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

Spatial and temporal variations in aerodynamic roughness length (z(o)) 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 z(o) measurements is established through comparison with independent vertical wind profile z(o) measurements over melting snow, slush and ice. The z(o) variations are explained through correlation and regression analyses, using independent measurements of meteorological and surface variables, and parameterizations are developed to calculate z(o) variations for use in surface energy-balance melt models. Several independent variables successfully explain snow z(o) 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 z(o) variation. The z(o) following a fresh snowfall on an ice surface is parameterized based on relationships with the underlying ice z(o), snow depth and accumulated daily maximum temperatures. None of the independent variables were able to successfully explain ice z(o) variation. Although further comparative studies are needed, the results lend strong support to the microtopographic technique of measuring z(o) over melting glacier surfaces.