Aerodynamic roughness length of crevassed tidewater glaciers from UAV mapping

Armin Dachauer,Richard Hann,Andrew J Hodson

doi:10.5194/tc-15-5513-2021

Abstract

Abstract. The aerodynamic roughness length (z0) is an important parameter in the bulk approach for calculating turbulent fluxes and their contribution to ice melt. However, z0 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-from-motion photogrammetry approach to build digital elevation models (DEMs). These DEMs were applied to five models (split across transect and raster methods) to estimate z0 values of the mapped area. The results point out that the range of z0 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 × 50 m), each producing one z0 value, accounts for the high spatial variability in z0 across the glacier. The z0 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 z0 values, again presenting a range of up to 1 order of magnitude each. On average, z0 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 z0 estimates of crevassed glaciers, which can be incorporated by models to improve the prediction of turbulent heat fluxes and ice melt rates.

Highlights

The aerodynamic roughness of a glacier influences the turbulent heat exchange between the glacier surface and the atmosphere (Rees and Arnold, 2006)
The digital elevation models (DEMs) obtained from the uncrewed aerial vehicles (UAVs)-based imagery and processed with the SfM-multi-view stereo (MVS) method illustrate that the crevasses of the mapped glaciers are in general aligned perpendicular to the glacier flow direction
The heavily crevassed terminus areas of the tidewater glaciers Fridtjovbreen, Heuglinbreen, Nordenskiöldbreen and Tunabreen were mapped with UAVs to build DEMs that revealed crevasse shape information

Summary

Introduction

The aerodynamic roughness of a glacier influences the turbulent heat exchange between the glacier surface and the atmosphere (Rees and Arnold, 2006). The bulk approach for the calculation of those turbulent fluxes is very popular due to its low requirements for data collection It only requires basic atmospheric measurements (e.g. wind speed, temperature), as well as the aerodynamic roughness length of the surface (Chambers et al, 2020). The aerodynamic roughness length, called z0, is a length scale that represents the height above the surface at which the wind speed drops to zero (Chappell and Heritage, 2007). It is often considered a surface characteristic and describes the loss of wind momentum that can be attributed to surface roughness (Smith, 2014)

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