Do all raindrops fall at terminal speed?

Abstract

[1] A unique relation between raindrop size and fall speed v t (D) is assumed throughout atmospheric science. Yet, our speed versus size measurements of millions of drops during natural rainfall events show that many intermediate sized raindrops fall up to an order of magnitude faster than expected. Furthermore, images of drop clusters reveal that these ''super-terminal drops'' are differently sized fragments of a recent break-up, moving with the speed of the parent drop and relaxing towards v t (D). Additional evidence of the break-up conjecture includes: (i) positive skewness in the distribution of fall speed deviations, (ii) strong size dependence of fall speed deviations and their maximum values and, (iii) preponderance of super-terminal drops in the presence of large raindrops (i.e., during periods of high rainfall rates). Citation: Montero-Martínez, G., A. B. Kostinski, R.

Further evidence for superterminal raindrops

“…The detection of only superterminal drops for this drop size is spurious; the 2DVD software automatically filters out any drops falling with a vertical velocity smaller than 0.5 m/s; thus, any detected drops less than 0.12 mm have to be superterminal. It is noteworthy that the plots for the two instruments largely agree with each other (and with the results given in Montero-Martinez et al [2009]) despite the fact that each instrument has a different form factor and sensing volume. This is encouraging.…”

“…This discovery has rapidly been noted and referenced in subsequent work [e.g., Niu et al, 2010;Villermaux and Eloi, 2011;Thurai et al, 2013]. The explanation given in Montero-Martinez et al [2009] asserts that these "superterminal" drops are likely the result of the breakup of larger, faster-moving drops that generate fragments that initially continue to fall at the speed of the larger, "parent" drop. These fragments presumably slow down in time, but if the breakup occurred shortly before detection, the fragments can still be moving faster than their terminal speed.…”

“…The data presented here come from six different rain events; summary of the event statistics are presented in Table 1. The 2DVD in these six events detected a total of over 1.5 million raindrops; the LPMs combined to detect over 22 million drops, making this data set over 2 orders of magnitude larger than that explored in Montero-Martinez et al [2009].…”

“…The authors of Montero-Martinez et al [2009] defined a superterminal drop as any drop exceeding its expected terminal velocity by at least 30%. We use the same definition here.…”

“…A recent study [Montero-Martinez et al, 2009] revealed that small raindrops often fall at speeds faster than the terminal velocity of liquid drops in still air. This discovery has rapidly been noted and referenced in subsequent work [e.g., Niu et al, 2010;Villermaux and Eloi, 2011;Thurai et al, 2013].…”