Lightning is the dominant natural source of impulsive electromagnetic radiation in Earth's atmosphere, a major driver of the global electric circuit, and a persistent hazard to infrastructure. Yet key aspects of lightning initiation and early development remain unresolved, as thundercloud electric fields are frequently measured far below the conventional breakdown, while X-rays, gamma-rays, and relativistic particles indicate that high-energy processes are involved. This thesis addresses these questions via observations of upward lightning at the Mt. Säntis Lightning Research Facility in northeastern Switzerland, where a 124 m tower atop a 2.5 km summit is struck over 100 times per year, and has been instrumented with synchronised sensors for the electric current and field, as well as high-speed cameras, a VHF interferometer, and X-ray detectors.

Treating the facility as a natural laboratory for studying stages of lightning flash development that are otherwise difficult to observe directly, this thesis combines a decade of measurements to investigate leader initiation and development, electromagnetic signatures of discharge processes, and energetic radiation. A key result is the first 3D reconstruction of an upward flash through parallel VHF interferometer and high-speed camera data, showing how complementary diagnostics recover channel geometry and transient development.

The early stages of upward positive flashes are analysed through current and electric-field pulses produced by upward negative leaders. Statistical analysis shows that two pulse categories, A and B, closely resemble the classical and narrow preliminary breakdown pulses observed in downward negative lightning, respectively. These findings support the use of upward positive flashes as a proxy for aspects of downward negative flash initiation and provide new constraints on lightning initiation models.

The thesis also identifies a physical mechanism for the characteristic main pulse of Type 1 upward positive flashes. Simultaneous current, electric-field, and optical measurements show that this pulse can be triggered by a downward-propagating recoil leader that retraces a decayed channel and connects to the grounded current-carrying branch at relatively low altitude. This observation reveals a mixed mode of charge transfer in an upward positive flash, and suggests that mixed-mode and M-component-type initial continuous current pulses are limiting cases of a single attachment process dependent on junction-point location.

Finally, the thesis reports the first direct observations of X-rays emitted during upward positive lightning. The detected X-rays are associated with upward negative leader stepping and occur more often when current and electric-field changes are larger. This, combined with their timing and relatively low energies, favor the cold electron runaway mechanism over relativistic avalanche-based models.

Together, the results show that upward lightning at instrumented towers provides a powerful diagnostic window into fundamental lightning physics. By linking direct current measurements with electric-field sensing, optical, VHF, and X-ray observations, this thesis constrains interpretations of lightning initiation, leader propagation, attachment, charge transfer, and energetic radiation.