Wind tunnel experiments of spontaneous and collision-induced breakup of raindrops and model simulations of the impact of breakup on precipitation

Abstract

Experiments were performed at the Mainz vertical wind tunnel where two types of drop breakup were investigated with freely floating drops. Spontaneous breakup under laminar conditions resulted in 5 % breakup events of drops with equivalent diameters between 8 and 13 mm by separating in the center due to strong oscillations in horizontal mode into two nearly equal sized fragments. In a turbulent airflow, several oscillation types mainly in vertical and transverse directions caused significant deformations of the drops, which seemed to promote the coherence of the drops. Collision-induced breakup occurs when two drops collide and break up into several fragments. To fill the gaps of missing drop sizes in earlier measurements, a set of relevant drop pairs with diameters of 4.0 to 5.2 mm and 1.9 to 3.1 mm was investigated. With a mean breakup efficiency of 0.55, approximately half of breakup events resulted in two or three fragments, the other half in the formation of more than 3 up to 9 fragments. The measured fragment size distributions were compared to results from earlier parameterizations so that modifications due to the new data set were identified. Accompanying the experiments, process studies with a 3D cloud model were performed to study the impact of breakup on drop size distributions and precipitation. It was found that breakup reduces the numbers of the largest raindrops, thereby delays the onset of precipitation, possibly reduces total precipitation amounts and changes the local distribution of precipitation. Important parameters in the process studies were the breakup efficiency as well as the fragment numbers and sizes.