The underside of the ice: Exploring the relationship between ice thickness and the roughness of the ice-water interface for freshwater lakes in Central Ontario

Seasonal lake ice coverage holds key importance for travel in northern communities and complex ecosystem processes. Lake ice thickness is an important safety consideration for the winter tourism economy in Ontario and a vital parameter for future lake ice coverage projections and broader-scale climatic changes. Radar remote sensing retrievals of lake ice thickness are being explored; however, this work is complicated by the lack of a physical exploration of the structure and scale of roughness features at the ice-water interface surface in a natural lake, which is the primary scattering mechanism of L-band over lake ice. We present a novel method for measuring the roughness at the underside of the ice and show that a statistically significant temporal relationship was observed where the roughness of the ice-water interface decreased as total ice thickness grew over each season. Interestingly, 17 spatially distributed roughness samples from both study lakes showed a different result: thicker total ice tended to have a rougher ice-water interface surface, but this relationship was not statistically significant. We highlight a potential relationship between under-ice water circulation and black ice roughness and present an average ice-water interface feature height of 0.988 mm, which can provide context and validation for previously modeled roughness. Our study represents the first known quantification of roughness from temperate region lakes. This work provides an exploration of an under-studied hydrological feature and forms a critical starting point as we build validation methods for the evolving satellite-based techniques required to monitor the changing lake ice globally. • First process-based examination of the in-situ ice-water interface roughness. • Average roughness height at the ice-water interface is 0.988 mm. • Ice-water interface roughness may be temporally related to total ice thickness. • Ice-water interface roughness is likely not spatially related to ice thickness.