What produces super-terminal raindrops?

Raindrop fall speed is an important parameter in cloud physics modeling and other meteorological applications. Over the last decades, various studies have accomplished fall-speed measurements of natural raindrops with different instruments and reported a large spread of values. In particular, it has been observed that small raindrops (diameters smaller than 0.5 mm) fall at larger velocities than expected. These so-called super-terminal drops may produce changes in the estimation of raindrop size distributions and rainfall intensity obtained with instruments such as disdrometers, as well as in the calculated breakup kinetic energy used, for example, in erosion effects studies. The hypotheses proposed to explain these deviations from theoretical — terminal — speeds, mostly refer to the production of satellite drops from breakup of larger raindrops, a “wake” effect or other effects due to turbulence.

This study presents observations of raindrop fall-speeds during natural rain events restricted to calm conditions, i.e., maximum horizontal wind speed of 2 m s^-1, gathered with two optical array probes (a 2D-C and a 2D-P) fixed at the ground in a vertical orientation. The measurements were made during 2002, 2004 and 2006 at the National Autonomous University of Mexico campus (2,280 masl), located in the southern end of Mexico City. The data show the simultaneous occurrence of super-terminal drops and drops falling near their predicted theoretical terminal speeds (v_t), with deviations from v_t becoming larger and more frequent as the rainfall rate increases.

In the present report, the three hypotheses mentioned above were considered to explain the origin of these observations.The plausibility for the discrepancies to be produced by instrumental and methodology errors was studied and discarded. Analyses of probability distributions carried out to the data allow one to conclude that the break-up of large raindrops constitutes the best explanation for the origin of super-terminal drops, as opposed to turbulent and flow-distortion processes.