Meteorological conditions during self-initiated upward lightning at the Säntis tower (Switzerland)

Meteorological conditions favoring the initiation of upward lightning from tall structures have been only poorly investigated so far. In addition to the scientific interest in this subject, it has practical applications to risk assessment for lightning occurrence to tall structures, particularly wind turbines. During the summer of 2017, a Lightning Mapping Array (LMA) network was deployed around the Säntis tower (northeastern Switzerland). This tower is located at 2502 m ASL and it is frequently struck by lightning, being one of the lightning hotspots in Central Europe. The tower is instrumented to measure lightning currents as well as electric and magnetic fields in its vicinity. With the addition of data from a C-band dual-polarimetric radar, this campaign allowed the gathering of a comprehensive set of observations of self-initiated upward lightning emerging from the tower. The observations revealed that most of the lightning flashes are self-initiated with upward-propagating positive leaders spreading mostly horizontally above the melting level, after an initial short vertical development from the tower tip. After this initial stage, upward leaders are followed by a sequence of negative return strokes. The inception of those upward lightning flashes, under a stratiform cloud, would be favored by the presence of negative charge at low levels close to the tower. The overall electrical structure would consist of a low positive charge in the isothermal layer near the 0ºC isotherm with the presence of negative charge above it (~4km / -5°C). In some cases, a positive charge layer above the negative charge layer has been identified.