3D-measurement of emission height
Till now, the measurement of intra-cloud strokes was only possible with the aid of extremely complicated and costly high-frequency measurement systems (VHF/HF). Moreover, the sensors within a high-frequency measurement network have to be spaced much more closely for physical reasons as high-frequency radiation diminishes quickly in the atmosphere. Hence, the coverage of large areas can prove to be difficult.
nowcast has developed a method (patented) which allows for the reliable identification of intra-cloud strokes even in the low-frequency range. Moreover, LINET is also able to determine the exact height of the intra-cloud stroke – with an average sensor spacing of around 200 km.
Large-area 3D-data opens up new possibilities
The systematic recording of the heights of intra-cloud strokes forms the basis for the assessment of the strength of storms. For the first time, storm situations can be extensively observed and analyzed over a large area for the duration of their entire life-cycles, from their formation to their decline. This is due to the fact that the recording of intra-cloud strokes and their heights constitute a substantial factor in evaluating the strength of storms. Apart from the ratio of intra-cloud to cloud-to-ground strokes, the height of a storm cell also provides information: Should the average height of a cell increase significantly, a strongly convective area may be assumed in which hail and heavy rain form in addition to lightning strokes. A clear danger signal for weather services, aviation or industrial installations which could initiate the necessary measures in good time thanks to this insight.
3D-measurement in detail
Intra-cloud strokes occur when a sufficient number of strong charge centers with opposite polarity form within a storm cloud. As a consequence, the radiation received by the lightning-measurement antennae is emitted from an area which is located several kilometers above the ground in a cloud. As such, the signal propagation delay of intra-cloud strokes from the point of emission within the cloud to the antenna is extended. A cloud-to-ground stroke however, which discharges very close to the ground given the same 2D-position, reaches the antenna faster. Although difference in the signal propagation delay is slight, it can nonetheless be reliably detected and recorded using the patented technique if all the components involved in the measurement system have been designed and developed with sufficient precision. For the reliable determination of the emission height, the sensors should be spaced within 150-200 km of each other. These requirements have been fulfilled in all the core regions of the European LINET measurement network of nowcast GmbH.
Alternative methods of discrimination between stroke types are based on the analysis of signal shapes and depend on the signal height and polarity. It has, however, been shown that considerable uncertainty with respect to discrimination arises, particularly in the case of weak and positive signals. This method by nowcast GmbH offers a reliable alternative.
nowcast GmbH has been successfully using this 3D-method in real-time operation since 2004 (Reference: Betz et al., “Lightning Detection with 3D-Discrimination of Intracloud and Cloud-to-Ground Discharges”, J. Geophys. Res. Lett., Vol. 31, L11108, doi:10.1029/ 2004GL019821, 2004). No other commercial lightning-detection system in the world offers such versatility.