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<br />mC€l.., Ti91 208 AT WE 89.00'llN M:lIlEl., TT9'1 208 AT Tll'E 89.00 MIN <br /> <br /> <br /> <br />CUARG& COULOMBS/M-.' <br />tOH1'~~ ~ ~2.86t.Ut... TO i.t9sgDC.tD <br />CQlrfl"OuR INTtR'/AL '.DaOOOt... <br />SCALEO BV I.OOOOOt.IZ <br /> <br />VSh'1'ICAL !LIC1'RIC PIl:tt.D VOLT/I <br />COtfTA,a ntoH -'7.'7I976E,+O' TO 1.'81&1[.0' <br />CCl/TM INlD\VIl. Maaaot.as <br />sum III '.oaaaaNl <br /> <br />Figure 1- a) Net total charge density, b) vertical <br />electric field. Shading indicates positive values. <br /> <br />alone is not sufficient to separate large charges, 3) <br />electrical forces in the vorticity equation produce <br />negligible influences at the maximum fields <br />obtained, 4) the life history of the electric fields <br />parallels that of the cloud, and 5) the electric field <br />magnitudes depend critically on the amount of <br />charge carried by the cloud and/or rain particles. <br />One other problem with the simulations <br />undertaken in this period was the difficulty of <br />obtaining initial profiles of the electrical variables <br />that were stable under the numerical integrations. <br />Although the appearance of Dr. Orville's name in <br />the published cloud electrification literature ends at <br />this point, his influence in the field did not. <br />In November 1974, Dr. Orville had the <br />foresight to facilitate the hiring of Dr. Chin-Shan <br />(Timothy) Chiu who had recently obtained his PhD <br />(Chiu, 1974) from the. New Mexico Institute of <br />Mining and Technology working under Professor <br />James Klett. Dr. Chiu had completed a study <br />similar to that of Ruhnke (1970) using an <br />analytical, steady-state, axisymmetric model to <br />study convective charging (Chiu, 1974; Chiu and <br />Klett, 1976). Dr. Orville, recognizing Dr. Chiu's <br />potential in the area of thunderstorm electrical <br />modeling, brought him to the Institute of <br />Atmospheric Sciences (IAS) to continue the work <br />initiated by Pringle and Stechmann. During his <br />time at the lAS, Dr. Chiu developed a 20, time- <br />dependent, axisymmetric model that was a <br />significant advance over the previous work, <br />although it did not initially include the ice phase. <br />Chiu developed several improvements in the <br />model representation of electrical interactions. <br />First, he included an explicit accounting of the <br />interaction of small ions with hydrometeors <br />through diffusion (based on work by Gunn,1954) <br />and conduction (following Whipple and Chalmers, <br /> <br />1944 and Gunn, 1956). This eliminated the <br />arbitrary assignment of ion charge to <br />hydrometeors used in Pringle et al. (1973). In a <br />similar vein, he accounted for hydrometeor charge <br />neutralization/enhancement when coalescing <br />interactions occurred. He also added an explicit <br />representation of the inductive charging process <br />between rain and cloud droplets based on theories <br />developed by Latham and Mason (1962), Davis <br />(1964), and Paluch and Sartor (1973). Finally, he <br />solved the electrical initiation problem by deriving <br />a self-consistent set of steady-state equations for <br />the vertical profiles of ions and the electric field. <br />The result of this work was a study of warm-cloud <br />electrification by inductive charging submitted to <br />the Journal of Geophysical Research in Nov. <br />1977. In a tragic accident, Dr. Chiu and his family <br />died before the manuscript was published. Dr. <br />Orville completed the revision process and the <br />manuscript was published in Oct. 1978 (Chiu, <br />1978). At the time of his death, Dr. Chiu was also <br />working on adding ice phase electrification to his <br />axisymmetric model. As a tribute to that effort, Dr. <br />Orville collected Chiu's notes and presented his <br />theoretical approach as a conference paper (Chiu <br />and Orville, 1978). <br />Chiu (1978) found that the application of <br />inductive charging to a warm-rain cloud could <br />result in strong electrification (10s to 100s of <br />kilovolts per meter) depending on the assumed <br />separation probability for drop-droplet collisions <br />and the number of cloud droplets per unit volume. <br />He also found that the frequently observed <br />positive-over-negative dipole charge structure was <br />reproduced as was the appearance of a lower <br />positive charge center in the latter part of the <br />simulations. Figure 2 shows a typical result for <br />300 cloud droplets cm-3 and a separation <br />probability of 0.04. Shown in the figure are: a) the <br />cloud water, b) the rainwater mixing ratios, c) the <br />charge on rain, and d) the vertical electric field <br />component. The largest electric field in this <br />simulation was -583 kV/m. When separation <br />probabilities of 0.02 were used, the field never <br />exceeded -32.8 kV/m showing the sensitivity of <br />the results to this important parameter. <br />There had been earlier modeling studies of <br />inductive charging, such as those of Ziv and Levin <br />(1974), Scott and Levin (1975), and Levin (1976), <br />but those models were all of a steady-state nature. <br />A steady-state model can predict the charging <br />resulting from particle interactions, but does not <br />account for the influence of changing cloud <br />dynamics on the resulting charging. Chiu (1978) <br />represents the first effort with fully coupled <br />microphysics, dynamics and electrification. <br /> <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 />ItI <br />I <br />I <br />I <br />I <br /> <br />36 <br />