The Effect of ABL Flow Characteristics on Rainfall Microphysics and UAS-based Wind Measurements

This study investigates the rainfall microphysical characteristics such as fall velocity, axis ratio, and canting angle during strong wind shear and high turbulence levels. These characteristics have a direct impact on the accuracy of the radar-based rainfall rate retrieval which indirectly influences urban hydrology, flash-flood warning systems, and many hydrological applications. Under natural precipitation, these characteristics were observed by using a novel instrument called the high-speed optical disdrometer (HOD), which has higher resolution and lower uncertainty levels than the conventional disdrometers. Simultaneously, the surrounding airflow was also observed by using a 3-D ultrasonic anemometer to be able to clarify the interaction between the atmospheric turbulence and raindrop characteristics. The results revealed that these characteristics of the raindrops were strongly affected by the atmospheric turbulence and wind shear. The fall velocities were found to be deviating from the correspondent terminal velocity and become either enhanced or reduced under windy conditions. The axis ratios of raindrops that are larger than 2.0 mm in diameter were augmented during high turbulent times. Furthermore, the standard deviation of the canting angle of the raindrops was also found to be increasing during windy times compared to relatively calm conditions. In addition, the atmospheric turbulence measurements by using a 3D ultrasonic anemometer attached to a small unmanned aircraft system (sUAS) were practiced to accompany the future planned sUAS -based rainfall microphysics measurements to elucidate the rainfall vertical profile of reflectivity.