Measurement and parameterization of aerodynamic roughness length variations at Haut Glacier d'Arolla, Switzerland

Spatial and temporal variations in aerodynamic roughness length (z0) on Haut Glacier d'Arolla, Switzerland, during the 1993 and 1994 ablation seasons are described, based on measurements of surface microtopography. The validity of the microtopographic z0 measurements is established through comparison with independent vertical wind profile z0 measurements over melting snow, slush and ice. The z0 variations are explained through correlation and regression analyses, using independent measurements of meteorological and surface variables, and parameterizations are developed to calculate z0 variations for use in surface energy-balance melt models. Several independent variables successfully explain snow z0 variation through their correlation with increasing surface roughness, caused by ablation hollow formation, during snowmelt. Non-linear parameterizations based on either accumulated melt or accumulated daily maximum temperatures since the most recent snowfall explain over 80% of snow z0 variation. The z0 following a fresh snowfall on an ice surface is parameterized based on relationships with the underlying ice z0, snow depth and accumulated daily maximum temperatures. None of the independent variables were able to successfully explain ice z0 variation. Although further comparative studies are needed, the results lend strong support to the microtopographic technique of measuring z0 over melting glacier surfaces.