Representing moisture fluxes and phase changes in glacier debris cover using a reservoir approach

E Collier,L I Nicholson,F Maussion,A B G Bush,R Essery,B W Brock

doi:10.5194/tc-8-1429-2014

Abstract

Abstract. Due to the complexity of treating moisture in supraglacial debris, surface energy balance models to date have neglected moisture infiltration and phase changes in the debris layer. The latent heat flux (QL) is also often excluded due to the uncertainty in determining the surface vapour pressure. To quantify the importance of moisture on the surface energy and climatic mass balance (CMB) of debris-covered glaciers, we developed a simple reservoir parameterization for the debris ice and water content, as well as an estimation of the latent heat flux. The parameterization was incorporated into a CMB model adapted for debris-covered glaciers. We present the results of two point simulations, using both our new "moist" and the conventional "dry" approaches, on the Miage Glacier, Italy, during summer 2008 and fall 2011. The former year coincides with available in situ glaciological and meteorological measurements, including the first eddy-covariance measurements of the turbulent fluxes over supraglacial debris, while the latter contains two refreeze events that permit evaluation of the influence of phase changes. The simulations demonstrate a clear influence of moisture on the glacier energy and mass-balance dynamics. When water and ice are considered, heat transmission to the underlying glacier ice is lower, as the effective thermal diffusivity of the saturated debris layers is reduced by increases in both the density and the specific heat capacity of the layers. In combination with surface heat extraction by QL, subdebris ice melt is reduced by 3.1% in 2008 and by 7.0% in 2011 when moisture effects are included. However, the influence of the parameterization on the total accumulated mass balance varies seasonally. In summer 2008, mass loss due to surface vapour fluxes more than compensates for the reduction in ice melt, such that the total ablation increases by 4.0%. Conversely, in fall 2011, the modulation of basal debris temperature by debris ice results in a decrease in total ablation of 2.1%. Although the parameterization is a simplified representation of the moist physics of glacier debris, it is a novel attempt at including moisture in a numerical model of debris-covered glaciers and one that opens up additional avenues for future research.

Highlights

Numerical modelling of debris-covered glaciers has received renewed scientific interest in recent years, because their contribution to changes in ice mass and water resources in many regions remains poorly understood (e.g. Kääb et al, 2012) and because the proportion of debris-covered glacier area is rising as glaciers recede (e.g. Stokes et al, 2007; Bolch et al, 2008; Bhambri et al, 2011).It is well established that supraglacial debris exerts an important control on glacier melt rates
We explore the utility of a reservoir scheme for parameterizing moisture fluxes and phase changes in a glacier debris layer that has been incorporated into a glacier climatic mass balance model
Within the context of the simplified parameterization, we show the influence of moisture on heat transfer in the debris layer, its physical properties, and subdebris ice melt, as well as assess the scale of the impact of phase changes

Summary

Introduction

Kääb et al, 2012) and because the proportion of debris-covered glacier area is rising as glaciers recede It is well established that supraglacial debris exerts an important control on glacier melt rates. Subdebris ice melt is strongly enhanced when the debris thickness is less than a few centimetres, due to a reduction in surface albedo, an increase in absorption of shortwave radiation, and the rapid transfer of energy to the underlying ice. Melt decreases exponentially as the thickness increases, as a result of insulation of the underlying glacier ice from the overlying atmosphere. E. Collier et al.: Representing moisture in glacier debris cover The presence of debris alters the glacier surface energy balance, by permitting surface temperatures to rise above the melting point and by altering surface heat and moisture exchanges with the atmosphere (e.g. Brock et al, 2010)

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