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The Cryosphere An interactive open-access journal of the European Geosciences Union
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© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 12 Nov 2018

Research article | 12 Nov 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal The Cryosphere (TC).

A multi-season investigation of glacier surface roughness lengths through in situ and remote observation

Noel Fitzpatrick1, Valentina Radic1, and Brian Menounos2 Noel Fitzpatrick et al.
  • 1Department of Earth, Ocean, and Atmospheric Sciences, University of British Columbia, Vancouver, V6T 1Z4, Canada
  • 2Natural Resources and Environmental Studies Institute and Geography Program, University of Northern British Columbia, Prince George, V2N 4Z9, Canada

Abstract. The roughness length values for momentum, temperature, and water vapour are key inputs to the bulk aerodynamic method for estimating turbulent heat flux. Measurements of site-specific roughness length are rare for glacier surfaces, and substantial uncertainty remains in the values and ratios commonly assumed when parameterising turbulence. Over three melt seasons, eddy covariance observations were implemented to derive the momentum and scalar roughness lengths at several locations on two mid-latitude mountain glaciers. In addition, two techniques were developed in this study for the remote estimation of momentum roughness length, utilising LiDAR-derived digital elevation models with a 1 × 1 m resolution. Seasonal mean momentum roughness length values derived from eddy covariance observations at each location ranged from 0.7–4.5 mm for ice surfaces, and 0.5–2.4 mm for snow surfaces. From one season to the next, mean momentum roughness length values over ice remained relatively consistent at a given location (0–1 mm difference between seasonal mean values), while within a season, temporal variability in momentum roughness length over melting snow was found to be substantial (> an order of magnitude). The two remote techniques were able to differentiate between ice and snow cover, and return momentum roughness lengths that were within 1–2 mm (<< an order of magnitude) of the in situ eddy covariance values. Changes in wind direction affected the magnitude of the momentum roughness length due to the anisotropic nature of features on a melting glacier surface. Persistence in downslope wind direction on the glacier surfaces, however, reduced the influence of this variability. Scalar roughness length values showed considerable variation (up to two and a half orders of magnitude) between locations and seasons, and no evidence of a constant ratio with momentum roughness length or each other. Of the tested estimation methods, the Andreas (1987) surface renewal model returned scalar roughness lengths closest to those derived from eddy covariance observations. Combining this scalar method with the remote techniques developed here for estimating momentum roughness length may facilitate the distributed parameterisation of turbulent heat flux over glacier surfaces without in situ measurements.

Noel Fitzpatrick et al.
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Noel Fitzpatrick et al.
Noel Fitzpatrick et al.
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Publications Copernicus
Short summary
Measurements of surface roughness are rare on glaciers, despite being an important control on heat exchange with the atmosphere and surface melt. In this study, roughness values were determined through measurements at multiple locations and seasons, and found to vary across glacier surfaces and to differ from commonly-assumed values in melt models. Two new methods to remotely determine roughness from digital elevation models returned good performance, and may facilitate improved melt modelling.
Measurements of surface roughness are rare on glaciers, despite being an important control on...