Effects of altitude on maximum raindrop size and fall velocity as limited by collisional breakup

Coalescence and breakup of drops are recognized as the main mechanisms determining raindrop size distributions on the ground. Full knowledge of these processes is hindered by the challenging difficulties both in the laboratory and tunnel experiments and during observations in the open air. In real rain breakup is mainly due to collision between drops of different sizes (collisional breakup) and occurs when the Collisional Kinetic Energy (CKE) is not absorbed by the colliding drops. In this work, we observe and measure the dependence on altitude of the occurrence of collisional breakup in real rainfall events, then we estimate the corresponding limit terminal velocities of drops and their size when breakup significantly takes place. Data from Pludix, an X-band microwave disdrometer, were collected at three locations at different elevations: collisional breakup position in the power spectrum of Pludix increases towards higher frequencies with increasing altitude.. Terminal velocities and sizes of the drops at breakup were determined consequently, with drop sizes resulting in 4.55  0.35 mm, 4.02  0.32 mm and 3.16  0.3 mm for altitudes of 15, 950 and 3300 m a.s.l. respectively. We computed the CKE of the colliding drops at the breakup, finding an upper limiting values of about 1.22  10-5 J for all three altitudes. This shows that most dominant collisional breakup signature occurs at similar CKE values for all three locations, corresponding to different drop diameters at different altitudes because of the effect of air density on the drop terminal velocity.