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<br />THE ROLE OF CONVECTION FOR SEEDING WINTER OROGRAPHIC STORMS <br /> <br />by <br /> <br />John D. Marwitz <br />University of Wyoming <br /> <br />1. Introduction <br /> <br />The Department of Atmospheric SeiencE~ at the University of l.Jyoming has been <br />studying the transport~ diffusion and cloud physics processes over Elk Mountain <br />and the Medicine Bow Mountains for a number of years. Beginning in 1973~ the <br />occasion presented itself to obtain similar transport~ diffusion and cloud physics <br />data over the San Juan Mountains. These studies began modestly in an attempt to <br />obtain comparison data with those OVI~r the Medicine Bow Mountains and Elk Mountain. <br />By late 1974~ it became apparent that the statistically designed Upper Colorado <br />River Cloud Seeding Project (referred to henceforth as the San Juan Project) <br />would not likely produce statistically significant results in the primary target <br />area by 1975~ its last of five scheduled years. These studies~ therefore~ took <br />on added importance. <br /> <br />II. Necessary Conditions for SuccesBful Modification of Orographic Clouds <br /> <br />In order to conduct a successful modi.fication of cold orographic clouds~ <br />there are three conditions which must exist. The conditions are as follows: <br />1. Presence of supercooled water. <br />2. Delivery of additional ice nuclei.. <br />3. Sufficient residence time. <br />The presence of supercooled water is the key condition without which there is <br />clearly no possible means of modifying clouds by these means. It appears~ however~ <br />that this key quantity is one about 1~hich we know very little. There are two <br />corollaries which follow from the supercooled water condition. They are (a) a <br />dearth of ice crystals~ and/or (b) high condensation rates. In the case of stably <br />stratified orographic clouds, it has been inferred from the Climax Project <br />(c.g., Grant and Mielke~ 1967) that the presence of supercooled water and hence, <br />a dearth of ice crystals (and hence, a high seeding potential) is related to the <br />500 mb temperature. The 500 mb templ~rature was chosen because it was the mean <br />cloud top height as observed by radar (Furman~ 1967). If the 500 mb temperature <br />is cold~ then many more natural ice nuclei. are activated and seeding is not <br />necessary. Warm 500 mb temperatures. on the other hand~ indicate that few natural <br />ice nuclei will be activated. Warm temperatures are also typically associated <br />with high condensation rates. and henee~ the seeding potential is high. Physical <br />observations in the cloud ,t-b' justify the importance which has been attributed to <br />the 500 mb temperature has been extr4~mely limited. <br />Field observations of convective clouds embedded in an orographic cloud are <br />quite limited. Vardiman and Grant (1972) have presented ice crystal concentration <br />data taken at the ground from convective clouds embedded in orographic clouds. <br />Their results indicate that abnormally large concentrations of ice crystals fall <br />from convective clouds. The anomalously large ice crystal concentrations were <br />attributed to an unspecified ice multiplication process. It appears that on the <br />basis of Vardiman and Grant's (1972) observations, Grant and Kahan (1974) have <br />concluded that orographic clouds with embedded convection have a low seeding <br />potential. <br /> <br />21 <br />