  Relativistic, charged particles present in extensive air showers lead to a
coherent emission of radio pulses which are measured to identify the shower
initiating high-energy cosmic rays. Especially during thunderstorms, there are
additional strong electric fields in the atmosphere, which can lead to further
multiplication and acceleration of the charged particles and thus have
influence on the form and strength of the radio emission. For a reliable energy
reconstruction of the primary cosmic ray by means of the measured radio signal
it is very important to understand how electric fields affect the radio
emission. In addition, lightning strikes are a prominent source of broadband
radio emissions that are visible over very long distances. This, on the one
hand, causes difficulties in the detection of the much lower signal of the air
shower. On the other hand the recorded signals can be used to study features of
the lightning development. The detection of cosmic rays via the radio emission
and the influence of strong electric fields on this detection technique is
investigated with the LOPES experiment in Karlsruhe, Germany. The important
question if a lightning is initiated by the high electron density given at the
maximum of a high-energy cosmic-ray air shower is also investigated, but could
not be answered by LOPES. But, these investigations exhibit the capabilities of
EAS radio antenna arrays for lightning studies. We report about the studies of
LOPES measured radio signals of air showers taken during thunderstorms and give
a short outlook to new measurements dedicated to search for correlations of
lightning and cosmic rays.
