Aerodynamic roughness length of crevassed tidewater glaciers from UAV mapping

• Main Authors: Dachauer, Armin, Hann, Richard, Hodson, Andrew J.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus 2021
• Subjects: The Cryosphere; Tidewater
• Online Access: https://hdl.handle.net/20.500.11850/521613; https://doi.org/10.3929/ethz-b-000521613

The aerodynamic roughness length (z(0)) is an important parameter in the bulk approach for calculating turbulent fluxes and their contribution to ice melt. However, z(0) estimates for heavily crevassed tidewater glaciers are rare or only generalised. This study used uncrewed aerial vehicles (UAVs) to map inaccessible tidewater glacier front areas. The high-resolution images were utilised in a structure-frommotion photogrammetry approach to build digital elevation models (DEMs). These DEMs were applied to five models (split across transect and raster methods) to estimate z(0) values of the mapped area. The results point out that the range of z(0) values across a crevassed glacier is large, by up to 3 orders of magnitude. The division of the mapped area into sub-grids (50 m x 50 m), each producing one z(0) value, accounts for the high spatial variability in z(0) across the glacier. The z(0) estimates from the transect method are in general greater (up to 1 order of magnitude) than the raster method estimates. Furthermore, wind direction (values parallel to the ice flow direction are greater than perpendicular values) and the chosen sub-grid size turned out to have a large impact on the z(0) values, again presenting a range of up to 1 order of magnitude each. On average, z(0) values between 0.08 and 0.88 m for a down-glacier wind direction were found. The UAV approach proved to be an ideal tool to provide distributed z(0) estimates of crevassed glaciers, which can be incorporated by models to improve the prediction of turbulent heat fluxes and ice melt rates. ISSN:1994-0416 ISSN:1994-0424