Employing terrestrial photogrammetry to determine surface roughness on a debris covered glacier

Abstract:

Aerodynamic surface roughness is an essential parameter in energy balance studies on glaciers. While actual measurements on bare ice glaciers are rare, a number of literature values exist for different types of ice and snow covers. There are only very few constant values suggested in the literature for debris covered glaciers and actual measurements are even scarcer. This is a significant shortcoming as the debris surface is often very heterogeneous, which results in variable turbulent fluxes. These fluxes, which use surface roughness as an input parameter, are also employed to derive debris thickness from surface temperature. The increased use of aerial and terrestrial photogrammetry on glaciers provides an opportunity to better account for this present shortcoming.

On a number of locations of Lirung Glacier in the Nepalese Himalayas we produced high resolution DEMs from terrestrial photogrammetry, from 1 x 1 m plots to a wider basin spanning more than 100 m. These images were then downsampled to different resolutions, ranging from one millimeter to a few centimeters. Employing different equations from the literature we determine surface roughness at different scales. This way we can discuss (1) the variability of results between different commonly used approaches, (2) the variability of surface roughness in space and (3) the impact of image resolution. From a tower with wind and temperature sensors at different heights we additionally infer surface roughness locally. We can then compare these values as well as see the effect of different wind speeds on the derivation of the value.

Employing a software originally developed to determine grain size distributions in river beds from optical imagery, we additionally determine rock shapes and size as well as provide an estimate for the grain size distribution of the debris cover. This could provide an initial step to a better estimation of the porous space of the debris cover, which is essential to determine energy flux from the surface to the ice below.