Glacier ice melt, a key driver of sea level rise, depends on how the ocean currents interact with ice. The roughness and shape of the ice on scales smaller than 10 m are important and remain poorly understood due to a lack of observations. We investigate submarine ice roughness using fine-resolution multibeam sonar measurements from 13 grounded icebergs and a drone survey of a recently capsized floating iceberg in the temperate tidewater glacial fjord Xeitl Geeyi’ (LeConte Bay), Alaska. From these 14 icebergs, 55 gridded iceberg surfaces (20–40 cm resolution) were derived. We apply a spectral, scale-resolved approach to quantify iceberg roughness. Spectral analysis shows that 40 of these surfaces were dominated by vertically oriented channels with wavelengths ranging from 0.9 m to 3.7 m, likely shaped by buoyancy-driven meltwater plumes. Statistical analyses reveal a mean peak wavelength of 1.9 m, RMS height of 0.3 m, skewness of -0.3 and kurtosis of 4.3. Roughness at medium- to small-scales $\mathcal{O}$(0.5-5 m) can nearly double the ice–ocean boundary surface area and, when combined with iceberg-scale morphology $\mathcal{O}$(10 m), underscores the need to integrate realistic roughness and morphology parameters into melt models, which may improve melt predictions.

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 (z0). 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 z0 and upscale these to map z0 across an ablating mountain glacier. At plot scales, we found spatial variability in z0 estimates of over two orders of magnitude with unpredictable z0 trajectories, even when classified into ice surface types. TLS-derived surface roughness exhibited strong relationships with plot-scale SfM z0 estimates. At the glacier scale, a consistent increase in z0 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 z0 over 60 d. These measurements permit us to propose a scale-dependent temporal z0 evolution model where unpredictable variability at the plot scale gives way to more predictable changes of z0 at the glacier scale. This model provides a critical step towards deriving spatially and temporally distributed representations of z0 that are currently lacking in the parameterisation of distributed glacier surface energy balance models.