The phenology of roughness length - long term analysis from a Michigan mixed forest

The surface roughness length is used in atmospheric modeling and interpretation of observation to vertically scale the wind profile, following similarity theory. It is also needed by models to calculate the drag force that the surface applies on the atmosphere in the models surface layer, and the effective frictional velocity, that determines the surface fluxes. In forests, roughness length is largely affected by the leaf area index (LAI), the mean tree height, the vertical distribution of leaf density and the horizontal heterogeneity of canopy structure. We calculated roughness length of a mixed forest using flux data from 1999 to 2009 at the University of Michigan Biological Station AmeriFlux site. Along with phenological data of leaf-on and leaf-off timing in different species at the site, we describe the phenology of roughness length and how it relates to the seasonal dynamics of LAI. There is a non-linear change in roughness length during leaf transitional stages, which cannot be described with LAI alone, due to change in canopy structure during the leaf-on and leaf-off processes.