[link to news.sciencemag.org]
" The idea that these air showers could cause lightning when they pass through a thundercloud has been around for 2 decades. In 1992, Russian physicist Alexandr Gurevich of the Lebedev Physical Institute in Moscow suggested that the high-energy particles produced by a cosmic ray strike ionize the air in thunderclouds, creating a region with a lot of free electrons. The thundercloud's electric field accelerates the electrons almost to the speed of light, boosting them to very high energies. Then the electrons collide with atoms in the air, generating even more electrons as well as x-rays and gamma rays. This avalanche of high-energy particles in the cloud—which Gurevich calls "runaway breakdown"—provides ideal conducting conditions for lightning. "
I think that aliens have the tecnology to obtain high energy from water and cosmic ray.