Walsh Medical Media

Heading : IC Lightning And Tornadogenesis

<p>One example of an alternative model description of tornado development within a supercell storm involves strong ground-level wind shear that tilts upward under appropriate condition to potentially produce multiple vortices surrounding a central circulation center [<a href="#53" title="53">53</a>]. The emphasis is on a continuous supply of lifted moisture inflow into a tornadic updraft from near ground level otherwise relying on a hydrodynamic-type description of CG wind velocity and cloud mechanics in the global manner outlined more broadly by Panofsky [<a href="#25" title="25">25</a>]. No obvious role of cloud electrification would be involved in the generation of these type tornadoes. More recently, a primary role for electrical force was brought in as a facilitator of IC tornadogenesis by Patton, Bothun and Sessions [<a href="#54" title="54">54</a>] with focus on the fact that many tornadic vortices begin at storm mid-level and build downward into ground level tornadoes, more or less in the same manner as originally described by Vonnegut [<a href="#2" title="2">2</a>,<a href="#3" title="3">3</a>]. Important connection was established with conditions of high wind shear and large CAPE, in the latter case as already mentioned for the description given in the detailed analysis reported by Carey and Buffalo [<a href="#20" title="20">20</a>]. In the Patton et al. model, a midlevel embryonic tornado structure, enhanced by electrical action, is taken to lead to a central downdraft that acts to build the vortex rotation in a downward direction. Historical electrical observations, even predating Benjamin Franklin, are quoted. Charge layer stratification in a cylindrically rotating system is an essential element of the model and local perturbation caused by downdraft or electromagnetic energy reduction is proposed to lead to cylindrical polarity with central negative charge. Latent heat from freezing provides buoyancy within the system. Reasonable negative core charge densities associated with mm size particles are described in the accompanying calculations; for example, a critical threshold core charge density [>~5 nC/m<sup>3</sup>] was estimated to lead with high probability to tornadogenesis and its maintenance on a time scale of a few minutes.</p>

<p>Armstrong and Glenn (AG) have given emphasis in their H<sub>2</sub>Obased model description of tornadogenesis to the role of lightning in generating OH<sup>-</sup> and H<sup>+</sup> particle densities driven in respective opposite up and down directions to provide wind enhancement and recombination energies for tornadic activity [<a href="#11" title="11">11</a>,<a href="#12" title="12">12</a>]; see <strong>Figure 3</strong>. A relevant reference on the issue is the reported study involving lightning and Doppler radar observations of two tornadic storms in Oklahoma in 1981 [<a href="#55" title="55">55</a>]. Emphasis was given to one of the storms reaching a peak in the IC lightning flash rate of ~0.3 s<sup>-1</sup> at a height of 6 km and occurring in association with build-up of cyclonic shear at an LCL height of 1.5 km. The obtained measurements and referenced others were taken “to suggest that 1) most tornadic storms (80% or more) have an increase in total flash rates near the time of the tornado, and 2) the increase in total flash rates is often dominated by intracloud flashes”. We suggest that the flash rate may be taken to be roughly analogous to the mechanical example of an initial push rate being required to generate a spinning top; and, the need for continued pushing for spin enhancement relates to Vonnegut’s admission of needed additional lightning flash enhancement to sustain a tornado [<a href="#2" title="2">2</a>,<a href="#3" title="3">3</a>]. <strong>Figure 5</strong> shows higher reported lightning flash rate results for a May 22 1997 severe storm near Orlando FL [<a href="#56" title="56">56</a>]. Williams et al. [<a href="#56" title="56">56</a>] and later Buechler et al. [<a href="#57" title="57">57</a>] had reported far greater IC flash rates as compared with CG values preceding tornadogenesis. In the first case, Williams et al. reported IC flash rate ‘jumps’ of 3 to 8 s<sup>-1</sup>, far larger than mentioned above, that were associated with follow-on sudden increases in ground level wind velocities of 50 to 70 mph. Very significantly, the wind velocity buildups were found to lag behind the flash rate jumps by 5 to 20 minutes. The results provide evidence for the AG model consideration that enhancement of wind velocity should follow the flash rate influence on ionization [<a href="#11" title="11">11</a>]. Anderson and Freier [<a href="#58" title="58">58</a>] have describe an asymmetry character for the CG generated positive or negative rates of charging while pointing out that positive lightning discharges release much more energy and that CG negative discharges remove only a small fraction of the total LCL negative charge. The authors point to the net charge level in a thunderstorm cloud being much greater than can be transformed by most lightning discharges. Comparison of the discussion given by Anderson and Freier may be made with the AG account for reduction of the LCL charge level through IC positive charge driven downward for recombination and consequent energy release. (<strong>Figure 5</strong>)</p>

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<p><strong>Figure 5:</strong> Correlation of reported lightning flash rate and ground level wind velocity [<a href="#56" title="56">56</a>].</p>
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<p>Williams et al. [<a href="#59" title="59">59</a>] provided an important assessment of atmospheric thermodynamics and electrical conditions favorable to the several conditions of thunderstorm updraft, its associated microphysics and the lightning flash rate. A tropical system was investigated with follow-on extension of the results to continental US consideration. Effects of temperature gradient, cloud water content and updraft velocity were described, particularly with concern for the lightning flash rate as a function of cloud height. Very interestingly, a Figure (4c) was presented of compiled CG and IC/CG positive lightning flash locations centered on the same US tornado incidence and severe storm coverages shown here in <strong>Figures 1</strong> and <strong>2</strong>. Very recently, Nishihashi et al. [<a href="#60" title="60">60</a>] have reported on both CG and IC lightning activity in connection with tornadogenesis occurring on 2 September 2013 on the Kanto Plain in Japan. Their measurements included observations made by the (Japan) Lightning Detection Network (LIDEN), coupled with radar reflectivity and Doppler velocity data. Rapid increases in both CG and total flash rates, comparable to those observed in the US, were recorded before tornadogenesis. Agreement was expressed with the results of Williams et al. [<a href="#56" title="56">56</a>] in that the lightning jumps were observed minutes before tornadogenesis. The CG flashes were essentially totally negative (96%). A reason suggested for the lightning jumps was updraft enhancement above the cloud freezing level caused by larger vorticity of the mesocyclone in the mid-cloud region compared to that at LCL.</p>

<p>It was suggested by Nishihashi et al. that detection of such rapid flash rate occurrences could be employed on a severe weather warning decision-making basis. In the US, Strader and Ashley [<a href="#61" title="61">61</a>] employed the national lightning detection network (NLDN), along with geographic information system techniques, to assess the relationship only of CG lightning and severe weather development. It was concluded that knowledge of the lightning flashes alone was not sufficient in themselves to detection of a ‘jump’ prior to tornadogenesis. Rudlosky and Fuelberg [<a href="#62" title="62">62</a>] have reported recently also on lightning detection for storm severity via the Automated Warning decision Support System (WDSS) and have given a description of correlation procedures for analysis of severe storm conditions. Documentation of more than 1200 severe and non-severe storms in the US mid-Atlantic region was achieved. It was concluded that tornadic storms exhibited much greater IC and CG flash rates. McCaul and Cohen in an earlier report [<a href="#63" title="63">63</a>] had contributed the important finding that deep IC moisture layers, at least 1.5-2.0 km thick, proved helpful to ‘maximizing updraft overturning efficiency’ and increasing the chances of large hail, frequent lightning and heavy precipitation. The authors discuss the importance of buoyancy with an updraft of ~55 m/s for a CAPE value of 2000 J/kg. The observation relates to the AG suggestion that intentional cloud seeding of the supersaturated IC structure would help to prevent tornadogenesis in favor of increased precipitation, and even should possibly involve research on seeding of appropriately charged particles [<a href="#11" title="11">11</a>].</p>

Electrical Role for Severe Storm Tornadogenesis (and Modification)

Heading : IC Lightning And Tornadogenesis