The Variety of Raindrop Shapes and Their Implications for Radar Estimates of Rain

A water drop falling in air is predicted by theory to have an equilibrium shape and axis ratio in response to a steady aerodynamic forcing. However, raindrops have a variety of shape perturbations, because of oscillations caused by both vortex shedding and drop collisions, and because of tilting and distortion caused by turbulence and wind shear. The difficulty has been to measure naturally occurring raindrop axis ratios and shape perturbations. Laboratory raindrop shape measurements in still air or a vertical wind tunnel fail to duplicate the interactions of raindrops. Field measurements of raindrop shape at the ground are compromised by enhanced shear near the surface. Similarly, shape measurements by aircraft are affected by accelerated airflow ahead of airplane. Indeed, tilted, nonequilibrium drop shapes seem to be the norm for camera data obtained in rain at the ground or 2DP data from aircraft, presumably because of the shear induced distortion.

We analyzed raindrop shapes from aircraft data obtained by 2DP probes during the 1991 Convection and Precipitation/Electrification Experiment (CaPE). The image canting angles of 20 - 45 degrees were found to significantly larger than the tilt of the images created by elliptical raindrops falling through the 2DP probe array. We also found a horizontal asymmetry in the images for all raindrop sizes in the form of a triangular distortion. The persistent horizontal asymmetry, the large canting angle, and the greater flattening of larger raindrops, all indicate a nontrivial influence on raindrop shape by the airflow disturbance ahead of the aircraft. In this paper, we will present results of model calculations of raindrop distortion in the accelerated airflow ahead of 2DP probes for comparison with aircraft-measured raindrop shape. The model results and aircraft observed raindrop shapes will be used to estimate errors in rainfall rates where these data are used to formulate radar algorithms.