Geometric versus anemometric surface roughness for a shallow accumulating snowpack

Abstract

When applied to a snow-covered surface, aerodynamic roughness length, z 0 , is typically considered as a static parameter within energy balance equations. However, field observations show that z 0 changes spatially and temporally, and thus z 0 incorporated as a dynamic parameter may greatly improve models. To evaluate methods for characterizing snow surface roughness, we compared concurrent estimates of z 0 based on (1) terrestrial light detection and ranging derived surface geometry of the snowpack surface (geometric, z 0G ) and (2) vertical wind profile measurements (anemometric, z 0A ). The value of z 0G was computed from Lettau’s equation and underestimated z 0A but compared well when scaled by a factor of 2.34. The Counihan method for computing z 0G was found to be unsuitable for estimating z 0 on a snow surface. During snowpack accumulation in early winter, z 0 varied as a function of the snow-covered area (SCA). Our results show that as the SCA increases, z 0 decreases, indicating there is a topographic influence on this relation.