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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />3. STORM EPISODES DURING 1991 ur/NOAA FIELD RESEARCH PERIOD <br /> <br />A. Introduction <br />This section describes the weather on cloudy or stormy periods during the 1991 <br />Utah/NOAA field program. Some research experiments aimed at investigating the transport and <br />dispersion (T and D) of cloud seeding aerosols were conducted during clear, non-storm periods. <br />These clear days will not be discussed here. <br />A review of radiometer, precipitation and satellite data was used to determine the subset <br />of days which had significant clouds and precipitation, or clouds which contained at least some <br />liquid water. Days which experienced cirrus or altostratus, but contained no detectable liquid <br />water, were not included in the subset of cloudy events. <br />A total of 14 cloudy or stormy periods consisting of 29 of the 60 days of the field <br />program were identified. In most cases multiple day episodes were blocked to encompass a <br />synoptic event. In some cases the separation between periods was not clearly evident in the <br />synoptic data, but was more apparent in distinct episodes of precipitation or liquid water. In the <br />following subsections the 14 Storm Periods are discussed in terms of synoptic characteristics, <br />local meteorological environment, and cloud characteristics. <br /> <br />B. Description of Stonn Periods <br />Stonn Period 1: 14-16 January 1991. This period was dominated by high pressure at <br />the surface over the Great Basin, and a 500 mb ridge whose axis was near the West Coast. The <br />Utah research area to the east of the ridge axis generally experienced northwesterly winds. This <br />pattern was persistent for nearly half of the field program. From 14 through 16 January a short <br />wave moved through the ridge and eventually deepened into a closed low over southern Arizona <br />on 17 January. Figures 3.1 and 3.2 show the positions of 700 mb and 500 mb short waves at <br />1700 (all times are MST) 14 January and 0500 on 15 January. <br />At 700 mb (Fig. 3.1) a relatively extensive region of moisture preceeded the short wave <br />axis and began advecting into Utah sometime on 14 January. By 1700 15 January the 700 mb <br />trough axis was east-west oriented and located over central Utah. The winds at 700 mb were <br />northwesterly in front of the trough, backed briefly to a more westerly direction as the trough <br />passed and then veered to north-northwesterly behind the short wave. At the surface there was <br />evidence of a trough modulating the Great Basin high, as shown in Fig. 3.2 near the Utah- <br />Nevada border. <br />Instrumentation for the Utah/NOAA program was not entirely in place for this frrst storm <br /> <br />3-1 <br />