On the behaviour of raindrop fall speed due to wind

Measurements of raindrop fall speed gathered at the ground over several years are presented and analysed in view of the horizontal wind intensity during sampling. It is confirmed that super-terminal drops occur only for diameters less than about 0.7 mm and that their percentage numbers increase during windy periods. Furthermore, through statistical and post hoc data analyses, it is shown that wind affects the fall speed of individual raindrops. A remarkable feature is the detection of sub-terminal raindrops, i.e. those having slower fall speeds than their calculated terminal velocities, smaller than 2 mm. Evidence that wind affects raindrop behaviour includes observations showing that under windy conditions: (i) super-terminal drops exhibit larger fall speed departures from the expected terminal speeds than in calm wind periods; and (ii) the percentage number of sub-terminal drops with diameters between 1 and 2 mm increases. These two findings imply a general broadening of the corresponding velocity distributions. Potential explanations for the occurrence of sub-terminal drops are given based on turbulence (small raindrops respond faster to changes in air conditions), drop oscillations (which produce larger transit times through the instrument sampling volume) and the enhancement of transverse drifts and trajectory deviations with respect to the vertical. However, other possibilities should be also addressed for drops with the largest fall speed anomalies (greater than two times the instrument uncertainty) as well as their implications for microphysical processes.