Collision efficiencies empirically determined from laboratory investigations of collisional growth of small raindrops in a laminar flow field

In laboratory experiments at the vertical wind tunnel of the University of Mainz the collisional growth of drops with radii between 70 and 170 μm in radius were observed while the collector drop freely floated in a cloud of droplets with radii ranging from 1 to 7 μm. Previously existing tables with collision efficiency values were interpolated and completed in such a way that drop growth rates calculated with these collision efficiencies match with observed growth rates. These new tables provide collision efficiency values for a wide range of drop sizes and radius ratios p including those ranges where efficiency values missed so far. This is of high importance for small p-ratios where the collision efficiency changes significantly. The empirically derived collision efficiencies can be used in cloud models. Growth rates observed during the laboratory experiments on drop collisions and those obtained by using model simulations of continuous growth using theoretically determined values of the collision efficiencies are in very good agreement.