Evolution of Raindrop Spectra with Collision-Induced Breakup

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Abstract
A numerical model was set up to study the evolution of raindrop spectra by collision-induced breakup as measured in the laboratory. The main conclusion is that drops with diameters larger than 2-3 mm, failing in a population of smaller drops typical of natural rain, break up in comparatively short times (1–5 min in rainfalls of 100 mm h−1). The presence of large drops (4–6 mm) in (cold) rain produced by the Wegener-Bergeron-Findeisen mechanism through melting of ice particles can be attributed to the short time available for large drops to break up in sufficient numbers during the time of fall after melting. Large drops are scarce in (steady-state) warm rain because they break up in collisions and rarely reach diameters larger than 2.5 mm. Hence, the standard notion of a critical diameter of 5–6 mm which raindrops are supposed to reach before breakup due to aerodynamic instability is no longer acceptable. Abstract A numerical model was set up to study the evolution of raindrop spectra by collision-induced breakup as measured in the laboratory. The main conclusion is that drops with diameters larger than 2-3 mm, failing in a population of smaller drops typical of natural rain, break up in comparatively short times (1–5 min in rainfalls of 100 mm h−1). The presence of large drops (4–6 mm) in (cold) rain produced by the Wegener-Bergeron-Findeisen mechanism through melting of ice particles can be attributed to the short time available for large drops to break up in sufficient numbers during the time of fall after melting. Large drops are scarce in (steady-state) warm rain because they break up in collisions and rarely reach diameters larger than 2.5 mm. Hence, the standard notion of a critical diameter of 5–6 mm which raindrops are supposed to reach before breakup due to aerodynamic instability is no longer acceptable.

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