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

Abstract

Raindrops are usually parameterized by oblate spheroids with axis ratios (ARs) determined from projected images. However, the adequacy of representing the 3-Dimensional (3D) nature of raindrops with their 2-Dimensional (2D) shadows remains an open question. This study reexamines raindrop AR parameterizations by applying a 3D oblate spheroidal reconstruction algorithm to 2D video disdrometer (2DVD) observations during the passages of 14 tropical cyclones. The results reveal an appreciable AR overestimation for large raindrops when retrieved from a single camera and especially for 2DVD raw products. This overestimation propagates into the simulated differential reflectivity (ZDR), resulting in an underestimation of ZDR by up to 0.5 dB at X-band when large raindrops dominate the raindrop size distribution (RSD), which would introduce an underestimation of more than 20% for heavy rain rate (>60 mm hr−1) estimation by polarimetric radar. In precipitating systems with drastically changing RSDs, the importance of adopting more realistic ARs is warranted.