A paper led by University of Alberta PhD student Emily Collier has just been accepted for open discussion in The Cryosphere Discussions:
Collier, E., Nicholson, L.I., Brock, B.W, Maussion, F., Essery, R. and Bush, A. B. G. Representing moisture fluxes and phase changes in glacier debris cover using a single-reservoir approach. The Cryosphere Discussions, 8, 1589-1629, 2014
This paper details a first attempt to include the role of moisture within supra-glacial debris cover. Below is a photo of a face dug into thin supra glacial debris overlying the Miage glacier that I took in the very hot European summer of 2003, showing that while the debris cover and the surface is dry and bright, beneath this surface the debris cover is wet, and the underlying ice is melting.
Although in this short study carried out at the Miage Glacier in Italy the treatment of moisture in the debris involves a lot of simplifications, and the impact of including this treatment of moisture in the calculating sub-debris ablation has a small affect on computed ablation at this site, the study is an interesting step forward as it (1) generates modelled latent heat fluxes at the surface associated with moisture transfer between the debris and the atmosphere above, and (2) includes the impact of moisture within the debris cover on the thermal properties of the debris.
With reference to point (1): In previous models of sub-debris ice ablation, moisture exchange at the debris surface is usually assumed to be zero, as the debris is assumed to be dry. However, that assumption poses a problem in the case of running coupled regional atmospheric models with glacier energy and mass balance models as Emily does, because assuming the debris surface to be dry effectively ‘switches off’ moisture transfer between the atmosphere and the debris covered glacier surface, and its not clear what impact this assumption can be expected to have within the coupled model system.
With reference to point (2): Data of the thermal properties of supra-glacial debris over annual timescales (Nicholson and Benn, 2013) indicated that the presence and location of moisture within the debris, whether it is ice or liquid water, and where and when it is changing phase between solid and liquid, affects the thermal properties of the supra glacial debris and therefore the efficiency with which it can transmit energy from atmosphere to the glacier ice beneath. Therefore including this in a model of sub-debris ice ablation might be important in obtaining accuracy over multi annual timescales.
There is still room for improvement and refinement, but its a step forwards.