Identification and Analysis of Collisional Breakup in Natural Rain

Abstract Numerous laboratory and numerical studies have been dedicated to understanding collisional breakup as one of the most important processes in rain formation. The present study aims to identify when, in natural rain, collisional breakup is dominant and thus able to modify the shape of the raindrop size distribution (DSD), up to the equilibrium DSD. To this end, an automated objective algorithm has been developed and applied to a total of more than 6000 two-minute-averaged DSDs. Since breakup is mostly observed in heavy precipitation, the method was applied to the DSDs where rain rate was above 5 mm h−1. The selected breakup DSDs had good agreement with those predicted to be the equilibrium DSD by different theoretical models. The equilibrium DSD was found in a variable fraction of the total samples (0%–7%), confirming that the onset of equilibrium is a rare event in natural rain. The occurrence of a DSD in which breakup is dominant and modifies the DSD but the equilibrium DSD is not reached is higher (15%–47%). The gamma distribution, which has been widely used in the parameterization of observed size spectra, had a poor fitting in breakup-induced DSD, especially in the 1.0–2.6-mm-diameter interval. This can impact applications for which the parameterization of DSD is needed, such as in the retrieval of a DSD integral parameter (such as rain rate) from active remote sensor data