High within towering thunderclouds, a distinct form of intracloud lightning, known as "narrow bipolar pulse" discharges, can occur. Like other forms of lightning, narrow bipolar events (NBE) can be either negative or positive discharges. These events are known for their high-powered, short-distance electrical discharges that produce strong emissions of very high frequency radio waves. Previous research has found that since NBEs take place at relatively high altitudes, it is possible to detect them remotely using satellites. To be able to use the detection of narrow bipolar events to measure cloud behavior or storm dynamics, however, requires a better understanding of the relationship between cloud properties and NBEs.
Using a high spatial resolution radar array and a low frequency lightning location system, Wu et al. measured the properties of the NBEs produced by 10 storms near Osaka, Japan, during the summer of 2012. From the sampled storms, the authors identified 232 instances of positive narrow bipolar lightning and 22 negative discharges. The authors find that positive narrow bipolar discharges are typically located deep within the cloud, either in or surrounding the region of deepest convection. Negative NBEs, on the other hand, almost exclusively occur near the cloud tops, with altitudes from 14 to 16 kilometers (about 8.5 to 10 miles).
Based on their observations, the authors suggest that there is a critical cloud height—around 15 kilometers (9 miles) altitude—below which negative narrow bipolar discharges will not occur. As a result of this finding, the authors suggest that the detection of negative NBEs could be used to estimate cloud top height remotely. Or, barring that, they say that the mere detection of negative NBEs can be used for a quick rough assessment of thundercloud height, and hence of its likely severity.
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More information: Spatial relationship between lightning narrow bipolar events and parent thunderstorms as revealed by phased array radar, Geophysical Research Letters, doi: 10.1002/grl.50112 , 2013. http://onlinelibrary.wiley.com/doi/10.1002/grl.50112/abstract
