Preferential Deposition of Snow and Dust Over Hills: Governing Processes and Relevant Scales

Abstract

Preferential deposition of snow and dust over complex terrain is responsible for a wide range of environmental processes and accounts for a significant source of uncertainty in the surface mass balances of cold and arid regions. Despite the growing body of literature on the subject, previous studies reported contradictory results on the location and magnitude of deposition maxima and minima. This study aims at unraveling the governing processes of preferential deposition in a neutrally stable atmosphere and to reconcile seemingly inconsistent results of previous works. For this purpose, a comprehensive modeling approach is developed, based on large eddy simulations of the turbulent airflow, Lagrangian stochastic model of particle trajectories, and immersed boundary method to represent the underlying topography. The model is tested against wind tunnel measurements of dust deposition around isolated and sequential hills. A scale analysis is then performed to investigate the dependence of snowfall deposition on the particle Froude and Stokes numbers, which fully account for the governing processes of inertia, flow advection, and gravity. Model results suggest that different deposition patterns emerge from different combinations of dimensionless parameters, with deposition maxima located either on the windward or the leeward slope of the hill. Additional simulations are performed, to test whether the often used assumption of inertialess particles yields accurate deposition patterns. Results indicate that this assumption can be justified when snowflakes present dendritic shape but may generate unrealistic results for rounded particles. We finally show that our scale analysis provides qualitatively similar results for hills with different aspect ratios.

Continuous Spatio-Temporal High-Resolution Estimates of SWE Across the Swiss Alps – A Statistical Two-Step Approach for High-Mountain Topography

“…This could be due to downward winds, which cause an increased deposition in the lee of mountain ridges, where winds are usually stronger than at lower elevations of glaciers. However, this relationship was not always confirmed for non-glacierized sites located in areas characterized by a different terrain morphology (e.g., Comola et al, 2019).…”

Gentle Topography Increases Vertical Transport of Coarse Dust by Orders of Magnitude

“…However, to ensure this time scale represents the inertial particle trajectory rather than the flow trajectory, the time scale is updated using the particle velocity and the correction formula in Sawford and Guest (1991). The time scale correction is also discussed elsewhere (see, e.g., Comola et al, 2019;Wilson, 2000). The scheme of Bailey ( 2017) is used to improve numerical stability when integrating the Langevin equation.…”

“…The immersed boundary and Lagrangian particle codes described above were previously used in studies of canopy turbulence (Chen et al, 2019(Chen et al, , 2020, and the inclusion of inertial effects follows Comola et al (2019). The particle parameters for this study are listed in Table 2, noting the density is based on silica dust.…”

Radar measurements of blowing snow off a mountain ridge

“…Nishimura et al (2019) recently applied 15 SPCs and ultra-sonic anemometers on a flat field to reveal the spatiotemporal structures of blowing snow near the surface and explore the interaction with the turbulent flow structures. Several studies have simulated wind-affected snow redistribution and accumulation by relating atmospheric wind fields to resulting snow deposition patterns in mountainous terrain (Dadic et al, 2010;Winstral et al, 2013;Mott et al, 2014;Vionnet et al, 2017;Gerber et al, 2017;Wang and Huang, 2017). Flow structures around a utility-scale 2.5 MW wind turbine have previously been measured by Hong et al (2014) using a field particle-imaging velocimetry (PIV) set-up with snow precipitation as the tracer particles.…”