Walsh Medical Media

Heading : Electrification Consequences Of Lightning

<p>(<strong>Figure 3</strong>) Grigor’ev and Shiryaeva [<a href="#35" title="35">35</a>] have provided a modern account of lightning initiation and growth as critically-dependent on water droplets/hail/graupel in an IC atmosphere. The analysis may be compared with an earlier experimental report given by Watkins et al. [<a href="#36" title="36">36</a>] leading to a different conclusion of relative unimportance of electrical discharges for vortex stabilization within an atmosphere of only air. For the case considered by Grigor’ev and Shiryaeva, initiation of the lightning discharge was quantitatively assessed in terms of the instability of negatively charged micrometer-scale water drops and water-ice crystals within the IC environment. The model calculations may be seen to add to the (un-referenced) earlier model descriptions given by Vonnegut [<a href="#2">2</a>] but utilizing essentially the same electrification parameters. Among the modeled results are descriptions of the formation of an initial plasma region coupled with the development of calculations supporting effective auto-electron emissions during an electrical discharge in an avalanche-type mechanism (of ~0.1 μs duration) from electrically-unstable heavily charged water particles. Consideration of particle size dependencies was found to be relatively important with regard to establishment of the generated plasma and consequent activity of electron emissions, for example, covering unstable particle dimensions between 30 μm radii, much smaller than covered in <strong>Figure 3</strong> taken from a compilation by Sin’kevich and Dovgalyuk [<a href="#37" title="37">37</a>] of critical electric field results.</p>

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<p><strong>Figure 3:</strong> Compilation of critical electric-field strength, Ecr, measurements resulting in a corona discharge as dependent on droplet radius, following the relationship: Ecr [SGS units] = 1.5 (γ [dyn/cm]/R [cm])<sup>1/2</sup>.</p>
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<p>The <strong>Figure 3</strong> compilation of measurements for dependence on particle size of critical electric field strength Ecr, as reported by Sin’kevich and Dovgalyuk, provides a broad review of the same subject at the larger particle sizes considered by Vonnegut [<a href="#2" title="2">2</a>,<a href="#3" title="3">3</a>] and included description of laboratory modeling of corona discharges. Focus was on mm-scale ice crystals, pellets, and hail stone particles taken to initiate corona discharges and consequent intense ionization. The IC rate of production of mainly negative ions during discharges was estimated to be ~107 per cm<sup>3</sup>s<sup>-1</sup> but otherwise had a charge sign dependence on the polarity of the external field. The discharge rate was taken to depend on the intensity of IC precipitation and when very high, the corresponding ion formation rate was proposed to increase by several orders of magnitude. Important ‘characteristics of the processes of hydrometeor charging’ involved particle collisions and break-ups. And in further relationship to the subject, Arseniev and Shelkovnikov [<a href="#38" title="38">38</a>] have provided a magnetohydrodynamic description of the field equations both for magnetic and electric characterization of a stable tornado. Excellent agreement was obtained between calculations of the wind velocity dependence on distance from the tornado core and reported measurements made for the devastating May 3, 1999, tornado in Oklahoma, already mentioned in regard to <strong>Figure 1</strong>. A matching calculation was made for the electric field dependence. Other agreement was described with observations of horizontal lightning occurring in the cores of tornadoes in Nebraska and in Kansas. As will be discussed, Rathbun [<a href="#39" title="39">39</a>] appears to have been first in pointing to the accompanying magnetic characteristics of tornadogenesis within a thunderstorm. (<strong>Figure 4</strong>)</p>

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<p><strong>Figure 4:</strong> Electrical force <em>vs</em>. particle size for important influence of ionization.</p>
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<p>The role of water vapor, water droplets, ice crystals, and graupel are all important to lightning generation and possible tornadogenesis in the IC environment. Armstrong and Glenn [<a href="#12" title="12">12</a>] sought to establish an important role for smaller particle lightning-induced ionization in vortex development through addition of such consideration to Vonnegut’s focus on cloud electrification. <strong>Figure 4</strong> is presented to show that such effect is important. In the figure, a closed diamond point is shown for Rathbun’s description of tornadogenesis associated with CG lightning strikes, involving ~10<sup>10</sup> positive ions/cm<sup>3</sup> being created and driven upward to lower cloud positive charge [<a href="#39" title="39">39</a>]; see the previous estimations made by Wilkins [<a href="#5" title="5">5</a>]. The hydrogen radius of ~10<sup>-15</sup> m is taken from Arrington and Sick [<a href="#40" title="40">40</a>] although perhaps a larger size should be taken as reflective of the distance of influence. The closed circle point applies for Vonnegut’s concern with micron-scale and larger particles within a cloud volume containing 2 g/m<sup>3</sup> water concentration and the linear dependence is his predicted relationship for dependence on particle size, including the largest open-circle particle for graupel; see http://wrh.noaa.gov/Flagstaff/science/cloud. htm for modern description of a typical moist cloud containing ~0.5 g/m<sup>3</sup> of water. Dashed lines show the dependencies for smaller and larger water contents. The top-most filled square point applies for 100% ionization of the corresponding ~6.7 × 10<sup>23</sup> OH- concentration/ m<sup>3</sup> driven upward in the IC environment to promote updraft wind and eventual molecular recombination. The appearance of the graph is relatively unchanged if the abscissa scale is changed to a plot on the basis of particle mass. The ionization consideration relates reasonably directly to the description given by Vonnegut [<a href="#3" title="3">3</a>] and Sin’kevich and Dovgalyuk [<a href="#37" title="37">37</a>]. An example of comparative precipitation charge, from 2 to 220 pC, and particle size, from 0.6 to 3.8 mm, measurements have been reported by Bateman, Marshall, Stolzenburg and Rust [1999] for a thunderstorm in New Mexico [<a href="#41" title="41">41</a>].</p>

Electrical Role for Severe Storm Tornadogenesis (and Modification)

Heading : Electrification Consequences Of Lightning