain-following mode, rather than flying at a <br />constant altitude (Super 1995), which required exceptional piloting and navigation by the NOAA pilots. <br />This practice and the special waivers resulted in lowest aircraft observations typically within 600 m of <br />the Plateau top. In spite of the unusually low level sampling, the aircraft often could not descend into <br />grourid-released seeding plumes or significant SL W cloud because both were in shallow layers over the <br />terrain. <br /> <br />The rela~ive uniformity of the Plateau, with the broad Sanpete Valley to the west and parallel barrier <br />farther westward (San Pitch Mountains), provided simplicities for airflow trajectories and numerical <br />modeling efforts compared with more complex and rugged terrain. Nevertheless, the Plateau is believed <br />to be reasonably typical of most of Utah's north-south oriented mountains targeted by the Utah <br />operational seeding program. <br /> <br />3 <br />