Revisiting Raindrop Axis Ratios Based on 3D Oblate Spheroidal Reconstruction: 2D Video Disdrometer Observations During Tropical Cyclone Passages

Abstract

Raindrops, forced by gravity, air resistance, surface tension, etc., are characterized by increasing asymmetry in nature as they grow. This asymmetrical shape can be approximately represented by an oblate spheroid with an axis ratio (AR) defined by the ratio of minor to major axes. Although the shapes of raindrops periodically change as they fall, which is known as drop oscillations (

ZDR Backwards Arc: Evidence of Multi‐Directional Size Sorting in the Storm Producing 201.9 mm Hourly Rainfall

“…To the best of our knowledge, this is the first report presenting this distinct feature. The MSS process is in line with underlying arc-shaped updrafts, which take smaller drops toward the rainfall center, leaving an arc-shaped region dominated by large raindrops with a high degree of non-sphericity (Zheng et al, 2023), as indicated in previous model simulations (Sun, Li, et al, 2023;Wei et al, 2022;Yin et al, 2022).…”

Raindrop Deformation in Turbulence

“…This study followed the shape correction method proposed by Schönhuber et al (2016). Then, the raindrop axis ratios were retrieved using the 3D oblate spheroidal reconstruction method based on the two orthogonal corrected shapes (Zheng et al, 2023). Although strong turbulence may cause raindrops to become irregular and deviate from oblate spheroidal shapes (Bolek & Testik, 2021;Thurai et al, 2019), our study still adopted the oblate spheroidal assumption, as this assumption satisfied our requirement for obtaining the core information (axis ratio) that characterizes raindrop shape.…”

“…Raindrops are three-dimensional (3D) objects in nature, while their characteristics are usually inferred from two-dimensional (2D) shadows measured by in situ sensors. In light of this challenge, we recently developed an algorithm to reconstruct "realistic" 3D raindrops by optimizing the information from two orthogonal 2D projections of raindrops (Zheng et al, 2023). In this study, this algorithm combined with collocated turbulence measurements was applied to more than 8 million raindrops with careful assessment of the wind effect to ascertain the impact of turbulence on raindrop canting angles, shapes and fall speeds.…”