Lightning Detection - Professional-quality Portable Lightning Detectors

Professional-quality Portable Lightning Detectors

Inexpensive portable lightning detectors as well as other single sensor lightning mappers, such as used on aircraft, have limitations including detection of false signals and poor sensitivity, particularly for intracloud (IC) lightning. Professional-quality portable lightning detectors improve performance in these areas by several techniques which facilitate each other, thus magnifying their effects:

• False signal elimination: A lightning discharge generates both a radio frequency (RF) electromagnetic signal – commonly experienced as “static” on an AM radio – and very short duration light pulses, comprising the visible “flash”. A lightning detector that works by sensing just one of these signals may misinterpret signals coming from sources other than lightning, giving a false alarm. Specifically, RF-based detectors may misinterpret RF noise, also known as RF Interference or RFI. Such signals are generated by many common environmental sources, such as auto ignitions, fluorescent lights, TV sets, light switches, electric motors, and high voltage wires. Likewise, light-flash-based detectors may misinterpret flickering light generated in the environment, such as reflections from windows, sunlight through tree leaves, passing cars, TV sets, and fluorescent lights.

However, since RF signals and light pulses rarely occur simultaneously except when produced by lightning, RF sensors and light pulse sensors can usefully be connected in a “coincidence circuit” which requires both kinds of signals simultaneously in order to produce an output. If such a system is pointed toward a cloud and lightning occurs in that cloud, both signals will be received; the coincidence circuit will produce an output; and the user can be sure the cause was lightning. When a lightning discharge occurs within a cloud at night, the entire cloud appears to illuminate. In daylight these intracloud flashes are rarely visible to the human eye; nevertheless, optical sensors can detect them. Looking through the window of the space shuttle in early missions, astronauts used optical sensors to detect lightning in bright sunlit clouds far below. This application led to development of the dual signal portable lightning detector which utilizes light flashes as well as the “sferics” signals detected by previous devices.

• Improved Sensitivity: In the past, lightning detectors, both inexpensive portable ones for use on the ground and expensive aircraft systems, detected low frequency radiation because at low frequencies the signals generated by cloud-to-ground (CG) lightning are stronger (have higher amplitude) and thus are easier to detect. However, RF noise is also stronger at low frequencies. To minimize RF noise reception, low-frequency sensors are operated at low sensitivity (signal reception threshold) and thus do not detect less intense lightning signals. This reduces the ability to detect lightning at longer distances since signal intensity decreases with the square of distance. It also reduces detection of intracloud (IC) flashes which generally are weaker than CG flashes.

• Enhanced Intracloud Lightning Detection: The addition of an optical sensor and coincidence circuit not only eliminates false alarms caused by RF noise; it also allows the RF sensor to be operated at higher sensitivity and to sense higher frequencies characteristic of IC lightning and enable the weaker high frequency components of IC signals and more distant flashes to be detected.

The improvements described above significantly extend the detector’s utility in many areas:

• Early warning: Detection of IC flashes is important because they typically occur from 5 to 30 minutes before CG flashes and so can provide earlier warning of developing thunderstorms, greatly enhancing the effectiveness of the detector in personal-safety and storm-spotting applications compared to a CG-only detector. Increased sensitivity also provides warning of already-developed storms which are more distant but may be moving toward the user.

• Storm location: Even in daylight, “storm chasers” can use directional optical detectors that can be pointed at an individual cloud to distinguish thunderclouds at a distance. This is particularly important for identifying the strongest thunderstorms which produce tornadoes, since such storms produce higher flash rates with more high frequency radiation than weaker non-tornadic storms.

• Microburst prediction: IC flash detection also provides a method for predicting microbursts. The updraft in convective cells starts to become electrified when it reaches altitudes sufficiently cold so that mixed phase hydrometeors (water and ice particles) can exist in the same volume. Electrification occurs due to collisions between ice particles and water drops or water coated ice particles. The lighter ice particles (snow) are charged positively and carried to the upper portion of the cloud leaving behind the negatively charged water drops in the central part of the cloud. These two charge centers create an electric field leading to lightning formation. The updraft continues until all the liquid water is converted to ice, which releases latent heat driving the updraft. When all the water is converted, the updraft collapses rapidly as does the lightning rate. Thus the increase in lightning rate to a large value, mostly due to IC discharges, followed by a rapid dropoff in rate provides a characteristic signal of the collapse of the updraft which carries particles downward in a downburst. When the ice particles reach warmer temperatures near cloudbase they melt causing atmospheric cooling; likewise, the water drops evaporate, also causing cooling. This cooling increases air density which is the driving force for microbursts. The cool air in “gust fronts” often experienced near thunderstorms is caused by this mechanism.

• Storm identification/tracking: Some thunderstorms, identified by IC detection and observation, make no CG flashes at all and would not be detected with a CG sensing system. IC flashes also are many times as frequent as CG so provide a more robust signal. The relative high density (number per unit area) of IC flashes allows convective cells to be identified when mapping lightning whereas CG lightning are too few and far between to identify cells which typically are about 5 km in diameter. In the late stages of a storm the CG flash activity subsides and the storm may appear to have ended—but generally there still is IC activity going on in the residue mid-altitude and higher cirrus anvil clouds, so the potential for CG lightning still exists.

• Storm intensity quantification: Another advantage of IC detection is that the flash rate (number per minute) is proportional to the 5th power of the convective velocity of the updrafts in the thundercloud. This non-linear response means that a small change in cloud height, hardly observable on radar, would be accompanied by a large change in flash rate. For example, a hardly noticeable 10% increase in cloud height (a measure of storm severity) would have a 60% change in total flash rate, which is easily observed. “Total lightning” is both the generally invisible (in daylight) IC flashes that stay within the cloud as well as the generally visible CG flashes that can be seen extending from cloud base to ground. Because most of the total lightning is from IC flashes, this ability to quantify storm intensity occurs mostly through detection of IC discharges. Lightning detectors that sense only low frequency energy detect only IC flashes that are nearby, so they are relatively inefficient for predicting microbursts and quantifying convective intensity.

• Tornado Prediction: Severe storms that produce tornadoes are known to have very high lightning rates and most lightning from the deepest convective clouds is IC, therefore the ability to detect IC lightning provides a method for identifying clouds with high tornado potential.