Charge Separation in Waterdrop Collisions and its Role in Warm-Cloud Electricity.
This work details the procedure, results and application of laboratory experiments conducted to determine whether a significant separation of electric charge accompanies the collision with fragmentation of two freely falling, uncharged raindrops. The waterdrops used in the experiments had diameters of 2.12 and 1.68 mm, were essentially uncharged, collided at inclinations of 0-3(DEGREES) from the vertical and attained pre-collision fallspeeds of 3.4 and 2.1 m s('-1) respectively. In an electric field of less than 100 V m('-1) no significant amount of charge was separated (< 5 x 10('-14) C) in any of over 500 collision-breakup events. In an electric field of 50 kV m('-1) charge of order (+OR-)10('-12) C was separated often (by induction). The most important conclusion from the experiments is that, based on the breakup modes observed, the inductive charging mechanism between raindrops dissipates vertical electric fields inside all-water clouds. To complement the experiments a numerical simulation was undertaken of the electrical feedback in a rainshaft between the microscale separation of charge by induction and the ambient electric field (vertical, uniform). Raindrops ranged in diameter from 100 (mu)m to 4 mm; 20 diameter categories were employed. The study indicates that the build-up of vertical electric fields inside all-water clouds must be associated with low rainfall rates. A supplementary investigation shows that rainshafts immersed in a vertical -350 kV m('-1) electric field for a period of 30 seconds carry down net positive charge in amounts ranging from less than +1 C km('-3) in rain of intensity below 7 mm h('-1) to +36 C km('-3) in 89-100 mm h('-1) rain. Similarities with the center of positive charge which is sometimes found near the base of thunderclouds were identified and analyzed.
- Pub Date:
- Physics: Atmospheric Science