The Possible Wind Effect on the Travelling Speed and Collisional Breakup of Raindrops in the Near-Surface of Landfalling Typhoons

Based on datasets from the laser-optical disdrometers and anemometers mounted at four altitudes(10m, 40m, 160m and 320m) of a 356-m meteorological tower located in Shenzhen, China, the possible wind effect on the travelling speed and collisional breakup of raindrops in the near-surface of two landfalling typhoons in 2017 are analyzed. It is interesting to note that larger raindrops are eroded with an increasing amounts of median-sized raindrops at 10m/40m height under stronger vertical wind shear (VWS). The observed vertical velocity of raindrops is significantly deviated from the terminal velocity for stagnant flows, especially for large ones with diameter exceeding 3mm, causing the reduction of relative speed of the binary raindrops at vertical plane. However, the relative speed of the colliding raindrops at horizontal direction can also be induced by VWS in terms of the theoretical calculation based on the observed vertical wind profile. Owing to the wind effect, the overall relative speed of colliding large raindrops with similar sizes is likely to be enhanced, leading to the increase of collisional kinetic energy and possibility of collisional-breakup. Therefore, wind effect should be accurately parameterized into the travelling velocity and collisional-coalescence and breakup of raindrops in the microphysical scheme to improve the simulated vertical evolution of raindrop size distribution in the near-surface and the ultimate precipitation feature on the ground.