Binary Collisions of Water Drops in Presence of Horizontal Electric Fields: A Wind Tunnel Study

Abstract

Manifestation of several dynamical, microphysical and electrical processes and their mutual interactions in the cloud result in rainfall, the most desired component of global water cycles. In warm clouds, raindrops are formed by a "Chain reaction'' involving several microphysical processes such as nucleation, condensation, collision, coalescence, and the breakup of drops (Langmuir, 1948). Drop breakup is an important process that limits the maximum size of the raindrops. Two mechanisms are usually considered to be responsible for the drop breakup process that controls the drop size inside the clouds: (a) collision-induced breakup following binary collisions of raindrops in which drops temporarily coalesce and then break up into several fragments, and (b) spontaneous breakup where a single large raindrop becomes hydro-dynamically unstable and breaks up spontaneously into smaller fragments in absence of collision. Generally, large raindrops are formed either as a result of the melting of large ice pellets or snowflakes originating from mixed-phase regions (Hobbs & Rangno, 2004). Both the breakup processes contribute to the evolution of raindrop size distribution (DSD) and may have different significance to drop spectrum evolution in different clouds. However, owing to difficulties in the formation of large raindrops in the clouds, several experiments and theoretical studies emphasize the collision-induced breakup process as the overwhelming cause of drop breakup and are considered to be as influential to the resulting DSD in warm rain processes (Pruppacher and Klett, 2010).