A scale-dependent model to represent changing aerodynamic roughness of ablating glacier ice based on repeat topographic surveys

• Published in: Journal of Glaciology
• Main Authors: Smith, Thomas, Smith, Mark W., Chambers, Joshua R., Sailer, Rudolf, Nicholson, Lindsey, Mertes, Jordan, Quincey, Duncan J., Carrivick, Jonathan L., Stiperski, Ivana
• Format: Article in Journal/Newspaper
• Language: English
• Published: Cambridge University Press (CUP) 2020
• Subjects: Journal of Glaciology
• Online Access: http://dx.doi.org/10.1017/jog.2020.56; https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0022143020000568

Abstract Turbulent fluxes make a substantial and growing contribution to the energy balance of ice surfaces globally, but are poorly constrained owing to challenges in estimating the aerodynamic roughness length ( z 0 ). Here, we used structure from motion (SfM) photogrammetry and terrestrial laser scanning (TLS) surveys to make plot-scale 2-D and 3-D microtopographic estimations of z 0 and upscale these to map z 0 across an ablating mountain glacier. At plot scales, we found spatial variability in z 0 estimates of over two orders of magnitude with unpredictable z 0 trajectories, even when classified into ice surface types. TLS-derived surface roughness exhibited strong relationships with plot-scale SfM z 0 estimates. At the glacier scale, a consistent increase in z 0 of ~0.1 mm d −1 was observed. Space-for-time substitution based on time since surface ice was exposed by snow melt confirmed this gradual increase in z 0 over 60 d. These measurements permit us to propose a scale-dependent temporal z 0 evolution model where unpredictable variability at the plot scale gives way to more predictable changes of z 0 at the glacier scale. This model provides a critical step towards deriving spatially and temporally distributed representations of z 0 that are currently lacking in the parameterisation of distributed glacier surface energy balance models.