Upward Lightning at the Gaisberg Tower: The Larger-Scale Meteorological Influence on the Triggering Mode and Flash Type

Upward lightning is rarer than downward lightning and requires tall (100+ m) structures to initiate. It may be either self-initiated or triggered by other lightning discharges. While conventional lightning location systems (LLSs) detect most of the upward lightning flashes superimposed by pulses or return strokes, they miss a specific flash type that consists only of a continuous current. Globally, only few specially instrumented towers can record this flash type. The proliferation of wind turbines in combination with damages from upward lightning necessitates an improved understanding under which conditions self-initiated upward lightning and the continuous-current-only subtype occur. This study uses a random forest machine learning model to find the larger-scale meteorological conditions favoring the occurrence of the different phenomena. It combines ground truth lightning current measurements at the specially instrumented tower at Gaisberg mountain in Austria with variables from larger-scale meteorological reanalysis data (ERA5). These variables reliably explain whether upward lightning is self-initiated or triggered by other lightning discharges. The most important variable is the height of the -10 degrees C isotherm above the tall structure: the closer it is, the higher is the probability of self-initiated upward lightning. For the different flash types, this study finds a relationship to the larger-scale electrification conditions and the LLS-detected lightning situation in the vicinity. Lower amounts of supercooled liquid water, solid, and liquid differently sized particles and no LLS-detected lightning events nearby favor the continuous-current-only subtype compared to the other subtypes, which preferentially occur with LLS-detected lightning events within 3 km from the Gaisberg Tower. Plain Language Summary Upward lightning (UL) is much rarer than downward lightning. It needs tall structures of 100+ m to initiate. However, UL has a much larger damage potential as it starts with a weak but long-lasting current. Lightning data from a specially instrumented tower reveal that (a) a large proportion of UL is initiated by the tall structure itself and (b) a subtype of UL, which only has a long-lasting current, cannot be detected by conventional lightning location system. Since so many tall wind turbines are built to generate electricity, correctly assessing the UL risk has become indispensable. Insights, which the networks cannot give, are gained using a large set of meteorological data and powerful machine learning models that combine it with the tower measurements. The most important variable for self-initiated UL is the height at which -10 degrees C are measured above the tower. The closer to the tower, the higher the chance that the tower itself triggers UL. The occurrence of the particular subtype of UL requires including larger-scale electrification conditions and information on the prevailing lightning activity. No LLS-detected lightning events or weak electrification conditions preferentially yield the long-lasting current type.