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

Abstract

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.

Keywords

• raindrop shape
• tropical cyclones
• rainfall microphysics
• polarimetric radar
• axis ratio parameterization