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I I I I I I I I I I I I I I I I I I I .. B """ . :>:~:~~::: :~: ~~_:~.:~ -~. tl C '2 0 " -.............. ': ~cc ~ · 5';;\\." . - /(~\ Figure 4':" A) net total charge density, B) potential, C) vertical, and D) horizontal electric field components. Solid-positive, dashed-negative. Lightning channel superimposed. Rawlins (1982) applied a simple charge neutralization scheme (reduce the magnitude of positive and negative charges by 70% when lightning occurs) and a threshold of 500 kV/m for lightning initiation. Takahashi (1987) used a similar procedure in his model, with an initiation threshold of 340 kV/m, to study lightning locations. Other similar approaches were devised by Ziegler and MacGorman (1994) for their 3D kinematic model and Baker et al. (1995) for their 1 D axisymmetric . model. The shortcoming' of this approach is that there are no physics attendant with the process, i.e., lightning is a function of arbitrarily specified parameters. We undertook to provide a physical basis for the incorporation of lightning within the model, by striving to produce an actual channel that would then manifest itself through the production of ions that would interact with hydrometeors in a physically consistent way. Wu (1986) developed the parameterization based on the theoretical work of Kasemir (1960, 1984). The scheme uses the electric field as the parameter determining the initiation, propagation, and termination of the modeled channel. From theory, we calculate the charge density deposited along the channel and, thus, the influence of the channel on the subsequent electrical development through the conversion of this charge to small ions. This also allows the calculation of the total charge transfer and the energy dissipation due to lightning in a physically meaningful way. The parameterization was developed for intracloud lightning, and needs modifications to the termination criteria to be suitable for calculating cloud-to-ground lightning. Although the underlying theory is 3D in nature, the scheme was originally developed for the 2D SEM. The first simulated lightning channel is shown in Fig. 4 for the 19 July CCOPE simulation. The advantage of this scheme is that it is physics based. Charge neutralization is accomplished by the injection of ions created along the channel into regions of opposite charge taking the arbitrariness out of the process. This is shown in Fig. 5, taken from Helsdon et a/. (1992) wherein the lightning scheme is outlined in detail. In Fig. 5 the net total charge density is shown (undisturbed in Fig. 4A) a) immediately after the discharge with the lightning-produced ions. present as the opposite charges within the main positive and negative charge regions, b) 15 s later, and c) 30 slater showing the recovery of the original charge structure as the charge separation process continues to operate. In this simulation the threshold for lightning initiation was set to 400 kV/m. This method of representing lightning allows the possibility of the creation of charge regions by lightning, something that observations are now beginning to show occurs. Using this scheme and simulating the flight track of the NCAR sailplane that observed this storm, we were able to explain how the sailplane recorded only a horizontal electric field change when the observed lightning flash occurred and not a vertical field change (channel geometry in conjunction with aircraft observation altitude relative to the termination region of the discharge). 11 Figure 5 - Net total charge density showing lightning-produced ions a) immediately after, b) 15 s after, and c) 30 s after the discharge. Solid contours-positive charge, dashed contours- negative charge. 2D simulation of the 19 July 1981 CCOPE case. 39