Highly variable aerodynamic roughness length (z₀) for a hummocky debris‐covered glacier

The aerodynamic roughness length ( z 0 ) is an essential parameter in surface energy balance studies, but few literature values exist for debris‐covered glaciers. We use microtopographic and aerodynamic methods to assess the spatial variability of z 0 for Lirung Glacier, Nepal. We apply structure from motion to produce digital elevation models for three nested domains: five 1 m 2 plots, a 21,300 m 2 surface depression, and the lower 550,000 m 2 of the debris‐mantled tongue. Wind and temperature sensor towers were installed in the vicinity of the plots within the surface depression in October 2014. We calculate z 0 according to a variety of transect‐based microtopographic parameterizations for each plot, then develop a grid version of the algorithms by aggregating data from all transects. This grid approach is applied to the surface depression digital elevation model to characterize z 0 spatial variability. The algorithms reproduce the same variability among transects and plots, but z 0 estimates vary by an order of magnitude between algorithms. Across the study depression, results from different algorithms are strongly correlated. Using Monin‐Obukov similarity theory, we derive z 0 values from the meteorological data. Using different stability criteria, we derive median values of z 0 between 0.03 m and 0.05 m, but with considerable uncertainty due to the glacier's complex topography. Considering estimates from these algorithms, results suggest that z 0 varies across Lirung Glacier between ∼0.005 m (gravels) to ∼0.5 m (boulders). Future efforts should assess the importance of such variable z 0 values in a distributed energy balance model.