Abstract
Abstract. Collision-induced water drop breakup in a vertical wind tunnel was observed using a high speed camera for interactions between larger drop sizes (up to 7 mm diameter) than have previously been experimentally observed. Three distinct collisional breakup types were observed and the drop size distributions from each were analysed for comparison with predictions of fragment distributions from larger drops by two sets of established breakup parameterisations. The observations showed some similarities with both parameterisations but also some marked differences for the breakup types that could be compared, particularly for fragments 1 mm and smaller. Modifications to the parameterisations are suggested and examined. Presented is also currently the largest dataset of bag breakup distributions observed. Differences between this and other experimental research studies and modelling parameterisations, and the associated implications for interpreting results are discussed. Additionally, the stochastic coalescence and breakup equation was solved computationally using a breakup parameterisation, and the evolving drop-size distribution for a range of initial conditions was examined. Initial cloud liquid water content was found to have the greatest influence on the resulting distribution, whereas initial drop number was found to have relatively little influence. This may have implications when considering the effect of aerosol on cloud evolution, raindrop formation and resulting drop size distributions. Calculations presented show that, using an ideal initial cloud drop-size distribution, ~1–3% of the total fragments are contributed from collisional breakup between drops of 4 and 6 mm.
Highlights
Raindrop breakup events are important to the evolution and formation of drop size distributions in precipitating clouds, for warm rain processes in which frequent collision, coalescence and breakup events play a major role in the production of raindrop-sized precipitation
To investigate the nature of this phenomenon, researchers have performed experimental laboratory studies to observe breakup events, modelling simulations of the evolution of cloud drop size distributions based on parameterisations from the experimental observations, and compared results to observations of drop size distributions in natural clouds
The disintegration of the filament bridge tends to depend on the thickness variations along its length; the thicker parts typically form larger drops as the filament destabilises and pinches during drop formation
Summary
Raindrop breakup events are important to the evolution and formation of drop size distributions in precipitating clouds, for warm rain processes in which frequent collision, coalescence and breakup events play a major role in the production of raindrop-sized precipitation. To investigate the nature of this phenomenon, researchers have performed experimental laboratory studies to observe breakup events, modelling simulations of the evolution of cloud drop size distributions based on parameterisations from the experimental observations, and compared results to observations of drop size distributions in natural clouds. Very few experimental laboratory investigations have been conducted to measure the drop size distributions resulting from collision-induced breakup. To our knowledge to date, these studies include McTaggart-Cowan and List (1975a), Low and List (1982a), and Barros et al (2008), and all three have employed an experimental setup which involved colliding falling drops. McTaggart-Cowan and List (1975a) and Low and List (1982a) used a drop acceleration system in which drops created at two sources, vertically accelerated toward terminal velocity as they fell. Of 25 000 drop collisions, it was reported that only 712 resulted in breakup, with the breakup type
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