Geometric versus anemometric surface roughness for a shallow accumulating snowpack

When applied to a snow-covered surface, aerodynamic roughness length, z₀, is typically considered as a static parameter within energy balance equations. However, field observations show that z₀ changes spatially and temporally, and thus z₀ incorporated as a dynamic parameter may greatly improve models. To evaluate methods for characterizing snow surface roughness, we compared concurrent estimates of z₀ 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_0Gwas 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 z₀ on a snow surface. During snowpack accumulation in early winter, z₀ varied as a function of the snow-covered area (SCA). Our results show that as the SCA increases, z₀ decreases, indicating there is a topographic influence on this relation.