Naar aanleiding van het bericht van gisteravond met beeldmateriaal van de vulkaanuitbarsting in Chili, heb ik gezocht naar artikelen welke een mogelijke verklaring geven voor de toch wel heftige bliksem welke soms gepaard gaat met zo'n uitbarsting.
Allereerst wat dingen over de pyrocumulonimbus. De pyrocumulonimbus is een buienwolk welke ontstaat ten gevolge van een sterke thermische stijgstroom gedreven door een bosbrand, of industriele activiteit. (Definitie van 'American Meteorological Society') Officieel gezien vallen de Cb's die soms ontstaan bij vulkaanuitbarstingen dus niet onder de Pyro-Cb's, maar veel gelijkenis is er wel.
Onderzoek van Rosenfeld et al. (2007) naar een specifieke Pyro-Cb boven een bosbrand in Canada wijst uit dat vooral veel positieve bliksem met opvallend hoge intensiteit plaatsvindt. Terwijl bij 'gewone' Cb's in de omgeving met name negatieve bliksem optreedt. Zie onderstaand citaat uit het
artikel De geinteresseerden raad ik aan om bovendien het abstract en de paragraaf 'Electrical Activity' en eventueel de 'Discussion' te lezen.
The pyro-Cb is characterized by a high density of high-intensity, positive lightning, while most lightning produced in the ambient clouds has negative polarity. This observation supports the hypothesis of Williams et al. (2005), who proposed that (1) the flash rate increases with increasing updraft speed and (2) inverted polarity in thunderstorms (leading to positive lightning) is the results of a high liquid water content in the mixed phase region of the cloud.
De hogere water content is het gevolg van het neerslagonderdrukkingsproces. De vele miniscule asdeeltjes afkomstig van de brand, vormen goede condensatiekernen (ik meen me ook te herinneren dat asdeeltjes sterk hydrofiel zijn, klopt dit?). De grote hoeveelheid condensatiekernen, zorgt ervoor dat zeer veel kleine druppeltjes ontstaan, deze druppeltjes zijn zo klein dat de kans dat ze botsen met andere druppeltjes erg klein is. Het gevolg is dat het coalescentieproces (ontstaan van grotere druppels door botsingen) sterk wordt gehinderd, en er weinig neerslagvorming plaatsvindt! Zie ook
Rosenfeld et al., 2003.
Onderstaand nog twee abstracts van artikelen over vulkanische bliksem, deze artikelen zijn volgens mij niet vrij beschikbaar op internet.
The electrification of volcanic plumes and volcanic lightning
AU: Harrison, R G
AF: Department of Meteorology, University of Reading, P.O. Box 243, Earley Gate, Reading, RG6 6BB United Kingdom
AU: * Mather, T A
EM: Tamsin.Mather@earth.ox.ac.uk
AF: Department of Earth Sciences, University of Oxford, Parks Road, Oxford, OX1 3PR United Kingdom
AB: We present a review of our current understanding of the electrification of volcanic plumes on Earth and discuss the possible implications both in terms of the volcanic monitoring, early Earth evolution and planetary exploration. We also present simple calculations to show how the global electrical circuit might be modified following a large volcanic eruption reaching the stratosphere. Volcanic lightning is perhaps the most spectacular consequence of the electrification of volcanic plumes. Recent years have seen remote-sensing measurements of volcanic lightning used as part of a portfolio of techniques to monitor volcanic eruptions. Surface observations of the atmospheric electric Potential Gradient (PG) and the charge carried on volcanic ash also show that many volcanic plumes, whilst not sufficiently electrified to produce lightning, have detectable electrification exceeding that of their surrounding environment. Electrification has only been observed associated with ash-rich explosive plumes, but there is little evidence that the composition of the ash is critical to its occurrence. Different conceptual theories for charge generation and separation in volcanic plumes have been developed to explain the disparate observations obtained, but the ash fragmentation mechanism appears to be key. It is unclear which mechanisms or combinations of electrification mechanisms dominate in different circumstances. Electrostatic forces play an important role in modulating dry fall-out of ash from a volcanic plume. Beyond the local electrification of plumes, the higher stratospheric particle concentrations following a large explosive eruption may affect the global atmospheric electrical circuit. It is possible that this might present another, if minor, way by which large volcanic eruptions affect global climate. Volcanic lightning has been implicated in a number of ways in the origin of life on Earth, and may also exist in other planetary atmospheres where measurements of its occurrence might give clues about the nature of volcanism on other planets. The direct hazard of volcanic lightning to communities on Earth is generally low compared to other aspects of volcanic activity.
Science 23 February 2007:
Vol. 315. no. 5815, p. 1097
DOI: 10.1126/science.1136091
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Brevia
Electrical Activity During the 2006 Mount St. Augustine Volcanic Eruptions
R. J. Thomas,1* P. R. Krehbiel,1 W. Rison,1 H. E. Edens,1 G. D. Aulich,1 W. P. Winn,1 S. R. McNutt,2 G. Tytgat,2 E. Clark2
By using a combination of radio frequency time-of-arrival and interferometer measurements, we observed a sequence of lightning and electrical activity during one of Mount St. Augustine's eruptions. The observations indicate that the electrical activity had two modes or phases. First, there was an explosive phase in which the ejecta from the explosion appeared to be highly charged upon exiting the volcano, resulting in numerous apparently disorganized discharges and some simple lightning. The net charge exiting the volcano appears to have been positive. The second phase, which followed the most energetic explosion, produced conventional-type discharges that occurred within plume. Although the plume cloud was undoubtedly charged as a result of the explosion itself, the fact that the lightning onset was delayed and continued after and well downwind of the eruption indicates that in situ charging of some kind was occurring, presumably similar in some respects to that which occurs in normal thunderstorms.
1 Langmuir Laboratory, New Mexico Tech, Socorro, NM 87801, USA.
2 Alaska Volcano Observatory, University of Alaska, Fairbanks, AK 99775, USA.
Groeten,
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