How does the rotation of particles affect precipitation settling velocity in the atmospheric surface layer*

The effect of particle rotation on the settling velocity of hydrometeors in the atmospheric surface layer (ASL) is not accounted for in numerical weather prediction (NWP) models. We present field study data obtained at a high-elevation mountain in Utah between December 2020 and April 2021. Several new instruments were deployed, including a particle tracking laser-camera system, a sonic anemometer, and a Differential Emissivity Imaging Distrometer (DEID). We characterize the first coincident measurement of individual particle trajectory, ASL turbulence, and the high-resolution shape/structure of the particle. Particle Tracking Velocimetry was used to measure fall and angular velocity. The turbulence levels were characterized by Taylor Reynolds numbers, which vary from 100 to 20,000. $Sv = v_t/u^\prime$ is large-scale settling parameter, where $v_t$is the terminal velocity in still air, $u^\prime$ is the turbulent velocity fluctuation of the air. We report sweeping at low turbulence case, $Sv>1$, and loitering at high turbulence case, $Sv<1$. The probability distribution function (pdf) of angular velocity was found to be near Gaussian distribution in low levels turbulence case and near Cauchy distribution in high levels turbulence case. The direct measurement of the particle's micro-physical properties, settling velocity, and turbulence around the particle is important to characterize the settling velocity near the surface.