Effects of Horizontal Wind on Raindrop Fall Speed

During the development of rain, a falling drop usually interacts with other hydrometeors. Because larger raindrops have larger terminal velocities, as they fall they catch-up and collide with smaller drops in their paths. The differential response of drops to gravitational force is considered to be the main cause of raindrop collisions.

Drop falling speeds are used for the calculation of the collision kinetic energy (CKE) involved in an interacting drop pair system. This parameter, along with the surface energy (SE), allows one to establish criteria to determine whether colliding drops would coalesce or break up. Raindrop fall speed is also 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 studies of the effects of soil erosion.

This study presents observations of raindrop fall-speeds during natural rain events gathered with two optical array probes (a 2D-C and a 2D-P) fixed at the ground in a vertical orientation. Raindrop fall-speed measurements were made during summer precipitation events that occurred in Mexico City over several years at the National Autonomous University of Mexico campus (2,280 m a.s.l.), 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. However, the measurements also suggest variations on the amount of large drops (diameters larger than 0.6 mm) falling at velocities different to v_t during windy periods. According to our results, even fall speeds of drops with diameters larger than 1 mm are affected by horizontal wind conditions and the velocity range spread is not uniform, i.e., it does not follow a normal distribution. The number and size range of non-terminal drops increases with wind speed. A plausible explanation for this phenomenon suggested here is the effect of turbulent flows, which is investigated as a part of the present work.