Laboratory studies of the influence of cloud droplet size on charge transfer during crystal-graupel collisions

E. E. AVILA; G. CARANTI; N. E. CASTELLANO; C. P. R. SAUNDERS

JOURNAL OF GEOPHYSICAL RESEARCH

AMERICAN GEOPHYSICAL UNION

Año: 1998 vol. 103 p. 8985 - 8996

0148-0227

Further laboratory measurements of charge transfer between ice crystals and riming graupel pellets, which are thoughtt o be associated with the electrification processes within thunderstorms have been carried out in the University of Manchester Institute of Science and Technology cloud chamber. In experimentsw ith cloudsi n the temperaturera nge -6 øC to -26 øC, the supercooled roplets pectrumh asb eene xtendedto larger droplets izes,a bove 60/xm maximumd iameter,r epresentativoe f the broadests pectrumo bservedi n some thunderstormc loud chargingr egions. The resultsi ndicatet hat at temperaturesfr om -6 øC to -18øC, broadeningt he droplets pectrumle adst o negativeg raupelc harginga t higher valueso f cloud effective liquid water content than has been reported in previous laboratory studies. The significanceo f the result is that in order to ensuret hat laboratorye xperimentss imulatea s closelya s possiblet he thunderstormc loud microphysicael nvironmenta, ttentionm ustb e paid to the spectrumo f dropletsu sed. Two mechanismso f charget ransfert hat may accountf or this behaviora re discussedt,h e relative growthr ate theorya nd the surfaces plintert heory, and both are found to be compatiblew ith the resultso n the assumptionth at the larger dropletsl ead to a reductioni n the rate of vapor depositionto the timing surface. Analysiso f the implicationso f theser esultst o thunderstormel ectrificationre quiresm ore detailso f the evolutiono f droplets pectrai n thundercloudsth, eir spatiala nd temporald evelopmenat nd location relative to observed regions of electrification