Earle Williams, MIT

Title: "Lightning Activity in Winter Storms: A Meteorological and Cloud Microphysical Perspective" (Video)

Abstract:  The meteorological context, predominant cloud microphysics, electrical charge separation and lightning in winter storms in a wide variety of geographical locations is reviewed. Both similarities and differences between summer and winter conditions are discussed. The observations on the electrical structure of summer thunderstorms are used as a guide for behavior in winter conditions, given the evidence for a key role for mixed phase (involving all three phases of water active) microphysics, but with a remaining puzzlement about the origin of the energetic positive ground flashes prone to generate sprites in the upper atmosphere in some winter storms. Particular emphasis is given to the ‘graupel dipole’, the dominant player in active phases of summer thunderstorms worldwide, and a ‘snow dipole’, whose existence is manifest in summer conditions in the End of Storm Oscillation and in the positive end of lightning ‘bipoles’ in the transition season between summer and winter, and in the trailing stratiform region of large continental squall lines. Outstanding problems in winter storm electrification are discussed.   The commonly used term “thundersnow” is in all likelihood a misnomer, as the initiation of lightning in winter is more commonly linked with graupel (an exceptional condition in winter) than with snow (a common condition in winter).  The measurement techniques that unveil the electrical structure of thunderstorms will be reviewed.