Let Us Change the Aerodynamic Roughness Length Hysteretically as a Function of Snow Depth

A shallow, seasonal snowpack is rarely homogeneous in depth, layer characteristics, or surface structure throughout an entire winter. Aerodynamic roughness length (z0) is typically considered a static parameter within hydrologic and atmospheric models. However, observations have shown that z0 is a dynamic variable, necessitating accurate spatial and temporal measurements of z0. Terrestrial LiDAR data were collected at nine different study sites in northwest Colorado from the 2019-2020 winter season to observe variability of the snowpack surface. Geometric z0 and snow depth (ds) observation over 112 site visits illustrated a change in z0 as a function of ds. Values of z0 decrease during initial snow accumulation, as the snow conforms to the underlying terrain. Once the snowpack is sufficiently deep, which depends on the height of the ground surface roughness features, the surface becomes more uniform. As melt begins, z0 increases, when the snow surface becomes more irregular. The correlation varied spatially and temporally though; it was obscured by snowpack surface modifications, such as the presence of vegetation, anthropogenic or ecologic influence. The rate of change in the z0 versus ds correlation was almost constant, regardless of the initial roughness conditions that only affected the initial z0.