Electrification in Mesoscale Updrafts of Deep Stratiform and Anvil Clouds in Florida

摘要

Airborne observations in deep stratiform and anvil clouds showed extensive layers of 10- to 40- kV/m electric fields colocated with highly stratified uniform radar reflectivity of 20 to 25 dBZ from 5- to 9-km altitude. Size distributions with numerous small- and intermediate-sized ice crystals (mostly aggregates) and large aggregates were observed in these regions. We infer that the uniform electric field, radar reflectivity, and broad particle size distributions were the result of mesoscale updrafts, confirmed by high-resolution images of particles showing diffusional growth and no riming. No measurable supercooled liquid water was found in these regions from -10 to -45 °C. Calculated particle collision rates from observed distributions were >5 × 10~3 collisions per cubic meter per second in this volume. Laboratory results show that weak charge separation occurs when ice particles collide and separate even in the absence of supercooled water. We infer that charge separation occurred in the mesoscale updrafts via a noninductive mechanism in which ice particles growing by diffusion collide and transfer charge without supercooled water being present. These regions with strong, uniform fields, stratified radar reflectivity, and broad size distributions also occurred in anvils that barely reached the melting zone. Thus, we deduce that the nonriming collisional mechanism acts at middle to upper cloud levels and is not dependent upon electrification occurring near the melting zone. This mechanism should produce two oppositely charged layers of charge with the top layer residing on smaller particles often existing near the top of the cloud.