Orientation angle distributions of drops after an 80-m fall using a 2D video disdrometer

This note reports on the use of a 2D video disdrometer to estimate the orientation of drops (> 2 mm) that were generated artificially and allowed to fall 80 m from a bridge with no obstruction and under calm conditions. This experimental setup enabled a large number of drops to be generated, up to 10 mm in horizontal dimension. The distribution of the canting angles for all drops > 2 mm was found to be nearly symmetric about 0 degrees with standard deviation between 7 degrees and 8 degrees. From the canting angle distributions derived from the two orthogonal camera view planes, the distributions Of the polar (theta) and azimuth (phi) angles were deduced: these two angles describe the 2D orientation of the symmetry axis. The azimuthal angle distribution was found to be nearly uniform in the range (0, 2 pi), whereas the distribution of p(Omega)(theta) = p(theta) sin theta was similar in shape to a special form of the Fisher distribution that is valid for describing the statistics on a spherical surface. The standard deviation of p(Omega)(theta) showed that larger drops are more stably oriented than smaller ones. This is in agreement with previous radar-based results of standard deviation of the canting angle decreasing with increasing Z(dr).