Abstract
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 to 4.5 mm for ice surfaces and 0.5 to 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 2.5 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.
Highlights
The turbulent fluxes of sensible and latent heat (QH and QL) can form a major component of the surface energy balance (SEB) of a glacier and substantially influence its rate of surface melt (Hock and Holmgren, 1996; Anderson et al, 2010; Fitzpatrick et al, 2017)
The second goal of this study is to develop remote methods for estimating momentum roughness lengths for a glacier surface, which would facilitate SEB modelling for glaciers without in situ observations and distributed modelling for glaciers with point measurements only
The geometric means of the roughness length values calculated from each EC data set are presented in Table 4, with separate z0v_ec values for periods with snow and ice surfaces
Summary
The turbulent fluxes of sensible and latent heat (QH and QL) can form a major component of the surface energy balance (SEB) of a glacier and substantially influence its rate of surface melt (Hock and Holmgren, 1996; Anderson et al, 2010; Fitzpatrick et al, 2017). With a lack of direct measurement on glaciers, the bulk aerodynamic method is commonly used to parameterise the turbulent fluxes, requiring input of roughness length values for momentum (z0v), temperature (z0t), and water vapour (z0q). Braun and Hock, 2004; Sicart et al, 2005) rather than obtaining site-specific measurements. This approach introduces uncertainty into turbulent flux estimation, as the transferability of roughness lengths between locations and seasons is unknown.
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