Feng and Beard [1991] developed a perturbation model of raindrop
oscillation which includes aerodynamic effects using the method of
multiple moments. They show that the characteristic frequencies for
the axisymmetric oscillation (oblate-prolate type oscillation) modes
increase with increasing flow past the drop, while the sectoral modes
(oscillation between ellipsoids with alternating major axis 90
apart in the horizontal plane) decrease. They also suggest that the
fine structure in the frequency spectrum of falling drops may play
a role in determining which of the two modes mentioned above are
dominant for a given raindrop.
A general theory for the drag and fall speed of hydrometeors has been developed by Bohm [1992a] which includes the effects of hydrometeor growth and melting on terminal velocity. This theory takes into account continuously varying particle shape and mass, including columnar and branched planar ice crystals, rimed and unrimed aggregates, lump, conical, and hexagonal graupel, hail, and raindrops.