Let Us Change the Aerodynamic Roughness Length 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. Here, we present observations showing z0 as a dynamic variable that is a function of snow depth (ds). This has a significant impact on sublimation modeling, especially for shallow snowpacks. Terrestrial LiDAR data were collected at nine different study sites in northwest Colorado from the 2019 to 2020 winter season to measure the spatial and temporal variability of the snowpack surface. These data were used to estimate the geometric z0 from 91 site visits. 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 value of z0 was altered by human disturbance at several of the sites. The z0 versus ds correlation was almost constant, regardless of the initial roughness conditions that only affected the initial z0.