Thunderstorm Detection

Thunderstorms pose a major risk to human activity and infrastructure, with hazardous conditions ranging from floods to hail, tornadoes and lightning. The best indicator of thunderstorm activity is the presence of lightning, which fortunately produces a powerful, broad spectrum of electromagnetic signals revealing its presence thousands of kilometers away.

Radio waves from lightning are modified during their journey from the storm and while characteristics such as signal amplitude reduce with distance, such signal changes are not usually sufficiently robust to allow accurate ranging to the storm. Lightning location methods therefore either require signals received by multiple sensors arranged as a large network, or if only a single site is available, a receiver and complex processing algorithms analyzing various waveform characteristics to determine lightning range and in some cases direction as well. Unfortunately, lightning is not the only source of radio signals, so additional processing or sensors must be incorporated to separate radio noise from genuine lightning signals.

In addition to being a source of radio frequency signals, lightning also produces changes in the atmospheric electric field. Such changes are much slower than those from radio waves, with the frequency of lightning-generated atmospheric electric field changes being approximately 1-20Hz and lasting about 0.2 seconds on average. It was these quasi-electrostatic signals that were first used to study lightning before the introduction of good-quality radio receivers, and are still used in modern thunderstorm research, such as for measuring the total charge neutralized during a lightning flash.

With an increasingly crowded radio communication spectrum and a diverse variety of man-made radio noise sources, from 50/60Hz electric power ‘hum’ through
to gigahertz frequency microwave transmissions, lightning detection and ranging methods that avoid radio waves altogether are appealing. One such sensor has been developed by Biral, exploiting the small currents induced on a conductor exposed to the quasi-electrostatic changes from lightning. The amplitude of these quasi-electrostatic signals is more strongly dependent on distance than is the case for radio frequency emissions, so they are well-suited to local, 100km or so, lightning detection. Such antennas have been used to study lightning for more than 100 years, but the unique feature of the Biral instrument is the use of two or more co-located antennas of different geometry. The outputs of these multiple antennas at a single site are used to effectively separate small and nearby electric field changes from sources such as raindrops or even birds flying overhead from the powerful but distant lightning flash. One of the major advantages of such a system is that all types of lightning can be detected – both cloud-to-ground and cloud-to-cloud – which is not always the case for detectors using radio frequency methods.

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