Volcanic lightning (Source: Wikipedia)
Met Office researchers have published new findings in ERL suggesting that the amount of lightning produced near a volcano each hour is roughly proportional to the plume height. They believe that the technique could ultimately be used to monitor volcanoes in remote locations or those that are obscured by rain.
To assess the number of lightning strokes the team used ATDnet, the UK Met Office‘s long-range lightning-location network, which has recently been updated.
“ATDnet is used to monitor thunderstorms over a vast area of the world extending from the US to China and the Arctic to the South Atlantic, centred over the UK,” siad Alec Bennett of the Met Office. “Lightning emits powerful electromagnetic pulses over a broad frequency range creating both the bright flash seen by an observer and the crackles on a nearby radio. The peak energy of this emission is found in the very low frequency (VLF) radio band at about 10 kHz, and it is this part of the spectrum that ATDnet uses to locate lightning, utilizing sensors positioned across Europe and beyond.”
The system has previously located lightning from volcanic plumes in southern Chile and earlier eruptions in Iceland. “The availability of quantitative observations of volcanic lightning is sparse, so our results are hoped to add new and detailed observational data to this research field,” said Bennett. “The eruption of Eyjafjallajökull in April–May 2010 provided an excellent opportunity for us to study both the performance of ATDnet after recent network upgrades and to use the lightning data to learn more about the relationship between volcanic activity and plume electrification, a process recognised for decades but still subject to ongoing scientific debate.”
Bennett and colleagues from the UK Met Office and Icelandic Meteorological Office found that plumes from Eyjafjallajökull that reached about 5 km above sea level generated lightning strong enough to be detected by ATDnet. Above this threshold, the rate of lightning production was approximately proportional to the plume height.
“Perhaps most importantly, it was found that this relationship did not exist all of the time, with some plumes higher than 5 km producing no lightning capable of being detected by ATDnet at all,” said Bennett. “This presents us with a unique case study whereby we have several days where a tall plume was not strongly electrified followed immediately by several days of strong electrification, enabling more research to be undertaken on the cause of vivid volcanic lightning displays.”
An eruption of Iceland’s Eyjafjallajökull volcano in late-March 2010 produced a large plume of ash and considerable disruption to flights across Europe.
The team used C-band radar data from the Icelandic Meteorological Office to quantify the plume height above the volcano. This allowed the scientists to combine detailed observations on lightning and plume height as well as modelled atmospheric parameters, such as wind speed, wind direction and ambient temperature. “With these data we can begin to quantify the effect of plume and atmospheric conditions on the generation of volcanic lightning,” said Bennett.
“By comparing the distance between the vent and lightning strokes it was also possible to gain evidence that a charging mechanism was present away from the vent, with many lightning strokes produced several kilometres downwind from the vent,” he added. “This finding suggests that two different charging mechanisms may be present in the plume, one immediately adjacent to the vent – producing numerous weak lightning strokes visible to an observer nearby but not strong enough to be detected by ATDnet – and a further strong charging mechanism operating within the plume, generating lightning with peak currents of at least 3 kA and detected by ATDnet.”