Settling Velocity of Snow With Varying Morphology In the Atmospheric Turbulence

Abstract

Here we present the field measurements of snow morphology as well as its corresponding settling velocity based on the data collected from multiple deployments at EOLOS Wind Energy Research Field Station at Rosemount, MN, USA. All the deployments were conducted at night, allowing us to implement the in situ large-scale particle image velocimetry (PIV) and particle tracking velocimetry (PTV) to quantify the turbulent flow field and snow particle settling velocity in a sampling area on the order of 10 m as reported by Nemes et al. [J. Fluid Mech. 2017] and Heisel et al. [J. Fluid Mech. 2018]. The general micrometeorological conditions were provided by a 130 m meteorological tower (met-tower) equipped with 4 sonic anemometers and 6 low frequency humidity and temperature sensors at the field site, and the turbulence characteristics were measured using both the met-tower sonics and in situ PIV. In addition, the snow particle size and morphology were captured using digital in-line holography (DIH). Snow particle terminal velocity in quiescent flow is calculated corresponding to the snow morphology based on the modified drag coefficient empirical formulas. The settling velocity of snow particles was captured using in situ PIV or PTV during each deployment. The turbulence conditions from the available deployments varied by an order of magnitude in the Taylor-scale Reynolds number, covering a range of Stokes number and different cases of snow particle-turbulence interaction phenomenology. The snow morphological effect on snow fall speed is quantitatively observed corresponding to the snow particle classification. The comparisons of snow particle fall speed and settling velocity quantify settling velocity enhancement by turbulence. Overall, the presented study focuses on a better understanding of the effects of snow morphology and atmospheric turbulence on the settling velocity of snow in nature.