Thunderstorm graupel charging in the absence of supercooled water droplets

LUQUE M.; BÜRGESSER R.; AVILA E. E.

QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY

JOHN WILEY & SONS LTD

Lugar: LOndres; Año: 2016

0035-9009

This study reports new measurements of the charge separation during ice crystal?graupelcollisions in the absence of supercooled water droplets. Experimental measurements of theaverage charge acquired by a simulated graupel, under condition of subsaturation withrespect to water and supersaturation with respect to ice, were performed. The relativehumidity of the air was controlled and measured during the experiments. The experimentswere carried out in the temperature range between −7 and −21 ◦C, with a relative velocityof 3ms−1 between the ice particles and the graupel. The aim of the measurements wasto study the dependence of the non-inductive mechanism on the presence/absence ofsupercooled water droplets. Present results together with previous findings indicate thatthe sign of the graupel charging depends on the presence/absence of supercooled waterdroplets and on the relative humidity (subsaturation/supersaturation) of the environmentin the case of absence of water droplets. This laboratory study is the first to map thenon-riming conditions with low velocity impact and these new results confirm the negativecharging of graupel under these conditions. It was found that the magnitude of the chargeseparated per collision in non-riming conditions is of the same order of magnitude as inriming conditions. This suggests that the non-inductive mechanism could be operating inthe storms even in the absence of water droplets, emphasizing the importance of knowledgeof the environmental thermodynamic conditions in order to know the charging behaviourof the graupel particles.