An Enhanced, Albedo Accounting Degree Day Melt Model for Distributed Application

In this paper, an enhanced degree-day melt model for the point scale is presented, in which the classical dependency on temperature is extended by considering albedo and global radiation. The standard approach has been recently improved by addition of a potential direct radiation term. Here, this approach is taken a step further by the inclusion of albedo, which controls the amount of global radiation available for conversion to melt energy at the snow or ice surface. An additive form of the degree-day model is proposed, aiming at clearly separating the two important contributions to melt energy, namely the longwave radiation and turbulent fluxes in one term and shortwave radiation in the other. The performance of the albedo enhanced degree-day model is tested against simulations obtained from a physically based energy-balance model. Hourly melt rates were calculated using the energy balance model at five sites on Haut Glacier d'Arolla, Switzerland, with data from a recent extensive field campaign. The results show that the enhanced degree-day model delivers significant improvements over other versions of the degree-day model, accounting for about 90% of the surface melt rate variation. In particular, including albedo enables the model to capture the major increases in the surface melt rate caused by metamorphism of new snow and the transition from a snow to an ice surface. Through a more physically-based representation of the surface melt process, the enhanced degree-day model offers a higher transferability than the simpler formulations. The potential for a distributed application of the enhanced degree-day model to Haut Glacier d'Arolla is discussed.