Aerodynamic roughness of glacial ice surfaces derived from high‐resolution topographic data

This paper presents new methods of estimating the aerodynamic roughness (z0) of glacier ice directly from three‐dimensional point clouds and digital elevation models (DEMs), examines temporal variability of z0, and presents the first fully distributed map of z0 estimates across the ablation zone of an Arctic glacier. The aerodynamic roughness of glacier ice surfaces is an important component of energy balance models and meltwater runoff estimates through its influence on turbulent fluxes of latent and sensible heat. In a warming climate these fluxes are predicted to become more significant in contributing to overall melt volumes. Ice z0 is commonly estimated from measurements of ice surface microtopography, typically from topographic profiles taken perpendicular to the prevailing wind direction. Recent advances in surveying permit rapid acquisition of high‐resolution topographic data allowing revision of assumptions underlying conventional z0 measurement. Using Structure from Motion (SfM) photogrammetry with Multi‐View Stereo (MVS) to survey ice surfaces with millimeter‐scale accuracy, z0 variation over 3 orders of magnitude was observed. Different surface types demonstrated different temporal trajectories in z0 through 3 days of intense melt. A glacier‐scale 2 m resolution DEM was obtained through terrestrial laser scanning (TLS), and subgrid roughness was significantly related to plot‐scale z0. Thus, we show for the first time that glacier‐scale TLS or SfM‐MVS surveys can characterize z0 variability over a glacier surface potentially leading to distributed representations of z0 in surface energy balance models. Key Points High‐resolution topographic data permit better glacier ice aerodynamic roughness (z0) estimates Spatial z0 variability over 3 orders of magnitude with different temporal trajectories Glacier topographic roughness used to upscale z0 measurements for distributed ablation modeling