Laboratory Observations of Preferential Flow Paths in Snow Using Upward-Looking Polarimetric Radar and Hyperspectral Imaging

Abstract

The infiltration of liquid water in a seasonal snowpack is a complex process that consists of two primary mechanisms: a semi-uniform melting front, or matrix flow, and heterogeneous preferential flow paths. Distinguishing between these two mechanisms is important for monitoring snow melt progression, which is relevant for hydrology and avalanche forecasting. It has been demonstrated that a single co-polarized upward-looking radar can be used to track matrix flow, whereas preferential flow paths have yet to be detected. Here, from within a controlled laboratory environment, a continuous polarimetric upward-looking C-band radar was used to monitor melting snow samples to determine if cross-polarized radar returns are sensitive to the presence and development of preferential flow paths. The experimental dataset consisted of six samples, for which the melting process was interrupted at increasing stages of preferential flow path development. Using a new serial-section hyperspectral imaging method, polarimetric radar returns were compared against the three-dimensional liquid water content distribution and preferential flow path morphology. It was observed that the cross-polarized signal increased by 13.1 dB across these experiments. This comparison showed that the metrics used to characterize the flow path morphology are related to the increase in cross-polarized radar returns spanning the six samples, indicating that the upward-looking polarimetric radar has potential to identify preferential flow paths.